ArticlePDF Available

Materials for the Spine: Anatomy, Problems, and Solutions

Authors:

Abstract and Figures

Disc degeneration affects 12% to 35% of a given population, based on genetics, age, gender, and other environmental factors, and usually occurs in the lumbar spine due to heavier loads and more strenuous motions. Degeneration of the extracellular matrix (ECM) within reduces mechanical integrity, shock absorption, and swelling capabilities of the intervertebral disc. When severe enough, the disc can bulge and eventually herniate, leading to pressure build up on the spinal cord. This can cause immense lower back pain in individuals, leading to total medical costs exceeding $100 billion. Current treatment options include both invasive and noninvasive methods, with spinal fusion surgery and total disc replacement (TDR) being the most common invasive procedures. Although these treatments cause pain relief for the majority of patients, multiple challenges arise for each. Therefore, newer tissue engineering methods are being researched to solve the ever-growing problem. This review spans the anatomy of the spine, with an emphasis on the functions and biological aspects of the intervertebral discs, as well as the problems, associated solutions, and future research in the field.
Content may be subject to copyright.
materials
Review
Materials for the Spine: Anatomy, Problems,
and Solutions
Brody A. Frost 1, Sandra Camarero-Espinosa 2, * and E. Johan Foster 1, *
1Department of Materials Science and Engineering, Macromolecules Innovation Institute, Virginia Tech,
Blacksburg, VA 24061, USA; bfrost12@vt.edu
2Complex Tissue Regeneration Department, MERLN Institute for Technology-inspired Regenerative
Medicine, Maastricht University, P.O. Box 616, 6200MD Maastricht, The Netherlands
*Correspondence: s.camarero-espinosa@maastrichtuniversity.nl (S.C.-E.); johanf@vt.edu (E.J.F.)
Received: 29 November 2018; Accepted: 5 January 2019; Published: 14 January 2019


Abstract:
Disc degeneration affects 12% to 35% of a given population, based on genetics, age, gender,
and other environmental factors, and usually occurs in the lumbar spine due to heavier loads and
more strenuous motions. Degeneration of the extracellular matrix (ECM) within reduces mechanical
integrity, shock absorption, and swelling capabilities of the intervertebral disc. When severe enough,
the disc can bulge and eventually herniate, leading to pressure build up on the spinal cord. This can
cause immense lower back pain in individuals, leading to total medical costs exceeding $100 billion.
Current treatment options include both invasive and noninvasive methods, with spinal fusion
surgery and total disc replacement (TDR) being the most common invasive procedures. Although
these treatments cause pain relief for the majority of patients, multiple challenges arise for each.
Therefore, newer tissue engineering methods are being researched to solve the ever-growing problem.
This review spans the anatomy of the spine, with an emphasis on the functions and biological aspects
of the intervertebral discs, as well as the problems, associated solutions, and future research in
the field.
Keywords:
spinal anatomy; intervertebral disc; degenerative disc disease; herniated disc; spinal
fusion; total disc replacement; tissue engineering
1. Human Spinal Anatomy
The spine, or vertebral column, is a bony structure that houses the spinal cord and extends the
length of the back, connecting the head to the pelvis [1].
The most important function of the spine is to protect the spinal cord, which is the nerve supply for
the entire body originating in the brain [
1
]. Along with this major function, others include supporting
the mass of the body, withstanding external forces, and allowing for mobility and flexibility while
dissipating energy and protecting against impact. The spine is connected to the muscles and ligaments
of the trunk for postural control and spinal stability [
2
]. It can be separated into five distinct sections,
the cervical spine, the thoracic spine, the lumbar spine, the sacrum, and the coccyx, all of which
are comprised of independent bony vertebrae and intervertebral discs [
3
], Figure 1. To describe the
differences between the spinal column sections, each one has been further discussed.
Materials 2019,12, 253; doi:10.3390/ma12020253 www.mdpi.com/journal/materials
Materials 2019,12, 253 2 of 41
Materials 2019, 12, x FOR PEER REVIEW 2 of 40
(a) (b)
Figure 1. Overview of the vertebral column with each specific section labeled for clarification (a). The
green highlighted section refers to the part of the spine that contain individual vertebrae, as well as
intervertebral discs (IVD). The structure of the vertebrae and IVD (green highlighted) have been
added for better visualization (b) [4].
1.1. Cervical Spine
The cervical section of the spine consists of seven vertebrae (C1–C7) and six intervertebral discs,
and extends from the base of the skull to the top of the trunk, where the thoracic vertebrae and rib
cage start [3] Figure 1. The cervical spine’s major functions include supporting and cushioning loads
to the head/neck while allowing for rotation, and protecting the spinal cord extending from the brain
[5].
Of these seven vertebrae, the atlas (C1) and the axis (C2) are among the most important for
rotation and movement of the head [6]. The atlas is the only cervical vertebra that does not contain a
vertebral body, but instead has a more ring-like structure for cradling the skull at the occipital bone,
creating the atlanto-occipital joint. This joint in particular makes up for about 50% of the head’s
flexion and extension range of motion [5–7]. The axis contains a large bony protrusion (the odontoid
process) that extends from the body, superiorly, into a facet on the ring-shaped atlas, forming the
atlanto-axial joint [5,6]. This connection allows the head and atlas to rotate from side to side as one
unit, and accounts for about 50% of the neck’s rotation, as well as having the function of transferring
the weight of the head through the rest of the cervical spine [5–7]. The rest of the vertebrae (C3–C7),
have significantly reduced mobility, however are mainly used as support for the weight bearing of
the head and other loads applied onto the neck.
The cervical spine protects both the efferent and afferent nerves that stem from the spinal cord
and, if damaged, can lead to dramatic effects on the nervous system eventually affecting the patient’s
daily activity, and even causing a potential paralysis [8]. The cushioning and support of loads by the
intervertebral discs are crucial to the longevity of vertebrae, and therefore, the nerves, since they run
through the same joint separation [9]. However, because of the extensive movement that occurs in
the cervical spine, the intervertebral discs go through drastic changes in stresses and strains causing
Figure 1.
Overview of the vertebral column with each specific section labeled for clarification (
a
).
The green highlighted section refers to the part of the spine that contain individual vertebrae, as well
as intervertebral discs (IVD). The structure of the vertebrae and IVD (green highlighted) have been
added for better visualization (b) [4].
1.1. Cervical Spine
The cervical section of the spine consists of seven vertebrae (C1–C7) and six intervertebral discs,
and extends from the base of the skull to the top of the trunk, where the thoracic vertebrae and rib cage
start [
3
] Figure 1. The cervical spine’s major functions include supporting and cushioning loads to the
head/neck while allowing for rotation, and protecting the spinal cord extending from the brain [5].
Of these seven vertebrae, the atlas (C1) and the axis (C2) are among the most important for
rotation and movement of the head [
6
]. The atlas is the only cervical vertebra that does not contain a
vertebral body, but instead has a more ring-like structure for cradling the skull at the occipital bone,
creating the atlanto-occipital joint. This joint in particular makes up for about 50% of the head’s
flexion and extension range of motion [
5
7
]. The axis contains a large bony protrusion (the odontoid
process) that extends from the body, superiorly, into a facet on the ring-shaped atlas, forming the
atlanto-axial joint [
5
,
6
]. This connection allows the head and atlas to rotate from side to side as one
unit, and accounts for about 50% of the neck’s rotation, as well as having the function of transferring
the weight of the head through the rest of the cervical spine [
5
7
]. The rest of the vertebrae (C3–C7),
have significantly reduced mobility, however are mainly used as support for the weight bearing of the
head and other loads applied onto the neck.
The cervical spine protects both the efferent and afferent nerves that stem from the spinal cord
and, if damaged, can lead to dramatic effects on the nervous system eventually affecting the patient’s
daily activity, and even causing a potential paralysis [
8
]. The cushioning and support of loads by the
intervertebral discs are crucial to the longevity of vertebrae, and therefore, the nerves, since they run
through the same joint separation [
9
]. However, because of the extensive movement that occurs in
the cervical spine, the intervertebral discs go through drastic changes in stresses and strains causing
Materials 2019,12, 253 3 of 41
them to be much more susceptible to injury, which can cause damage to or impingements on these
nerves [9]. This can lead to feelings of weakness, numbness, tingling, and potentially loss of feeling.
1.2. Thoracic Spine
The thoracic section of the spine consists of twelve vertebrae (T1–T12) and twelve intervertebral
discs, and extends from the bottom of the cervical spine to the beginning of the lumbar spine [
3
],
Figure 1. The thoracic spine’s major functions include heavy load bearing and protection of the spinal
cord, supporting posture and stability throughout the trunk, and connection of the rib cage that houses
and protects vital organs, such as the heart and lungs [10].
This connection poses a significant decrease in mobility, as compared to the cervical spine
section, and a greater stability and support of the entire trunk, usually leading to fewer cases of
disc degeneration [
10
,
11
]. The vertebrae that make up the thoracic spine have body sizes (thickness,
width, and depth) that drastically increases descending from T1 to T12, corresponding to an increased
load bearing that is transferred from the vertebra above [
12
]. All other features stay relatively the same
throughout, except for the T11 and T12 vertebrae, in which no ribs are connected. Along with this
change towards the end of the thoracic spine, the T12 plays an interfacial role and has distinct thoracic
characteristics superiorly and lumbar characteristics inferiorly for articulation with the L1 vertebra,
allowing rotational movements with T11 while disallowing movements with L1 [12].
The thoracic spine contains nerves that are much less specialized per vertebrae like that of the
cervical and lumbar spine, however they are no less important. The afferent and efferent nerves
that stem from the spinal cord in this section power the muscles that lie around (major back, chest,
and abdominal muscles) and between (intercostal muscles) the ribs [
13
]. The sympathetic nervous
system, which stems from the entire thoracic spine and top two lumbar vertebrae and help power
the intercostal muscles, is necessary for vital involuntary functions such as increasing heart rate,
increasing blood pressure, controlling breathing rate, regulating body temperature, air passage dilation,
decreasing gastric secretions, bladder function (bladder muscle relaxation, and storage of urine),
and sexual function [
13
]. The thoracic spine and sacrum are the only sections of the spinal cord
that these involuntary nervous systems stem from, and if impinged, can cause similar problems as
discussed for the cervical spine. As mentioned previously, with these nerves passing through the same
proximity as the intervertebral discs, cushioning of loads and proper weight dissipation is crucial for
disc health and nerve protection, although the structural support of the ribcage makes damage to these
discs much less prevalent [11].
1.3. Lumbar Spine
The lumbar section of the spine consists of five vertebrae (L1–L5) and five intervertebral discs,
and extends from the bottom of the thoracic spine to the beginning of the sacrum, which attaches
the spine to the pelvis [
3
], Figure 1. The lumbar spine’s major functions include heavy load bearing
and protection of the spinal cord during locomotion and bending/torsion of the trunk, providing
maximum stability while maintain crucial mobility of the trunk about the hips/pelvis [14].
This particular section of the spine needs to be the most resilient due to the vital functions it
provides. Not only does it need to support all of the transferred weight from the previous spinal
sections (virtually the entire human body), but it also needs to be able to retain its mobility under
these strenuous conditions. The lumbar spine, from bending over to standing straight, can go through
more than a 50
range for the average person (
±
28.0
from 0
bend) [
15
]. As well as bending motion,
rotation becomes a big factor, with each normal lumbar segment having the ability to undergo up to
7
–7.5
of rotation [
16
]. When weight is added to these conditions, such as bending over to pick up a
backpack or a weight from the floor, an immense amount of stress and strain is induced into the lumbar
spine [
17
]. Because of this, the vertebrae and intervertebral discs in the lumbar spine are the greatest
in thickness, width, and depth [
18
]. The L1 vertebra starts out with a thickness, width, and depth
greater than any of the cervical or thoracic vertebrae, and the trend only continues as the lumbar spine
Materials 2019,12, 253 4 of 41
continues to descend to the L5 vertebra [
18
]. Although the vertebrae increase in size as the lumbar
spine descends, none of the vertebrae themselves are specialized in any way like the aforementioned
atlas and axis of the cervical spine. The L5 vertebra is not much different to the others other than in
size, but since it is the most inferior vertebra in the spine, it takes more load bearing responsibility
than any other vertebra in the spine making it a necessity to be the biggest and strongest [19,20].
The lumbar spine contains afferent and efferent nerves that are much more similar to those of the
cervical spine, in that each one that comes out of the different levels have very specialized functions,
which if damaged, can hinder an individual’s daily life and potentially leave them paralyzed from
the waist down [
21
,
22
]. These nerves control mainly the front of the lower extremities, and when
impinged can lead to loss of feeling, mobility, weakness, isolated lower back pain, and extending
leg pain [
23
]. With all of the load bearing, torsion, and bending, these nerves tend to have the most
significant chance to be impinged or damaged (roughly 95% in individuals aged 25–55 years, further
discussed in Section 3.4), compared to any other spinal section [22,24].
1.4. Sacrum
The sacrum consists of five fused vertebrae (S1–S5) that connect to the pelvis at the sacro-iliac
joint, and acts as the only skeletal connection between the trunk and the lower body [
3
]. While in
adolescence, the sacrum remains unfused, as an individual grows into adulthood, the sacrum begins to
fuse together. The fusion of the sacrum tends to begin with the lateral elements fusing around puberty,
and the vertebral bodies fusing at about 17 or 18 years of age, becoming fully fused by 23 years of
age [
3
], Figure 1. The sacrum has few active roles in the body, however one of those roles are incredibly
vital, being the bridge between the hips with the rest of the spine [25].
Although the sacrum has no intervertebral discs, it does have very important afferent and efferent
nerves that stem from the spinal cord, going through the entire lower extremity. The most important
and commonly injured of these nerves travels through the L5/S1 space, which is more commonly
known as the sciatic nerve. When this nerve is damaged or impinged it leads to pain and numbness
down the legs hindering much of an individual’s way of life [26].
1.5. Coccyx
The coccyx consists of three to five fused vertebrae depending on the individual (four is most
common) that are connected to the bottom of the sacrum, and is usually referred to as the tail bone [
3
],
Figure 1. The coccyx’s major functions include acting as an attachment site for pelvic tendons, ligaments,
and muscles, mainly those of which make up the pelvic floor, and supporting and stabilizing the body
while in a sitting position [27].
The coccyx has no intervertebral discs nor do any nerves pass through it, therefore it is insignificant
with regards to disc degeneration and disc damage.
2. Intervertebral Discs
Every vertebra in the cervical (excluding the C1 and C2 vertebrae), thoracic, and lumbar spines
is separated by intervertebral discs, each named for the two vertebrae they sit between (e.g., C6–C7,
T7–T8, and L4–L5, also sometimes denoted as L4/L5). These discs make up about 20–30% of the total
length of the spine, and have incredibly important functions including load cushioning, reducing stress
caused by impact (shock absorber), weight dispersion, allowing for movement of individual vertebrae,
and allowing for the passage of nutrients and fluid to the spine and spinal cord [
28
]. Although
each disc grants almost identical functions to the spine, based on their location, their structure and
mechanical properties change to adapt to the different loads, stresses, and strains produced [
29
].
For example, as the expected weight-bearing role of each disc increases, descending from the base of
the skull along the length of the spine, the transverse cross-sectional area of the discs also increases.
The pressure exerted on the discs however, does not increase to the same extent due to the fact that the
cross-sectional area increases in the inferior direction [29].
Materials 2019,12, 253 5 of 41
Along with the changes in the cross-sectional areas of the discs, the height (thickness) of each disc
changes throughout the spine as well. The cervical and lumbar spines have been shown to have much
thicker discs than that of the thoracic spine, most likely being adapted to the higher range of motion
expected from these sections, for both flexion-extension and torsion [
29
]. All cross-sectional areas and
thicknesses for the continuation of this review will be associated with the transverse plane and disc
height, respectively.
On a smaller scale, the three components that form the disc, the annulus fibrosus, the nucleus
pulposus, and the vertebral endplates, (further discussed in Section 2.2), change throughout the spinal
sections as well [
28
]. For example, as the discs increase in thickness, the length of reinforcing fibers
of the annulus fibrosus increase as well. This change allows for a decrease in fiber strain caused by a
given movement for thicker discs compared to thinner discs [
29
]. Although there is a general trend
between the structural and mechanical properties of the intervertebral discs and the spinal sections
they belong to, each individual disc of the same section have their differences as well.
2.1. Classification of Intervertebral Discs
2.1.1. Cervical Discs
The cervical spine consists of six intervertebral discs (C2/C3–C7/T1), with the absence of a disc
between the atlas (C1) and the axis (C2) [
3
]. These discs are smaller in cross-sectional area than any
of the other discs in the spine, due to the load bearing role of the cervical spine being much less
than that in any other section, therefore decreasing the need for load distribution [
29
]. The average
cross-sectional areas and thicknesses taken from 70 cervical discs range from 190–440 mm
2
and 3.5 to
4.5 mm, respectively, shown in Table 1[29,30].
Table 1.
Average dimensions of the intervertebral discs in the cervical, thoracic, and lumbar spine [
29
33
].
Cervical IVD
Dimensions C2/C3 C3/C4 C4/C5 C5/C6 C6/C7 C7/T1
Area (mm2)190 ±10 280 ±40 240 ±20 300 ±30 460 ±5 440 ±5
Thickness (mm)
3.51
±
0.71
3.74 ±0.36 4.07 ±0.36 4.45 ±0.21 4.11 ±0.28
4.50
±
0.53
Thoracic IVD
Dimensions T1/T2 T2/T3 T3/T4 T4/T5 T5/T6 T6/T7
Area (mm2)510 ±50 490 ±5 485 ±5 450 ±40 605 ±20 750 ±10
Thickness (mm)
4.40
±
0.65
3.50 ±0.69 3.30 ±0.50 3.20 ±0.47 3.50 ±0.47
4.10
±
0.47
Thoracic IVD
Dimensions T7/T8 T8/T9 T9/T10 T10/T11 T11/T12 T12/L1
Area (mm2)710 ±30 900 ±10 840 ±30 1080 ±20 1170 ±30 1190 ±40
Thickness (mm)
3.90
±
0.72
5.30 ±0.80 4.80 ±1.07 6.50 ±0.97 5.40 ±0.95 6.8 ±0.21
Lumbar IVD
Dimensions L1/L2 L2/L3 L3/L4 L4/L5 L5/S1
Area (mm2)1400 ±20 1640 ±50 1690 ±40 1660 ±30 1680 ±30
Thickness (mm) 7.65 ±0.57 8.90 ±0.25 9.25 ±0.29 9.90 ±0.49 9.35 ±1.06
(From References [
29
33
]). Cervical disc thicknesses were taken from 19 Chinese cadaveric humans of no specified
age or gender. Standard deviations were estimated from graphical error bars. Thoracic disc thicknesses were taken
from 15 healthy female and male cadaveric humans with average ages of 58.67
±
10.74 years and 56.20
±
11.65 years,
respectively. Lumbar disc thicknesses were taken from 607 female and 633 male human spines with age ranges from
20–92 years and 20–87 years, respectively. Standard deviations were estimated from graphical error bars. All of the
cross-sectional areas were taken from 4 full human cadaver spines of the following demographics: Male of 73 years,
female of 86 years, female of 85 years, and female of 80 years.
In adults, the maximum flexion and extension of the cervical spine occurs around the C5/C6
disc, therefore its thickness is representative of such and will be, on average, thicker than the others.
The cervical discs also show a maximum thickness in the anterior section and a minimum height in
the posterior section, giving it a natural convex curvature [
30
]. Because of the mobility of the cervical
Materials 2019,12, 253 6 of 41
spine, its discs have a significantly higher risk of damage from bending and torsion, making it the
second most common spinal section for disc injury [34].
2.1.2. Thoracic Discs
The thoracic spine consists of twelve intervertebral discs (T1/T2–T12/L1) [
3
]. These discs are
greater in cross-sectional area than the cervical discs, however are still less than that of the lumbar
discs. This is due to the amount of extra load transferred to the thoracic spine from the vertebrae above,
therefore increasing the need for greater load distribution [29]. The average cross-sectional areas and
thicknesses taken from 72 thoracic discs range from 500–1200 mm
2
and 4.4 to 6.8 mm, respectively,
shown in Table 1[31,32].
Although the thoracic discs are greater in cross-sectional area than the cervical discs, they are still
thinner in comparison. This is because the thoracic spine does not go through as much flexion/extension
and rotation as the other sections of the spine, mainly due to the attachment of the rib cage [
29
].
The majority of the thoracic discs also show a greater height in the anterior section as opposed to the
posterior section (exception of T4/T5, T5/T6, and T10/T11), like that of the cervical discs, however,
the difference is not to the same extent as the other sections of the spine [31,32].
Because of the lack of mobility throughout the thoracic spine, its discs tend to have very little
torsional stress, giving them a very low chance to become injured from degradation. However, if a
high impact is sustained in the thoracic spine, there is a possibility of disc damage, although it is much
more common for one of the vertebra to fracture before damage to the disc occurs [35].
2.1.3. Lumbar Discs
The lumbar spine consists of five intervertebral discs (L1/L2–L5/S1) [
3
]. These discs have the
greatest cross-sectional area out of all of the spinal sections, with L2/L3–L5/S1 being virtually equal.
This is because the lumbar discs need to withstand the greatest amount of load without building up
too much pressure and failing [
29
]. The average cross-sectional areas taken from roughly 1200 lumbar
discs range from 1400–1700 mm2and 7.6 to 9.4 mm, respectively, shown in Table 1[32,33].
Like the cervical spine, the lumbar spine goes through a large amount of flexion/extension and
torsion causing a high stress and strain on the discs. Due to these factors, they are the thickest discs
and they have the largest surface area [
32
]. The lumbar discs also have a high ratio of anterior disc
thickness to posterior disc thickness, the greatest being the L5/S1 disc, causing the lumbar spine’s
natural convex curvature similar to the cervical spine [
32
,
33
]. Because of the mobility of the lumbar
spine and the high loads applied to it, sometimes being in the order of thousands of newtons, its discs
have a significantly higher chance of becoming damaged from bending and torsion, making it the
most common spinal section for disc injury [36].
2.2. Intervertebral Disc Physiology
Each intervertebral disc is a complex structure comprised of three main components, a thick outer
ring of fibrous cartilage called the annulus fibrosus, a more gelatinous core called the nucleus pulposus,
and the cartilage vertebral endplates. All together, they bring structural and mechanical integrity to
the organ. These components combine to give the necessary structural and mechanical properties to
the intervertebral discs as a whole (further discussed in Sections 2.2.12.2.3) [37], Figure 2.
