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The Physiology of Sports Injuries and Repair Processes



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Chapter 2
The Physiology of Sports Injuries and Repair Processes
Kelc Robi, Naranda Jakob, Kuhta Matevz and
Vogrin Matjaz
Additional information is available at the end of the chapter
1. Introduction
Sports injuries are among the most common injuries and therefore present a significant
public health problem. Physiologic processes after injuries are often neglected while
much more attention is being paid to the management of symptoms. However, compre‐
hension of these processes is becoming more and more important as therapies are get‐
ting increasingly focused on specific molecular and cellular processes. In recent decades,
extensive research of tissue regeneration after injury and degeneration, including molecu‐
lar pathways in healing, helped towards better understanding of this process and led to
discoveries of new potential therapeutic targets. In this chapter physiology of sports inju‐
ries and the latest advances in understanding pathophysiological processes after injury
will be discussed.
2. Physiology of tendon and ligament injury and repair
For skeletal muscles to act properly they must be attached to the bone. Tendons serve as
mediators of force transmission that results in joint motion, but they also enable that the
muscle belly remains at an optimal distance from the joint on which it acts. Tendons act
as springs, which allows them to store and recover energy very effectively. Ligaments on
the other hand attach bone to bone and therefore provide mechanical stability of the
joint, guide joint motion through their normal range of motion when a tensile load is ap‐
plied and prevent excessive joint displacement. Although tendons and ligaments differ in
function, they share similar physiological features with a similar hierarchical structure
and mechanical behavior.
© 2013 Robi et al.; licensee InTech. This is an open access article distributed under the terms of the Creative
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distribution, and reproduction in any medium, provided the original work is properly cited.
2.1. Histoanatomical features of tendons and ligaments
Tendons are made up predominantly of collagen fibers embedded in proteoglycan matrix
that attracts water and elastin molecules with a relatively small number of fibroblasts.
Fibroblasts are the predominant cell type in tendons. They are spindle shaped and arranged
in fascicles with surrounding loose areolar tissue called peritenon. Cells are orientated in the
direction of muscle loading. In mature tendon tissue they are arranged in parallel rows
along the force transmitting axis of the tendon. Long cytoplasmic processes extend between
the intratendinous fibroblasts, enabling cell-to-cell contact by gap-junctions.
Fibroblasts are connected to the extra cellular matrix (ECM) via integrins that permit the
cells to sense and respond to mechanical stimuli which appears vital for their function be‐
cause this way the mechanical continuum is established along which forces can be transmit‐
ted from the outside to the inside of the cell and vice versa. Integrins are also likely
candidates for sensing tensile stress at the cell surface. It is also speculated that integrin-as‐
sociated proteins are involved in signaling adaptive cellular responses upon mechanical
loading of the tissue [1-5].
Type I collagen is the major constituent of tendons, accounting for about 95% of the dry ten‐
don weight. Collagen type III accounts for about 5% of the dry tendon weight, but smaller
quantities of other collagens are also present, including types V, VI, XII and type II collagen.
The latter is primarily found in regions that are under compression [1-3].
Fibroblasts secrete a precursor of collagen, called procollagen, which is cleaved extracelul‐
larly to form type I collagen. The synthesis of collagen fibrils occurs in two stages: intracellu‐
lar and extracellular. The pro α-chains are initially synthesized with an additional signal
peptide at the aminoterminal end with the function to direct movement of the polypeptides
into the rough endoplasmic reticulum where it is cleaved off. Triple helix with three poly‐
peptide chains wound together to form a stiff helical structure is formed intracellularly.
Then the procollagen is secreted into the extracellular matrix where it is converted to colla‐
gen. Finally, collagen molecules aggregate and the cross-links responsible for its stable struc‐
ture are formed [1-4].
The parallel arrangement of the collagen fibers in tendons enables them to sustain high ten‐
sile loads. Collagen molecules group together to form microfibrils, which are defined as 5
collagen molecules stacked in a quarter-stagger array. Microfibrils combine to form subfi‐
brils, and those combine further to form fibrils (50-200 nm in diameter). Fibrils combine to‐
gether to form fibers (3-7 µm in diameter) which further combine to form fascicles, and
these group together to form a tendon. Fascicles are separated by endotenon and surround‐
ed by epitenon. At the level of fascicles, the characteristic »crimp« pattern can be seen histo‐
logically (discussed later in this chapter) (Figure 1) [1-4].
Proteoglycans (PGs) account for 1-5% of the dry weight of the tendon. PGs are highly hydro‐
philic they attract water molecules. The predominant proteoglycans in the tendon are decor‐
in and lumican. Biglycan and decorin (and collagen type V) regulate collagen fiber diameter
in fibrillogenesis. Because decorin molecules form cross-links between collagen fibers they
Current Issues in Sports and Exercise Medicine
may increase the stiffness of the fibrils. Proteoglycans are also responsible for lubricating
collagen fibers and thus allowing them to glide over each other [2-4]. Aggrecan, a normal
structure of articular cartilage, in found in tendons that are under compression [5].
Figure 1. Structure of a tendon. See text for details. Adopted from Kastelic et al. [6]
Although tendons and ligaments are very similar in structure, there are some differences be‐
tween them. (1) Ligaments consist of lower percentage of collagen molecules, but a higher
percentage of the proteoglycans and water. (2) Collagen fibers are more variable and have
higher elastin content and (3) fibroblasts appear rounder. (4) Furthermore, ligaments receive
blood supply from insertion sites (Table 1) [1, 2].
Content / Feature
Ligaments Tendons
Fibroblasts 20% 20%
Ground substance 20-30% lower
Collagen 70-80% Slightly higher
Collagen type I 90% 95-99%
Collagen type III 10% 1-5%
Elastin Up to 2x collagen scarce
Water 60-80% 60-80%
Organisation More random Organized
Orientation Weaving pattern Long axis orientation
Table 1. Differences between tendon and ligament structure
2.1.1. Vascular supply
There are two types of tendons: (1) tendons covered with paratenon, and (2) sheathed tendons.
They mainly differ in vascular supply. In sheathed tendons a mesotenon (vincula) carries a ves‐
sel that supplies only one part of the tendon. Therefore, parts of the tendon are relatively avas‐
The Physiology of Sports Injuries and Repair Processes
cular and their nutrition depends on diffusion. On the other hand, paratenon-covered tendons
receive their blood supply from vessels entering the tendon surface and forming a rich capilla‐
ry system. Because of the difference in the vasculature, paratenon-covered tendons heal better.
As stated above, ligaments receive their blood supply from insertion sites [2, 3].
There is still an ongoing debate about the efficiency of the blood supply to tendons dur‐
ing exercise. Experiments showed that although the increase in tendon blood flow is
somehow restricted during exercise, there is no indication of any major ischemia in the
tendon region. The question remains how blood flow to the tendon region is regulated.
Several candidates as regulators of blood flow in skeletal muscle have been proposed,
and it is possible that similar substances and metabolites are vasoactive also in the ten‐
don region suca as bradykinin [2].
2.1.2. Insertion sites
As tendons attach skeletal muscles to bony structures, two types of tendinous junction are to
be distinguished – osteotendinous where tendon attaches to the bone and musculotendinous
where it attaches to the muscle. Four distinct zones have been observed at the osteotendi‐
nous junction, with a gradual change between them (Figure 2). (1) The first zone is structur‐
ally similar to the tendon propter, but with smaller amounts of PG decorin. This zone is
followed by (2) fibrocartilage, where mostly collagen type II and III are found, but also small
amounts of types I, IX and X. Furthermore, there is less PGs aggrecan and decorin. In the
third zone, (3) mineralized fibrocartilage is made up of mainly collagen type II, but large
quantities of collagen X and aggrecan are also present. The fourth zone is (4) bone, build up
mainly of collagen type I and minerals [1-3].
Figure 2. Diagram of a osteotendinous junction; B bone; MF minarelized fibrocartilage; F fibrocartilage; T tendon.
At musculotendinous junction, muscle cells are involuted and folded to provide maximal
surface for attachment where fibrils attach. Sarcomeres of the fast contracting muscles are
shortened at the junction, which may reduce the force intensity within the junction [3].
