The role of the nucleus pulposus in neutral zone human lumbar intervertebral disc mechanics.
ABSTRACT To study the effect of denucleation on the mechanical behavior of the human lumbar intervertebral disc through a 2mm incision, two groups of six human cadaver lumbar spinal units were tested in axial compression, axial rotation, lateral bending and flexion/extension after incremental steps of "partial" denucleation. Neutral zone, range of motion, stiffness, intradiscal pressure and energy dissipation were measured; the results showed that the contribution of the nucleus pulposus to the mechanical behavior of the intervertebral disc was more dominant through the neutral zone than at the farther limits of applied loads and moments.
Article: Reduced nucleus pulposus glycosaminoglycan content alters intervertebral disc dynamic viscoelastic mechanics.[show abstract] [hide abstract]
ABSTRACT: The intervertebral disc functions over a range of dynamic loading regimes including axial loads applied across a spectrum of frequencies at varying compressive loads. Biochemical changes occurring in early degeneration, including reduced nucleus pulposus glycosaminoglycan content, may alter disc mechanical behavior and thus may contribute to the progression of degeneration. The objective of this study was to determine disc dynamic viscoelastic properties under several equilibrium loads and loading frequencies, and further, to determine how reduced nucleus glycosaminoglycan content alters dynamic mechanics. We hypothesized that (1) dynamic stiffness would be elevated with increasing equilibrium load and increasing frequency, (2) the disc would behave more elastically at higher frequencies, and finally, (3) dynamic stiffness would be reduced at low equilibrium loads under all frequencies due to nucleus glycosaminoglycan loss. We mechanically tested control and chondroitinase ABC injected rat lumbar motion segments at several equilibrium loads using oscillatory loading at frequencies ranging from 0.05 to 5Hz. The rat lumbar disc behaved non-linearly with higher dynamic stiffness at elevated compressive loads irrespective of frequency. Phase angle was not affected by equilibrium load, although it decreased as frequency was increased. Reduced glycosaminoglycan decreased dynamic stiffness at low loads but not at high equilibrium loads and led to increased phase angle at all loads and frequencies. The findings of this study demonstrate the effect of equilibrium load and loading frequencies on dynamic disc mechanics and indicate possible mechanical mechanisms through which disc degeneration can progress.Journal of biomechanics 07/2009; 42(12):1941-6. · 2.66 Impact Factor
Article: The effect of nucleotomy and the dependence of degeneration of human intervertebral disc strain in axial compression.[show abstract] [hide abstract]
ABSTRACT: Biomechanics of human intervertebral discs before and after nucleotomy. To noninvasively quantify the effect of nucleotomy on internal strains under axial compression in flexion, neutral, and extension positions, and to determine whether the change in strains depended on degeneration. Herniation and nucleotomy may accelerate the progression of disc degeneration. Removal of nucleus pulposus (NP) tissue has resulted in altered disc mechanics in vitro, including a decrease in internal pressure and an increase in the deformations at physiologically relevant strains. We recently presented a technique to quantify internal disc strains using magnetic resonance imaging (MRI). Degeneration was quantitatively assessed by the T1ρ relaxation time in the NP. Samples were prepared from human levels L3-L4 and/or L4-L5. A 1000-N compressive load was applied while in the magnetic resonance scanner. Nucleotomy was performed by removing 2 g of NP through the posterior-lateral annulus fibrosus (AF). The discs were rehydrated, reimaged, and retested. The analyzed parameters include axial deformation, AF radial bulge, and strains. RESULTS.: The axial deformation was more compressive after nucleotomy. In the neutral position, the axial deformation after nucleotomy correlated with degeneration (as quantified by T1ρ in the NP), with minimal alteration in nondegenerated discs. Nucleotomy altered the radial displacements and strains in the neutral position, such that the inner AF radial bulge decreased and the radial strains were more tensile in the lateral AF and less tensile in the posterior AF. In the bending loading positions the radial strains were not affected by nucleotomy. Nucleotomy alters the internal radial and axial AF strains in the neutral position, which may leave the AF vulnerable to damage and microfractures. In bending, the effects of nucleotomy were minimal, likely due to more of the applied load being directed over the AF. Some of the nucleotomy effects are modulated by degeneration, where the mechanical effect of nucleotomy was magnified in degenerated discs and may further induce mechanical damage and degeneration.Spine 03/2011; 36(21):1765-71. · 2.08 Impact Factor