The intervertebral discs are the among the largest avascular tissues within the body, due to the
lack of vessel penetration throughout the internal sections. Therefore, a flow of nutrients occurs via
diffusion from the pre-disc vessels that reach into the outer most layers of the disc [
37
]. The increase in
vascularization into the inner parts of the discs are contributed to their degeneration (further discussed
in Sections 2.2.4 and 3). To better understand the functions and properties of each component, they will
be further described in detail.
Materials 2019,12, 253 7 of 41
Figure 2.
Pictured (
a
) is a cut out portion of a normal disc depicting the nucleus pulposus, vertebral
endplates, and annulus fibrosus. The chosen intervertebral disc is 4 cm wide and 7–10 mm thick [
37
].
Depicted in the lower image (
b
) is a diagram showing the detailed structure of the annulus fibrosus,
with its 15–25 lamellae comprised of 20–60 collagen fiber bundles. Also shown, is the angle
α
,
which correlates to the directionality of the fibers’ bundles in relation to the vertebrae [38].
2.2.1. Annulus Fibrosus
The annulus fibrosus is a fibrocartilaginous tissue that is structured as concentric rings, or lamellae,
surrounding the nucleus pulposus (Figure 2), and is referred to as having two main sections, the inner
and outer annulus fibrosus. Both of these sections are composed of mostly water (70–78% inner and
55–65% outer wet weight), collagens (type I and type II collagen, 25–40% inner and 60–70% outer
dry weight), proteoglycans (11–20% inner and 5–8% outer dry weight), and other minor proteins
building-up the extracellular matrix (ECM). The composition of the ECM varies gradually with
increasing radial distance from the nucleus, mainly the type of collagen (having more collagen type I
as the distance increases) and decrease of proteoglycans [
37
,
39
,
40
]. These ECM components help create
the more rigid structure of the annulus fibrosus necessary to withstand the loads and strains applied.
The annulus fibrosus accounts for a multi-layered structure with alternating collagen fiber angles
(varying in degrees throughout the lamella) that help creating a structurally stable material, housing
the nucleus pulposus, keeping it under pressure and from impinging on the spine, and enabling
the disc to withstand complex loads with its inhomogeneous, anisotropic, and nonlinear mechanical
behaviors [41].
Materials 2019,12, 253 8 of 41
(1) Composition
The annulus fibrosus is a unicellular tissue comprising of annulus fibrosus cells embedded in
an ECM composed mainly of collagen types I and II, and proteoglycans, which are responsible for
the high load-bearing properties of the tissue [
41
]. Collagens play structural roles, contributing to the
mechanical properties, tissue organization, and shape of the annulus fibrosus. Many different isoforms
of collagen exist, more than 28 of which have already been identified. It is one of the most abundant
ECM proteins in the body, and can take varying structures such as fibrils, short-helix or globular
structures [
42
]. The annulus fibrosus contains only fibril forming collagen, collagen type I and type II,
which form the fibrocartilage of the lamellae, Table 2. The collagen types I and II replace one another
in a smooth gradient, transitioning from 100% type I in the furthest outer lamella, to 100% type II in
the furthest inner lamella [
40
]. However, based on discs of different individuals, some might include
minute amounts of the opposing collagen in the inner and outer lamellae. Not only does the type of
collagen change as radial distance increases, but the concentration of collagen as well, increasing from
inner annulus to outer annulus [
40
]. This creates a smooth transition zone between the softer nucleus
pulposus and the stronger outer annulus fibrosus [43].
Table 2. Types of collagen found in lamellae of the annulus fibrosus [4244].
Collagen Type Structure Genes Alpha Chains % Collagen Distribution
Collagen I Large diameter, 67-nm
banded fibrils
COL1A1
COL1A2
α1(I)
α2(I)
Increases from 0100 from inner
to outer regions
Collagen II 67-nm banded fibrils COL2A1 α1(II)
Decreases from 100
0 from inner
to outer regions
All collagen consists of a triple helix structure comprised of three polypeptide chains [
45
].
These polypeptide chains, called alpha (
α
) chains (procollagens), further diversify the collagen family
by creating several molecular isoforms for the same collagen, as well as hybrid isoforms comprised
of two different collagen types. The size of these
α
chains can vary from 662 to 3152 amino acids
for humans, and can either be identical to form homotrimers or different to form heterotrimers [
42
].
Collagen type I is considered a heterotrimer consisting of
α
1(I) and
α
2(I), while collagen type II is
considered a homotrimer consisting of only α1(II), both of which are found in the annulus fibrosus.
After the transcription and translation of the procollagen
α
chains, four distinct stages occur
for the assembly of collagen fibrils. The first stage is transportation of the
α
chains into the rough
endoplasmic reticulum, where they are modified to form the triple-helical procollagen. The second
stage is the modification of the procollagen in the Golgi apparatus and its packaging into secretory
vesicles. The third stage is the formation of the collagen molecule in the extracellular space by cleavage
of the procollagen. The final stage is the crosslinking between the collagen molecules to stabilize
the supramolecular collagen structure [
45
], Figure 3. These collagen fibrils are vital to the structure,
strength, and flexibility of the fibrocartilage in the annulus fibrosus lamellae.
Proteoglycans are glycosylated proteins which have covalently attached highly anionic
glycosaminoglycans (GAGs). Major GAGs include heparin sulphate, chondroitin sulphate, dermatan
sulphate, hyaluronan, and keratin sulphate [
45
]. They are less abundant glycoproteins found in
the annulus fibrosus ECM, and instead of being predominantly fibrillar in structure, like collagen,
they form higher ordered brush-like ECM structures around cells. The main proteoglycans present in
the annulus fibrosus are aggrecan and versican, which promote hydration and mechanical strength
within the tissue. The keratin sulphate and chondroitin sulphate attached to their protein cores provide
the ability to aggregate to hyaluronic acid, resulting in substantial osmotic swelling pressure crucial
for the biomechanical properties of the tissue [
45
,
46
]. To clarify, their major biological function is to
bind water to provide hydration and swelling pressure to the tissue, giving it compressive resistance.
More specifically, the negative charges of the sulfated and carboxylated GAGs help trap water within
the brushes, generating large drag forces when a load is applied to the tissue, as well as creating osmotic
Materials 2019,12, 253 9 of 41
pressure for added resistance [
46
]. Inverse of the collagen, the proteoglycan concentration has an
increasing gradient from outer annulus to inner annulus, or transition zone [
40
]. Other proteoglycans
present in smaller amount on the ECM are the small leucine-rich repeat proteoglycans (SLRPs), such as
decorin and byglycan, which are implicated in fibrillary collagen assembly.
Materials 2019, 12, x FOR PEER REVIEW 9 of 40
concentration has an increasing gradient from outer annulus to inner annulus, or transition zone [40].
Other proteoglycans present in smaller amount on the ECM are the small leucine-rich repeat
proteoglycans (SLRPs), such as decorin and byglycan, which are implicated in fibrillary collagen
assembly.
Figure 3. Construction of fibrillary collagen as described above [45].
(2) Structure
The annulus fibrosus has a unique structure consisting of anywhere from 15 to 25 distinct layers
(lamellae), depending on the circumferential location, the spine level, and the individual’s age, with
the thickness of these individual lamellae varying both circumferentially and radially, increasing as
age increases [47]. Each adjacent lamella is held together by discrete collagenous bridging structures
comprised of type VI collagen, and aggrecan and versican, which are orientated radially to wrap
around individual collagen fibers and prevent severe delamination [48,49]. Based on the location of
the disc, the amount of collagen fibril bundles in each lamella can vary from 20 to 60 bundles over
the total height of the disc, with an average inter-bundle spacing of 0.22 mm and bundle thickness of
roughly 10 microns [47], Figure 2. These bundles sit at different angles ranging anywhere from 55° to
20°, alternating direction every other layer, and have a planar zig-zag (crimped) structure. This allows
them to be stretched and extend more as the crimps straighten out, resulting in the rotational and
flexion/extension mobility of the spine [48,50]. Although the components within the annulus fibrosus
are relatively the same, as previously stated, the organization of components such as microfibrils,
collagen fibers, and elastin fibers differ with respect to the outer and inner annulus fibrosus [51]. This
gives rise to different mechanical properties throughout the structure, detailed in Table 3 and Figure
4 [50,52,53].
The annulus fibrosus’ unique structure helps give it its mechanical functions of containing the
radial bulge of the nucleus, enabling a uniform distribution and transfer of compressive loads
between vertebral bodies, and to distend and rotate, allowing and facilitating joint mobility [40].
(3) Mechanical Properties
Like the collagen and proteoglycan concentration, the mechanical properties of the annulus
fibrosus differ with an increase in radial distance, usually becoming stronger and stiffer towards the
outer annulus. These mechanical properties are highly anisotropic and nonlinear in uniaxial tension,
compression, and shear, and have a high tensile modulus in the circumferential direction [52]. In
particular, the tensile properties of the lamella show drastic differences depending on the tested
samples and the orientation at which they are tested. When testing parallel to the alignment of the
Figure 3. Construction of fibrillary collagen as described above [45].
(2) Structure
The annulus fibrosus has a unique structure consisting of anywhere from 15 to 25 distinct layers
(lamellae), depending on the circumferential location, the spine level, and the individual’s age, with the
thickness of these individual lamellae varying both circumferentially and radially, increasing as age
increases [
47
]. Each adjacent lamella is held together by discrete collagenous bridging structures
comprised of type VI collagen, and aggrecan and versican, which are orientated radially to wrap
around individual collagen fibers and prevent severe delamination [
48
,
49
]. Based on the location of
the disc, the amount of collagen fibril bundles in each lamella can vary from 20 to 60 bundles over
the total height of the disc, with an average inter-bundle spacing of 0.22 mm and bundle thickness of
roughly 10 microns [
47
], Figure 2. These bundles sit at different angles ranging anywhere from 55
to
20
, alternating direction every other layer, and have a planar zig-zag (crimped) structure. This allows
them to be stretched and extend more as the crimps straighten out, resulting in the rotational and
flexion/extension mobility of the spine [
48
,
50
]. Although the components within the annulus fibrosus
are relatively the same, as previously stated, the organization of components such as microfibrils,
collagen fibers, and elastin fibers differ with respect to the outer and inner annulus fibrosus [
51
].
This gives rise to different mechanical properties throughout the structure, detailed in Table 3and
Figure 4[50,52,53].
The annulus fibrosus’ unique structure helps give it its mechanical functions of containing the
radial bulge of the nucleus, enabling a uniform distribution and transfer of compressive loads between
vertebral bodies, and to distend and rotate, allowing and facilitating joint mobility [40].
(3) Mechanical Properties
Like the collagen and proteoglycan concentration, the mechanical properties of the annulus
fibrosus differ with an increase in radial distance, usually becoming stronger and stiffer towards
the outer annulus. These mechanical properties are highly anisotropic and nonlinear in uniaxial
tension, compression, and shear, and have a high tensile modulus in the circumferential direction [
52
].
Materials 2019,12, 253 10 of 41
In particular, the tensile properties of the lamella show drastic differences depending on the tested
samples and the orientation at which they are tested. When testing parallel to the alignment of the
collagen fiber bundles as opposed to perpendicular, the strength and modulus increases due to the
strength and reinforcement given by the fibers, and the same correlation can be found when testing
the outer lamellae as opposed to the inner lamellae, Table 3[50,5255].
Table 3. Mechanical properties of the annulus fibrosus and nucleus pulposus [50,5255].
Tensile Properties of the Annulus Fibrosus
Sample Sample
Specification
Ultimate Stress,
MPa
Elastic
Modulus, MPa
Yield
Strain, %
Ultimate
Strain, %
Stiffness,
N/m
Bulk Annulus
Outer, A 3.9 ±1.8 16.4 ±7.0 20–30 * 65 ±16 5.7 ±3.4
Outer, P 8.6 ±4.3 61.8 ±23.2 20–30 * 34 ±11 5.7 ±3.4
Inner 0.9 20–30 * 33 1.2 ±1.1
Single
Lamella
Parallel 80–120 – –
Perpendicular 0.22 – – –
Compressive Properties of the Annulus Fibrosus
Section Swell Pressure, (Psw),
MPa Modulus, (HA), MPa Permeability, (k),
(×1015 m4/N-s)
Anterior 0.11 ±0.05 0.36 ±0.15 0.26 ±0.12
Posterior 0.14 ±0.06 0.40 ±0.18 0.23 ±0.09
Outer 0.11 ±0.07 0.44 ±0.21 0.25 ±0.11
Middle 0.14 ±0.04 0.42 ±0.10 0.22 ±0.06
Inner 0.12 ±0.04 0.27 ±0.11 0.27 ±0.13
Compressive Properties of the Nucleus Pulposus
Sample Swell Pressure, (Psw),
MPa Modulus, (HA), MPa Permeability, (k),
(×1016 m4/N-s)
Nucleus
Pulposus 0.138 1.0 9.0
A/P, anterior/posterior section of the annulus. Parallel/Perpendicular, alignment of testing in relation to the fiber
orientation. * Only one value was ascertained for entirety of the annulus fibrosus. Tensile properties for the bulk
annulus fibrosus were taken from 7 cadaveric human lumbar spines. Tensile properties for the single lamella
were taken from 8 male and 3 female cadaveric human lumbar spines with an average age of 57.9
±
15.4 years.
The spines were harvested within 24 h of death. Compressive properties of the annulus fibrosus were taken from
cadaveric humans of no specified age or gender. Compressive properties of the nucleus pulposus were taken from
10 IRB-approved cadaveric human lumbar spines with ages between 19–80 years (average of 57.5 years) and of no
specified gender.
Although the elastic modulus of the lamella differs by a factor of roughly 500, with respect
to fiber orientation, when tested as a whole, the tensile elastic modulus instead hovers around
18–45 MPa
[52,53]
. As the stress induced on the annulus fibrosus increases, the rigidity of the system
increases. This mechanical behavior is the result of the un-crimping of the collagen fibers that leads to
the stiffening of the intervertebral disc tissue for larger strains. Not only does the stiffness relate to
amount of strain on the annulus fibrosus, but also the load rate of the induced stress [54].
The annulus fibrosus is the only section of the disc that undergoes tensile stress, and it is usually
due to these stresses that the collagen fibrils breakdown and deteriorate, making its unique tensile
properties a focus when studying disc degeneration. However, while tensile properties are important
for the understanding of how much stress and strain the annulus fibrosus can withstand, the injuries
sustained are rarely due to a single impact, but more often the cyclic loading or wear and tear of
the spine that causes deterioration of the collagen fibrils [
50
,
53
]. Therefore, cyclic loading tests are
crucial for the understanding of the annulus fibrosus’ mechanical integrity and resiliency of the tissue.
For example, both the anterior and posterior sections of a healthy annulus fibrosus have been shown
to withstand more than 10,000 applied cycles with a stress magnitude of 45% or less of its ultimate
tensile strength [50].
Materials 2019,12, 253 11 of 41
Although not as important for the annulus fibrosus as it is for the nucleus pulposus, compressive
stresses and strains still occur on the lamellae, Table 3. However, they have very little effect on
the degradation of the annulus fibrosus. Most often only the swell pressure (P
sw
), modulus (H
A
),
and permeability (k) are characterized [55].
2.2.2. Nucleus Pulposus
The nucleus pulposus resides in the middle of the disc surrounded by the annulus fibrosus, which
keeps it from leaking into the spinal canal. It consists of randomly organized collagen type II fibers
(15–20% dry weight) and radially arranged elastin fibers, housed in a proteoglycan hydrogel (50% dry
weight), with chondrocyte-like cells interspersed at a low density of approximately 5000/mm
3
[
37
,
56
].
The nucleus is an incompressible structure that it is made up of about 80–90% water, which helps it
carry out its vital roles in the intervertebral disc of compressive load dispersion, compressive shock
absorption, and keeping the inside of the disc swollen for necessary internal pressure [57].
(1) Composition
There are four main components found in the nucleus pulposus; collagen type II fibrils and elastin
fibers (roughly 150 micrometers in length), proteoglycans, and chondrocyte-like cells. Each play a vital
role in the performance and health of the nucleus pulposus, providing it with the necessary mechanical
properties to serve its functions [
58
]. For a description of collagen and proteoglycan formation and
structure, the reader is referred to Section 2.2.1, (1).
Unlike the annulus fibrosus, the collagen in the nucleus forms a loose network, which is joined by
the network of elastin fibers. The elastin fibers are necessary for maintaining collagen organization and
recovery of the disc size and shape after the disc deforms under various loads. It accomplishes this
with its unique structure of microfibrils forming a meshwork around a central elastin core, Figure 4.
These microfibrils are structural elements of the nucleus’ ECM, and have been found distributed in
connective and elastic tissues such as blood vessels, ligament, and lung [51].
The microfibrils play vital roles in the properties of the elastic fibers, such as conferring mechanical
stability and limited elasticity to tissues, contributing to growth factor regulation, and playing a role
in tissue development and homeostasis. Microfibrils are made up of a multicomponent system,
consisting of a glycoprotein fibrillin core (three known types), microfibril associated proteins (MFAPs),
and microfibril associated glycoproteins (MAGPs). The MFAPs and MAGPs, as well as a few other
peripheral molecules, contribute to link microfibrils to elastin, to other ECM components, and to
cells [59].
In the nucleus pulposus, the chondrocyte-like cells act as metabolically active cells that
synthesize and turnover a large volume of ECM components, mainly collagen and proteoglycans [
60
].
They produce and maintain the ECM with the presence of Golgi cisternae and well-developed
endoplasmic reticulum, and are able to withstand very high compressive loads and help with the
movement of water and ions within the matrix [
61
]. They also maintain tissue homeostasis, play a
role in the physio-chemical properties of cartilage-specific macromolecules, and prevent degenerative
diseases like degenerative disc disease and osteoarthritis. However, with age these cells start to become
necrotic, increasing from about 2% at birth to 50% in most adults. This can lead to cartilage/collagen
degradation, abnormal bone growth formation on the vertebrae (osteophyte) where bone on bone
friction occurs, and stiffening of joints [58,60].
Materials 2019,12, 253 12 of 41
Materials 2019, 12, x FOR PEER REVIEW 12 of 40
Figure 4. Fluorescence microscopic images of stained components in the outer annulus fibrosus (a),
inner annulus fibrosus (b), and nucleus pulposus (c). The microfibrils in relation to cell distribution
(blue) and collagen fiber organization (red) indicates the organization of the microfibrils within the
ECM of the outer annulus fibrosus. Opposite however, the microfibrils (red) and elastin fibers (green)
in the inner annulus fibrosus do not demonstrate any organization or co-localization to any great
degree within the ECM. These two distinct characteristics of organization give rise to the varying
mechanical properties of each, Section 2.2.1, (3). The microfibrils (red) show a tendency to
hover/organize around the nucleus pulposus cells (blue), while the elastin fibers (green) have a
tendency to stay dispersed through the entire ECM [51].
(2) Structure
The nucleus pulposus is a soft, gelatinous mass that is irregularly ovoid and is found under
pressure in the center of the disc. Because it is mostly water (between 80–90%), it does not have a
definite structure or form, but like a liquid, takes the shape of wherever it is confined [62]. From birth
to adolescence, the nucleus pulposus is a semi-fluid mucoid mass formed by proliferation and
degeneration of embryological notochord cells with a few scattered chondrocytes and collagen fibers.
As age increases into adulthood, the notochord cells completely degenerate and become replaced by
chondrocyte-like cells, which deposit a specialized ECM to provide the nucleus tissue with its
structure and mechanical properties. Also with age, the nucleus becomes less fluid-like and more
cartilaginous as the collagen fibrils start to crosslink together forming fibers like the collagen type II
fibers of the annulus [63].
(a)
(b)
(c)
Figure 4.
Fluorescence microscopic images of stained components in the outer annulus fibrosus (
a
),
inner annulus fibrosus (
b
), and nucleus pulposus (
c
). The microfibrils in relation to cell distribution
(blue) and collagen fiber organization (red) indicates the organization of the microfibrils within the
ECM of the outer annulus fibrosus. Opposite however, the microfibrils (red) and elastin fibers (green) in
the inner annulus fibrosus do not demonstrate any organization or co-localization to any great degree
within the ECM. These two distinct characteristics of organization give rise to the varying mechanical
properties of each, Section 2.2.1, (3). The microfibrils (red) show a tendency to hover/organize around
the nucleus pulposus cells (blue), while the elastin fibers (green) have a tendency to stay dispersed
through the entire ECM [51].
(2) Structure
The nucleus pulposus is a soft, gelatinous mass that is irregularly ovoid and is found under
pressure in the center of the disc. Because it is mostly water (between 80–90%), it does not have a
definite structure or form, but like a liquid, takes the shape of wherever it is confined [
62
]. From birth
to adolescence, the nucleus pulposus is a semi-fluid mucoid mass formed by proliferation and
degeneration of embryological notochord cells with a few scattered chondrocytes and collagen fibers.
As age increases into adulthood, the notochord cells completely degenerate and become replaced by
chondrocyte-like cells, which deposit a specialized ECM to provide the nucleus tissue with its structure
and mechanical properties. Also with age, the nucleus becomes less fluid-like and more cartilaginous
as the collagen fibrils start to crosslink together forming fibers like the collagen type II fibers of the
annulus [63].
Materials 2019,12, 253 13 of 41
(3) Mechanical Properties
Being a virtually incompressible liquid, the nucleus pulposus does not endure any tensile stresses
or strains, and the loads it can withstand in compression are largely due to the force that the annulus
fibrosus can resist radially. The natural swell pressure of the nucleus at rest is 0.138 MPa, which is
correlated to the water uptake and retention during resting periods. However, as compressive forces
are introduced to the nucleus, the swell pressure increases to withstand the loads within the confined
space of the annulus fibrosus [
52
]. When testing for compressive properties, the nucleus is confined
so that accurate measurements can be taken, Table 3. Confining the nucleus during testing allows
for a more accurate resemblance of the resistance towards outward deformation controlled by the
annulus fibrosus, as well as keeping the nucleus from being infinitely compressed, since it is a virtually
incompressible liquid.
During everyday activities, the lumbar compressive forces can fluctuate between 800 N and
3000 N. This causes the nucleus to become pressurized up to 0.4 MPa while lying down, 1.5 MPa while
standing or sitting, and up to 2.3 MPa while actively lifting, however these stresses can vary slightly
due to the different dimensional areas of the disc [
64
]. Although the mechanical testing of the nucleus
pulposus is not quite as extensive as that of the annulus fibrosus, it does not make it any less important
to the structural and mechanical properties of the disc as a whole.