Ligaments insert into bone in two ways: through indirect or direct insertions. In indirect in‐
sertions the superficial layer is continued at with the periosteum and the deeper layer an‐
chores to bone via Sharpey’s fibers. In direct insertions, fibers attach to bone at 90° angle.
Four distinct zones have been observed, with a gradual change between ligament midsub‐
stance, fibrocartilage, mineralized fibrocartilage, and bone [2].
Current Issues in Sports and Exercise Medicine
2.1.3. Biomechanics of tendons and ligaments
Typical parameters describing the tendon/ligament mechanical properties are strain, which
describes the elongation/deformation of the tendon (ΔL) relative to the normal length (L0);
stress, the tendon force (Ft) relative to the tendon cross-sectional area (CSA), stiffness, the
change in tendon length (ΔL) in relation to the force applied (ΔFt) and modulus, which de‐
scribes the relation between tendon stress and tendon strain and represents the properties
independently of the CSA (Figure 3 and 4). High modulus indicates stiffer tissue [7-9].
Figure 3. Structural properties of the bone-ligament-bone complex - A load/elongation curve; stiffness is represented
by the slope of the curve; ultimate load is the highest load applied to the bone-ligament-bone complex before failure;
the dashed area under the curve is the maximum energy stored by the complex [7, 9].
Figure 4. Mechanical properties of the bone-ligament-bone complex A stress/strain curve; modulus is represented
by the slope of the curve; tensile strength is the maximum stress of the bone-ligament-bone complex before failure;
the dashed area under the curve represents the strain energy density [7, 9].
The Physiology of Sports Injuries and Repair Processes
The biomechanics of ligaments is similar to tendon biomechanics. The biomechanical prop‐
erties of ligaments are described as either structural properties of the bone-ligament–bone
complex or the material properties of the ligament midsubstance itself. Structural properties
of the bone-ligament-bone complex depend on the size and shape of the ligament, therefore
they are extrinsic measures. They are obtained by loading a ligament to failure and therefore
represented as a load-elongation curve between two defined limits of elongation. Mechani‐
cal properties are intrinsic measures of the quality of the tissue substance and are represent‐
ed by a stress-strain curve [7, 8].
A tendon is the strongest component in the muscle-tendon-bone unit. It is estimated that
tensile strength is about one-half of stainless steel (e.g. 1 cm
cross-section of a tendon can
bear weight of 500-1000 kg) [3, 9].
2.1.4. Non-linear elasticity and viscoelasticity
There are three distinct regions of the stress/strain curve: (1) the toe region, (2) the linear re‐
gion, and (3) the yield and failure region (Figure 5). In normal activity, most ligaments and
tendons exist in the toe and somewhat in the linear region. This region is responsible for
nonlinear stress/strain curve, because the slope of the toe region is not linear. The toe region
represents "un-crimping" of the collagen fibrils. Since it is easier to stretch out the crimp of
the collagen fibrils, this part of the stress strain curve shows a relatively low stiffness com‐
pared to linear portion. The toe region ends at about 2% strain when all crimpled fibers
straighten. When all collagen fibrils become uncrimped, the collagen fibers stretch. The ten‐
don deforms in a linear fashion due to the inter-molecular sliding of collagen triple helices.
If strain is less than 4%, the tendon will return to its original length when unloaded, there‐
fore this portion is elastic and reversible and the slope of the curve represents an elastic
modulus. When a tendon/ligament is stretched beyond physiological limits, some fibrils be‐
gin to fail. Micro failure accumulates, stiffness is reduced and the ligament/tendon begins to
fail. This occurs when intramolecular cross-links between collagen fibers fail. The tendon
therefore undergoes irreversible plastic deformation. When the tendon/ligament is stretched
to more than 8-10% of its original length, macroscopic failure follows [2, 3, 7].
Viscoelasticity refers to time dependent mechanical behavior. In other words, the relationship
between stress and strain is not constant but depends on the time of displacement or load.
There are three major characteristics of a viscoelastic material of ligaments and tendons:
creep, stress relaxation, and hysteresis or energy dissipation. Creep indicates increasing de‐
formation under constant load. This is in contrast with the usual elastic material, which does
not elongate, no matter how long the load is applied (Figure 6). Stress relaxation is a feature
of a ligament or tendon meaning that stress acting upon them will be eventually reduced
under a constant deformation (Figure 7). When a viscoelastic material is loaded and unload‐
ed, the unloading curve is different from the loading curve. This is called hysteresis. The dif‐
ference between the two curves represents the amount of energy that is dissipated or lost
during loading (Figure 8). If loading and unloading are repeated several times, different
curves are obtained. However, after about 10 cycles, the loading and unloading curves do
not change anymore, but they are still different. In other words, the amount of hysteresis un‐
Current Issues in Sports and Exercise Medicine
der cyclic loading is reduced and the stress-strain curve becomes reproducible (Figure 9).
This behavior is called pseudo-elasticity to represent the nonlinearity of ligament/tendon
stress strain behavior [7].
Figure 6. Creep is increasing deformation under constant load.
2.1.5. The influence of loading and gender on tendon and ligament size
Ligaments and tendons are adapted according to changes in mechanical stiffness. However,
changes occur slowly, partly due to the fact that tendons and ligaments are relatively avas‐
cular tissues. There is strong evidence that tendons undergo hypertrophy, at least after long-
term mechanical loading.
Figure 5. There are three distinct regions of the stress/strain curve: (1) the toe region, (2) the linear region, and (3) the
yield and failure region. The toe region represents "un-crimping" of the collagen fibrils; toe region ends at about 2%
of strain when all crimpled fibers straighten. It os followed by linear region, in which the collagen fibers respond line‐
arly to load. If strain is less than 4%, the tendon will return to its original length when unloaded. Between 4 to 8 per
cent of strain the collagen fibers begin to slide past one another as the cross-links start to fail which results in micro‐
scopic failure. If strain is more than 8%, macroscopic failure results.
The Physiology of Sports Injuries and Repair Processes
Male runners were found to have about larger Achilles tendon cross-sectional areas than
non-runners. Furthermore, greater cross-sectional area (CSA) of patella tendons in the
leading leg of male athletes competing for at least 5 years in sports with a side-to-side
difference was demonstrated; an almost 30% difference in the cross-sectional area of the
proximal part of the tendon between the leading and non-leading leg was observed [8,
10]. When subjected to short-term loading, only certain parts of tendons hypertrophied.
It appears that tendons undergo hypertrophy in response to both long- and short-term
loading, but that short-term changes in CSA are relatively small and seemingly occur on‐
ly in specific regions of the tendon [8].
Interestingly, findings described above seem to be gender specific since marked differences
in tendon CSA were not consistently found between female athletes and sedentary controls.
Figure 7. Stress relaxation - the stress will be reduced under a constant deformation.
Figure 8. Hysteresis or energy dissipation when tendon or ligament is loaded and unloaded, the unloading curve
will not follow the loading curve. The energy is lost as heat (dashed area).
Current Issues in Sports and Exercise Medicine
Some other studies do in fact indicate that the exercise related adaptation of the tendon tis‐
sue is lower when levels of estrogen are high but the mechanism of this is not clear [8, 11].
Similary, premenopausal women were found to have lower risk for developing lower leg
tendinopathies than men. The risk for developing lower leg tendinopathy in women increas‐
es in the post-menopausal period and is probably influenced by hormone-replacement ther‐
apy and activity levels. The mechanism behind this observations is not clear [12].
2.1.6. The effect of aging and immobilization ligament and tendon structure and function
With age there is an increase in the mechanical properties of ligaments and tendons up to
the young adulthood when a decrease in the mechanical properties follows. Woo and collea‐
gues tested femur-acl-tibia complex from young cadaver knees with the average age of 35
and older cadaver knees with the age of 76. They found that the linear structural stiffness of
the ACL decreased both when tested at 30 degrees of knee flexion and when tested along
the axis of the ligament complex [13].
Immobilization has a negative impact on tendons and ligaments [14]. Corresponding to the
reduction in mechanical properties, there is a reduction in the ligament structure. Immobili‐
zation has a more rapid effect on mechanical properties than increased load from exercise. It
was established that during immobilization,the cross sectional area of the ACL is reduced,
which is believed to be a consequence of a loss in collagen fibrils as well as glycosaminogly‐
can that form the ground substance of the ligament. In addition, there might be alterations
in collagen fibril orientation reducing the ligament properties. Upon remobilization, it ap‐
peared that the mechanical properties normalized first, followed by the structural proper‐
ties. It is also believed that structural loss at the ligament insertion site may take longer to be
removed than changes in ligament substance [7].