Article: Axial creep loading and unloaded recovery of the human intervertebral disc and the effect of degeneration.[show abstract] [hide abstract]
ABSTRACT: The intervertebral disc maintains a balance between externally applied loads and internal osmotic pressure. Fluid flow plays a key role in this process, causing fluctuations in disc hydration and height. The objectives of this study were to quantify and model the axial creep and recovery responses of nondegenerate and degenerate human lumbar discs. Two experiments were performed. First, a slow compressive ramp was applied to 2000 N, unloaded to allow recovery for up to 24 h, and re-applied. The linear-region stiffness and disc height were within 5% of the initial condition for recovery times greater than 8 h. In the second experiment, a 1000 N creep load was applied for four hours, unloaded recovery monitored for 24 h, and the creep load repeated. A viscoelastic model comprised of a "fast" and "slow" exponential response was used to describe the creep and recovery, where the fast response is associated with flow in the nucleus pulposus (NP) and endplate, while the slow response is associated with the annulus fibrosus (AF). The study demonstrated that recovery is 3-4X slower than loading. The fast response was correlated with degeneration, suggesting larger changes in the NP with degeneration compared to the AF. However, the fast response comprised only 10%-15% of the total equilibrium displacement, with the AF-dominated slow response comprising 40%-70%. Finally, the physiological loads and deformations and their associated long equilibrium times confirm that diurnal loading does not represent "equilibrium" in the disc, but that over time the disc is in steady-state.Journal of the mechanical behavior of biomedical materials. 10/2011; 4(7):933-42.
THE ROLE OF THE NUCLEUS PULPOSUS IN NEUTRAL ZONE HUMAN
LUMBAR INTERVERTEBRAL DISC MECHANICS
*Cannella, M; **Arthur, A; *Allen, S; *Joshi, A; ***Vresilovic, E; +*Marcolongo, M
+*Department of Materials Science and Engineering, Drexel University, Philadelphia, PA
Removal of the nucleus pulposus is common in discectomy
procedures. While the clinical outcome of these procedures is generally
good, a comprehensive understanding of the role of the nucleus in
intervertebral disc mechanics is still warranted. Numerous groups have
examined the role of the nucleus in mechanical behavior of the disc,
indicating that the removal of the nucleus leads to increased
displacement and that the more nucleus removed, the more displacement
increases. Koebbe et al. state that high clinical success rates are possible
with full disc decompression . We hypothesize that the contribution
of the nucleus pulposus to the mechanical behavior of the disc is more
dominant through the neutral zone than at the farther limits of applied
loads and moments. In this work, we investigated the effect of partial
denucleation through a 2 mm incision on the compressive, tensile,
bending and torsional behavior of the intervertebral disc as examined
through anterior column units (ACUs) of human lumbar intervertebral
Two separate groups of six ACUs were harvested from six human
lumbar spines (3M/3F). The average specimen age was 47±19years,
ranging from 30 to 64years, for the first group and 49±13years, ranging
from 26 to 65years, for the second group. Each ACU was prepared,
potted, and kept moist throughout testing. Initial intervertebral disc
height under zero load was measured by a calibrated X-ray image
acquired with an FIS-Fluoroscan III (Fluoroscan Imaging Systems,
Northbrook, IL). The average intact disc height was 9.1±2.1mm,
ranging from 5.4mm to 10.9mm, and 7.2±1.3mm, ranging from 6.2mm
to 9.5mm, for the first and second groups, respectively.
The first group of specimens (Data Set 1) was tested in axial loading
after several incremental steps of partial denucleation. Preconditioning
was performed in displacement control at 3% of initial disc height (DH)
for 50 cycles with a sawtooth wave shape at 1Hz using a servohydraulic
dynamic test system (Model no. 8874, Instron, Corp., Norwood, MA).
The intact disc was then tested in load control with a 5 cycle sawtooth
waveform at 0.1Hz from 150N tension to 1500N compression.
Intradiscal pressure was collected inside the nucleus pulposus through
the anterior annular wall. Under a 50N compressive load (to simulate
lying prone), a portion of the nucleus tissue was removed by an
automated vacuum tissue removal system (Nucleotome, Model 22500,
Clarus Medical, LLC, Minneapolis, MN). Tissue removal was
performed using a posterolateral approach in four 5 minute intervals. At
the end of each denucleation period, the tissue removed from the disc
was collected, dried and weighed, and the disc was tested with the same
protocol described above.