2.2.3. Vertebral Endplates
The vertebral endplates are situated on the top and bottom of each intervertebral disc, and are
comprised of hyaline cartilage [
65
]. Their main function is to function as an interface between the dense,
harder cortical bone shell of the vertebrae and the annulus and nucleus via mechanical interlocking,
and to keep the nucleus pressurized and from bulging into the soft, spongy/cancellous trabecular bone
center of the vertebrae, Figure 5. The vertebral endplates are the strongest part of the intervertebral
disc, and usually fail after the vertebral body has already fractured [38].
Materials 2019, 12, x FOR PEER REVIEW 13 of 40
(3) Mechanical Properties
Being a virtually incompressible liquid, the nucleus pulposus does not endure any tensile
stresses or strains, and the loads it can withstand in compression are largely due to the force that the
annulus fibrosus can resist radially. The natural swell pressure of the nucleus at rest is 0.138 MPa,
which is correlated to the water uptake and retention during resting periods. However, as
compressive forces are introduced to the nucleus, the swell pressure increases to withstand the loads
within the confined space of the annulus fibrosus [52]. When testing for compressive properties, the
nucleus is confined so that accurate measurements can be taken, Table 3. Confining the nucleus
during testing allows for a more accurate resemblance of the resistance towards outward deformation
controlled by the annulus fibrosus, as well as keeping the nucleus from being infinitely compressed,
since it is a virtually incompressible liquid.
During everyday activities, the lumbar compressive forces can fluctuate between 800 N and 3000
N. This causes the nucleus to become pressurized up to 0.4 MPa while lying down, 1.5 MPa while
standing or sitting, and up to 2.3 MPa while actively lifting, however these stresses can vary slightly
due to the different dimensional areas of the disc [64]. Although the mechanical testing of the nucleus
pulposus is not quite as extensive as that of the annulus fibrosus, it does not make it any less
important to the structural and mechanical properties of the disc as a whole.
2.2.3. Vertebral Endplates
The vertebral endplates are situated on the top and bottom of each intervertebral disc, and are
comprised of hyaline cartilage [65]. Their main function is to function as an interface between the
dense, harder cortical bone shell of the vertebrae and the annulus and nucleus via mechanical
interlocking, and to keep the nucleus pressurized and from bulging into the soft, spongy/cancellous
trabecular bone center of the vertebrae, Figure 5. The vertebral endplates are the strongest part of the
intervertebral disc, and usually fail after the vertebral body has already fractured [38].
Figure 5. The connection of the hyaline cartilage vertebral endplate to the perforated cortical bone of
the vertebral body and collagen fibers of the annulus and nucleus. The arrows in the figure refer to
the direction of nutrients and blood flow through the different components of the disc, mainly coming
from the bone through the vertebral endplates [37].
Figure 5.
The connection of the hyaline cartilage vertebral endplate to the perforated cortical bone of
the vertebral body and collagen fibers of the annulus and nucleus. The arrows in the figure refer to the
direction of nutrients and blood flow through the different components of the disc, mainly coming
from the bone through the vertebral endplates [37].
Materials 2019,12, 253 14 of 41
The vertebral endplates also have the unique role of acting as the main transport for nutrients
in and out of the disc. This provides the nucleus and annulus with the cells and other required
components that keep the disc alive, and from degenerating [64].
(1) Composition
The vertebral endplates are composed of an osseous and a cartilaginous component. The hyaline
cartilage within differs from the articular cartilage of the joins on its structure. While both are composed
of chondrocytes, proteoglycans and a string collagenous network, the former is not connected to
the underlying bone [
65
]. The hyaline cartilage of the vertebral endplates maintains very similar
macromolecules in their ECM as that of the nucleus pulposus, however the ratios of proteoglycan to
collagen content differs drastically. The typical ratio of glycosaminoglycan to collagen in the endplates
is roughly 2:1, providing to the tissue with higher mechanical properties than the nucleus pulposus
with a ratio of 27:1 [
66
]. Also, distinctively different from the annulus fibrosus’ fibrocartilage which
contain large collagen fiber bundles, the endplates have fine collagen fibers similar to the nucleus,
but they are closely packed together. The hyaline cartilage in the endplates are made up of multiple
types of collagen. Collagen Type II is the main collagenous component on the endplates. Collagens are
often employed as a measure of the degeneration state (hypertrophy of chondrocytes and ossification)
of the endplate, being the downregulation of collagen II and upregulation of collagen X the most
characteristic markers. [
65
]. The other collagens, Type I, III, V, VI, IX, and XI are present in small
amounts, and only contribute to a minor portion of the cartilage with the main functions of forming
and stabilizing the collagen Type II fibril network [6769].
All of the collagen structures and cellular make-up are the same for the hyaline cartilage as
previously discussed in the annulus fibrosus (Section 2.2.1, (1)).
(2) Structure
Two major structures can be distinguished in the vertebral endplates, the collagen fibers of the
cartilaginous section (roughly 0.1 to 0.2 mm thick) that connect to the annulus fibrosus and the bony
layer of the vertebral section (roughly 0.2 to 0.8 mm thick) that connect to the vertebrae. For the
cartilaginous section, the proteoglycan hydrogel-enveloped collagen fibers run horizontal and parallel
to the vertebral bodies, however the fibers then continue into the annulus fibrosus at an angle parallel
to the currently residing fibers [
37
]. The integration between the collagen fibers in the nucleus and
the endplates is more convoluted. For the vertebral section, the bony component of the endplate is
a porous layer of fused trabecular bone with osteocytes embedded within saucer-shaped lamellar
packets, resembling the structure of the vertebral cortex [64].
The most important structural features of the endplate biomechanical functions are the thickness,
porosity, and curvature. For example, thick, dense endplates with a high degree of curvature are
stronger than thin, porous, and flat endplates [
64
]. They are typically less than 1.0 mm thick, and cover
the entire surface area of the top and bottom of the intervertebral disc. The thickness across the width
of the disc is not uniform, varying considerably, while tending to be the thinnest in the central region
adjacent to the nucleus [65]. The density tends to increase towards the vertebral periphery where the
subchondral bone growth starts, however porosity can increase up to 50–130% with aging and disc
degeneration. Due to the variations throughout the structure of the vertebral endplate, its mechanical
properties vary as well [70,71].
(3) Mechanical Properties
The mechanical properties of the vertebral endplates vary with the region on which the endplate
is tested, as well as the region of the spine from which they are extracted. The central area of the
endplates tends to be the weakest, and increases in strength and stiffness radially towards the outer
annulus [
70
,
71
]. When tested in different sections of the spine, the endplates show a significant increase
in strength and stiffness from superior to inferior sections of the spine. Not only do the properties
Materials 2019,12, 253 15 of 41
change between spinal sections, but also within the same section, such as the stiffness and strength
increasing as the lumbar spine descends (L1/L2–L5/S1) [
70
]. Due to the unique structure of the
endplates, they are able to withstand high loads, outlasting the vertebral body more often than not.
The failure of the vertebral endplate tends to occur at around 10.2 kN, however the failure of the
vertebral body, usually due to fracture, occurs around 4.2 kN in individuals 60 years of age or older,
and around 7.6 kN in individuals 40 years or younger [
47
,
72
]. Not only do the endplates have great
strength, but they also possess great stiffness (1965
±
804 N/mm) that allow it to be semi-flexible
during the loads put onto the spine. This helps the nucleus move and cushion loads more readily
inside of the disc, while also protecting the endplates from tensile damage, of which they are most
likely to fail [64,73].
2.2.4. Blood Vessels and Nerve Supply
Because the intervertebral disc is one of the most avascular tissues in the human body, in a healthy
adult, it tends to have very few microvessels. However, during early stages of skeletal development,
blood and lymph vessels are present throughout the majority of the disc with the exception of the
nucleus. With maturation of the skeleton, blood and lymph vessels found within the disc start to
decrease and migrate towards the outer parts of the annulus fibrosus. These blood vessels extend
through the cartilaginous endplates into the inner and outer annulus and slightly into the nucleus up
to 12 months of age. However, as age increases past 12 months into skeletal maturity (around 20 years
of age), the blood vessels start to recede from the nucleus and inner annulus, until they only remain in
the outer annulus and endplates, Figure 6[37,74].
Given the size of the tissue, once the blood vessels retract from the disc in adulthood, the discs
rely on diffusion through the endplates and annulus for the nutritional supply of the disc cells [
75
].
This reduced nutrient supply is thought to contribute to the degeneration of the discs and to be
responsible of the lower regenerative potential of the tissue during aging, giving a reason for the low
structural and functional restoration properties of the tissue during aging [75].
The intervertebral discs are innervated organs with some of the most important nerves residing
in the cervical and lumbar spine. Recurrent sinuvertebral nerves innervate the posterior and some of
the posterolateral aspects of the disc, and the posterior longitudinal ligament, branching off the dorsal
root ganglion extending from the spinal cord. The other posterolateral aspects receive branches from
the adjacent ventral primary rami and from the grey rami communicants [
76
]. Lateral aspects of the
disc receive other branches from the rami communicantes, some of which cross the intervertebral disc
and are embedded within the surrounding connective tissue of the disc, such as the origin of the psoas
for the lumbar spine. Lastly, the anterior aspects along with the anterior longitudinal ligament are
innervated by recurrent branches of rami communicantes, Figure 7[76].
Opposite of blood vessels, in a healthy young adult, the sensory nerve endings of the disc can be
found on the superficial layers of the annulus and in the outer third of the annulus, only extending
about 3 mm into the disc [
77
]. With age and degeneration, the nerves tend to creep into the inner
parts of the disc by means of neoinnervation, arising from granulation tissue growing in the disc.
This can cause innervation of the middle and inner annulus, and potentially of the nucleus pulposus.
As innervation progresses, significant problems with regards to lower back pain can arise from the
amount of pressure being induced onto the discs, and therefore pressure onto the nerves [77,78].
Materials 2019,12, 253 16 of 41
Materials 2019, 12, x FOR PEER REVIEW 16 of 40
Figure 6. (a) Schematic representation of the multiple longer and thicker vascular channels
throughout the intervertebral disc on a 10-month old female; while (b) represents the vascular
channels throughout the disc of a 50-year old adult, showing the retraction and thinning of the
channels [37].
(a)
(b)
Figure 6.
(
a
) Schematic representation of the multiple longer and thicker vascular channels throughout
the intervertebral disc on a 10-month old female; while (
b
) represents the vascular channels throughout
the disc of a 50-year old adult, showing the retraction and thinning of the channels [37].
Materials 2019,12, 253 17 of 41
Materials 2019, 12, x FOR PEER REVIEW 17 of 40
Figure 7. The innervation of a healthy intervertebral disc, showing the sinuvertebral nerves and rami
communicantes extending into the vertebral foramen and the outer annulus of the disc [37].
Opposite of blood vessels, in a healthy young adult, the sensory nerve endings of the disc can
be found on the superficial layers of the annulus and in the outer third of the annulus, only extending
about 3 mm into the disc [77]. With age and degeneration, the nerves tend to creep into the inner
parts of the disc by means of neoinnervation, arising from granulation tissue growing in the disc.
This can cause innervation of the middle and inner annulus, and potentially of the nucleus pulposus.
As innervation progresses, significant problems with regards to lower back pain can arise from the
amount of pressure being induced onto the discs, and therefore pressure onto the nerves [77,78].
3. Spinal Degeneration and Lower Back Pain
Back pain is a major health problem in Western industrialized societies, inflicting suffering and
distress on a large number of patients, especially those of old age, increasing with the increased aged
population. The effects of this problem are vast, with a study in the year 2000 in the UK showing
prevalence rates ranging from 12% to 35%, and around 10% of sufferers becoming chronically
disabled [79]. With total costs, including direct medical costs, insurance, lost production, and
disability benefits, reaching into the billions of dollars, an enormous economic burden is placed on
society [79]. In the United States alone, costs associated with lower back pain exceeds $100 billion per
year, two-thirds resulting from lost wages and reduced productivity [80]. Among the other third are
direct costs for medical treatments of back pain diagnoses, estimated at $34 billion out of the total $47
billion for all treatments for pain diagnoses in 2010. These costs include office-based visits, hospital
outpatients, emergency services, hospital inpatients, and prescription drugs [81]. This back pain is
strongly associated with disc degeneration and injury, the majority of the time occurring in the
lumbar spine due to the increased stresses, strains, and torsion compared to other sections, and the
thoracic spine being the least affected [11].
Intervertebral discs can degenerate due to injury or due wear and tear, as a result of the stress
and strain to which the tissue is exposed to on a daily basis. However, intervertebral discs are among
the most avascular tissues in the human body and together with the low proliferative potential of
cells within, being almost quiescent, results in a tissue that is unable to adequately self-regenerate.
[82]. Multiple factors promote the degeneration of the tissue other than just wear and tear, such as
genetic predisposition, impaired metabolite transport, altered levels of enzyme activity, cell
Figure 7.
The innervation of a healthy intervertebral disc, showing the sinuvertebral nerves and rami
communicantes extending into the vertebral foramen and the outer annulus of the disc [37].
3. Spinal Degeneration and Lower Back Pain
Back pain is a major health problem in Western industrialized societies, inflicting suffering and
distress on a large number of patients, especially those of old age, increasing with the increased
aged population. The effects of this problem are vast, with a study in the year 2000 in the UK
showing prevalence rates ranging from 12% to 35%, and around 10% of sufferers becoming chronically
disabled [
79
]. With total costs, including direct medical costs, insurance, lost production, and disability
benefits, reaching into the billions of dollars, an enormous economic burden is placed on society [79].
In the United States alone, costs associated with lower back pain exceeds $100 billion per year,
two-thirds resulting from lost wages and reduced productivity [
80
]. Among the other third are
direct costs for medical treatments of back pain diagnoses, estimated at $34 billion out of the total
$47 billion for all treatments for pain diagnoses in 2010. These costs include office-based visits, hospital
outpatients, emergency services, hospital inpatients, and prescription drugs [
81
]. This back pain is
strongly associated with disc degeneration and injury, the majority of the time occurring in the lumbar
spine due to the increased stresses, strains, and torsion compared to other sections, and the thoracic
spine being the least affected [11].
Intervertebral discs can degenerate due to injury or due wear and tear, as a result of the stress
and strain to which the tissue is exposed to on a daily basis. However, intervertebral discs are among
the most avascular tissues in the human body and together with the low proliferative potential of
cells within, being almost quiescent, results in a tissue that is unable to adequately self-regenerate [
82
].
Multiple factors promote the degeneration of the tissue other than just wear and tear, such as genetic
predisposition, impaired metabolite transport, altered levels of enzyme activity, cell senescence
and death, changes in matrix macromolecules and water content, osteoarthritis, structural failure,
and neurovascular ingrowth. Although genetic inheritance is the greatest risk factor, it does not cause
discs to degenerate by itself, but instead increases their susceptibility to environmental factors such as
high and repetitive mechanical loading and smoking cigarettes [83].
Materials 2019,12, 253 18 of 41
3.1. Degenerative Disc Disease
Degenerative disc disease is defined by the degeneration of intervertebral discs due to aging
and other environmental factors, with genetic inheritance playing a significant role in the rate of
degradation. Approximately 50–70% of the variability in disc degeneration is caused by an individual’s
genetic inheritance [
83
,
84
]. The inherited genes associated with disc degeneration include those
for collagen type I and IX (COL1A1, and COL9A2 and COL9A3, respectively), aggrecan, vitamin
D receptor, matrix mettalopeptidase-3 (MMP3), and cartilage intermediate layer protein (CILP).
The strength of musculoskeletal tissue, like that of intervertebral discs, is affected by the composition
of the ECM, such as the strength of the collagen fibrils throughout the annulus fibrosus, which is
regulated by the aforementioned genes (and others) [
85
]. Although an unfavorable genetic inheritance
is present at birth, disc degeneration only becomes prevalent and common in the individual’s 40’s,
and usually only in the lower lumbar spine [
83
,
84
]. Some individuals however, can become inflicted
by this disease much earlier than the norm, depending on both the severity of their genetic deficiencies
and lifestyles.
Degeneration of intervertebral discs can occur at faster rates than for other tissues and is sometimes
presented on individuals as young as 11–16 years of age, usually found in the lumbar section [
79
].
Degenerative disc disease affects about 20% of people in their teens, showing mild signs of degeneration
before their second decade of life. However, because the discs have yet to undergo progressive
innervation, most cannot feel the pain and disabilities associated with degeneration until it propagates
through to the later years of life. Therefore, this disease increases drastically with age, causing the
discs of around 10% of 50-year-old population and 60% of 70-year-old population to become severely
degenerated, significantly hindering daily activities [79].
Degenerative disc disease can affect the tissue in many ways, causing it to undergo striking
alterations in volume, shape, structure, and composition that result on a decreased motion and an
altered biomechanical properties of the nucleus pulposus and annulus fibrosus tissues, thus altering
the mechanics of the spine [
84
]. Both the nucleus pulposus and annulus fibrosus experience changes
individually, mainly in the ECM composition and structure. Consequentially, due to the compositional
changes on the discs ECM, such as collagen, proteoglycan, and water content, the major structural
properties become hindered as well. The main structural effects tend to be the loss of swelling ability,
and therefore volume of the nucleus, and tears or fissures forming in the annulus [
86
]. When these
fissures are formed in the annulus, there is also frequently a cleft formation of some sort, particularly
in the nucleus, and the morphology becomes more and more disorganized, Figure 8. The vertebral
endplates also go through some deformation and changes, such as an increase of porosity from 50 to
130%, the natural curvature becoming less apparent and flattening out, and a significant decrease in
the thickness by roughly 20 to 50% [
64
,
71
]. These changes make the vertebral endplate much more
likely to fracture under the stresses of the spine and tensile stresses induced by the nucleus.
Along with major structural changes, many biochemical changes occur throughout the disc
as well. With age and degeneration, comes an increased incidence in these changes, including cell
proliferation and death, mucous degeneration, decrease in proteoglycan content, increase in collagen
fibril cross-linking (mainly nucleus), granular changes, and concentric tears in the annulus [
79
].
Innervation and vascularization of the disc are thought to cause the increase in cell proliferation in the
nucleus, which leads to the formation of clusters of living, necrotic, and apoptotic cells. The appearance
of these apoptotic and necrotic cells can promote cell death in the healthy living cells. Unfortunately,
these mechanisms tend to be very common with age, with more than 50% of cells in adult discs being
necrotic [79].
Materials 2019,12, 253 19 of 41
Materials 2019, 12, x FOR PEER REVIEW 19 of 40
Figure 8. A healthy, normal intervertebral disc on the left, shows a distinct difference between the
swollen, softer looking nucleus and the ringed annulus. However, during growth and skeletal
maturation, the boundary between these components becomes less obvious, and with the nucleus
generally becoming more fibrotic and less gel-like, like the highly degenerate disc on the right [79].
Along with major structural changes, many biochemical changes occur throughout the disc as
well. With age and degeneration, comes an increased incidence in these changes, including cell
proliferation and death, mucous degeneration, decrease in proteoglycan content, increase in collagen
fibril cross-linking (mainly nucleus), granular changes, and concentric tears in the annulus [79].
Innervation and vascularization of the disc are thought to cause the increase in cell proliferation in
the nucleus, which leads to the formation of clusters of living, necrotic, and apoptotic cells. The
appearance of these apoptotic and necrotic cells can promote cell death in the healthy living cells.
Unfortunately, these mechanisms tend to be very common with age, with more than 50% of cells in
adult discs being necrotic [79].
As degeneration progresses, compositional and structural changes to the discs become more and
more apparent. The status of the degeneration is commonly studied via Magnetic Resonance Imaging
(MRI) and evaluated with the Magnetic Resonance Classification System with rankings from Grade
I to Grade V [87]. The ranks are based on disc structure, signal intensity, distinction between the
nucleus and annulus, and the height of the disc, Table 4.
Table 4. Distinction between different grades of disc degeneration based on magnetic resonance
imaging (MRI) scans [87].
Grade Structure
Distinction of
Nucleus and
Annulus
Signal Intensity Height of
Intervertebral Disc
I Homogenous, bright
white Clear Hyperintense, isointense to
cerebrospinal fluid Normal
II Inhomogeneous with or
without horizontal bands Clear Hyperintense, isointense to
cerebrospinal fluid Normal
III Inhomogeneous, gray Unclear Intermediate Normal to slightly
decreased
IV Inhomogeneous, gray to
black Lost Intermediate to hypointense
Normal to
moderately
decreased
V Inhomogeneous, black Lost Hypointense Collapsed disc
space
Although the grading scale has shifted from the previous radiographic imaging systems, which
focuses on the antero-posterior abnormalities of the discs, distinguishing among bulging, protrusion,
and extrusion, (Grade I through Grade III respectively), the MRI images used for the Magnetic
Resonance Classification System still show the symptoms of all three past grades, Figure 9. It can be
Figure 8.
A healthy, normal intervertebral disc on the left, shows a distinct difference between
the swollen, softer looking nucleus and the ringed annulus. However, during growth and skeletal
maturation, the boundary between these components becomes less obvious, and with the nucleus
generally becoming more fibrotic and less gel-like, like the highly degenerate disc on the right [79].
As degeneration progresses, compositional and structural changes to the discs become more and
more apparent. The status of the degeneration is commonly studied via Magnetic Resonance Imaging
(MRI) and evaluated with the Magnetic Resonance Classification System with rankings from Grade I
to Grade V [
87
]. The ranks are based on disc structure, signal intensity, distinction between the nucleus
and annulus, and the height of the disc, Table 4.
Table 4.
Distinction between different grades of disc degeneration based on magnetic resonance
imaging (MRI) scans [87].
Grade Structure
Distinction of
Nucleus and
Annulus
Signal Intensity Height of Intervertebral
Disc
I
Homogenous, bright white
Clear Hyperintense, isointense to
cerebrospinal fluid Normal
II Inhomogeneous with or
without horizontal bands Clear Hyperintense, isointense to
cerebrospinal fluid Normal
III Inhomogeneous, gray Unclear Intermediate Normal to slightly
decreased
IV Inhomogeneous, gray to
black Lost Intermediate to hypointense Normal to moderately
decreased
V Inhomogeneous, black Lost Hypointense Collapsed disc space
Although the grading scale has shifted from the previous radiographic imaging systems,
which focuses on the antero-posterior abnormalities of the discs, distinguishing among bulging,
protrusion, and extrusion, (Grade I through Grade III respectively), the MRI images used for the
Magnetic Resonance Classification System still show the symptoms of all three past grades, Figure 9.