Figure 9. During cyclic loading and unloading, the stress/strain curve shifts to the right. After 10 repetitions, the curve
becomes reproducible. The amount of hysteresis under cyclic loading is reduced.
The Physiology of Sports Injuries and Repair Processes
2.2. Tendon and ligament injury mechanisms
Tendon injury occurs because of direct trauma (i.e. penetrating, blunt, etc.) or indirect tensile
overload. Acute tensile failure occurs if strain is more than 10%. However, lesser strain can
cause tendon failure due to pre-existing chronic repeated insult and degeneration. Musculo‐
tendinous junction is the weakest link, especially during eccentric contractions. Maximum
tension is created in forceful contractions. Furthermore, greater speed of eccentric contrac‐
tion will increase the force developed. If the loading rate is slow, avulsion fracture is likely
to occur. If loading is fast, tendon failure is more likely, especially if degenerated [3].
Tendon overuse injuries are a source of major concern in competitive and recreational ath‐
letes. It is estimated that 30% to 50% of all sport injuries are due to overuse [15, 16]. Studies
from primary care show that 16% of general population suffers from shoulder pain, which
rises to 21% in the elderly. The prevalence of Achilles tendinopathy in runners has been esti‐
mated at 11%. Tendinopathy of the forearm extensor tendons affects 1-2% of the population,
most commonly occurring in the fourth and fifth decade of life. The overall prevalence of
patellar tendinopathy among elite and non-elite athletes is high and varies between 3% and
45% [17]. Quadriceps tendon and tibialis posterior tendon are also often affected [15]. In the
great majority of patients with spontaneous tendon rupture, the ruptured tendon shows de‐
generative lesions present before the rupture [16].
The term »tendinitis« has been widely used to describe a combination of tendon pain, swel‐
ling, and impaired performance. It is believed to be an inflammatory condition, although
histopathological studies show degeneration rather than inflammation and therefore the
term »tendinopathy« has been suggested as a more appropriate term [16, 18]. The term ten‐
dinopathy encompasses a spectrum of disorders, including lesions of the tenosynovium, the
paratenon, the entesis, or tendon proper. Lesions can coexist and the tendon can tear partial‐
ly or completely. Tendinopathies can be divided according to the duration of symptoms into
acute (up to 2 weeks in duration), subacute (2-4 weeks), and chronic (over 6 weeks) [18].
There are multiple theories for the mechanism of tendon degeneration: (1) mechanical, (2)
vascular, (3) neural, and (4) alternative theory.
In the mechanical theory of tendon injury, the overload of the tendon tissue is blamed for the
pathologic process. Towards the higher end of the physiologic range, a microscopic failure
may occur within a tendon and repetitive microtrauma can lead to matrix and cell changes,
altered mechanical properties of the tendon, and symptoms development. Non-uniform
stress within a tendon may produce localized fiber degeneration and damage without a his‐
tory of a specific injury [15]. Studies have shown that cyclic mechanical stretching of cells
can cause changes in cell morphology and alteration of both DNA and protein syntheses. In
situ cell nucleus deformation does occur during tensile loading of tendons which may play a
significant role in the mechanical signal transduction pathway in the affected tendon [19].
The production of prostaglandin E2 (PGE2) in tendon fibroblasts increases in a stretching
magnitude-dependent manner for which cyclooxygenase (COX) is responsible [20]. Studies
also showed that asymptomatic pathologic changes were common in the Achilles and patel‐
lar tendons in elite soccer players and that a greater number of hours per week resulted in a
Current Issues in Sports and Exercise Medicine
higher prevalence of patellar tendinopathy. However, »underuse« may also be the cause of
tendon degeneration because the etiopathogenic stimulus for the degenerative cascade is the
catabolic response of tendon cells to mechanobiological understimulation [19].
The vascular theory of tendinopathy suggests that tendons generally have poor blood supply,
especially the Achilles tendon and those of tibialis posterior and supraspinatus muscle. The
Achilles tendon should have a hypovascular region 2-6 cm proximal to its calcaneal inser‐
tion. In such tendons overuse may lead to injury.
However, studies on the Achilles blood flow show that blood supply along the whole
tendon is in fact evenly distributed throughout the tendon, but is significantly lower at
the distal insertion. Blood flow in the symptomatic tendons was significantly elevated as
compared with the controls, demonstrated a similar vascular response to physical load‐
ing with a progressive decline in blood flow with increasing tension [21]. Male gender,
advancing age, and mechanical loading of the tendon are associated with diminished
tendon blood flow [22]. Therefore, vascular theory may be more important in the lesions
of fibrocartilagenous entheses that are relatively avascular, and this may contribute to a
poor healing response. Angiogenesis is mediated by angiogenic factors such as vascular
endothelial growth factor (VEGF). VEGF is highly expressed in degenerative Achilles ten‐
dons, whereas its expression is nearly completely downregulated in healthy tendons.
Several factors are able to upregulate VEGF expression in tenocytes: hypoxia, inflamma‐
tory cytokines, and mechanical load. Since VEGF has the potential to stimulate the ex‐
pression of matrix metalloproteinases and inhibit the expression of tissue inhibitors of
matrix metalloproteinases (TIMP), this cytokine might play a significant role in the path‐
ogenetic processes during degenerative tendon disease [23].
The neural theory suggests that neurally mediated mast cell degranulation could release me‐
diators such as substance P, which is contained in primary afferent nerves. Its quantity
could be related to chronic pain. The increased amount of substance P in the subacromial
bursa and nerve fibers immunoreactive to substance P were localized around the vessels of
rotator cuff, especially in patients with the non-perforated rotator cuff injury [24]. Inflamma‐
tory cytokines, proteinases, and cyclooxygenase enzymes, have been shown to be present in
the subacromial bursa of patients with rotator cuff tear [25]. However, neural theory does
not explain why morphologically pathologic tendons are not always painful [15].
The alternative theory suggests that exercise induced localized hyperthermia may be detri‐
mental to tendon cell survival. Tendons that store energy during locomotion, such as the
equine superficial flexor digitorum tendon and the human Achilles tendon, suffer a high
incidence of central core degeneration which is thought to precede tendon rupture. Stud‐
ies have shown that the central core of equine tendon reaches temperatures as high as
45°C during high-speed locomotion, but temperatures above 42.5°C are known to result
in fibroblast death In vitro [26]. Temperatures experienced in the central core of the ten‐
don In vivo are unlikely to result in tendon cell death, but repeated hyperthermic insults
may compromise cell metabolism of matrix components, resulting in tendon central core
degeneration [27].
The Physiology of Sports Injuries and Repair Processes
Although exact mechanism or their combination has not been determined yet, some factors
influencing the development of tendinopathy have been. There is some evidence for genetic
correlation, especially with target genes close to ABO gene on chromosome 9 like COL5A1
and TNC gene [28]. Women seem to have less tendinopathy than men, especially prior to
menopause. Although tendons do not degenerate with age as such, a reduction in proteo‐
glycans and an increase in cross-links with increasing age make tendon stiffer and less capa‐
ble in tolerating load. Decreased flexibility, training on harder surface, and even drugs such
as corticosteroids and quinolone antibiotics have been reported to be associated with the de‐
velopment of tendinopathy [15].
Ligament injuries are classified into three grades. (1) Grade I injury – mild sprain. Clinically,
there is minimal pain present over the injured ligament and no joint instability can be detected
by clinical examination despite the microfailure of collagen fibers. (2) Grade II injurymoder‐
ate sprain or partial tear of the ligament. There is severe pain present and minimal instability
detected by clinical testing. Ligament strength and stiffness decrease by 50%. (3) Grade III in‐
jury a complete ligament tear. Most collagen fibers have ruptured and the joint is completely
unstable. Another type of injury is ligament avulsion from its bony insertion. Midsubstance
ruptures are more common in45 adults; avulsion injuries are more common in children. Avul‐
sion occurs between unmineralized and mineralized fibrocartilage layers [2, 3].