The second group of specimens (Data Set 2) was tested in axial
loading, axial rotation, lateral bending and flexion/extension after full
denucleation. After pre-conditioning (as above), the ACUs were loaded
using a custom test jig modeled after that reported by Spenciner et al.
. The test specimen and a six degree-of-freedom (DOF) load cell
(Model MC3A, AMTI, Inc., Watertown, MA) were positioned above an
XY table that was used to relieve shear forces. Although off-axis
moments were in general less than 10% of the applied moment (peak at
19%), the difference in off-axis moments for any test condition on a
given disc was less than 5%. Five cycles of tension/compression (-150N
to 1000N, sinusoidal waveform, 0.1Hz), axial rotation, lateral bending
and flexion/extension (±7.5Nm, sinusoidal waveform, 0.1Hz) were
applied separately to each intact ACU. Each disc was then denucleated
as above for 20min. and dry mass was measured. The full loading
regime was re-applied to the denucleated specimens.
For each data set, range of motion (ROM), neutral zone (NZ) and
stiffness were calculated for the intact and denucleated states of each
ACU using the fifth loading cycle. Paired t-tests were used to determine
whether the denucleated state was significantly different from the intact
state using p < 0.05.
The effect of partial denucleation on compressive biomechanics is
shown in Figure 1. The disc height was reduced with denucleation,
while the compressive neutral zone increased significantly (p<0.05) for
each denucleation point except 5 min, ranging from about 1.5-2.0 times
that of the intact discs (Figure 1a). The compressive stiffness followed
an interesting trend where low loading levels (under 400 N) showed a
reduction in compressive stiffness compared to intact levels, the higher
loading regimes (greater than 400 N) generally showed stiffness values
that were not different from intact specimens. Intradiscal pressure
dropped 10-20% from the intact condition, but did not vary with applied
load. However intradiscal pressure showed some dependence on
Figure 1: Axial loading with increased denucleation times shows
increased motion (a) and reduced compressive stiffness (b) through
the neutral zone in comparison to the intact ACUs.
Norm DHNorm cROM Norm tROMNorm cNZ
Normalized to Intact
50N200N400N 800N 1400N
For fully denucleated ACUs that were subjected to axial, bending and
torsional loading, there was a large increase in normalized neutral zone
displacement for each mode of loading. The stiffness values for each
loading mode were below the level of the intact specimens in the lower
loading regime, but equal to or higher than the intact levels as the
loading increased (Figure 2).
Figure 2: Axial loading, bending and torsion lead to an increase in
displacement through the neutral zone in comparison to the intact
condition for ACUs (a); stiffness values in bending and torsion were
reduced below that of the intact condition in lower load levels, but
not at higher loading (b).
Disc HeightCompressionAxial RotationLateral BendingFlexion/Extension
Normalized to intact
Axial Rotation Lateral BendingFlexion/Extension
It can be hypothesized that the initial loading of the disc, through the
neutral zone, relies on the intradiscal pressure created by the nucleus to
tense the annulus fibrosus. With this tensioning, the disc can operate in
the normal, intact condition. However, when the intradiscal pressure is
compromised (with denucleation) there is then a lag where the load-
displacement curves translate along the displacement axis. By the time
the disc is loaded to a high level, the excessive displacement is
minimized or reduced completely bringing the levels of stiffness of the
denucleated conditions approximately equivalent to the levels of the
intact condition for the ACUs examined in this study. The long-term
clinical consequences of partial or total removal of the disc are unclear to
date. While used as a method to alleviate pain, the long term quasi-static
effects of an unstable disc (as well as the dynamic effects that were not
addressed here) are not well-understood. The relationship, if any,
between mechanical instability of the disc and a clinically painful disc
are worthy of continued study as are the role that the nucleus plays in
creating pain and accelerated degeneration.
1. Koebbe, CJ, et al. Neurosurg Focus, 2002. 13(2): p. E3.
2. Spenciner, D., et al. Spine J, 2006. 6(3): p. 248-57.
AFFILIATED INSTITUTIONS FOR CO-AUTHORS
**Synthes Spine, West Chester, PA; ***Department of Biomedical
Engineering, Drexel University, Philadelphia, PA
Synthes Spine for Funding.
53rd Annual Meeting of the Orthopaedic Research Society
Poster No: 1062