It can be seen that Grade II–III shows a slight bulging of the nucleus (more prominent in Grade III),
Grade IV shows the beginning stages of protrusion of the disc, and Grade V shows a fully blown-out
disc in which the entire nucleus has been extruded into the spinal canal [87].
Materials 2019,12, 253 20 of 41
Materials 2019, 12, x FOR PEER REVIEW 20 of 40
seen that Grade II–III shows a slight bulging of the nucleus (more prominent in Grade III), Grade IV
shows the beginning stages of protrusion of the disc, and Grade V shows a fully blown-out disc in
which the entire nucleus has been extruded into the spinal canal [87].
Figure 9. MRI scans showing the different grades of disc degeneration based on the Pfirrmann
grading system, (IV) referring to Grades (IV): I is representative of Grade (I) degeneration, (II) is
representative of Grade (II) degeneration, (III) is representative of Grade (III) degeneration, (IV) is
representative of Grade (IV) degeneration, and (V) is representative of Grade (V) degeneration [88].
3.2. Osteoarthritis
Although not as common of a cause for disc degeneration as degenerative disc disease,
osteoarthritis can have a significant impact on the structural changes of the intervertebral discs,
causing major problems at long term. Osteoarthritis is a degenerative disorder of the articular
cartilage affecting over 30% of the population above the age of 65 and is associated with hypertrophic
changes of the tissue affecting the facet joints and vertebrae of the spine, especially the lumbar spine
[89,90]. Many risk factors can affect the probability as well as severity of osteoarthritis including
genetic inheritance, female gender, past physical trauma, increased age, and obesity. Symptoms
usually include joint pain that increases with movement, trouble or disability with activities of daily
living, and lower back pain associated with narrowing disc space. With the current U.S. population
living longer and becoming more obese, osteoarthritis has become more common than it ever has
before, affecting an estimated 27 million adults in the U.S. [89,91].
Peripheral joints such as hips, knees, and hands, were most commonly thought of with regards
to osteoarthritis, with prevalence in the spine often being ignored. However, the prevalence of
disabilities and functional distress caused to the spine by osteoarthritis are actually quite high. In the
lumbar spine, it is a very common condition, with a prevalence range of roughly 40–85% based on
age, weight, and other factors. The spinal degeneration process has been partly linked to both
osteoarthritis and changes in facet joint structure. Osteoarthritis leads to the narrowing of disc
spacing from the formation of vertebral osteophytes introducing increased pressure to the disc. Being
Figure 9.
MRI scans showing the different grades of disc degeneration based on the Pfirrmann
grading system, (
I
V
) referring to Grades (
I
V
): I is representative of Grade (
I
) degeneration, (
II
) is
representative of Grade (
II
) degeneration, (
III
) is representative of Grade (
III
) degeneration, (
IV
) is
representative of Grade (IV) degeneration, and (V) is representative of Grade (V) degeneration [88].
3.2. Osteoarthritis
Although not as common of a cause for disc degeneration as degenerative disc disease,
osteoarthritis can have a significant impact on the structural changes of the intervertebral discs,
causing major problems at long term. Osteoarthritis is a degenerative disorder of the articular cartilage
affecting over 30% of the population above the age of 65 and is associated with hypertrophic changes
of the tissue affecting the facet joints and vertebrae of the spine, especially the lumbar spine [
89
,
90
].
Many risk factors can affect the probability as well as severity of osteoarthritis including genetic
inheritance, female gender, past physical trauma, increased age, and obesity. Symptoms usually
include joint pain that increases with movement, trouble or disability with activities of daily living,
and lower back pain associated with narrowing disc space. With the current U.S. population living
longer and becoming more obese, osteoarthritis has become more common than it ever has before,
affecting an estimated 27 million adults in the U.S. [89,91].
Peripheral joints such as hips, knees, and hands, were most commonly thought of with regards to
osteoarthritis, with prevalence in the spine often being ignored. However, the prevalence of disabilities
and functional distress caused to the spine by osteoarthritis are actually quite high. In the lumbar
spine, it is a very common condition, with a prevalence range of roughly 40–85% based on age, weight,
and other factors. The spinal degeneration process has been partly linked to both osteoarthritis and
changes in facet joint structure. Osteoarthritis leads to the narrowing of disc spacing from the formation
of vertebral osteophytes introducing increased pressure to the disc. Being comprised of the same
type of cartilage as appendicular joints, facet joints have similar pathological degenerative processes,
Materials 2019,12, 253 21 of 41
such as crystal deposition within the cartilage, degradation from high impact and torsional loads,
and joint instability, which all can cause additional stress to the discs [
91
]. Both the intervertebral
discs and facet joints play vital roles in the motion of the spine, especially in the cervical and lumbar
spines, therefore when they are heavily affected by osteoarthritis, the mobility of the spine can decrease
significantly, and pain can ensue from even the slightest of movements.
Three main components are observed with regards to osteoarthritis in spine, referred to as the
“three joint complex”. These components include the structure of vertebral osteophytes, facet joint
osteoarthritis, and disc space narrowing. With the amount of nerve supply running through all of
these spinal structures, lower back pain can be generated by any of them [
91
]. With further progression
of disc degeneration in the spine, the facet joints as well as vertebrae further degenerate, due to disc
space narrowing, which in turn puts even more stresses onto the intervertebral discs. Facet joint
osteoarthritis is a multifactorial process that is highly affected by disc degeneration, leading to greater
loads and motions endured by the joints [
92
]. This, consequently, leads to the breakdown of the layer
of hyaline cartilage between the two subchondral bones, creating friction and grinding between them,
and finally abnormal bone growth and pressure. However, facet joint osteoarthritis can still occur in
the absence of disc degeneration, in which case it causes more stress and motion on the intervertebral
disc leading to quicker degeneration [93].
Changes in the structure of the vertebral osteophytes on the shape of formation of bony
outgrowths which arise from the periosteum at the junction of the bone and cartilage, lead to disc
space narrowing, Figure 10. Although it is highly correlated to disc degeneration, like that of the
osteoarthritis in the facet joints, osteophyte formation in the vertebral column can occur without
any signs of cartilage damage, implying that with the general aging process, they may form in an
otherwise healthy joint [
91
]. In this case, the vertebral osteophytes can cause extra stresses on the
discs, mainly in the annulus fibrosus, potentially weakening it for further degeneration, damage,
and tears/fissures [91,94].
Materials 2019, 12, x FOR PEER REVIEW 21 of 40
comprised of the same type of cartilage as appendicular joints, facet joints have similar pathological
degenerative processes, such as crystal deposition within the cartilage, degradation from high impact
and torsional loads, and joint instability, which all can cause additional stress to the discs [91]. Both
the intervertebral discs and facet joints play vital roles in the motion of the spine, especially in the
cervical and lumbar spines, therefore when they are heavily affected by osteoarthritis, the mobility
of the spine can decrease significantly, and pain can ensue from even the slightest of movements.
Three main components are observed with regards to osteoarthritis in spine, referred to as the
“three joint complex”. These components include the structure of vertebral osteophytes, facet joint
osteoarthritis, and disc space narrowing. With the amount of nerve supply running through all of
these spinal structures, lower back pain can be generated by any of them [91]. With further
progression of disc degeneration in the spine, the facet joints as well as vertebrae further degenerate,
due to disc space narrowing, which in turn puts even more stresses onto the intervertebral discs.
Facet joint osteoarthritis is a multifactorial process that is highly affected by disc degeneration,
leading to greater loads and motions endured by the joints [92]. This, consequently, leads to the
breakdown of the layer of hyaline cartilage between the two subchondral bones, creating friction and
grinding between them, and finally abnormal bone growth and pressure. However, facet joint
osteoarthritis can still occur in the absence of disc degeneration, in which case it causes more stress
and motion on the intervertebral disc leading to quicker degeneration [93].
Changes in the structure of the vertebral osteophytes on the shape of formation of bony
outgrowths which arise from the periosteum at the junction of the bone and cartilage, lead to disc
space narrowing, Figure 10. Although it is highly correlated to disc degeneration, like that of the
osteoarthritis in the facet joints, osteophyte formation in the vertebral column can occur without any
signs of cartilage damage, implying that with the general aging process, they may form in an
otherwise healthy joint [91]. In this case, the vertebral osteophytes can cause extra stresses on the
discs, mainly in the annulus fibrosus, potentially weakening it for further degeneration, damage, and
tears/fissures [91,94].
Figure 10. Sagittal computerized axial tomography (CT scan) image of the cervical spine showing
large anterior osteophytes (indicted by the arrows) extending from C5 to C7, which affect the
intervertebral disc space [95].
Osteoarthritis, along with the aforementioned degenerative disc disease and mechanical loading
factors endured by the spine, can cause severe lower back pain because of the potential impingement
and injury that can happen to the spinal cord in a couple ways such as bulging discs, disc prolapse
and protrusion, and finally disc herniation/rupture and extrusion [94].
Figure 10.
Sagittal computerized axial tomography (CT scan) image of the cervical spine showing large
anterior osteophytes (indicted by the arrows) extending from C5 to C7, which affect the intervertebral
disc space [95].
Materials 2019,12, 253 22 of 41
Osteoarthritis, along with the aforementioned degenerative disc disease and mechanical loading
factors endured by the spine, can cause severe lower back pain because of the potential impingement
and injury that can happen to the spinal cord in a couple ways such as bulging discs, disc prolapse
and protrusion, and finally disc herniation/rupture and extrusion [94].
3.3. Bulging Disc
Bulging discs are considered the starting stage for problems with impingement to the spine and
are generally associated with fatigue failure from mechanical loading and disc degeneration of Grade 0
(negligible degeneration), Grade I, and Grade II [
96
]. In the early stages of disc degeneration, when the
annulus fibrosus starts to dry out and become more fibrous, the amount of mechanical strain it can take
decreases. With high compressive loads that are put onto the discs that require the nucleus to push
out causing pressure to the annulus, this can cause problems such as small tears part way through the
lamellae. When some of these lamellae tear, usually in the posterior section of the disc, the pressure
from the nucleus can make the discs bulge outwards due to the lack of support from the annulus,
Figure 11 [97].
Materials 2019, 12, x FOR PEER REVIEW 22 of 40
3.3. Bulging Disc
Bulging discs are considered the starting stage for problems with impingement to the spine and
are generally associated with fatigue failure from mechanical loading and disc degeneration of Grade
0 (negligible degeneration), Grade I, and Grade II [96]. In the early stages of disc degeneration, when
the annulus fibrosus starts to dry out and become more fibrous, the amount of mechanical strain it
can take decreases. With high compressive loads that are put onto the discs that require the nucleus
to push out causing pressure to the annulus, this can cause problems such as small tears part way
through the lamellae. When some of these lamellae tear, usually in the posterior section of the disc,
the pressure from the nucleus can make the discs bulge outwards due to the lack of support from the
annulus, Figure 11 [97].
(a)
(b)
Figure 11. MRI image showing a slight bulge of the annulus into the spinal canal without severe
impingement (a). MRI image showing a full lumbar disc herniation with substantial spinal stenosis
and nerve-root compression (b) [97].
When the disc bulges into the spinal canal, it can put pressure onto the spinal cord and other
spinal nerves, one of the most prominent being the sciatic nerve, causing pain and sometimes even
numbness [22]. Although the pain from these bulging discs is bearable, if left untreated, they can lead
to even more severe problems such as disc herniation.
Figure 11.
MRI image showing a slight bulge of the annulus into the spinal canal without severe
impingement (
a
). MRI image showing a full lumbar disc herniation with substantial spinal stenosis
and nerve-root compression (b) [97].
When the disc bulges into the spinal canal, it can put pressure onto the spinal cord and other
spinal nerves, one of the most prominent being the sciatic nerve, causing pain and sometimes even
numbness [
22
]. Although the pain from these bulging discs is bearable, if left untreated, they can lead
to even more severe problems such as disc herniation.
3.4. Disc Herniation (Prolapse/Rupture)
Disc herniation, also referred to as disc prolapse, rupture, and extrusion, occurs in later stages of
disc degeneration, Grades III–V, and is brought about by increased mechanical loading and fatigue of
Materials 2019,12, 253 23 of 41
the annulus that has typically already started to bulge [
96
]. As the annulus becomes more and more
fibrous with degeneration, there is an increase in tears through the lamellae due to the forces of the
nucleus. When the tears penetrate all the way through the annulus, the nucleus starts to push out
and leak into the spinal canal, Figure 11 [
98
]. Unlike bulging discs, because the nucleus actually leaks
into the spinal canal, it tends to have much more significant impacts on an individual’s life due to the
severe impingement on nerves of the spinal cord, causing pain, numbness, tingling, and weakness [
99
].
The most common area for disc herniation is in the lumbar spine, particularly in the lower lumbar,
with roughly 56% of herniations occurring in the L4/L5 disc and roughly 41% occurring in the L5/S1
disc [
99
]. Both of these disc herniations can play significant roles in the quality of an individual’s
life, since they both are involved with the sciatic nerve. The sciatic nerve, as mentioned in the above
anatomy, runs all the way from the lower spine down through the back of the leg. When impinged,
this can cause severe problems with motions such as standing up from a seated position, walking,
bending over, and twisting of the upper body, and can cause pain, numbness, weakness and general
discomfort throughout the entire low extremity. With disc herniation, surgery is very often required
to fix it, however with a bulging disc or other lower back pain, some other less invasive procedures
exist [100].
4. Current Treatment Techniques
Depending on the severity of disc degeneration, and whether or not a disc is bulging or herniated,
there are multiple treatment options, both invasive (surgical) and noninvasive (nonsurgical). The most
common treatments include physical therapy, epidural injections, and medications for noninvasive,
and radiofrequency ablation, spinal fusion surgery, synthetic total disc replacements, and annulus
fibrosus repair for invasive. Although pain and disability are usually relieved for a period of time,
the effectiveness of these treatments are less than ideal, due to certain problems associated with
each, further discussed below. Therefore, along with the invasive and noninvasive options, other
less-traditional treatments are being researched such as the use of stem cells, growth factors, and gene
therapy with the theoretical potential to prevent, slow, or even reverse disc degeneration, as well as
tissue engineered scaffolds in order to completely replace degenerated discs [101].
4.1. Nonsurgical Treatments
4.1.1. Physical Therapy
With disc degeneration, comes lack of support and stability of the spine due to the decreasing
biomechanical functions of the intervertebral disc. In order to regain this loss of function, the muscles
surrounding the spine and supporting spinal loads must increase in strength and stability, therefore
decreasing the need for intervertebral disc support for the spine. A solution to this problem is physical/
functional therapy, of which benefits include increased strength, flexibility, and range of motion [
102
].
Improving motion in a joint is one of the optimal ways to relieve pain. This can be accomplished by
stretching and flexibility exercises which improve mobility in the joints and muscles of the spine and
extremities. The next is increasing strength with exercises for the trunk muscles, providing greater
support for the spinal joints, and arm and leg muscles, reducing the workload required by the spinal
joints. Aerobic exercising has also been shown to relieve lower back pain by promoting a healthy
body weight and improving overall strength and mobility [
102
]. Other therapies include deep tissue
massaging, posture and movement education for daily life (functional therapy), and special treatments
such as ice, electrical stimulation, traction, and ultrasound. Ultrasound treatment, in particular,
has been shown to significantly improve lower back pain for individuals suffering from degenerative
and even prolapsed discs, although it is only a temporary solution [
103
]. Physical therapy does not
reverse the age-related disc degenerative changes, however, healing should be promoted by stimulating
cells, boosting metabolite transport, and preventing adhesions and re-injury, which in turn will relieve
pain caused by degenerative disc disease [104].
Materials 2019,12, 253 24 of 41
4.1.2. Epidural Steroid Injections
Epidural steroid injections are one of the most common injections for relief of pain, by reducing
inflammation caused by degenerative disc disease. The injections consist of cortisone, which has
anti-inflammatory properties reducing and further preventing additional inflammation, combined
with a local anesthetic, which offers immediate short-term pain relief. Both of these components help
to turn off the inflammatory chemicals produced by the body’s immune system that can lead to future
flare-ups [105]. It is injected into the epidural space that surrounds the membrane covering the spine
and nerve roots. Because it is administered so close to the area of pain, this treatment tends to have
better effects and outcomes than that of oral and topical medications, however it can only be performed
three times a year due to the negative side effects of the steroids in the body and the effects only last
1–2 months. Also, it does not reverse the changes of degenerative disc disease already caused by aging,
with over two-thirds of patients undergoing an additional invasive treatment within two years of the
epidural injections [106].
4.1.3. Medications
For low to moderate lower back pain caused by degeneration of the discs and spine, oral and
topical medications can be prescribed. These medications include over-the-counter acetaminophen
(Tylenol) and non-steroidal anti-inflammatory drugs (NSAIDs), anti-depressants, skeletal muscle
relaxants, neuropathic agents, opioids (narcotics), and prescription NSAIDs, each having individual
and unique benefits depending on the severity and type of pain [107].
The acetaminophen and NSAIDs are usually taken for very low, dull chronic pain. Acetaminophen
such as Tylenol is used to essentially block the brain’s pain receptors, while NSAIDs such as ibuprofen,
naproxen, or aspirin are used to reduce inflammation. The NSAIDs however, need to be taken on a
daily basis because they work to build up an anti-inflammatory effect in the immune system [
108
].
This means that only taking them when pain is present does not work to limit inflammation as well as
taking them regularly. Tricyclic anti-depressants are usually given for chronic lower back pain as well.
These anti-depressants work similarly to acetaminophen, blocking pain messages on their way to the
brain. They also help to increase the body’s production of endorphins, a natural painkiller, and help
individuals sleep better, allowing the body to regenerate and recover [
107
,
108
]. Skeletal muscle
relaxants, such as tizanidine and cyclobenzaprine, are needed for individuals who have acute back
pain due to muscle spasms. When their muscles spasm, they put additional stresses onto the discs
and spinal nerves causing intense pain through the spine. Neuropathic agents, such as Neurontin
and Lyrica, are used when the nerves of the spine are impinged due to a bulging or herniated discs.
These medications allow for the specific targeting of nerves to block signals sent to the brain in order
to prevent pain. Opioids (narcotics), such as Vicodin and Percocet, are used in extreme cases of spinal
pain given their addictive qualities. They work by attaching to receptors in the brain, similar to
acetaminophen, however with much higher strength and effect, tending to cause side effects such as
slow breathing, general calmness/drowsiness, and an anti-depressant effect. Prescription NSAIDs
work exactly the same as over-the-counter NSAIDs, however they tend to work better given their
increased strength and potency [107,108].
4.2. Surgical Treatments
4.2.1. Radiofrequency Ablation
Radiofrequency ablation is a technique that uses heat put through the tip of a needle, either
by continuous or pulsed radiofrequency, to denervate an injured disc causing pain to an individual.
Nerves of which can be denervated to help with low back pain are the facet nerves, sympathetic
nerves, communicating rami, and nerve branches in the disc itself. After anesthesia is administered
to the procedure site, a needle or electrode is inserted into the disc or near the small nerve branch,
under X-ray, fluoroscopy, computerized axial tomography, or magnetic resonance guidance [
109
,
110
].
Materials 2019,12, 253 25 of 41
When in the right position, the tip of the needle or electrode is heated up to the point in which it causes
damage or heat lesions to the nerves, destroying them to the point that back pain is relieved. Pain can
be relieved usually for 6 to 12 months, and in some cases can last for a few years. It is one of the less
invasive operations, and therefore is considered an outpatient surgery, in which the patient is put
under local anesthesia and can go home that day without being hospitalized [
109
,
110
]. This procedure
is usually recommended for patients who have already undergone procedures such as epidural steroid
injections, facet joint injections, sympathetic nerve blocks, or other nerve blocks with pain relief lasting
shorter than desired. The average cost of this procedure ranges anywhere from $2000 to $5000 based
on practitioner, amount of nerves destroyed, and location of spine. If, however degenerative disc
disease becomes too severe, this method will not be suitable for long term, and other surgeries or total
disc replacements will have to be considered.
4.2.2. Spinal Fusion Surgery
Spinal fusion surgery has been widely accepted as a useful treatment option for correcting severe
disc degeneration disease, however its efficacy and success remain controversial. Multiple approaches
for this procedure can be taken such as posterolateral fusion, anterior lumbar interbody fusion,
posterior lumbar interbody fusion, and lateral lumbar interbody fusion, each being a minimally
invasive technique to lumbar spinal fusion [
101
]. For this treatment, the damaged disc is completely
removed from the spine and replaced with either an osteoconductive-filled titanium cage or a
hydroxyapatite bone graft extender that sits in between the two vertebrae [
111
,
112
]. Titanium plates
are then attached to the vertebrae above and below the titanium cage, using titanium pedicle screws as
fasteners, to offer additional support to the spine after surgery, Figure 12. This allows for stability of
the spine and correct anatomic alignment of the spinal segments by sharing the loads acting on the
spine, until the point in which solid biological fusion occurs into a single bone [
113
]. This is important
because if the adjacent segment motion is altered, it can lead to further degeneration of additional
discs and motion segments [
101
]. Once this occurs, the patient can opt to have the plates and screws
removed via another surgery.
Materials 2019, 12, x FOR PEER REVIEW 25 of 40
nerves, communicating rami, and nerve branches in the disc itself. After anesthesia is administered
to the procedure site, a needle or electrode is inserted into the disc or near the small nerve branch,
under X-ray, fluoroscopy, computerized axial tomography, or magnetic resonance guidance
[109,110]. When in the right position, the tip of the needle or electrode is heated up to the point in
which it causes damage or heat lesions to the nerves, destroying them to the point that back pain is
relieved. Pain can be relieved usually for 6 to 12 months, and in some cases can last for a few years.
It is one of the less invasive operations, and therefore is considered an outpatient surgery, in which
the patient is put under local anesthesia and can go home that day without being hospitalized
[109,110]. This procedure is usually recommended for patients who have already undergone
procedures such as epidural steroid injections, facet joint injections, sympathetic nerve blocks, or
other nerve blocks with pain relief lasting shorter than desired. The average cost of this procedure
ranges anywhere from $2000 to $5000 based on practitioner, amount of nerves destroyed, and
location of spine. If, however degenerative disc disease becomes too severe, this method will not be
suitable for long term, and other surgeries or total disc replacements will have to be considered.