2.3. Pathophysiology of tendon and ligament repair
The process of tendon healing follows a pattern similar to that of other healing tissues. There
are three phases of healing: (1) hemostasis/inflammation, (2) reparative phase, and (3) re‐
modeling and maturation phase. Ligament healing goes through the same stages as tendon
healing. However, there are differences among different ligaments. A classic model for liga‐
ment healing is the rupture of medial collateral ligament of the knee (MCL). MCL has a
good tendency to heal spontanelously. In contrast, the anterior cruciate ligament of the knee
(ACL) does not show any tendency to heal spontaneously, which is believed to be the conse‐
quence of synovial fluid interrupting the healing process between the ruptured ends of the
ligament. Therefore, an ACL reconstruction is a treatment of choice [2, 3].
After the injury, the wound site is infiltrated by inflammatory cells. Platelets aggregate at
the wound and create a fibrin clot to stabilize the torn tendon edges. The clot contains cells
and platelets that immediately begin to release a variety of molecules, most notably growth
factors (such as platelet-derived growth factor, transforming growth factor β, and insulin-
like growth factor -I and –II) causing acute local inflammation. During this inflammatory
phase that usually lasts three to five days, there is an invasion of extrinsic cells such as neu‐
trophils and macrophages which clean up necrotic debris by phagocytosis and together with
intrinsic cells (such as endotenon and epitenon cells) produce a second pool of cytokines to
initiate the reparative phase [2-4].
In reparative phase (three to six weeks) large amounts of disorganized collagen are deposit‐
ed at the repair site with granulation tissue formation, together with neovascularization, ex‐
trinsic fibroblast migration, and intrinsic fibroblast proliferation. After four days fibroblasts
infiltrate the wound site and proliferate. They produce extracellular matrix, including large
amounts of collagen III and glycosaminoglycan [2-4].
Current Issues in Sports and Exercise Medicine
In the remodeling phase, there is a decrease in the cellular and vascular content of the re‐
pairing tissue, and an increase in collagen type I content and density. Eventually, the colla‐
gen becomes more organized, properly orientated, and cross-linking with the healthy matrix
outside the injury takes place. Matrix metalloproteinase degrade the collagen matrix, replac‐
ing type II collagen with type I collagen. The remodeling stage can be divided into a consoli‐
dation and maturation phase. At the end of the consolidation phase, at about 10–12 weeks,
and with the beginning of the maturation phase, the brous tissue is converted to a stronger
scar tissue. Around the fourth week collagen fibers are being longitudinally reorganized so
that they are aligned in the direction of muscle loading. During the next three months the
individual collagen fibers form bundles identical to the original ones. After the healing proc‐
ess is complete, cellularity, vascularity, and collagen makeup will return to something ap‐
proximating that of the normal tendon, but the diameters and cross-linking of the collagen
will often remain inferior after healing. This phase lasts for months or years, usually be‐
tween 6 weeks and 9 months or more. However, the tissue continues to remodel for up to 1
year. The structural properties of the repaired tendon typically reach only two thirds of nor‐
mal, even years after injury [2-4].
There are slight differences in the way different tendons heal. Extrasynovial tendons can be
easily influenced by growth factors and cytokines produced by extrinsic cells (e.g. paratenon),
but intrasynovial tendons are more reliant on intrinsic cells (e.g. epitenon and endotenon) [3].
2.4. Treatment of tendon and ligament injuries
According to stages of healing response, a proper rehabilitation program time frame can be
introduced. During the inflammatory phase of 3-5 days rehabilitation program should avoid
excess motion because it can disrupt the healing process. During the repair phase a gradual
introduction of motion can be introduced to prevent excessive muscle atrophy and prevent
the diminishing of range of motion (ROM). Later progressive stress can be applied, howev‐
er, tendons can require up to one year to get close to normal strength levels [3, 29].
Proper postsurgical rehabilitation strategies are being debated. Rehabilitation protocols dif‐
fer due to anatomical site, because different tendons have different healing characteristics.
There is even a difference in the rehabilitation protocol between sheathed tendons and ten‐
dons that are not enclosed in sheaths. In sheathed tendons, early mobilization is crucial to
prevent scar formation between tendon sheath, therefore diminishing ROM. The response of
healing tendons to mechanical load varies depending on anatomical location. Flexor tendons
require motion to prevent adhesion formation, yet excessive force results in gap formation
and subsequent weakening of the repair [2, 3].
2.4.1. Immobilization and early remobilization
Ruptured and immobilized ligaments heal with a fibrous gap between the ruptured ends,
whereas sutured ligaments heal without fibrous gap. The mechanical properties of scars are
inferior to normal ligaments, which may lead to joint dysfunction by abnormalities in joint
kinematics [30]. In spite of this, many ligaments are not repaired routinely[3].
The Physiology of Sports Injuries and Repair Processes
Protective immobilization may enhance tendon-to-bone healing compared with other post
repair loading regimens like exercise or complete tendon unloading. In the repaired rotator
cuff, immobilization has shown to be benecial in tendon-to-bone healing. A complete re‐
moval of loading is detrimental to rotator cuff healing. However, immobilization is not a
proper treatment for all repaired tendons; some require early passive motion [4].
Tendons requiring long excursions for function (e.g. the exor tendons) are typically en‐
cased in synovial sheaths. To maintain gliding after injury, adhesions between the tendon
surface and its sheath must be prevented. Passive mechanical rehabilitation methods have
shown to be beneficial to prevent brotic adhesions [4, 31].
The optimal time for the initiation of such treatment is about 5 days after tendon repair
[31]. Controlled loading can enhance healing in most cases, but a ne balance must be
reached between loads that are too low (leading to a catabolic state) or too high (leading
to micro damage).
2.4.2. Surgical reconstruction
There is still a debate when ligament or tendon injuries should be treated conservatively and
when surgical repair is indicated. In practice the »50% rule« is commonly used [32]. The »
50% rule« suggests that tendon/ligament injuries with structural involvement of less than
50% should be treated conservatively, but damage greater than 50% should be treated by
surgical repair or reconstruction. This rule applies to a variety of orthopedic conditions, like
partial fractural involvement of less than 50%, anterior cruciate ligament, partial-thickness
injuries of the rotator cuff, and partial tears of the long head of the biceps tendon. However,
there is very little evidence for accuracy, reproducibility, or predictive power and this rule
has to be used with caution. It is maybe better to individualize the treatment according to a
patient's clinical and physical status, expectations, and demands after the treatment [32].
2.5. The role of corticosteroid injection therapy
At the cellular level, anti-inflammatory and immunosuppressive actions of corticosteroids
are the consequence of inhibition of cytokine-genes and pro-inflammatory mediators’ syn‐
thesis, such as nitric oxide and prostaglandins. The immunosuppressive and anti-inflamma‐
tory actions of corticosteroids are mediated through the interference of two transcription
factors: activating protein-1 (AP-1) and nuclear factor-κB (NF-κB) [16]. The exact mechanism
by which corticosteroids inhibit the transcriptional activity of AP-1 is not fully understood.
However, the activation of the cell by immune signals leads to degradation of IκB inhibitory
protein from NF-κB, allowing nuclear translocation of NF-κB and consequently the tran‐
scription of multiple target genes. Corticosteroids induce the production of IκB and there‐
fore provide efficient inactivation of NF-κB [16].
Besides the anti-inflammatory action, corticosteroids decrease the production of collagen
and extracellular matrix proteins by the fibroblasts and enhance bone resorption. Further‐
more, the production of extracellular matrix degrading enzymes MMP-3 (stromelysin-1),
MMP-13 (collagenase-3), and MMP-1 (collagensae-1) in ligaments and other tissues is also
Current Issues in Sports and Exercise Medicine
suppressed. Whether this is beneficial when treating chronic tendon lesions is unknown, but
some reports indicate the overexpression of MMPs in the Achilles tendinopathy [16].
Corticosteroids alter mechanical properties of tendons. Incubation of tendon fibrils in cortico‐
steroids resulted in a significant reduction in tensile strength after only 3 days [33, 34]. It is pos‐
sible, that corticosteroid injection affect the component of the extracellular matrix in a way that
influences tensile strength. They may reduce decorin gene expression and inhibit the prolifera‐
tion and activity of tenocytes, which leads to suppression in collagen production [34]. Howev‐
er, the magnitude of reduction in collagen type 1 and decorin gene expression appeared to be
smaller when corticosteroid treatment was combined with mechanical strain [35].
Recommendations for the use of local corticosteroid injections are still not clear. Application
should be peritendinous rather than intratendinous due to the demonstrated deleterious ef‐
fect of corticosteroid on tendon tissue. Short or moderate acting, more soluble preparations
are recommended because in theory they cause fewer side effects (hydrocortisone, methyl‐
prednisolone). Local anesthetics are usually mixed with the corticosteroid injection for wid‐
er dispersion and more comfortable procedure; but some manufacturers warn against
mixing because of theoretical risk of precipitation. Corticosteroid injections in »high strain
« tendons, especially the Achilles tendon or patellar tendon, are discouraged due to the pos‐
sible and well documented risk of tendon rupture [18]. This therapy should be reserved only
for chronic tendon injuries after the intensive use of other approaches for at least 2 months;
injections should be peritendinous only. One study showed an increased rupture risk only
when corticosteroids were injected intratendionously, but not when injected in peritendi‐
nous tissue. A maximum of three injections at one site should be given with a minimum in‐
terval between injections of 6 weeks. If two injections do not provide at least 4 week's relief,
they should be discontinued [18].
2.6. Future therapies to improve tendon and ligament healing
Injection of growth factors, especially those derived from activated thrombocytes, and tis‐
sue-engineering strategies, such as (1) the development of scaffold microenvironment, (2) re‐
sponding cells, and (3) signaling biofactors are generating potential areas for additional
prospective investigation in tendon or ligament regeneration. Tissue engendering is a prom‐
ising field to enhance tendon and ligament repair. Nevertheless, signicant challenges re‐
main to accomplish a complete and functional tendon or ligament repair that will lead to a
clinically effective and commercially successful application. More will be discussed in the
following sections.
3. Skeletal muscle damage and repair
Musculoskeletal injuries resulting in the necrosis of muscle fibers are frequently encoun‐
tered in clinical and sports medicine [36] and are the most common cause of severe long-
term pain and physical disability, affecting hundreds of millions of people around the world
and accounting for the majority of all sport-related injuries [37].
The Physiology of Sports Injuries and Repair Processes
The annual direct and indirect costs for musculoskeletal conditions in the United States
were estimated at USD $849 billion or ~ 8% of the gross domestic product. Similarly, a study
published in 2009 by Fit for Work Europe, examining musculoskeletal disorders in 23 Euro‐
pean countries, reported that > 44 million members of the European Union workforce had a
long-standing health problem or disability that affected their ability to work and that mus‐
culoskeletal disorders accounted for a higher proportion of sickness absence from work than
any other health condition. In 2009, the total cost of musculoskeletal disorders in European
workforce was estimated at €240 billion a year [38, 39].
Injured skeletal muscle can undergo repair spontaneously via regeneration; however, this
process often is incomplete because the overgrowth of extracellular matrix and the deposi‐
tion of collagen lead to significant fibrous scarring [40, 41].
Muscle injuries therefore frequently result in significant morbidity, including early function‐
al and structural deficits, contraction injury, muscle atrophy, contracture, and pain.
By neutralizing pro-fibrotic processes in injured skeletal muscle, it is possible to prevent fib‐
rosis and enhance muscle regeneration, thereby improving the functional recovery of the in‐
jured muscle [40].
3.1. Muscle structure and mechanism of action
A number of non-contractile connective tissue elements are necessary for the organization of
the contractile muscle fibers into effective mechanical stress. Thus the fibers are bound to‐
gether into fascicles by the fibroelastic perimysium; the ends of the muscle are attached to
the bones by tendons and aponeuroses, and the whole muscle is held in its proper place by
the connective tissue sheets called fasciae [42].
The arrangement of muscle fascicles, and the manner in which they approach the tendons,
has many variations. In some muscles, the fascicles are parallel with the longitudinal axis
and terminate at either end in flat tendons. In case of the converging fascicles to one side of
a tendon the muscle is called penniform, like the semimembranosus muscle. If muscles con‐
verge to both sides of a tendon, they are called bipenniform, or if they converge to several
tendons, they are called multipenniform, as in case of deltoid muscle. The nomenclature of
striated muscle is based on different parameters describing their properties (Table 2).
The arrangement of fascicles and the power of muscles are positively correlated. Those with
comparatively few fascicles, extending the length of the muscle, have a greater range of mo‐
tion but not as much power. Penniform muscles, with a large number of fascicles distributed
along their tendons, have a greater power but a smaller range of motion (ROM).
Molecular basis of muscle contraction is in the interaction between actin and myosin, fuelled by
ATP and initiated by the increase in [Ca
. Skeletal muscle possesses an array of transverse T-
tubules extending into the cell from the plasma membrane, through which the action potential
is spread into the inner portion of the muscle fiber (Figure 10), followed by releasing a short
puff of Ca
from the sarcoplasmic reticulum (SR) into the sarcoplasm. Ca
binds to troponin, a
protein that normally blocks the interaction between actin and myosin. When Ca
binds, tro‐
ponin moves out of the way and allows the contractile machinery to operate.
Current Issues in Sports and Exercise Medicine
Muscle, named by Muscle
∙ brachialis
∙ supraspinatus
∙ rectus abdominis
∙ obliquus abdominis
∙ flexor hallucis
∙ extensor digitorum
∙ deltoideus
∙ trapezius
attachment points
∙ sternocleidomastoideus
∙ omohyoideus
Table 2. Muscle nomenclature according to different parameters.
Figure 10. Molecular basis of muscle contraction.
3.2. Muscular injury mechanisms
Muscle injuries can be a consequence of a variety of causes: during the exercise, on the
sports field, in the workplace, during surgical procedures, or in any kind of accidents.
Regarding the mechanism, they are classified as direct and indirect. Direct injuries in‐
The Physiology of Sports Injuries and Repair Processes
clude lacerations and contusions, whereas the indirect class involves complete or incom‐
plete muscle strain [43].
The current classification of muscle injuries distinguishes mild injuries from moderate and
severe, based on the clinical symptoms. In a mild muscle injury, a strain or contusion is
characterized by a tear of only a few muscle fibers with minor swelling and discomfort ac‐
companied with no or only minimal loss of strength and restriction of movement. Moderate
injury is represented by greater muscle damage with a clear loss of function, whereas a tear
across the entire cross-section of the muscle resulting in a virtually complete loss of muscle
function, is termed a severe injury [44, 45].
Muscle strain injuries after eccentric contractions are the most common type of muscle in‐
jury in athletes and are especially common in sports that require sprinting or jumping [46].
Submaximal lengthening contractions are used in everyday life, but it is well known that
high-force lengthening contractions are associated with muscle damage and pain [47, 48].
Muscle strains are divided into three grades according to severity (Table 3) [43].
Muscle strains classification according to clinical severity
Grade Clinical Manifestation
Tear of new muscle fibers with minimal swelling and discomfort
Minimal loss of strength with almost no limitation of movements
A greater damage of muscle
Partial loss of strength and limitation of movements
A severe tear across the whole section of the muscle
Total loss of the muscle function
Table 3. Classification of muscle strains according to clinical manifestation [43].
3.3. Pathophysiology of muscle damage and repair
The cellular and molecular mechanisms of muscle regeneration after injury and degenera‐
tion have been described extensively in recent decades [39, 49, 50]. Physiologically, healing
progresses over a series of overlapping phases [43]. These stages include: (a) hemostasis,
which usually starts with the formation of a blood clot and is followed by the local degranu‐
lation of platelets, which release several granule constituents; (b) the acute inflammatory
phase is characterized by peripheral muscle fiber contraction, formation of edema and cell
damagen and death; and (c) the remodeling phase that lasts from 48 hrs up to 6 wks; ana‐
tomic structures are restored and tissue regeneration occurs. Several cell types are involved
in this phase and fibroblasts start to synthesize scar tissue.
Only local necrosis affects the injured ends of the myofibres because the torn sarcolemma is
rapidly resealed, allowing the rest of the ruptured myofibres to survive [51]. Debris is re‐
moved by macrophages that secrete growth factors and activate the satellite cells. These are
regenerative mononucleated stem cells of muscle tissue that normally lie between the basal
Current Issues in Sports and Exercise Medicine
lamina and plasma membrane of the muscle fiber [52]. First, they form myoblasts which
then begin to produce muscle specific proteins and finally mature into muscle fibers with
peripherally located nuclei [49].