4.2.2. Spinal Fusion Surgery
Spinal fusion surgery has been widely accepted as a useful treatment option for correcting severe
disc degeneration disease, however its efficacy and success remain controversial. Multiple
approaches for this procedure can be taken such as posterolateral fusion, anterior lumbar interbody
fusion, posterior lumbar interbody fusion, and lateral lumbar interbody fusion, each being a
minimally invasive technique to lumbar spinal fusion [101]. For this treatment, the damaged disc is
completely removed from the spine and replaced with either an osteoconductive-filled titanium cage
or a hydroxyapatite bone graft extender that sits in between the two vertebrae [111,112]. Titanium
plates are then attached to the vertebrae above and below the titanium cage, using titanium pedicle
screws as fasteners, to offer additional support to the spine after surgery, Figure 12. This allows for
stability of the spine and correct anatomic alignment of the spinal segments by sharing the loads
acting on the spine, until the point in which solid biological fusion occurs into a single bone [113].
This is important because if the adjacent segment motion is altered, it can lead to further degeneration
of additional discs and motion segments [101]. Once this occurs, the patient can opt to have the plates
and screws removed via another surgery.
Figure 12. Example image of spinal fusion surgery using titanium cages loaded with hydroxyapatites
and pedicle screws and rods to keep stability and anatomic alignment in spinal segment [114].
Figure 12.
Example image of spinal fusion surgery using titanium cages loaded with hydroxyapatites
and pedicle screws and rods to keep stability and anatomic alignment in spinal segment [114].
Materials 2019,12, 253 26 of 41
Although spinal fusion surgery tends to alleviate discogenic pain associated with degenerative
changes, due to eliminating motion between certain vertebrae, some other problems can arise that could
potentially be more detrimental in the long run. When two vertebrae are fused together, there becomes
no load absorbing center, which severely limits shock absorption and increases loads and stresses on
surrounding tissues and discs, as well as limiting mobility [
101
,
113
]. This gives way to additional
intervertebral disc degeneration in the adjacent levels, which will then potentially need to be fused
as well. However, since the lumbar is the main contributor to the mobility of the spine, preserving
that mobility is vital to everyday activity. For this reason, most doctors refuse to fuse more than
three levels of the spine together so to not hinder the movements of everyday life and cause more
problems than leaving the damaged disc in the spine [
115
]. It is estimated that over 137,000 cervical and
162,000 lumbar spinal fusion surgeries are performed every year in the United States alone, totaling
over 325,000 fusions, each costing over $34,000 for the average hospital bill, excluding professional
fees and equipment fees [
116
,
117
]. In the last few years however, interest in total disc replacement
instead of spinal fusion surgery has grown due to their ability to retain motion of the lumbar motion
segments [116].
4.2.3. Total Disc Replacement
Total disc replacements (TDR) is a treatment option that consists of the removal of the degenerative
native disc and replacing it with a synthetic implant. This option offers the mobility that is required
for the lumbar section that spinal fusion surgery does not, however, they are still not as mainstream
as fusion surgery [
116
]. In order for a TDR to be considered effective, the implant must fulfill four
main requirements: (1) a solid, nondestructive interface with the adjacent vertebral bodies; (2) provide
mobility to mimic the range of motion of the natural disc; (3) resist wear and tear in the body to reduce
debris contamination in the body; (4) have the ability to absorb shock and distribute loads evenly and
effectively [
118
]. In all of these requirements, the lumbar spine TDR must perform at a more demanding
level than that of the cervical spine due to the extra loads it must bear. Therefore, fabrication of TDRs for
the lumbar spine have proven to be much more difficult when compared to those for the cervical spine.
Lumbar TDR can be classified according to their configuration, materials, bearing type, and regulatory
status, Table 5. The configurations of the TDR devices are designed to maximize the range of motion
within the realm of natural disc mobility and permit the most freedom. Each configuration of TDR
is dependent upon the type of modules involved in the working disc, therefore current designs are
built around a bearing for maximum mobility [
118
]. The bearing systems used includes one-piece (1P),
Metal-on-Metal (MoM), or Metal-on-Polymer (MoP), with MoM and MoP bearings using a ball and
socket design to allow for motion in all directions. Only two lumbar disc prostheses have currently
been approved for use by the Food and Drug Administration (FDA), the Charite
®
from DePuy Spine
and the Prodisc
®
L from DePuy Synthes, although many more are becoming prevalent through trial
testing such as MaverickTM, Kineflex®, Freedom®, and Mobidisc®[118].
Although there are a lot of different TDR options, each has their disadvantages, with only the
two previously mentioned even being FDA approved. Ball and socket bearing systems give way to
the possibility of hypermobility within the motion segment, greater amounts of debris from wear,
and stress concentration within bearing itself, which causes higher stresses to act on the vertebrae. It has
also been shown that these systems show no elastic shock absorption properties, even between MoM
and ultra-high molecular weight polyethylene (UHMWPE) cores (MoP) [119]. The one-piece bearing
systems were designed to potentially counteract the above flaws by adequately mimicking the natural
disc behavior; reducing the number of surfaces on which wear can occur, reducing the hypermobility
of the joint, and distributing load and absorbing shock [
118
,
119
]. The flaws with the one-piece systems,
however, are that the elastomer core used suffers greater chance of material tears either within the
material or at the adhesion interface between the different materials. They experience short fatigue
life and are still recent designs, needing further evaluation of wear and corrosion resistance [
118
].
Creep deformations and hysteresis properties of the elastomeric material may be limiting factors
Materials 2019,12, 253 27 of 41
as well [
119
]. Each TDR system experiences failure through two mechanisms of degradation of the
implant, wear and corrosion. These degradations are to be expected with articulating bearings in harsh
environments, however act more heavily on some materials as opposed to others, Table 6.
Table 5.
Summary of current total disc replacement (TDR) classification, materials, bearing type, and
regulatory status [118].
Device Classification Biomaterials Bearing Design Examples of
Manufacturer
CHARITE MoP CoCr-UHMWPE Mobile DePuy Spine
Prodisc-L MoP CoCr-UHMWPE Fixed DePuy Synthes
Activ-L MoP CoCr-UHMWPE Mobile Aesculap
Mobidisc MoP CoCr-UHMWPE Mobile LDR Medical
Baguera MoP DLC coated Ti-UHMWPE Fixed Spineart
NuBlac PoP PEEK-PEEK Fixed Pioneer
Maverick MoM CoCr-CoCr Fixed Medtronic
Kineflex MoM CoCr-CoCr Mobile SpinalMotion
Flexicore MoM CoCr-CoCr Constrained Stryker
XL-TDR MoM CoCr-CoCr Fixed NuVasive
CAdisc-L 1P PU-PC graduated modulus 1P
Rainier Technology
Freedom 1P Ti plates; silicone PU-PC core 1P Axiomed
eDisc 1P Ti plates; elastomer core 1P Theken
Physio-L 1P Ti plates; elastomer core 1P NexGen Spine
M6-L 1P Ti plates; PU-PC core with
UHMWPE fiber encapsulation 1P Spinal Kinetics
LP-ESP
(elastic spine pad) 1P Ti endplates; PU-PC coated
silicone gel with microvoids 1P FH Orthopedics
CoCr—Cobalt-chromium alloy. UHMWPE—Ultra-high molecular weight polyethylene. DLC—Diamond-like
carbon. Ti—Titanium. PEEK—Polyether ether ketone. PU-PC—Polyurethane-polycarbonate elastomer.
Table 6. Common problems of different implant materials and their effects leading to failure [118].
Bearing Type Material Problems Effects
Ball and Socket
CoCr
Reactive wear ions and
fibrous particles
Metal sensitivity reactions, Inflammation,
Osteolysis
Metallosis –
No shock absorption Compressive stresses on vertebral bodies
UHMWPE
Large wear volume and
wear debris Bone resorption, Osteolysis
Plastic deformation
Increased range of
motion (hypermobility) Facet and ligament loading
No shock absorption Compressive stresses on vertebral bodies
PEEK Prosthesis migration Biomechanical incompatibility, Stress on
remaining annulus, Total rejection of device
Endplate reaction Severe biological rejection
1P PUPC More studies necessary
Note: The effects stated are correlated to the problems directly next to it.
When using MoM devices, the degradation due to wear is minimal when compared to MoP
devices and PEEK-on-PEEK devices (PoP), however the toxicity introduced to the body is relatively the
same. Although the volume of wear particles might be smaller, the CoCr wear particles are chemically
reactive within the body causing corrosion, tribocorrosion, and toxic and biological responses, such as
metallosis, biological reactions, osteolysis, and inflammation. When MoP devices wear, the particles
produced tend to be fine, needle and fiber-shaped particles which are less chemically reactive than
Materials 2019,12, 253 28 of 41
the metal particles although bigger in size. The PoP devices shows properties of resisting expulsion
of nucleus particles, and superior fatigue resistance and wear resistance, however severe biological
reactions occur causing device rejection and migration of device into surrounding muscle tissue [
118
].
Each of these systems have their benefits and disadvantages when compared to each other, however
when compared to spinal fusion surgery, shows great advantages in the range of mobility. If a disc has
undergone some degeneration, but is not yet to the point of spinal fusion or total disc replacement,
other actions can be taken such as annulus fibrosus repair.
4.2.4. Repair of Annulus Fibrosus
The annulus fibrosus is involved in almost any pathological condition of the degenerating spine,
therefore when its function becomes impaired, plays a fundamental role in two specific clinical
situations. It acts as the main source of discogenic low back pain, and as the origin of disc herniation
due to its insufficiency caused by degenerative disc disease. As previously discussed, when small
fissures occur in the annulus, a repair process takes place in which granulation tissue is formed along
with neovascularization and concomitant ingrowth of nerve fibers. This causes chronic discogenic
pain throughout the disc due to the pressure being sustained by the nerves. Annulus fibrosus repair
is the procedure to fix those tears before the disc herniates, and is usually performed in relatively
young patients with very minor degenerative changes. Efficient annulus repair could significantly limit
the need for future surgeries in certain cases in which there is potential of disc herniation, however
no herniation has currently occurred. When the ruptures are treated, the focus is on improving
cell-biomaterial interaction, using an initial implant to provide immediate closure of the tear and
maintain mechanical properties of the disc, while the cellular component starts the regenerative
process within the disc. This process, however, is not complete or satisfactory when it comes to
being a permanent solution, but instead is a preventive measure for disc prolapse [
120
]. The most
straight-forward solution is suturing the annulus tear shut, helping give the disc a stronger tendency
to heal itself. However, its sole purpose is the containment of the nucleus pulposus and does not
compensate for the loss of the annulus nor reverse the biomechanical changes [
121
]. One way to adjust
for the lack of compensation could be the addition of growth factors in order to enhance the regenerative
process of the annulus tissue [
122
]. Vadala et al. studied the potential of Transforming Growth Factor-
β
(TGF-
β
) loaded microfibrous poly(L-lactide) scaffold
in vitro
. The biological evaluation of the scaffolds
was performed using bovine annulus fibrosus cells that were cultured on the scaffold for up to three
weeks [
122
]. These electrospun scaffolds allowed for the closure of the defect site while releasing the
TGF-
β
, inducing an anabolic stimulus on the annulus cells, mimicking the ECM environment of the
tissue [
122
]. The scaffolds, together with the TGF-
β
release, promoted rapid cell growth compared to
the control, resulting in the deposition of significantly greater amounts of GAGs and total collagen
within the annulus tissue, as well as a higher neo-ECM thickness [
122
]. Another method studied by
Cruz et al., focuses on the repair of annulus fibrosus defects through a cell-seeded adhesive biomaterial,
further detailed in Section 5[123].
Annulus fibrosus repair gives great advantages to those who have yet to have a full disc herniation,
as well as those only experiencing minimal degeneration, giving them the opportunity to forgo the
potential chance for surgery in the future. It should be noted however, that this treatment option is not
a cure for degenerative disc disease, but a preventative measure taken to increase the longevity of the
native disc, potentially permanently, depending on an individual’s particular life style.
Materials 2019,12, 253 29 of 41
5. Tissue Engineering and Regeneration Strategies
With all of these options facing difficult challenges, tissue engineering and regenerative strategies
stand out as potential solutions. These include some form of gene therapy, regeneration strategies
via delivery of bioactive molecules, e.g., growth factors, or a material scaffold with or without cells.
Gene therapy and regeneration with growth factors, cells, or enzymes, such as ADAMTS5, have been
researched in rats for early stage trials, showing greater GAGs and total collagen deposition for the
TGF-
β
1 treated animals, and successful suppression of the degradation of the nucleus pulposus for
ADAMTS5 treated ones [
122
,
124
]. Along with these, a similar approach was studied by Guterl et al. to
target caspase 3 in rabbit models, in order to disrupt the execution of apoptosis [
125
]. A direct injection
of Alexa Fluor 555-caspase 3 small interfering RNA (siRNA) into the rabbit intervertebral disc was
used to determine the effect on suppression of degenerative changes within the disc. Compared to the
caspase 3 siRNA control, the Alexa Fluor 555-caspase 3 siRNA resulted in a significant decrease in
serum-starved apoptotic cells, as well as a significant suppression of the degenerative changes to the
disc [125].
In more current regeneration efforts, FDA-cleared Phase III adult stem cells were used in a
test study to treat chronic lower back pain associated with degenerative disc disease. The use of
mesenchymal precursor cells directly injected into the lumbar disc will hopefully show some ability
to regenerate lost tissue of the disc [
126
]. Another system studied by Alini et al. uses implanted
intervertebral disc cells in a scaffold of collagen and hyaluronan, or entrapped into a chitosan gel,
with either fetal calf serum (FCS) or growth factors (TGF-
β
1, bFGF, and IGF-1) to modulate ECM
synthesis [124]. The FCS and TGF-β1 were able to induce proteoglycan synthesis, while the presence
of bFGF and IGF-1 reduced proteoglycan synthesis. However, the IGF-1 was shown to stimulate cell
division by the greatest extent [
124
]. By day 20 of the culture, in FCS and the varying growth factors,
not only did the matrices contain aggrecan, but also other small leucine-rich repeat proteoglycan found
in the normal disc and both collagen type I and II [
124
]. Although all proteoglycans found in a normal
disc were synthesized, the construct was not able to retain the majority of its proteoglycans, resulting
in the inability to withstand the compressive loads normally subjected to an intervertebral disc [
124
].
Another future direction is to look more into the regeneration and repair of the annulus tissues as
opposed to the nucleus tissues. Efforts for novel therapies have mainly been directed towards nucleus
tissue regeneration and replacement, however a main challenge is the development of strategies
and techniques that deal with the degenerated annulus, preferably in a combined approach with the
nucleus [
121
]. A recent study performed by Cruz et al. shows the possibility to help repair damaged
annulus fibrosus tissue through a cell-seeded adhesive biomaterial [
123
]. Multiple genipin-crosslinked
fibrin adhesive cell carriers were developed with varying genipin to fibrin ratios, to determine the
optimal composition for mimicking natural annulus fibrosus tissue. Among the adhesive cell carrier,
were encapsulated bovine annulus fibrosus cells to show the feasibility of cell delivery to the injured
tissue [
123
]. The cell-seeded adhesive demonstrated shear and compressive properties matching
those of the annulus fibrosus tissue, while significantly improving failure strength in situ. As well,
the adhesive showed increased cell viability and GAGs production [
123
]. These efforts could propel
the future of annulus repair, offering successful preventative methods, as opposed to perpetuating the
need for herniation surgeries and issues associated with them.
In the last few years, tissue engineered scaffolds for total intervertebral disc replacement have
risen to the forefront of current biomaterial literature and research in order to address the challenges
mentioned above [
123
,
127
144
]. Scaffolds have been fabricated using both natural and synthetic
materials, as well as most containing embedded cells for natural tissue growth and integration [
127
].
Two recent studies by Iu et al. and Yang et al. focus on total intervertebral disc replacement using a
hierarchically organized annulus fibrosus and a hydrogel-like nucleus pulposus, however differ with
respect to materials, procedures, and objectives [
129
,
130
]. Iu et al. focuses on an
in vitro
generated
intervertebral disc with the ability of tissue integration between the fabricated annulus and nucleus to
better mimic the natural disc [
129
]. The annulus fibrosus was created using six lamellae comprised
Materials 2019,12, 253 30 of 41
of aligned nanofibrous polycarbonate urethane scaffolds cultured with annulus fibrosus cells in a
Dulbecco’s modified Eagle’s medium (DMEM) with 20% fetal bovine serum (FBS) for three weeks to
produce an integrated type I collagen-rich ECM [
129
]. This surrounded the nucleus pulposus tissue
comprised of a type II collagen- and aggrecan-rich ECM hydrogel which was cultured with nucleus
pulposus cells in DMEM with 20% FBS solution for four weeks [
129
]. Both tissues were then combined
and co-cultured to create the full intervertebral disc model with integration between the tissues [
129
].
It was shown that this system successfully integrated the annulus fibrosus lamellae not only to the
nucleus tissue, but also to each other, allowing for interlamellar connectivity. When biologically and
mechanically tested both
in vitro
and
in vivo
, the tissue engineered intervertebral disc showed no
inflammatory reaction and was able to stand up to the interlamellar and annulus-nucleus interface
shear forces experienced by the disc in the spine [
129
]. The
in vitro
studies demonstrated the possibility
to create an intervertebral disc with mechanically stable tissue integration that was able to grow similar
ECMs as the natural tissue. The
in vivo
studies demonstrated the ability of the engineered nucleus
pulposus to form tissue
in vivo
, as well as test the disc’s ability to develop intradiscal swelling pressure
under load [129]. However, this experiment was evaluated using a bovine caudal spine rather than a
human spine, resulting in a smaller intervertebral disc model, and would therefore need to pursue
further research to evaluate the scalability and suitability of the system for human biological disc
replacement [
129
]. Yang et al., on the other hand, focused on creating a total intervertebral disc
replacement that can integrate natural tissue
in vivo
, and demonstrates excellent hydrophilicity and
functional performance [
130
]. The hydrophilicity property of this scaffold is highly important for
not only the swelling properties of the disc for mechanical stability, but also diffusion of nutrients
through the disc for cell viability. The annulus fibrosus mimicry was fabricated using electrospun
polycaprolactone/poly(D,L-lactide-co-glycolide)/collagen type I nanofibers to create a hierarchically
organized, concentric ring-aligned structure, and the nucleus pulposus mimicry was fabricated
using an alginate hydrogel [
130
]. Both components were cultured for 3 days in a DMEM/F12, 10%
FBS, and 1% penicillin-streptomycin with a seeded cell density of 2500 cells/cm, and tested for
biocompatibility and mechanical integrity before being implanted into rat caudal spine models [
130
].
In vivo
, the replacement discs demonstrated excellent hydrophilicity, mimicking the highly hydrated
native tissue, as well as shape maintenance, integration with surrounding natural tissue, acceptable
mechanical support, and flexibility [
130
]. This study shows the potential of scaffold materials as
intervertebral disc tissue engineering and regeneration platforms
in vivo
. However, like the previous
study, the fabricated disc was smaller than that of a human, since it was synthesized for a rat model,
resulting in the need for much lower mechanical properties for the materials. Therefore, further
research would be needed to scale this approach to human trials [130].
Bhunia et al. studied a method for correcting degenerative disc disease that lies between
annulus fibrosus repair and total intervertebral disc replacement [
131
]. Bhunia et al. focused
on the recapitulation of form and function of the intervertebral disc through a silk protein-based
multilayered, disc-like angle-ply annulus fibrosus scaffold comprising of multiple concentric lamellae.
The scaffold was fabricated to resemble the hierarchical structure of the natural tissue, which was
verified through electron microscopy [
131
]. These “biodiscs” demonstrated mechanical properties
similar to those of the native tissue, as well as support of human mesenchymal stem cell proliferation
and differentiation, and deposition of a sufficient amount of ECM after 14 days of culture. A section of
the biodisc was implanted subcutaneously in a mice model and retrieved after one and four weeks
of implantation, showing negligible immune response [
131
]. However, the proposed system lacked
the ability to replace the entirety of the intervertebral disc, leading to the need for further research
with the addition of an implantable nucleus. A recent study by Ghorbani et al. shows a promising
method for nucleus pulposus replacement utilizing an injectable hydrogel [
132
]. The hydrogel was
comprised of chitosan-
β
-glycerophosphate-hyaluronic acid, chondroitin-6-sulfate, type II collagen,
gelatin, and fibroin silk, in order to replicate the complexity of the natural nucleus pulposus ECM.
The synthesized nucleus demonstrated ideal hydrophilicity, stability, and strength when subjected
Materials 2019,12, 253 31 of 41
to loads, with the storage modulus remaining nearly constant over a wide range of strain.
In vitro
tests were conducted using MTT and trypan blue to quantify and qualify cell growth and cytotoxicity,
revealing the hydrogel to be cytocompatible with good cell attachment and growth [
132
]. Like the
study performed by Bhunia et al., the solution only focuses on nucleus replacement/regeneration,
therefore further research is needed with the combination of a tissue engineered annulus fibrosus.
As pointed out earlier, many of these studies use relatively weak electrospun scaffolds or
combinations thereof with hydrogels that lead to mechanical properties that are on the range of
rat native IVD but that are far from recapitulating those of human IVD. Recent studies have focused
their attention on the development of sophisticated scaffolds and materials that can better mimic
the outstanding mechanical properties of human IVD (Table 7). Novel materials and composites
on the form of hydrogels have been investigated for the replacement of the nucleus pulposus.
These include interpenetrating networks based on dextran, gelatin and poly (ethylene glycol) [
133
];
cross-linked collagen-II, aggrecan and hyaluronan [
134
]; and silk-fibrin and hyaluronic acid composite
hydrogels [
135
], among others. For the annulus fibrosus scaffolds on the shape of fibrous matts or polymer
films are generally preferred, mimicking the structure of the native tissue. To this end, novel materials
have also been investigated such as nanocellulose reinforced gellan-gum hydrogels [
136
]; electrospun
aligned polyurethane scaffolds or poly(trimethylene carbonate) structures prepared by lithography and
covered with a polyester urethane membrane [
137
], among others [
138
]. However, these studies report
on the fabrication of individual tissues rather than the recapitulation of the whole organ, what makes
difficult the extrapolation of these results to a more complete and practicable approach.
Hu et al. recently reported on the fabrication of 3D printed scaffolds based on the combination of
poly (lactic acid) (PLA) and gellan gum-poly (ethylene glycol) diacrylate (GG-PEGDA) double network
hydrogel [
139
]. This combination allowed fine tuning of the mechanical properties of the overall organ
by changing the infill patterns and the density of the PLA framework. Initial studies with in-situ
bioprinted human mesenchymal stem cells (hMSCs) show good cell viability and spreading within
the constructs. Although this first study shows an interesting 3D printing approach, the final scaffold
represents a rather homogeneous construct and not the clearly compartmented native organ.
Yang et al.
overcame this issue by designing and fabricating a triphasic scaffolds that aimed at recapitulating
the three main structures of native IVD, the nucleus pulposus and the inner and outer rings of the
annulus fibrosus, while targeting the mechanical properties of human IVD [
140
]. The authors used a
chitosan hydrogel to mimic the inner nucleus pulposus that was then surrounded by a poly(butylene
succinate-co-terephthalate) (PBST) fiber film and a poly(ether ether ketone) (PEEK) ring to mimic
the inner and outer annulus fibrosus, respectively. This multi-layered structure was seeded with
porcine IVD cells and used on an
in vivo
porcine spine model. After 4 and 8 weeks of implantation
the cell-scaffold construct retained its original height and showed a histological gross appearance that
resembled that of the native tissue. Moreover, the compressive Young’s modulus of the construct
was 58.4
±
12.9 MPa, similar to that measured for the native tissue (71.5
±
18.2 MPa) [
140
]. Under a
similar concept, Choy et al. studied the potential of biphasic scaffolds for full IVD tissue engineering.
They developed a collagen and GAG hydrogel core that was encapsulated on a multiple lamella of
photochemically cross-linked collagen membranes, mimicking the nucleus pulposus and the annulus
fibrosus, respectively [
141
]. These constructs were capable of recovering up to 87% their original size
after compression and showed a dynamic mechanical stiffness similar of that of the native rabbit IVD.
Although this studied showed great promise in terms of mechanical properties and shape recovery of
the constructs, a detailed biological study is still missing [141].
Materials 2019,12, 253 32 of 41
Other studies, such as those by Hudson et al., have focused more on the adaptation of tissue
engineered intervertebral discs when exposed to certain environments and conditions [
142
,
143
]. In both
studies, the intervertebral disc was fabricated by floating an injection molded alginate hydrogel
nucleus pulposus in a collagen type I annulus fibrosus that contracted around the nucleus given
ample time. This study showed that hypoxic expansion of human mesenchymal stem cells enhances
the maturation of the tissue engineered disc, as opposed to normoxic environments [
142
]. Hypoxic
conditions, which correlated to 1 to 5% oxygen content, resulted in an increase in ECM production,
as well as driving chondrogenesis of the embedded stem cells, when compared to normoxic conditions
(21% oxygen). Also, the hypoxic discs were stiffened up to 141%, and showed an increase in GAGs
and collagen content within the nucleus, compared to normoxic [
142
]. The results obtained in this
study show the benefit of hypoxic maturation of stem cells within the tissue engineered disc before
implantation, however, to fully grasp the effectiveness of this scaffold,
in vivo
tests will need to be
performed. Another study focused on the potential of dynamic unconfined compressive loading on
the tissue regeneration/deposition rate [
143
]. Each tissue engineered disc was subjected to mechanical
stimulation from a strain amplitude range of 1–10% for two weeks with a cycle of one hour on,
one hour off
, one hour on. The discs were then evaluated for biochemical and mechanical properties,
which showed an increase in GAGs and hydroxyproline content, and equilibrium and instantaneous
modulus for both the nucleus and annulus [
143
]. These results suggest that dynamic loading increases
the functionality of the tissue engineered disc, with each section experiencing region dependent
responses, which could be used to expedite maturation for implantation. Although promising, further
research would need to be performed
in vivo
as well as on a larger scale models, bearing a closer
resemblance to the natural intervertebral disc [143].
Altogether, these studies show the need of further development and study of materials and
scaffolds fabrication techniques for the regeneration of full IVD. The outstanding mechanical properties
and complexity of the multi-phasic structure of the native organ will require the development of also
complex systems that can recapitulate these features.
Materials 2019,12, 253 33 of 41
Table 7. Materials, scaffold architecture, mechanical properties, and cell types used in tissue engineering approaches for IVD.
Targeted
Tissue Material Structure Mechanical Properties Cells Comments Reference
Total IVD AF: Poly caprolactone urethane
NP: Collagen II and aggrecan
AF: Nanofibrous, aligned
NP: Hydrogel
Compressive modulus of
17.2 ±7.5 kPa AF and NP cells
Integration between the two
compartments. Tested in vitro and
in vivo in a bovine model
[129]
Total IVD
AF: Polycaprolactone/poly(D,L-lactide-co-
glycolide)/collagen type I
NP: Alginate
AF: Electrospun nanofibers to
create a concentric
ring-structure
NP: Hydrogel
Tensile Young’s modulus
of 380 MPa Rat AF and NP cells
Integration with host tissue and
between compartment in in vivo
rat caudal spine model
[130]
AF Silk
Concentric layers of lamella
sheets on an angle-ply
construct
499.18 ±86.45 kPa Porcine AF cells and
human MSCs
Subcutaneous implantation in rat
showed negligible immune
response
[131]
NP
chitosan-
β
-glycerophosphate-hyaluronic acid,
chondroitin-6-sulfate, type II collagen, gelatin,
and fibroin silk
Hydrogel 50 Pa Rabbit NP cells Preliminary study with in vitro
cell compatibility assays [132]
IVD PLA and GG_PEGDA 3D printed Compressive Young’s
modulus of 400 MPa hMSCs Preliminary study on cell viability [133]
IVD NP: Chitosan; inner AF: PBST and outer AF:
PEEK
NP: hydrogel and AF fiber film
and ring
Compressive Young’s
modulus of 58.4 ±
12.9 MPa
Porcine IVD cells In vivo implantation on a porcine
spine model [134]
NP Dextran, gelatin and poly (ethylene glycol); Hydrogel
Compressive Young’s
modulus of 15.86 ±
1.7 kPa
Porcine NP cells In vivo
subcutaneous implantation
in Lewis rats [135]
NP Cross-linked collagen-II, aggrecan and
hyaluronan Hydrogel Storage modulus of
1.25 kPa Bovine NP cells 7 days in vitro cell culture studies [136]
NP Silk-fibrin and hyaluronic acid
composite hydrogels Hydrogel Compressive modulus of
5–7 kPa
Human primary
chondrocytes
Full in vitro study with up to 4
weeks cell culture [137]
AF Nanocellulose reinforced
gellan-gum hydrogels Hydrogel Compressive modulus of
45–55 kPa Bovine AF cells Preliminary in vitro studies [138]
AF Electrospun aligned polyurethane scaffolds Fibrous scaffold N/A Rabbit AF derived
progenitor cells 7 days in vitro cultures [139]
AF poly(trimethylene carbonate) and
polyester urethane Fibrous scaffold Yield strength of 4.9 ±
1.4 MPa Human MSCs
In vitro bovine caudal spine organ
culture model with or without
dynamic load.
[140]
NP: nucleus pulposus and AF: annulus fibrosus.
Materials 2019,12, 253 34 of 41
6. Conclusions
Although great strides have been made in the field of degenerative disc disease, there is still
a lot more progress to be made, given the challenges faced with every treatment option currently
available. In early stages of degenerative disc disease, noninvasive treatments or treatments such as
radiofrequency ablation and annulus fibrosus repair can be of great help, however they only mitigate
the symptoms instead of the actual cause. Noninvasive treatments face the challenges of only dealing
with some of the symptoms of the pain rather than dealing with the actual degeneration of the discs,
therefore allowing the discs to continue to degrade to the future point of needing invasive treatments.
Radiofrequency, although good for reducing pain, has the challenge of only lasting short term, a few
months to a year in most cases. Also, it is an expensive procedure that has to be repeated every six
months [
109
]. Annulus repair seems to be a better option for young adults with degeneration to the
point just before herniation to significantly reduce the need for future surgery, but faces challenges
of, again, only fixing the symptoms of the main problem as well as not being to mend any biological
changes/losses within the annulus [
120
]. When disc degeneration gets even worse, greater procedures
need to take place, such as spinal fusion surgery and TDR. Spinal fusion surgery is, as of today,
the most common life-long solution to severe disc degeneration, however it is struck with multiple
challenges such as significantly limiting mobility and adding additional stresses to the adjacent motion
segments potentially causing greater degeneration in other intervertebral discs [
113
]. TDR has been
shown to help retain the mobility that spinal fusion cannot, but can sometimes lead to hypermobility of
the joint, can wear and corrode causing a biological reaction in the body, and more often than not, does
not distribute load nor absorb shock, but rather transfers it directly into the adjacent vertebrae [
118
].
These challenges have led to vast research in the field of tissue engineering for disc degeneration.
Even though scaffolds for disc regeneration are taking strides in the right direction, many still remain in
a premature state. Each have their own benefits, but also complications, including scalability, tunability,
tissue integration, or optimal mechanical properties. Therefore, the gap between the translation of
this research to the clinic still remains fairly large with many hurdles to overcome, leading to the
need for future research [
127
,
144
]. With degenerative disc disease posing such a large problem for
individuals and society, and no current ideal treatment options that come without complications,
there is a welcoming for future research in the field of tissue engineered biomaterials for the solution
of total intervertebral disc replacement [128144].
Author Contributions:
The following contributions for this review are associated with the authors listed:
Writing-Original Draft Preparation, B.F.; Writing-Review & Editing, S.C.E. and E.J.F.
Funding: This research received no external funding.
Acknowledgments:
The authors would like to thank Priya Venkatraman for her comments and input on
this review.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
Vertebral Column. In Encyclopaedia Britannica; Britannica, T.E.o.E. (Ed.) Encyclopaedia Britannica, Inc.:
Chicago, IL, USA, 2014.
2.
Kibler, W.B.; Press, J.; Sciascia, A. The role of core stability in athletic function. Sports Med.
2006
,36, 189–198.
[CrossRef] [PubMed]
3.
Agur, A.M.R.; Dalley, A.F. Grant’s Atlas of Anatomy, 12th ed.; Lipincott Williams & Wilkins: Pennsylvania, PA,
USA, 2009; p. 841.
4.
Vertebral Column. Available online: https://www.kenhub.com/en/start/c/vertebral- column (accessed on
14 April 2016).
5.
Bogduk, N.; Mercer, S. Biomechanics of the cervical spine. I: Normal kinematics. Clin. Biomech.
2000
,15,
633–648. [CrossRef]
Materials 2019,12, 253 35 of 41
6.
Swartz, E.E.; Floyd, R.T.; Cendoma, M. Cervical spine functional anatomy and the biomechanics of injury
due to compressive loading. J. Athl. Train. 2005,40, 155–161. [PubMed]
7.
Panjabi, M.M.; Crisco, J.J.; Vasavada, A.; Oda, T.; Cholewicki, J.; Nibu, K.; Shin, E. Mechanical properties
of the human cervical spine as shown by three-dimensional load-displacement curves. Spine
2001
,26,
2692–2700. [CrossRef]
8.
Caridi, J.M.; Pumberger, M.; Hughes, A.P. Cervical radiculopathy: A review. HSS J.
2011
,7, 265–272.
[CrossRef] [PubMed]
9.
Yeung, J.T.; Johnson, J.I.; Karim, A.S. Cervical disc herniation presenting with neck pain and contralateral
symptoms: A case report. J. Med. Case Rep. 2012,6, 166. [CrossRef] [PubMed]
10.
Edmondston, S.J.; Singer, K.P. Thoracic spine: Anatomical and biomechanical considerations for manual
therapy. Man. Ther. 1997,2, 132–143. [CrossRef]
11.
Son, E.S.; Lee, S.H.; Park, S.Y.; Kim, K.T.; Kang, C.H.; Cho, S.W. Surgical treatment of t1-2 disc herniation
with t1 radiculopathy: A case report with review of the literature. Asian Spine J.
2012
,6, 199–202. [CrossRef]
12.
Goh, S.; Tan, C.; Price, R.I.; Edmondston, S.J.; Song, S.; Davis, S.; Singer, K.P. Influence of age and gender
on thoracic vertebral body shape and disc degeneration: An MR investigation of 169 cases. J. Anat.
2000
,
197 Pt 4, 647–657. [CrossRef]
13.
Cervero, F.; Tattersall, J.E. Somatic and visceral sensory integration in the thoracic spinal cord. Prog. Brain
Res. 1986,67, 189–205.
14.
Boszczyk, B.M.; Boszczyk, A.A.; Putz, R. Comparative and functional anatomy of the mammalian lumbar
spine. Anat. Rec. 2001,264, 157–168. [CrossRef]
15.
Troup, J.D.; Hood, C.A.; Chapman, A.E. Measurements of the sagittal mobility of the lumbar spine and hips.
Ann. Phys. Med. 1968,9, 308–321. [CrossRef]
16.
Haughton, V.M.; Rogers, B.; Meyerand, M.E.; Resnick, D.K. Measuring the axial rotation of lumbar vertebrae
in vivo with MR imaging. Am. J. Neuroradiol. 2002,23, 1110–1116.
17.
Granhed, H.; Jonson, R.; Hansson, T. The loads on the lumbar spine during extreme weight lifting. Spine
1987,12, 146–149. [CrossRef]
18.
Tan, S.H.; Teo, E.C.; Chua, H.C. Quantitative three-dimensional anatomy of cervical, thoracic and lumbar
vertebrae of Chinese Singaporeans. Eur. Spine J. 2004,13, 137–146. [CrossRef]
19.
Crawford, R.P.; Cann, C.E.; Keaveny, T.M. Finite element models predict
in vitro
vertebral body compressive
strength better than quantitative computed tomography. Bone 2003,33, 744–750. [CrossRef]
20.
Shah, J.S.; Hampson, W.G.; Jayson, M.I. The distribution of surface strain in the cadaveric lumbar spine.
J. Bone Jt. Surg. 1978,60, 246–251. [CrossRef]
21. Bogduk, N. The innervation of the lumbar spine. Spine 1983,8, 286–293. [CrossRef]
22.
Luoma, K.; Riihimaki, H.; Luukkonen, R.; Raininko, R.; Viikari-Juntura, E.; Lamminen, A. Low back pain in
relation to lumbar disc degeneration. Spine 2000,25, 487–492. [CrossRef]
23.
Nygaard, O.P.; Mellgren, S.I. The function of sensory nerve fibers in lumbar radiculopathy—Use of
quantitative sensory testing in the exploration of different populations of nerve fibers and dermatomes.
Spine 1998,23, 348–352. [CrossRef]
24.
Takahashi, I.; Kikuchi, S.; Sato, K.; Sato, N. Mechanical load of the lumbar spine during forward bending
motion of the trunk—A biomechanical study. Spine 2006,31, 18–23. [CrossRef]
25.
Bogduk, N. Clinical and Radiological Anatomy of the Lumbar Spine, 5th ed.; Elsevier Health Sciences:
Philadelphia, PA, USA, 2012.
26.
Koes, B.W.; van Tulder, M.W.; Peul, W.C. Diagnosis and treatment of sciatica. BMJ
2007
,334, 1313–1317.
[CrossRef]
27.
Lirette, L.S.; Chaiban, G.; Tolba, R.; Eissa, H. Coccydynia: An overview of the anatomy, etiology, and
treatment of coccyx pain. Ochsner J. 2014,14, 84–87.
28.
Humzah, M.D.; Soames, R.W. Human intervertebral disc: Structure and function. Anat. Rec.
1988
,220,
337–356. [CrossRef]
29.
Pooni, J.S.; Hukins, D.W.; Harris, P.F.; Hilton, R.C.; Davies, K.E. Comparison of the structure of human
intervertebral discs in the cervical, thoracic and lumbar regions of the spine. Surg. Radiol. Anat.
1986
,8,
175–182. [CrossRef]
Materials 2019,12, 253 36 of 41
30.
Mahendra, K.A.; Rajani, J.A.; Shailendra, J.S.; Narsinh, H.G. Morphometric study of the cervical intervertebral
disc. Int. J. Anat. Phys. Biochem. 2015,2, 22–26.
31.
Kunkel, M.E.; Herkommer, A.; Reinehr, M.; Bockers, T.M.; Wilke, H.J. Morphometric analysis of the
relationships between intervertebral disc and vertebral body heights: An anatomical and radiographic
study of the human thoracic spine. J. Anat. 2011,219, 375–387. [CrossRef]
32. Shao, Z.; Rompe, G.; Schiltenwolf, M. Radiographic changes in the lumbar intervertebral discs and lumbar
vertebrae with age. Spine 2002,27, 263–268. [CrossRef]
33.
Twomey, L.; Taylor, J. Age changes in lumbar intervertebral discs. Acta Orthop. Scand.
1985
,56, 496–499.
[CrossRef]
34.
Davis, H. Increasing Rates of Cervical and Lumbar Spine Surgery in the United-States, 1979–1990. Spine
1994,19, 1117–1124. [CrossRef]
35.
Williams, M.P.; Cherryman, G.R.; Husband, J.E. Significance of thoracic disc herniation demonstrated by MR
imaging. J. Comput. Assist. Tomogr. 1989,13, 211–214. [CrossRef] [PubMed]
36.
Adams, M.A.; Roughley, P.J. What is intervertebral disc degeneration, and what causes it? Spine
2006
,31,
2151–2161. [CrossRef] [PubMed]
37.
Raj, P.P. Intervertebral disc: Anatomy-physiology-pathophysiology-treatment. Pain Pract.
2008
,8, 18–44.
[CrossRef] [PubMed]
38.
Adams, M.A. Intervertebral Disc Tissues. In Mechanical Properties of Aging Soft Tissues; Derby, B., Akhtar, R.,
Eds.; Springer International Publishing: Cham, Switzerland, 2015; pp. 7–35.
39.
Galante, J.O. Tensile properties of the human lumbar annulus fibrosus. Acta Orthop. Scand.
1967
,
38 (Suppl. 100), 1–91. [CrossRef]
40.
Guerin, H.L.; Elliott, D.M. Quantifying the contributions of structure to annulus fibrosus mechanical function
using a nonlinear, anisotropic, hyperelastic model. J. Orthop. Res. 2007,25, 508–516. [CrossRef] [PubMed]
41.
Smith, L.J.; Fazzalari, N.L. The elastic fibre network of the human lumbar anulus fibrosus: Architecture,
mechanical function and potential role in the progression of intervertebral disc degeneration. Eur. Spine J.
2009,18, 439–448. [CrossRef]
42. Ricard-Blum, S. The Collagen Family. CSH Perspect. Biol. 2011,3, a004978. [CrossRef]
43.
Eyre, D.R.; Muir, H. Types I and II collagens in intervertebral disc. Interchanging radial distributions in
annulus fibrosus. Biochem. J. 1976,157, 267–270. [CrossRef]
44.
Kielty, C.M.; Grant, M.E. The Collagen Family: Structure, Assembly, and Organization in the Extracellular
Matrix. In Connective Tissue and Its Heritable Disorders: Molecular, Genetic, and Medical Aspects, 2nd ed.;
Royce, P.M., Steinmann, B., Eds.; Wiley-Liss: Hoboken, NJ, USA, 2003; pp. 159–221.
45.
Mouw, J.K.; Ou, G.; Weaver, V.M. Extracellular matrix assembly: A multiscale deconstruction. Nat. Rev. Mol.
Cell Biol. 2014,15, 771–785. [CrossRef]
46.
Yanagishita, M. Function of proteoglycans in the extracellular matrix. Pathol. Int.
1993
,43, 283–293.
[CrossRef]
47.
Marchand, F.; Ahmed, A.M. Investigation of the laminate structure of lumbar disc anulus fibrosus. Spine
1990,15, 402–410. [CrossRef] [PubMed]
48.
Melrose, J.; Smith, S.M.; Appleyard, R.C.; Little, C.B. Aggrecan, versican and type VI collagen are components
of annular translamellar crossbridges in the intervertebral disc. Eur. Spine J.
2008
,17, 314–324. [CrossRef]
[PubMed]
49.
Hickey, D.S.; Hukins, D.W.L. Relation between the Structure of the Annulus Fibrosus and the Function and
Failure of the Intervertebral-Disk. Spine 1980,5, 106–116. [CrossRef] [PubMed]
50.
Green, T.P.; Adams, M.A.; Dolan, P. Tensile properties of the annulus fibrosus II. Ultimate tensile strength
and fatigue life. Eur. Spine J. 1993,2, 209–214. [CrossRef]
51.
Yu, J.; Tirlapur, U.; Fairbank, J.; Handford, P.; Roberts, S.; Winlove, C.P.; Cui, Z.; Urban, J. Microfibrils, elastin
fibres and collagen fibres in the human intervertebral disc and bovine tail disc. J. Anat.
2007
,210, 460–471.
[CrossRef] [PubMed]
52.
Nerurkar, N.L.; Elliott, D.M.; Mauck, R.L. Mechanical design criteria for intervertebral disc tissue engineering.
J. Biomech. 2010,43, 1017–1030. [CrossRef] [PubMed]
Materials 2019,12, 253 37 of 41
53.
O’Connell, G.D.; Sen, S.; Elliott, D.M. Human annulus fibrosus material properties from biaxial testing and
constitutive modeling are altered with degeneration. Biomech. Model. Mechan.
2012
,11, 493–503. [CrossRef]
54.
Ambard, D.; Cherblanc, F. Mechanical behavior of annulus fibrosus: A microstructural model of fibers
reorientation. Ann. Biomed. Eng. 2009,37, 2256–2265. [CrossRef] [PubMed]
55.
Best, B.A.; Guilak, F.; Setton, L.A.; Zhu, W.B.; Saednejad, F.; Ratcliffe, A.; Weidenbaum, M.; Mow, V.C.
Compressive Mechanical-Properties of the Human Anulus Fibrosus and Their Relationship to Biochemical-
Composition. Spine 1994,19, 212–221. [CrossRef]
56.
Perie, D.S.; MacLean, J.J.; Owen, J.P.; Iatridis, J.C. Correlating material properties with tissue composition in
enzymatically digested bovine annulus fibrosus and nucleus pulposus tissue. Ann. Biomed. Eng.
2006
,34,
769–777. [CrossRef]
57.
Iatridis, J.C.; Setton, L.A.; Weidenbaum, M.; Mow, V.C. Alterations in the mechanical behavior of the human
lumbar nucleus pulposus with degeneration and aging. J. Orthop. Res.
1997
,15, 318–322. [CrossRef]
[PubMed]
58.
Trout, J.J.; Buckwalter, J.A.; Moore, K.C. Ultrastructure of the human intervertebral disc: II. Cells of the
nucleus pulposus. Anat. Rec. 1982,204, 307–314. [CrossRef] [PubMed]
59.
Bonetti, M.I. Microfibrils: A cornerstone of extracellular matrix and a key to understand Marfan syndrome.
Ital. J. Anat. Embryol. 2009,114, 201–224.
60.
Akkiraju, H.; Nohe, A. Role of Chondrocytes in Cartilage Formation, Progression of Osteoarthritis and
Cartilage Regeneration. J. Dev. Biol. 2015,3, 177–192. [CrossRef]
61.