Figure 11. Role of satellite cells in muscle regeneration after acute injury. (a) quiescent satellite cells in a normal mus‐
cle just above sarcolemma; (b) mechanical stress and growth factors released from macrophages activate satellite cells
that begin to express myogenic proteins which further stimulate proliferation; (c) in early differentiation phase, myo‐
blasts express myogenin and MRF4, factors that promote further differentiation and the fusion of mononucleated
cells; (d) in the late differentiation phase polynucleated myotubes begin to express factors that promote the final fu‐
sion and definite differentiation of myotubes into mature myofibres; (e) although muscle tissue is capable of self-re‐
generation, partial fibrosis contributes to function loss.
The Physiology of Sports Injuries and Repair Processes
A typical feature during muscle differentiation is the variation in expression of various genes
along with myogenic factors [53]. Sequence-specific myogenic regulatory factors (MRFs) are
expressed exclusively in skeletal muscle and regulate the process of muscle development [54]
(Figure 11). It is their role to govern the expression of multiple genes in myogenesis, from the
engagement of mesodermal cells in the muscle lineage, to the differentiation of somatic cells
and the terminal differentiation of myocytes into myofibres [55].
The MRFs consist of a group of transcription factors. They have been divided into two func‐
tional groups: The primary MRFs, MyoD, and Myf-5 required for the determination of skele‐
tal myoblasts; and the secondary MRFs, myogenin and MRF4 that act later in the program,
most likely as differentiation factors [54]. Activated satellite cells first express either Myf-5
or MyoD followed soon by co-expression of Myf-5 and MyoD. After the proliferation, myo‐
genin and MRF4 are expressed in cells and begin their differentiation program [53].
The cellular process required for degeneration and regeneration may be affected by altera‐
tions in the inflammatory response. Although strained skeletal muscle is capable of self-re‐
generation, the healing process is slow and often incomplete, resulting in strength loss and a
high rate of reinjury at the site of the initial injury [40]. Unfortunately, the muscle repair
process involves a complex balance between muscle fiber regeneration and scar-tissue for‐
mation [39].
3.4. TGF-β and myostatin – a key factors in muscular scarring
TGF-β is a cytokine with numerous biologic activities related to wound-healing, including
fibroblast and macrophage recruitment, stimulation of collagen production, downregulation
of proteinase activity, and increases in metalloproteinase inhibitor activity. There are three
mammalian isoforms of TGF-β: TGF-β1, TGF-β2, and TGF-β3. All three isoforms are poten‐
tially produced by most cells active in wound-healing, with platelets being a major contribu‐
tor [56]. The major functions of TGF-β are listed in Table 4.
Activity of TGF-β
Stimulation of mesenchymal cell proliferation
Regulation of endothelial cells and fibroblasts
Promotion of extracellular matrix production
Stimulation of endothelial chemotaxis and angiogenesis
Inhibition of macrophage and lymphocyte proliferation
Inhibition of satellite cell differentiation
Table 4. Activity of TGF-β summarized by Borrione et al. [43]
TGF-β is a potent stimulator of fibrosis in the kidneys, liver, heart, and lungs [57-59] and is
closely associated with skeletal muscle fibrosis as well where it plays a significant role in
both the initiation of fibrosis and the induction of myofibroblastic differentiation of myogen‐
Current Issues in Sports and Exercise Medicine
ic cells in injured skeletal muscle [41]. Many reports indicate that the overproduction of
transforming growth factor TGF-β1 in response to injury and disease is a major cause of tis‐
sue fibrosis both in animals and humans [36, 57].
Muscle-derived stem cells (MDSDs) are populations of stem cells that appear to be distinct
from satellite cells and can differentiate into myofibroblasts after muscle injury [41]. But my‐
oblasts can also differentiate into fibrotic cells where TGF-β is a key factor that stimulates
fibrotic differentiation [36].
Inhibition of TGF-β has been shown to decrease collagen deposition and scarring. For exam‐
ple, the application of neutralizing antibodies to TGF-β in rat incisional wounds successfully
reduced cutaneous scarring [53].
However, it is not yet clear whether TGF-β acts alone or requires an interaction with other
molecules during the development of muscle fibrosis. Recent studies have shown that myo‐
statin may also be involved in fibrosis formation within skeletal muscle [60, 61].
Over the last years, the TGF-β member myostatin (MSTN) has gained particular relevance
because of its ability to exert a profound effect on muscle metabolism, by regulating the my‐
ofibre size in response to physiological or pathological conditions [62]. Myostatin or GDF8
(Growth differentiation factor 8) is a TGF-β protein family member that inhibits muscle dif‐
ferentiation and growth [63] and is expressed specifically in developing and adult skeletal
muscle [62]. It inhibits the activity of satellite cells during muscle regeneration due to its
control of the movement of macrophages, and also inhibits the multiplication of myoblasts
and their differentiation [64]. In myogenic cells, myostatin induces down-regulation of Myo-
D, an early marker of muscle differentiation, and decreases the expression of Pax-3 and
Myf-5, which encode transcriptional regulators of myogenic cell proliferation [65]. Its ex‐
pression is restricted initially to the myotome compartment of developing somites and con‐
tinues to be limited to the myogenic lineage at later stages of the development and in adult
animals [53]. Major functions of myostatin are summarized in Table 5.
Activity of myostatin
Inhibition of satellite cell activity
Control of macrophage movement
Down-regulation of MyoD
Inhibition of transcriptional regulators of proliferation
Inhibition of myoblast multiplication in differentiation
Regulation of myofibre size
Table 5. Activity of myostatin.
Myostatin loss-of-function due to naturally occurring mutations into its gene triggers mus‐
cle mass increase in cattle [66], dogs [67], and humans as well [68]. Jarvnien et al. reported
that the injection of a neutralizing monoclonal antibody to myostatin led to increased skele‐
The Physiology of Sports Injuries and Repair Processes
tal muscle mass in mice without side effects [51]. This method was found to be safe in a sub‐
sequent clinical trial, although dose escalation was limited by cutaneous hypersensitivity
restricting potential efficacy [69]. Blocking of the MSTN signaling transduction pathway by
specific inhibitors and genetic manipulations has been shown to result in a dramatic in‐
crease of skeletal muscle mass [70]. In principle, blocking of MSTN signaling can be ach‐
ieved by three different pharmacological strategies: blocking MSTN gene expression
(knocking out, inactivating the MSTN gene by viral-based gene overexpression, and anti‐
sense technologies); blocking the synthesis of the MSTN protein; and blocking of the MSTN
receptor (small molecules, specific blocking antibodies) [71].
3.5. Therapeutic standards and controversies in treatment of muscle injuries
Despite the clinical significance of muscle injuries, the current treatment principles for in‐
jured skeletal muscle lack a firm scientific basis and are based on performing RICE (Rest,
Ice, Compression, and Elevation). These four methods are supposed to limit the hematoma
formation, though there are no randomized studies confirming their true value in the man‐
agement of soft tissue injuries [72].
The most convincing is the effect of “rest” on muscle regeneration [73]. Limb immobilization
prevents further retraction of the injured muscle and thereby greater discontinuity of the tis‐
sue, enlargement of hematoma, and the consequential scar tissue formation. Putting „ice“al‐
so limits the formation of the hematoma, additionally impairs inflammation, and accelerates
early tissue regeneration [74]. Concerns about the limited perfusion in the damaged muscle
because of the limb „compression“ are putting it under question while its „elevation“ above
the level of the heart follows the basic physiological principles as the hydrostatic pressure in
the elevated tissue falls, followed by lesser interstitial fluid accumulation and the formation
of edema. In this phase it is recommended to maintain the cardiovascular fitness without the
risk for reinjury like cycling or swimming [51].
Although lacking scientific background, therapeutic ultrasound is a widely accepted adju‐
vant method for treating muscle injuries [75]. Micro massage with high-frequency waves
has a pain relieving effect and it is supposed to act proregeneratory, especially in the early
phase after an injury [51]. Despite promoting proliferation, therapeutic ultrasound does not
seem to have a positive effect on the final outcome of muscle healing [76, 77].