Muir, H. The chondrocyte, architect of cartilage. Biomechanics, structure, function and molecular biology of
cartilage matrix macromolecules. Bioessays 1995,17, 1039–1048. [CrossRef] [PubMed]
62.
Calve, J.; Galland, M. The intervertebral nucleus pulposus—Its anatomy, its physiology, its pathology. J. Bone
Jt. Surg. 1930,12, 555–578.
63.
Keyes, D.C.; Compere, E.L. The normal and pathological physiology of the nucleus pulposus of the
intervertebral disc—An anatomical, clinical, and experimental study. J. Bone Jt. Surg. 1932,14, 897–938.
64.
Lotz, J.C.; Fields, A.J.; Liebenberg, E.C. The Role of the Vertebral End Plate in Low Back Pain. Glob. Spine J.
2013,3, 153–163. [CrossRef]
65.
Moore, R.J. The vertebral endplate: Disc degeneration, disc regeneration. Eur. Spine J.
2006
,15, S333–S337.
[CrossRef]
66.
Mwale, F.; Roughley, P.; Antoniou, J. Distinction between the extracellular matrix of the nucleus pulposus
and hyaline cartilage: A requisite for tissue engineering of intervertebral disc. Eur. Cell Mater.
2004
,8, 58–63.
[CrossRef]
67.
Sophia Fox, A.J.; Bedi, A.; Rodeo, S.A. The basic science of articular cartilage: Structure, composition, and
function. Sports Health 2009,1, 461–468. [CrossRef] [PubMed]
68.
Miller, E.J.; Rhodes, R.K. Preparation and characterization of the different types of collagen. Methods Enzym.
1982,82 Pt A, 33–64.
69.
Kuhn, K.; Schmid, T.M.; Linsenmayer, T.F.; Rest, M.; Mayne, R. Structure and Function of Collagen Types, 1st ed.;
Mayne, R., Burgeson, R.E., Eds.; Academic Press: New York, NY, USA, 1987; pp. 1–281.
70.
Grant, J.P.; Oxland, T.R.; Dvorak, M.F. Mapping the structural properties of the lumbosacral vertebral
endplates. Spine 2001,26, 889–896. [CrossRef] [PubMed]
71.
Rodriguez, A.G.; Rodriguez-Soto, A.E.; Burghardt, A.J.; Berven, S.; Majumdar, S.; Lotz, J.C. Morphology of
the human vertebral endplate. J. Orthop. Res. 2012,30, 280–287. [CrossRef] [PubMed]
72.
Herkowitz, H.N.; Spine, I.S.S.L. The Lumbar Spine; Lippincott Williams & Wilkins: Philadelphia, PA, USA,
2004; pp. 1–943.
73.
Nekkanty, S.; Yerramshetty, J.; Kim, D.G.; Zauel, R.; Johnson, E.; Cody, D.D.; Yeni, Y.N. Stiffness of the
endplate boundary layer and endplate surface topography are associated with brittleness of human whole
vertebral bodies. Bone 2010,47, 783–789. [CrossRef] [PubMed]
74.
Rudert, M.; Tillmann, B. Lymph and Blood-Supply of the Human Intervertebral Disc—Cadaver Study of
Correlations to Discitis. Acta Orthop. Scand. 1993,64, 37–40. [CrossRef] [PubMed]
75.
Nerlich, A.G.; Schaaf, R.; Walchli, B.; Boos, N. Temporo-spatial distribution of blood vessels in human lumbar
intervertebral discs. Eur. Spine J. 2007,16, 547–555. [CrossRef]
Materials 2019,12, 253 38 of 41
76.
Bogduk, N.; Tynan, W.; Wilson, A.S. The Nerve Supply to the Human Lumbar Intervertebral Disks. J. Anat.
1981,132, 39–56.
77.
Edgar, M.A. The nerve supply of the lumbar intervertebral disc. J. Bone Jt. Surg. Br.
2007
,89, 1135–1139.
[CrossRef]
78.
Freemont, A.J.; Peacock, T.E.; Goupille, P.; Hoyland, J.A.; OBrien, J.; Jayson, M.I.V. Nerve ingrowth into
diseased intervertebral disc in chronic back pain. Lancet 1997,350, 178–181. [CrossRef]
79. Urban, J.P.G.; Roberts, S. Degeneration of the intervertebral disc. Arthritis Res. 2003,5, 120–130. [CrossRef]
80.
Gaskin, D.J.; Richard, P. Relieving Pain in America: A Blueprint for Transforming Prevention, Care, Education, and
Research; National Academies Press: Washington, DC, USA, 2011.
81.
Crow, W.T.; Willis, D.R. Estimating cost of care for patients with acute low back pain: A retrospective review
of patient records. J. Am. Osteopath. Assoc. 2009,109, 229–233. [PubMed]
82.
Roberts, S.; Evans, H.; Trivedi, J.; Menage, J. Histology and pathology of the human intervertebral disc.
J. Bone Jt. Surg. Am. 2006,88 (Suppl. 2), 10–14.
83.
Battie, M.C.; Videman, T.; Levalahti, E.; Gill, K.; Kaprio, J. Genetic and environmental effects on disc
degeneration by phenotype and spinal level: A multivariate twin study. Spine
2008
,33, 2801–2808. [CrossRef]
[PubMed]
84.
Buckwalter, J.A. Aging and degeneration of the human intervertebral disc. Spine
1995
,20, 1307–1314.
[CrossRef] [PubMed]
85.
Chan, D.; Song, Y.; Sham, P.; Cheung, K.M. Genetics of disc degeneration. Eur. Spine J.
2006
,15 (Suppl. 3),
S317–S325. [CrossRef]
86.
Inoue, N.; Espinoza Orias, A.A. Biomechanics of intervertebral disk degeneration. Orthop. Clin. North. Am.
2011,42, 487–499. [CrossRef]
87.
Pfirrmann, C.W.A.; Metzdorf, A.; Zanetti, M.; Hodler, J.; Boos, N. Magnetic Resonance Classification of
Lumbar Intervertebral Disc Degeneration. Spine 2001,26, 1873–1878. [CrossRef]
88.
Radek, M.; Pacholczyk-Sienicka, B.; Jankowski, S.; Albrecht, L.; Grodzka, M.; Depta, A.; Radek, A. Assessing
the correlation between the degree of disc degeneration on the Pfirrmann scale and the metabolites identified
in HR-MAS NMR spectroscopy. Magn. Reson. Imaging 2016,34, 376–380. [CrossRef]
89. Sinusas, K. Osteoarthritis: Diagnosis and treatment. Am. Fam. Physician 2012,85, 49–56.
90.
Maetzel, A.; Li, L.C.; Pencharz, J.; Tomlinson, G.; Bombardier, C.; The Community Hypertension and
Arthritis Project Study Team. The economic burden associated with osteoarthritis, rheumatoid arthritis,
and hypertension: A comparative study. Ann. Rheum. Dis. 2004,63, 395–401. [CrossRef]
91.
Goode, A.P.; Carey, T.S.; Jordan, J.M. Low Back Pain and Lumbar Spine Osteoarthritis: How Are They
Related? Curr. Rheumatol. Rep. 2013,15, 305. [CrossRef] [PubMed]
92.
Dunlop, R.B.; Adams, M.A.; Hutton, W.C. Disc space narrowing and the lumbar facet joints. J. Bone Jt.
Surg. Br. 1984,66, 706–710. [CrossRef]
93.
Fujiwara, A.; Tamai, K.; An, H.S.; Kurihashi, A.; Lim, T.H.; Yoshida, H.; Saotome, K. The relationship between
disc degeneration, facet joint osteoarthritis, and stability of the degenerative lumbar spine. J. Spinal Disord.
2000,13, 444–450. [CrossRef] [PubMed]
94.
Fujiwara, A.; Lim, T.H.; An, H.S.; Tanaka, N.; Jeon, C.H.; Andersson, G.B.J.; Haughton, V.M. The effect of
disc degeneration and facet joint osteoarthritis on the segmental flexibility of the lumbar spine. Spine
2000
,
25, 3036–3044. [CrossRef] [PubMed]
95.
Horkoff, M.; Maloon, S. Dysphagia secondary to esophageal compression by cervical osteophytes: A case
report. BCMJ 2014,56, 442–444.
96.
Milette, P.C.; Fontaine, S.; Lepanto, L.; Cardinal, E.; Breton, G. Differentiating lumbar disc protrusions, disc
bulges, and discs with normal contour but abnormal signal intensity. Magnetic resonance imaging with
discographic correlations. Spine 1999,24, 44–53. [CrossRef] [PubMed]
97.
Rim, D.C. Quantitative Pfirrmann Disc Degeneration Grading System to Overcome the Limitation of
Pfirrmann Disc Degeneration Grade. Korean J. Spine 2016,13, 1–8. [CrossRef]
98. Adams, M.A.; Hutton, W.C. Gradual disc prolapse. Spine 1985,10, 524–531. [CrossRef] [PubMed]
99.
Kortelainen, P.; Puranen, J.; Koivisto, E.; Lahde, S. Symptoms and Signs of Sciatica and Their Relation to the
Localization of the Lumbar-Disk Herniation. Spine 1985,10, 88–92. [CrossRef]
100. Frymoyer, J.W. Back Pain and Sciatica. N. Engl. J. Med. 1988,318, 291–300. [CrossRef] [PubMed]
Materials 2019,12, 253 39 of 41
101.
Taher, F.; Essig, D.; Lebl, D.R.; Hughes, A.P.; Sama, A.A.; Cammisa, F.P.; Girardi, F.P. Lumbar Degenerative
Disc Disease: Current and Future Concepts of Diagnosis and Management. Adv. Orthop.
2012
,2012, 970752.
[CrossRef] [PubMed]
102.
Physical Therapist’s Guide to Degenerative Disc Disease. Available online: http://www.moveforwardpt.com/
symptomsconditionsdetail.aspx?cid=514086b4-1272-4584-8742-ec6d2aa8f8cb (accessed on 8 March 2017).
103.
Nwuga, V.C. Ultrasound in treatment of back pain resulting from prolapsed intervertebral disc. Arch. Phys.
Med. Rehabil. 1983,64, 88–89. [PubMed]
104.
Adams, M.A.; Stefanakis, M.; Dolan, P. Healing of a painful intervertebral disc should not be confused with
reversing disc degeneration: Implications for physical therapies for discogenic back pain. Clin. Biomech.
2010,25, 961–971. [CrossRef] [PubMed]
105.
Will Steroid Injections Help My Degenerative Disc Disease? Available online: http://www.arksurgicalhospital.
com/will-steroid-injections-help-my-degenerative-disc-disease/ (accessed on 8 March 2017).
106.
Buttermann, G.R. The effect of spinal steroid injections for degenerative disc disease. Spine J.
2004
,4, 495–505.
[CrossRef] [PubMed]
107.
Chou, R.; Huffman, L.H. Medications for acute and chronic low back pain: A review of the evidence for an
American Pain Society/American College of Physicians clinical practice guideline. Ann. Int. Med.
2007
,147,
505–514. [CrossRef] [PubMed]
108.
Drugs, Medications, and Spinal Injections for Degenerative Disc Disease. Available online:
https://www.spineuniverse.com/conditions/degenerative-disc/drugs-medications-spinal-injections-
degenerative-disc-disease (accessed on 8 March 2017).
109.
Sluijter, M.E.; Cosman, E.R. Method and Apparatus for Heating an Intervertebral Disc for Relief of Back
Pain. U.S. Patent 5433739A, 18 July 1995.
110.
Sluijter, M.E.; Cosman, E.R. Thermal Denervation of an Intervertebral Disc for Relief of Back Pain. U.S.
Patent 5571147A, 5 November 1996.
111.
Sulaiman, S.B.; Keong, T.K.; Cheng, C.H.; Saim, A.B.; Idrus, R.B. Tricalcium phosphate/hydroxyapatite
(TCP-HA) bone scaffold as potential candidate for the formation of tissue engineered bone. Indian J. Med.
Res. 2013,137, 1093–1101.
112.
Spivak, J.M.; Hasharoni, A. Use of hydroxyapatite in spine surgery. Eur. Spine J.
2001
,10 (Suppl. 2),
S197–S204. [CrossRef]
113.
Nouh, M.R. Spinal fusion-hardware construct: Basic concepts and imaging review. World J. Radiol.
2012
,4,
193–207. [CrossRef] [PubMed]
114.
Confusion about Spinal Fusion. Available online: https://www.spineuniverse.com/treatments/surgery/
lumbar/confusion-about-spinal-fusion (accessed on 22 March 2017).
115.
Multilevel Spinal Fusion for Low Back Pain. Available online: https://www.spine-health.com/treatment/
spinal-fusion/multilevel-spinal-fusion-low-back-pain (accessed on 21 March 2017).
116.
Quirno, M.; Goldstein, J.A.; Bendo, J.A.; Kim, Y.; Spivak, J.M. The Incidence of Potential Candidates for Total
Disc Replacement among Lumbar and Cervical Fusion Patient Populations. Asian Spine J.
2011
,5, 213–219.
[CrossRef] [PubMed]
117.
Deyo, R.A.; Nachemson, A.; Mirza, S.K. Spinal-fusion surgery—The case for restraint. N. Engl. J. Med.
2004
,
350, 722–726. [CrossRef] [PubMed]
118.
Reeks, J.; Liang, H. Materials and Their Failure Mechanisms in Total Disc Replacement. Lubricants
2015
,3,
346–364. [CrossRef]
119.
Serhan, H.; Mhatre, D.; Defossez, H.; Bono, C.M. Motion-preserving technologies for degenerative lumbar
spine: The past, present, and future horizons. Int. J. Spine Surg. 2011,5, 75–89. [CrossRef] [PubMed]
120.
Guterl, C.C.; See, E.Y.; Blanquer, S.B.; Pandit, A.; Ferguson, S.J.; Benneker, L.M.; Grijpma, D.W.; Sakai, D.;
Eglin, D.; Alini, M.; et al. Challenges and strategies in the repair of ruptured annulus fibrosus. Eur. Cell
Mater. 2013,25, 1–21. [CrossRef]
121.
Bron, J.L.; Helder, M.N.; Meisel, H.J.; Van Royen, B.J.; Smit, T.H. Repair, regenerative and supportive
therapies of the annulus fibrosus: Achievements and challenges. Eur. Spine J. 2009,18, 301–313. [CrossRef]
122.
Vadala, G.; Mozetic, P.; Rainer, A.; Centola, M.; Loppini, M.; Trombetta, M.; Denaro, V. Bioactive electrospun
scaffold for annulus fibrosus repair and regeneration. Eur. Spine J. 2012,21 (Suppl. 1), S20–S26. [CrossRef]
Materials 2019,12, 253 40 of 41
123.
Cruz, M.A.; Hom, W.W.; DiStefano, T.J.; Merrill, R.; Torre, O.M.; Lin, H.A.; Hecht, A.C.; Illien-Junger, S.;
Iatridis, J.C. Cell-Seeded Adhesive Biomaterial for Repair of Annulus Fibrosus Defects in Intervertebral
Discs. Tissue Eng. Part A 2018,24, 187–198. [CrossRef]
124.
Alini, M.; Roughley, P.J.; Antoniou, J.; Stoll, T.; Aebi, M. A biological approach to treating disc degeneration:
Not for today, but maybe for tomorrow. Eur. Spine J. 2002,11 (Suppl. 2), S215–S220.
125.
Sudo, H.; Minami, A. Caspase 3 as a therapeutic target for regulation of intervertebral disc degeneration in
rabbits. Arthritis Rheum. 2011,63, 1648–1657. [CrossRef] [PubMed]
126.
Sakai, D.; Mochida, J.; Iwashina, T.; Hiyama, A.; Omi, H.; Imai, M.; Nakai, T.; Ando, K.; Hotta, T. Regenerative
effects of transplanting mesenchymal stem cells embedded in atelocollagen to the degenerated intervertebral
disc. Biomaterials 2006,27, 335–345. [CrossRef] [PubMed]
127.
D’Este, M.; Eglin, D.; Alini, M. Lessons to be learned and future directions for intervertebral disc biomaterials.
Acta Biomater. 2018,78, 13–22. [CrossRef] [PubMed]
128.
Bowles, R.D.; Setton, L.A. Biomaterials for intervertebral disc regeneration and repair. Biomaterials
2017
,129,
54–67. [CrossRef] [PubMed]
129.
Iu, J.; Massicotte, E.; Li, S.Q.; Hurtig, M.B.; Toyserkani, E.; Santerre, J.P.; Kandel, R.A. In Vitro Generated
Intervertebral Discs: Toward Engineering Tissue Integration. Tissue Eng. Part A
2017
,23, 1001–1010.
[CrossRef] [PubMed]
130.
Yang, J.C.; Yang, X.L.; Wang, L.; Zhang, W.; Yu, W.B.; Wang, N.X.; Peng, B.A.; Zheng, W.F.; Yang, G.;
Jiang, X.Y. Biomimetic nanofibers can construct effective tissue-engineered intervertebral discs for therapeutic
implantation. Nanoscale 2017,9, 13095–13103. [CrossRef]
131.
Bhunia, B.K.; Kaplan, D.L.; Mandal, B.B. Silk-based multilayered angle-ply annulus fibrosus construct to
recapitulate form and function of the intervertebral disc. Proc. Natl. Acad. Sci. USA
2018
,115, 477–482.
[CrossRef]
132.
Ghorbani, M.; Ai, J.; Nourani, M.R.; Azami, M.; Beni, B.H.; Asadpour, S.; Bordbar, S. Injectable natural
polymer compound for tissue engineering of intervertebral disc:
In vitro
study. Mater. Sci. Eng. C Mater.
2017,80, 502–508. [CrossRef]
133.
Gan, Y.; Li, P.; Wang, L.; Mo, X.; Song, L.; Xu, Y.; Zhao, C.; Ouyang, B.; Tu, B.; Luo, L.; et al. An interpenetrating
network-strengthened and toughened hydrogel that supports cell-based nucleus pulposus regeneration.
Biomaterials 2017,136, 12–28. [CrossRef]
134.
Halloran, D.O.; Grad, S.; Stoddart, M.; Dockery, P.; Alini, M.; Pandit, A.S. An injectable cross-linked scaffold
for nucleus pulposus regeneration. Biomaterials 2008,29, 438–447. [CrossRef]
135.
Park, S.-H.; Cho, H.; Gil, E.S.; Mandal, B.B.; Min, B.-H.; Kaplan, D.L. Silk-Fibrin/Hyaluronic Acid Composite
Gels for Nucleus Pulposus Tissue Regeneration. Tissue Eng. Part A
2011
,17, 2999–3009. [CrossRef] [PubMed]
136.
Pereira, D.R.; Silva-Correia, J.; Oliveira, J.M.; Reis, R.L.; Pandit, A.; Biggs, M.J. Nanocellulose reinforced
gellan-gum hydrogels as potential biological substitutes for annulus fibrosus tissue regeneration. Nanomed.
Nanotechol. 2018,14, 897–908. [CrossRef] [PubMed]
137.
Liu, C.; Zhu, C.; Li, J.; Zhou, P.; Chen, M.; Yang, H.; Li, B. The effect of the fibre orientation of electrospun
scaffolds on the matrix production of rabbit annulus fibrosus-derived stem cells. Bone Res.
2015
,3, 15012.
[CrossRef] [PubMed]
138.
Pirvu, T.; Blanquer, S.B.G.; Benneker, L.M.; Grijpma, D.W.; Richards, R.G.; Alini, M.; Eglin, D.; Grad, S.; Li, Z.
A combined biomaterial and cellular approach for annulus fibrosus rupture repair. Biomaterials
2015
,42,
11–19. [CrossRef] [PubMed]
139.
Hu, D.; Wu, D.; Huang, L.; Jiao, Y.; Li, L.; Lu, L.; Zhou, C. 3D bioprinting of cell-laden scaffolds for
intervertebral disc regeneration. Mater. Lett. 2018,223, 219–222. [CrossRef]
140.
Yang, F.; Xiao, D.; Zhao, Q.; Chen, Z.; Liu, K.; Chen, S.; Sun, X.; Yue, Q.; Zhang, R.; Feng, G. Fabrication of a
novel whole tissue-engineered intervertebral disc for intervertebral disc regeneration in the porcine lumbar
spine. RSC Adv. 2018,8, 39013–39021. [CrossRef]
141.
Choy, A.T.H.; Chan, B.P. A Structurally and Functionally Biomimetic Biphasic Scaffold for Intervertebral
Disc Tissue Engineering. PLoS One 2015,10, e0131827. [CrossRef]
142.
Hudson, K.D.; Bonassar, L.J. Hypoxic Expansion of Human Mesenchymal Stem Cells Enhances
Three-Dimensional Maturation of Tissue-Engineered Intervertebral Discs. Tissue Eng. Part A
2017
,23,
293–300. [CrossRef]
Materials 2019,12, 253 41 of 41
143.
Hudson, K.D.; Mozia, R.I.; Bonassar, L.J. Dose-Dependent Response of Tissue-Engineered Intervertebral
Discs to Dynamic Unconfined Compressive Loading. Tissue Eng. Part A 2015,21, 564–572. [CrossRef]
144.
Buckley, C.T.; Hoyland, J.A.; Fujii, K.; Pandit, A.; Iatridis, J.C.; Grad, S. Critical aspects and challenges for
intervertebral disc repair and regeneration—Harnessing advances in tissue engineering. JOR Spine
2018
,
1, e1029. [CrossRef]
©
2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).
... Surgical decompression can relieve the compression of nerve roots or the spinal cord by degenerative tissues and palliate the clinical symptoms. 1 However, the frequent occurrence of failed back surgery syndrome (FBSS, 4%~48%) seriously affects the quality of life of postoperative patients. 2 Epidural adhesion is considered the most common cause of FBSS and involves various degrees of inflammatory infiltration, scar tissue formation, and tissue adhesions in the epidural space. ...