Another adjuvant therapeutic option for improving muscle repair is hyperbaric oxygen ther‐
apy (HBO), which has shown to have positive effects during the early phase of repair by ac‐
celerating the recovery of the injured muscle [78]. However, not a single randomized
prospective study has been performed on the treatment of severe skeletal muscle injuries by
HBO, which might increase the sensation of pain in less severe forms of injuries like delayed
onset muscle soreness (DOMS) [79]. In case of both mild and severe muscle injuries there is
a lack of clinical studies confirming the real place of this therapeutic option in athletes.
The use of non-steroidal anti-inflammatory drugs (NSAID’s) in the treatment of muscle inju‐
ries is common, but controversial. The most commonly prescribed are COX-2 inhibitors ad‐
ministered either via intramuscular, oral or transdermal route [39]. While the first studies
Current Issues in Sports and Exercise Medicine
reported on the positive effects of NSAID’s on muscle regeneration without compromising
muscle contractility or stem cell proliferation, the more recent showed the importance of the in‐
flammatory process after injury and by inhibiting it the NSAID’s promote scar tissue formation
[80, 81]. Incomplete muscle fiber regeneration and fibrotic infiltration can lead to long-term
functional deficits and physical incapacitation [39]. The use of glucocorticoids in case of muscle
injuries is even more questionable as the elimination of the hematoma and necrotic tissue
seems to be slower and biomechanical strength of the injured muscle reduced [66, 82].
The identification of MRFs allows researchers a new and more detailed insight into the proc‐
esses of muscle regeneration which is crucial for developing novel therapeutic targets. In re‐
cent years many studies using antifibrotic agents have been performed in patients with
different heart and kidney diseases or systemic sclerosis. In vitro and In vivo studies showed
important antifibrotic effects of platelet-rich plasma derived growth factors, recombinant
proteins such as decorin, follistatin, γ-interferon, suramin, relaxin, and other biologically ac‐
tive agents like mannose-6-phosphate, N-acetylcysteine, and angiotensin-receptor blockers.
Although none of these has yet been tested on humans, their promising effects may signifi‐
cantly alter the therapeutic options of muscle injuries in the future. Furhter discussion on
these bioactive agents will follow in Chapter XX (numer needed: Latest advances).
4. Articular cartilage damage and repair
Cartilage comprises of inherited limited healing potential and thus remains a challenging
tissue to repair and reconstruct. Traumatic and degenerative cartilage defects occur fre‐
quently in the knee joint and represent difficult clinical dilemma. Articular cartilage has a
limited capacity to self-repair principally due to its avascular nature and the limited ability
of mature chondrocytes to produce a sufficient amount of extracellular matrix. Untreated
cartilage injuries therefore lead to the development of arthritis. Current first line treatment
options for smaller and mid-sized lesions in lower-demand patients are debridement or lav‐
age and bone marrow-stimulating techniques (microfracture) which promote a fibrocarti‐
lage healing response. On the other hand, restorative treatment options such as
osteochondral autologous graft transplantation (OATS) are limited by the amount of donor
tissue availability and the size and depth of the defect. Regenerative treatment techniques
such as autologous chondrocyte implantation (ACI) are promising treatment options for
large full thickness articular cartilage defects where cells from healthy non-weight bearing
areas are multiplied In vitro and implanted into such defects. Opposed to the traditional rep‐
arative procedures (e.g. bone marrow stimulation microfracture), which promote a fibro‐
cartilage formation with lower tissue biomechanical properties and poorer clinical results,
ACI is capable to restore hyaline-like cartilage tissue in damaged articular surfaces. This
technique has undergone several advances and is constantly improving. Indeed, there are
numerous studies exploring new biomaterials; applications of various growth factors; the
synergistic effects of mechanical stimulation in terms of tissue engineering In vitro, In vivo,
and in animal models in order to stimulate the formation of hyaline-like cartilage.
The Physiology of Sports Injuries and Repair Processes
4.1. Cartilage structure
Articular (hyaline) cartilage is a specific and well-characterized tissue with remarkable me‐
chanical properties consisting of exclusively one cell type - chondrocytes which are embed‐
ded in the extracellular matrix (ECM). The principal function of articular cartilage is to
withstand mechanical loads, facilitate smooth and perfect glide among articular surfaces,
and enable painless and low friction movements of synovial joints. The articular cartilage is
an aneural, avascular and alymphatic structure. The nutrition of chondrocytes occurs via
diffusion between synovial fluid and cartilage matrix.
The only resident cells in articular cartilage (chondrocytes) contribute to only 1-5 % of tissue
volume. The remaining 99 % represent the extracellular matrix (ECM) structural compo‐
nents that mainly consist of water, collagen, and proteoglycans (PGs). ECM works as a bi‐
phasic structure composed of a fluid phase (water and electrolytes) and solid phase
consisting mainly of collagen and proteoglycans. The solid phase comprises of low permea‐
bility due to the high resistance of a fluid flow which causes a high rate of fluid pressuriza‐
tion and contributes to the load transmission of cartilage. Together, both solid and fluid
phase establish the stiffness and viscoelastic properties of a cartilage [83, 84].
4.1.1. Structural layers
The structure of cartilage matrix varies with the depth; four different zones (superficial,
transitional, radial, and calcified) are distinguished based upon the cell morphology, matrix
composition, and collagen fibril orientation (Figure 12). Chondrocytes change their confor‐
mation from parallel to vertical in deep zones. Similarly, collagen fibers alignment becomes
parallel in deeper zones of cartilage tissue. There is also an increase in the overall volume,
water content, and overall biological activity in deeper zones [85].
Figure 12. Structural layers of articualar cartilage.
Current Issues in Sports and Exercise Medicine
Chondrocytes are specialized cells and basic structural cells in the articular cartilage, which
are sparsely spread within the matrix and altogether form only 1-5 % of cartilage volume.
They are deprived of blood supply and obtain the nutrients by diffusion from synovial fluid.
The formation of cartilage tissue and maturation of chondrocytes follows a multi-step proc‐
ess called chondrogenesis. In general it comprises of mesenchymal stem cell proliferation
and their differentiation into mature chondrocytes capable to synthesize structural compo‐
nents of ECM (type II collagen, PG and non-collagenous proteins) and to maintain its contin‐
uous formation and restoration.
Each step of chondrogenesis can be classified according to the expression of different sets of
transcription factors, cell adhesion molecules and extracellular matrix components. Chon‐
drocytes have no cell-to-cell contacts, are highly metabolically active (however, due to low
overall cell volume the total activity appears low) and are exposed to low oxygen environ‐
ment and anaerobic metabolism. Mature chondrocytes are in the continuous communication
with ECM and hence respond to changes in ECM and regulate its metabolism [85, 86].
Cartilage tissue is under constant impact of anabolic and catabolic cellular activity in re‐
sponse to extracellular environment and exposure to different cytokines and growth factors.
Anabolic proteins such as tumor growth factor beta (TNF-beta), insulin growth factor
(IGF-1), bone morphogenic protein (BMP), and fibroblast growth factor (FGF) stimulate ma‐
trix formation and promote the anabolic activity of chondrocytes. On the other hand, catabo‐
lic proteins such as tumor necrosis factor alpha (TNF-α) and interleukin 1 beta (IL-1β)
inhibit protein synthesis and promote matrix degeneration. The constant equilibrium in the
functioning of all signaling pathways is of crucial importance for the proper function and
maintenance of cartilage tissue. The modern concept of cartilage tissue engineering is based
on the imitation of the cartilage natural environment and the process of chondrogenesis to
try to stimulate the formation of such a cartilage, which contains all the structural and bio‐
mechanical properties of native cartilage [87].
Extracellular matrix (ECM) is consists of water, collagen, and proteoglycans. All together wa‐
ter represents 60-85 % of the weight of the cartilage. The water content varies with the depth
of the tissue; near the articular surface the water content is the highest and PG concentration
is relatively low; vice versa is found in a deeper zone near subchondral bone, where the wa‐
ter content is the lowest but the PG concentration is the greatest. A high amount of water
content in cartilage tissue is important for nutrition, lubrication, and for creating a low-fric‐
tion gliding surface. In diseased cartilage such as osteoarthritis, the water content amounts
to more than 90% as a result of matrix disruption and increased permeability. This leads to
the decreased modulus of elasticity and reduction in load bearing capability.