Article
Full-text available
Background: The frequent occurrence of failed back surgery syndrome (FBSS) seriously affects the quality of life of postoperative lumbar patients. Epidural adhesion is the major factor in FBSS. Purpose: A safe and effective antiadhesion material is urgently needed. Methods: A superhydrophilic PLGA-g-PVP/PC nanofiber membrane (NFm) was prepared by electrospinning. FTIR was performed to identify its successful synthesis. Scanning electron microscopy, thermogravimetric analysis, differential scanning calorimetry, and water contact angle measurement were performed. CCK-8 assays were performed in primary rabbit fibroblasts (PRFs) and RAW264.7 cells to explore the cytotoxicity of PLGA-g-PVP/PC NFm. Calcein-AM/PI staining was used to measure the adhesion status in PRFs. ELISA was performed to measure the concentrations of TNF-α and IL-10 in RAW264.7 cells. In addition, the anti-epidural adhesion efficacy of the PLGA-g-PVP/PC NFm was determined in a rabbit model of lumbar laminectomy. Results: The PLGA-g-PVP/PC NFm exhibited ultrastrong hydrophilicity and an appropriate degradation rate. Based on the results of the CCK-8 assays, PLGA-g-PVP/PC NFm had no cytotoxicity to PRFs and RAW264.7 cells. Calcein-AM/PI staining showed that PLGA-g-PVP/PC NFm could inhibit PRF adhesion. ELISAs showed that PLGA-g-PVP/PC NFm could attenuate lipopolysaccharide-induced macrophage activation. In vivo experiments further confirmed the favorable anti-epidural adhesion effect of PLGA-g-PVP/PC NFm and the lack of a strong inflammatory response. Conclusion: In this study, PLGA-g-PVP/PC NFm was developed successfully to provide a safe and effective physical barrier for preventing epidural adhesion. PLGA-g-PVP/PC NFm provides a promising strategy for preventing postoperative adhesion and has potential for clinical translation.
... In addition, the smallest height available for the M6-C prosthesis is 6 mm, while the height of a natural disc may vary between 3.5 and 6.1 mm. [31][32][33] As such, the height of the prosthesis and the need for significant tissue removal during endplate preparation likely contributed to the greater anterior and posterior disc height observed for the M6-C prosthesis. It has been postulated that excessive anterior/posterior disc height (over-distraction) may result in worsened clinical outcomes through stretching of facet joints. ...
Article
Full-text available
Introduction: Cervical total disc replacement (CTDR) is an alternative to anterior cervical discectomy and fusion for select patients that may preserve range of motion and reduce adjacent segment disease. Various CTDR prostheses are available; however, comparative data are limited. This study aimed to compare the short-term kinematic and radiological parameters of the M6-C, Mobi-C, and the CP-ESP prostheses. Methods: This retrospective cohort study included patients treated with CTDR between March 2005 and October 2020 at a single institution. Patients were included if their follow-up assessment included lateral erect and flexion/extension radiographs. The primary outcome assessed at 3-months postoperatively was range of motion, measured by the difference in functional spinal unit angle between flexion and extension. Results: A total of 131 CTDR levels (120 patients, 46.2 ± 10.1 years, 57% male) were included. Prostheses implanted included the M6-C (n = 52), Mobi-C (n = 54), and CP-ESP (n = 25). Range of motion varied significantly (8.2° ± 4.4° vs. 10.9° ± 4.7° vs. 6.1° ± 2.7°, P < 0.001). On post hoc analysis, the Mobi-C prosthesis demonstrated a significantly greater range of motion than either the M6-C prosthesis (P = 0.003) or CP-ESP (P < 0.001). Conclusion: Although the optimal range of motion for CTDR has not been established, short-term differences in the range of motion may guide the selection of CTDR prosthesis. Further studies with longer follow-up and consideration of clinical outcome measures are necessary.
... For example, joint braking and non-steroidal antiinflammatory drugs are early treatments for osteoarthritis (133,134). Patients with lumbar spine degeneration should rest in bed to relieve spinal pressure (135,136). Stent implantation can correct local disordered hemodynamics and effectively delay the progression of atherosclerosis (62,137). Piezo1 channel provides solid theoretical support for these clinical prevention measures. ...
Article
Full-text available
Mechanical damage is one of the predisposing factors of inflammation, and it runs through the entire inflammatory pathological process. Repeated or persistent damaging mechanical irritation leads to chronic inflammatory diseases. The mechanism of how mechanical forces induce inflammation is not fully understood. Piezo1 is a newly discovered mechanically sensitive ion channel. The Piezo1 channel opens in response to mechanical stimuli, transducing mechanical signals into an inflammatory cascade in the cell leading to tissue inflammation. A large amount of evidence shows that Piezo1 plays a vital role in the occurrence and progression of chronic inflammatory diseases. This mini-review briefly presents new evidence that Piezo1 responds to different mechanical stresses to trigger inflammation in various tissues. The discovery of Piezo1 provides new insights for the treatment of chronic inflammatory diseases related to mechanical stress. Inhibiting the transduction of damaging mechanical signals into inflammatory signals can inhibit inflammation and improve the outcome of inflammation at an early stage. The pharmacology of Piezo1 has shown bright prospects. The development of tissue-specific Piezo1 drugs for clinical use may be a new target for treating chronic inflammation.
... The intervertebral disc has a complex structure, which results in a redistribution of stresses in the vertebrae. Such damping properties of the intervertebral disc prevent premature wear of the vertebrae (Frost et al., 2019). Usually, such in-vitro methods as tensile, compression, and indentation tests are used to study the mechanical properties of the materials making up the spine. ...
Article
Full-text available
Degenerative diseases of the spine significantly reduce the quality of human life. The spine consists of vertebral bodies and intervertebral discs. The most degraded are intervertebral discs. To study the factors affecting the damageability of the discs, we propose assessing the stress and strain fields by numerical simulation. The vertebral body consists of a shell (cortical bone tissue) and internal contents (cancellous bone tissue). The intervertebral disc is a complex structural element of the spine, consisting of the nucleus pulposus, annulus fibrosus, and cartilaginous plates. To develop numerical models for vertebrae and intervertebral disc, first, it is necessary to verify and validate the material models. This paper for the first time presents new numerical models based on the movable cellular automation method for the materials of the constituent elements of the lumbar spine, their validation, and verification. The models are validated using tensile, compression, and indentation experiments. A good qualitative and quantitative agreement was found with the data of field experiments from the literature.
... However, only a small percentage of patients require surgery. The vast majority qualify for conservative treatment aimed at reducing both the pain and the negative consequences to physical fitness (Colombini, Lombardi, Corsi, Banfi, 2008;Roughley, 2004;Frost, Camarero-Espinosa, Foster, 2019). When long-term pain contributes to the occurrence of chronic discomfort in the sacral spine and the entire back, treatment and therapy require more time and increased economic outlays (Hodgkinson, Shen, Diwan, Hoyland, Richardson, 2019). ...
Article
Degenerative disc disease (DDD) in the lumbosacral spine is one of the most common causes of pain and the significant associated limitations in physical activity and daily functioning, with the vast majority of patients requiring long-term physiotherapy. Hence, the significance of proper diagnostics, locating the cause of the ailment, implementation of appropriate therapy and prevention. The aim of the study was to investigate the efficacy of outpatient physiotherapy on reducing pain and improving the function of the lumbosacral spine. The research group comprised 95 people (50 women and 45 men) with an average age of 53 years, all patients with DDD in the lumbosacral spine. They underwent 3 physical treatments: magnetotherapy, laser therapy, and systemic cryotherapy, as well as gymnastic exercises, aimed at improving physical fitness, and strengthening the muscular corset. The research methods included the Schober test, the Thomayer test (finger-ground test), the Visual Analogue Scale scale, Laitinen's pain questionnaire, and calculation of BMI. Physiotherapeutic treatments significantly reduced the patients' pain symptoms, significantly increased the range of motion in the lumbosacral spine and improved physical fitness. Better results of the therapy were observed in patients with lower BMI.
... Its kyphotic curve allows the spine to bear loads anteriorly and to resist tension posteriorly, protecting the spinal cord while moving or bending the body. Since the thoracic spine provides most of the stability and support for the entire trunk [1], spine deformity resulting from poor posture often occurs in this segment. ...
Article
Full-text available
Inadequate sitting posture can cause imbalanced loading on the spine and result in abnormal spinal pressure, which serves as the main risk factor contributing to irreversible and chronic spinal deformity. Therefore, sitting posture recognition is important for understanding people’s sitting behaviors and for correcting inadequate postures. Recently, wearable devices embedded with microelectromechanical systems (MEMs) sensors, such as inertial measurement units (IMUs), have received increased attention in human activity recognition. In this study, a wearable device embedded with IMUs and a machine learning algorithm were developed to classify seven static sitting postures: upright, slump, lean, right and left bending, and right and left twisting. Four 9-axis IMUs were uniformly distributed between thoracic and lumbar regions (T1-L5) and aligned on a sagittal plane to acquire kinematic information about subjects’ backs during static-dynamic alternating motions. Time-domain features served as inputs to a signal-based classification model that was developed using long short-term memory-based recurrent neural network (LSTM-RNN) architecture, and the model’s classification performance was used to evaluate the relevance between sensor signals and sitting postures. Overall results from performance evaluation tests indicate that this IMU-based measurement and LSTM-RNN structural scheme was appropriate for sitting posture recognition.
Article
Purpose: To evaluate the biomechanics effect of modified cortical bone screw technique (MCBT) with other traditional internal fixation systems on lumbar osteoporotic wet specimen. Methods: Four different finite element models were established using CT data: (1) lumbar osteoporosis model without internal fixation system; (2) traditional pedicle screw technology (TT) model; (3) traditional cortical bone screw technology (CBT) model; (4) MCBT model. The changes of global displacement, intervertebral disc displacement of all models and internal fixation system Von Mises stress among the three models were compared under the same physiological load. Results: Compared with the other three models, the total displacement of the modified CBT screw model was the smallest, with the maximum displacement of 0.216 mm; The intervertebral disc displacement of the modified CBT screw model was the smallest, with the maximum displacement of 0.149 mm; the internal fixation system Von Mises stress of the modified CBT screw technique model was the largest compared with the other three models, The maximum Von Mises stress is 232.73 MPa. Conclusion: Compared to traditional pedicle screw and traditional CBT, MCBT has better mechanical stability, and it is of certain clinical application value.
Article
Full-text available
Lower back pain is a leading cause of disability and is one of the reasons for the substantial socioeconomic burden. The etiology of intervertebral disc (IVD) degeneration is complicated, and its mechanism is still not completely understood. Factors such as aging, systemic inflammation, biochemical mediators, toxic environmental factors, physical injuries, and genetic factors are involved in the progression of its pathophysiology. Currently, no therapy for restoring degenerated IVD is available except pain management, reduced physical activities, and surgical intervention. Therefore, it is imperative to establish regenerative medicine-based approaches to heal and repair the injured disc, repopulate the cell types to retain water content, synthesize extracellular matrix, and strengthen the disc to restore normal spine flexion. Cellular therapy has gained attention for IVD management as an alternative therapeutic option. In this review, we present an overview of the anatomical and molecular structure and the surrounding pathophysiology of the IVD. Modern therapeutic approaches, including proteins and growth factors, cellular and gene therapy, and cell fate regulators are reviewed. Similarly, small molecules that modulate the fate of stem cells for their differentiation into chondrocytes and notochordal cell types are highlighted.
Article
Full-text available
The study was conducted to find out the effect of secondary compounds extracted from Lepedium sativium seeds on the phenotypic structure of the cervical vertebrae by measuring the weight and lengths of each cervical vertebra. The study was performed in the animal house of Department of Biology, College of Education for Girls, University of Kufa, it included the use of 40 Albino rat of the Sprague Dawley strain, one months old, the average weights range (200-250g).the were divided in to 4 equal groups, the first group included the control group, which was dosed orally with distilled water only, the second group was treated with cold aqueous extract of the Lepedium sativium seeds 50 mg / kg, the third group was treated with cold aqueous extract of the Lepedium sativium seeds 100 mg / kg, the forth group was treated with cold aqueous extract of the Lepedium sativium seeds 150 mg / kg of body weight. Each group was administrated from the first day to experiment and until the sacrifice of animals, which was in two stages on 30 and 45 days for each group. The study included measuring the weights of the cervical vertebrae, as well as measuring the lengths (length, width, dimeter) of the vertebrae. The statistical analysis of the current study results was showed significant increase in weights of the cervical vertebrae in the group treated with cold eqeous extract of Lepedium sativium seed at a concentration 150 mg / kg, compared to the control group. As the Average weight increases with increasing the concentration of the extract. Also, The statistical analysis of the current study results was showed significant increase in lengths of the cervical animals treated with extract of Lepedium sativium seed at a concentration 150 mg / kg, for periods of 30 and 45 days compared to the control group.
Article
Full-text available
A novel whole tissue-engineered IVD consisting of a triphasic scaffold demonstrated excellent biocompatibility and mechanical properties in the porcine lumbar spine.
Article
Full-text available
Low back pain represents the highest burden of musculoskeletal diseases worldwide and intervertebral disc degeneration is frequently associated with this painful condition. Even though it remains challenging to clearly recognize generators of discogenic pain, tissue regeneration has been accepted as an effective treatment option with significant potential. Tissue engineering and regenerative medicine offer a plethora of exploratory pathways for functional repair or prevention of tissue breakdown. However, the intervertebral disc has extraordinary biological and mechanical demands that must be met to assure sustained success. This concise perspective review highlights the role of the disc microenvironment, mechanical and clinical design considerations, function versus mimicry in biomaterial‐based and cell engineering strategies, and potential constraints for clinical translation of regenerative therapies for the intervertebral disc. This article is protected by copyright. All rights reserved.
Article
Full-text available
We present a total tissue engineered (TE) intervertebral disc (IVD) to address IVD degradation which is a major cause of chronic neck and back pain. The TE IVD comprises an alginate hydrogel-based nucleus pulposus (NP) and hierarchically organized, concentric ring-aligned electrospun (ES) polycaprolactone (PCL)/poly (D, L-lactide-co-glycolide) (PLGA)/Collagen type I (PPC)-based annulus fibrosus (AF). The TE IVD exhibits excellent hydrophilicity to simulate highly hydrated native IVD. Long-term in vivo implantation assays demonstrate the excellent structural (shape maintenance, hydration, and integration with surrounding tissues) and functional (mechanical supporting and flexibility) performances of the TE IVD. Our study provides a novel approach for treating IVD degeneration.
Article
Biomaterials science has achieved significant advancements for the replacement, repair and regeneration of intervertebral disc tissues. However, the translation of this research to the clinic presents hurdles. The goal of this paper is to identify strategies to recapitulate the intrinsic complexities of the intervertebral disc, to highlight the unresolved issues in basic knowledge hindering the clinical translation, and finally to report on the emerging technologies in the biomaterials field. On this basis, we identify promising research directions, with the hope of stimulating further debate and advances for resolving clinical problems such as cervical and low back pain using biomaterial-based approaches. Statement of significance: Although not life-threatening, intervertebral disc disorders have enormous impact on life quality and disability. Disc function within the human body is mainly mechanical, and therefore the use of biomaterials to rescue disc function and alleviate pain is logical. Despite intensive research, the clinical translation of biomaterial-based therapies is hampered by the intrinsic complexity of this organ. After decades of development, artificial discs or tissue replacements are still niche applications given their issues of integration and displacement with detrimental consequences. The struggles of biological therapies and tissue engineering are therefore understandable. However, recent advances in biomaterial science give new hope. In this paper we identify the most promising new directions for intervertebral disc biomaterials.
Article
Mimicking the three-dimensional (3D) biological structure of native tissues and organs has remained a challenge for tissue engineering. The current use of hydrogels for intervertebral disc (IVD) repair is not ideal for insufficient mechanical properties. To overcome this limitation, we combine the excellent mechanical performance of poly(lactic acid) (PLA) with the biocompatibility and bioprintability of gellan gum-poly (ethylene glycol) diacrylate (GG-PEGDA) double network hydrogel to meet the necessary requirement of IVD regeneration. The cell-laden constructs were fabricated using 3D bioprinting technology. Mechanical and degradation properties of the dual printed scaffolds can be regulated by controlling the infill patterns and density of the PLA frameworks. Bone marrow stromal cells co-printed into the PLA/GG-PEGDA scaffolds remained high viability and showed excellent spreading within the hydrogels. Considering positive biocompatibility accompanied with suitable mechanical properties, this hybrid scaffolds have the potential to assist IVD regeneration.
Article
Recapitulation of the form and function of complex tissue organization using appropriate biomaterials impacts success in tissue engineering endeavors. The annulus fibrosus (AF) represents a complex, multilamellar, hierarchical structure consisting of collagen, proteoglycans, and elastic fibers. To mimic the intricacy of AF anatomy, a silk protein-based multilayered, disc-like angle-ply construct was fabricated, consisting of concentric layers of lamellar sheets. Scanning electron microscopy and fluorescence image analysis revealed cross-aligned and lamellar characteristics of the construct, mimicking the native hierarchical architecture of the AF. Induction of secondary structure in the silk constructs was confirmed by infrared spectroscopy and X-ray diffraction. The constructs showed a compressive modulus of 499.18 ± 86.45 kPa. Constructs seeded with porcine AF cells and human mesenchymal stem cells (hMSCs) showed ∼2.2-fold and ∼1.7-fold increases in proliferation on day 14, respectively, compared with initial seeding. Biochemical analysis, histology, and immunohistochemistry results showed the deposition of AF-specific extracellular matrix (sulfated glycosaminoglycan and collagen type I), indicating a favorable environment for both cell types, which was further validated by the expression of AF tissue-specific genes. The constructs seeded with porcine AF cells showed ∼11-, ∼5.1-, and ∼6.7-fold increases in col Iα 1, sox 9, and aggrecan genes, respectively. The differentiation of hMSCs to AF-like tissue was evident from the enhanced expression of the AF-specific genes. Overall, the constructs supported cell proliferation, differentiation, and ECM deposition resulting in AF-like tissue features based on ECM deposition and morphology, indicating potential for future studies related to intervertebral disc replacement therapy.
Article
Defects in the annulus fibrosus (AF) of intervertebral discs allow nucleus pulposus tissue to herniate causing painful disability. Microdiscectomy procedures remove herniated tissue fragments but unrepaired defects remain allowing reherniation or progressive degeneration. Cell therapies show promise to enhance repair but methods are undeveloped and carriers are required to prevent cell leakage. To address this challenge, this study developed and evaluated genipin-crosslinked fibrin (FibGen) as an adhesive cell carrier optimized for AF repair that can deliver cells, match AF material properties and have low risk of extrusion during loading. Part 1 determined feasibility of bovine AF cells encapsulated in high concentration FibGen (F140G6: 140mg/ml fibrinogen; 6mg/ml genipin) for 7 weeks could maintain high viability, but had little proliferation or matrix deposition. Part 2 screened tissue mechanics and in situ failure testing of nine FibGen formulations (fibrin: 35-140mg/ml; genipin: 1-6 mg/ml). F140G6 formulation matched AF shear and compressive properties and significantly improved failure strength in situ. Formulations with reduced genipin also exhibited satisfactory material properties and failure behaviors warranting further biological screening. Part 3 screened AF cells encapsulated in four FibGen formulations for one-week and found reduced genipin concentrations increased cell viability and glycosaminoglycan production. F70G1 (70mg/ml fibrinogen; 1mg/ml genipin) demonstrated balanced biological and biomechanical performance warranting further testing. We conclude that FibGen has potential to serve as an adhesive cell carrier to repair AF defects with formulations that can be tuned to enhance biomechanical and biological performance; future studies are required to develop strategies to enhance matrix production.
Article
Intervertebral disc (IVD) degeneration is related to both structural damage and aging. Annulus fibrosus (AF) defects such as annular tears, herniation and discectomy require novel strategies to functionally repair AF tissue. An ideal construct will repair the AF by providing physical and biological support and by facilitating regeneration. The present strategy proposes a gellan gum-based construct reinforced with cellulose nanocrystals (nCell) as a biological self-gelling AF substitute. Nanocomposite hydrogels were fabricated and characterized for their swelling capacity and stability and physico-chemical properties. Rheological evaluation on the nanocomposites demonstrated the GGMA reinforcement with nCell and thus matrix entanglements with higher stiffness upon ionic crosslinking. Compressive mechanical tests demonstrate values close to those of the human AF tissue. Furthermore, cell culture studies by means of gel loading with bovine AF cells indicated that the construct promoted cell viability and a physiologically relevant morphology for up to fourteen days in vitro.
Article
Intervertebral disc degeneration is recognized to be the leading cause for chronic low-back pain. Injectable hydrogel is one of the great interests for tissue engineering and cell encapsulation specially for intervertebral (IVD) affecting rate of regeneration success, in this study we assessed viscoelastic properties of a Chitosan-β glycerophosphate-hyaluronic acid, Chondroitin-6-sulfate, type 2 of Collagen, gelatin, fibroin silk (Ch-β-GP-HA-CS-Col-Ge-FS) hydrogel which was named as NP hydrogel that is natural extracellular matrix of IVD. Chitosan-based hydrogel was made in the ratio of 1.5%: 7%: 1%:1%:1%–1.5%–1% (Ch: β-GP: HA-CS-Col-Ge-FS). Gelation time and other rheological properties were studied using amplitude sweep and frequency sweep tests. Also, the cytotoxicity of the hydrogel invitro assessed by MTT and trypan blue tests. Morphology of the hydrogel and attachment of NP cells were evaluated by SEM. Our result showed that NP hydrogel in 4 °C is an injectable transparent solution. It started gelation in 37 °C after about 30 min. Gelation temperature of NP hydrogel was 37 °C. Storage modulus (G′) of this hydrogel at 37 °C was almost constant over a wide range of strain. MTT and trypan blue tests showed hydrogel was cytocompatible. The obtained results suggest that this hydrogel would be a natural and cytocompatible choice as an injectable scaffold for using in vivo study of IVD regeneration.
Article
Hydrogel is a suitable scaffold for the nucleus pulposus (NP) regeneration. However, its unmatched mechanical properties lead to implant failure in late-stage disc degeneration because of structural failure and implant extrusion after long-term compression. In this study, we evaluated an interpenetrating network (IPN)-strengthened and toughened hydrogel for NP regeneration, using dextran and gelatin as the primary network while poly (ethylene glycol) as the secondary network. The aim of this study was to realize the NP regeneration using the hydrogel. To achieve this, we optimized its properties by adjusting the mass ratios of the secondary/primary networks and determining the best preparation conditions for NP regeneration in a series of biomechanical, cytocompatibility, tissue engineering, and in vivo study. We found the optimal formulation of the IPN hydrogel, at a secondary/primary network ratio of 1:4, exhibited high toughness (the compressive strain reached 86%). The encapsulated NP cells showed increasing proliferation, cell clustering and matrix deposition. Furthermore, the hydrogel could support long-term cell retention and survival in the rat IVDs. It facilitated rehydration and regeneration of porcine degenerative NPs. In conclusion, this study demonstrates the tough IPN hydrogel could be a promising candidate for functional disc regeneration in future.