Collagen is the main component of ECM. This fibrous protein represents 60 to 70% of the
dry weight of the tissue. Type II collagen is the predominant collagen (90–95%) of ECM and
provides a tensile strength to the articular cartilage. The high rate of cross-linkage between
collagen molecules provides cartilages its resistance against traction forces. Other types of
collagen molecules are also found in cartilage tissue in smaller amounts, these are types V,
VI, IX, X and XI. Type IX and XI are most abundant in minor types collagen. Type XI partici‐
The Physiology of Sports Injuries and Repair Processes
pates in cross-linkage with type II collagen, integrins, and proteoglycans, whereas type XI is
important in regulating the fibril diameter of type II collagen. Collagen architecture varies
through the depth of the tissue. On the sliding surface of entire cartilage (tangential zone)
collagen fibers are oriented parallel to the cartilage surface.
Proteoglycans (PGs) are protein polysaccharides and form 10–20% dry weight of the articu‐
lar cartilage. Their primary function is to provide compressive strength to cartilage tissue. In
articular cartilage they can be classified in two major classes, large aggregating proteoglycan
monomers (aggrecans) and small proteoglycan molecules (decorin, biglycan, and fibromo‐
dulin). PG are composed of glycosaminoglycans (GAG) subunits (chondroitin and keratin
sulfate) which are bound to a central core protein via sugar bonds to form proteoglycan ag‐
grecan, which is highly characteristic for hyaline cartilage. Aggrecan, 250 kDa protein repre‐
sents more than 80 % of all PG molecules in cartilage tissue. It binds to hyaluronic acid to
form high molecular weight aggregates with more than 3.5 x 106 kDa. In the cartilage tissue
these aggregates are located within the collagen type II fibril network resulting in densely
packed negative charge which interacts with water via hydrogen bond and causing electro‐
static repulsion. This key feature enables cartilage tissue to resist deformation under com‐
pression and to withstand and redistribute mechanical [83] [84].
4.2. Cartilage lesions
Injuries to articular cartilage are observed with an increasing frequency in athletes. In partic‐
ular participation in pivoting sports such as football, basketball, and soccer they are associat‐
ed with a rising number of sport-related cartilage injuries. The exact incidence of the
cartilage damage is not known since they mostly appear asymptomaticly. However, during
a review of 25,124 and 31.516 knee arthroscopies the injury of articular cartilage was found
in 60 - 63 % [88, 89]. The incidence of 5 11 % was reported for full-thickness cartilage le‐
sions (ICRS grade III and IV) [90]. Additionally, cartilage injuries of the knee joint are often
accompanied with other acute injures such as ligament and meniscal injuries, traumatic pa‐
tellar dislocation, osteochondral injuries, etc. [91].
The main symptom in patients with cartilage defects is the joint pain. Patients may also ex‐
perience swelling and mechanical symptoms. Traumatic cartilage injury in the athletic pop‐
ulation may progress to chronic pathological loading patterns such as joint instability and
axis deviation. Although intact articular cartilage has the ability to adjust to the increasing
weight bearing activity in athletes by increasing cartilage volume and thickness recent stud‐
ies indicated that the degree of adaptation is limited [92]. Any activity beyond a threshold
value may therefore result in maladaptation and cartilage damage. It has been shown that
high impact joint loading above the adaptation limit causes decreased PGs content and leads
to increase of degradative enzymes release and chondrocytes apoptosis [93]. Eventually, the
integrity of functional weight bearing unit of cartilage is disrupted and leads to the loss of
articular cartilage volume and stiffness, elevation of pressure and further articular cartilage
damage in the long run.
Clinically, focal lesions are ranked according to the appearance of superficial zone of articu‐
lar cartilage and are generally small (<1cm2) and sub-chondral and therefore asymptomatic.
Current Issues in Sports and Exercise Medicine
It is difficult to predict whether the chondral lesion will progress to the more extensive deg‐
radation. However, in animal studies it was observed that smaller defects have the potential
of spontaneous healing while the inverse relationship to repair potential was revealed in
larger defects [94]. Once a patient becomes symptomatic due to cartilage damage, the lesion
is likely to progress. A mechanical injury to articular cartilage can be acute, chronic, or acute
and chronic. Cartilage loss often occurs after single or repeated impact loading due to trau‐
ma or misalignment. An increase in shear forces as a consequence of chronic abnormal load‐
ing of a joint surface results in irreversible changes in the biochemical composition of
articular cartilage. Loading studies reported of significant swelling of articular cartilage (in‐
creased water content) and changes in the proteoglycans content only two weeks after ab‐
normal loading [95].
Cartilage tissue has a limited intrinsic capacity of healing response after cartilage damage,
thus cannot fully regenerate and often leads to secondary degenerative disease. Early recog‐
nized and treated cartilage lesions might therefore prevent the secondary damage and pro‐
gression to the osteoarthritis. The main raisons for limited capacity to self-repair and
regeneration seem to be the avascular nature of cartilage tissue and inability for clot forma‐
tion, which is the basic step in the healing cascade. That is why progenitor cells in blood and
bone marrow and resident chondrocytes are unable to migrate to sites of the cartilage lesion
[96]. Generally, intrinsic cartilage repair does not follow the main steps that usually occur
after an injury in the other tissue: necrosis, inflammation, and repair or remodeling. Further‐
more, mature chondrocytes own limited proliferative capacity and have the limited ability
to produce a sufficient amount of extracellular matrix to cover the defect. However, several
cells are mobilized to the cartilage surfaces after an injury and can produce the repair ma‐
trix, although this matrix is morphologically and mechanically inferior to the original native
cartilage tissue. Such a spontaneous healing was observed in small sub-chondral defects of
fetal lambs and partial healing was also detected in small (less than 3 mm diameter) full-
thickness lesions in rabbits [97]. However, larger cartilage defects of more than 6 mm rarely,
if ever, show intrinsic healing potential but lead to progressive degenerative disease [94].
4.2.1. Partial and full thickness defects
Cartilage lesions can be divided into partial thickness defects which do not penetrate the
subchondral bone and do not repair spontaneously, and full thickness defects which do pen‐
etrate subchondral bone have a partial repair potential, depending on the size and locations
of the defect (Figure 13) [98]. The nature of the partial thickness defects has been studied
and it was observed that the cells adjacent to the wound margin undergo cell death. Howev‐
er, there is an increase in cell proliferation, chondrocyte cluster formation, and matrix syn‐
thesis, but this repair is short-lived and eventually fails to repair the defect. It was also
documented that the cells from synovia can migrate to the lesion in the presence of growth
factors and can fill the defect with repair tissue. Due to anti-adhesive properties of PG and
the absence of fibrin matrix these cells usually fail to adhere to the surface of defect [99].
The potential of cartilage repair in full thickness lesions is due to breaching of subchondral
bony plate which leads to local influx of blood and undifferentiated mesencyhmal cells and
The Physiology of Sports Injuries and Repair Processes
hematoma formation containing fibrin clot, platelet, red and white blood cells. The blood
clot can only fill the smaller defects < 2-3mm in diameter from the subchondral bone mar‐
row. However, mobilized cells in the newly formatted blood clot are not capable to replace
the defect with native hyaline cartilage, but produce fibrocartilage tissue, composed of high‐
er collagen type I to collagen type II ratio and less proteoglycan, which has as mentioned
already inferior properties compared to native hyaline cartilage. Several surgical techniques
used the same attempt to treat full thickness defects such as micfrofracture which penetrate
the subchondral bone in order to stimulate the clot formation and immobilize cells to the
side of cartilage lesion [98].
Figure 13. Partial and full thickness defects of articular cartilage.
4.2.2. Cartilage lesion classification
There are several classification systems to access cartilage lesion used in clinical practice. A
number of elements are important in deciding what intervention might be the most helpful
in trying to restore cartilage tissue such as: the size and area of cartilage damage, the depth
of the damage, the degree of functional disability, patients' age, etc. However, not enough is
known about a proper treatment of particular cartilage. Therefore, more objective data,
methods and operative outcomes are required for good decision making regarding the treat‐
ment modalities since new procedures are rather expensive. Currently, the structural classi‐
fications such as Outerbridge and ICRS Classification (Table 6) are commonly used
involving the examination of the extent and the depth of the cartilage lesion that helps sur‐
geons to follow progression and improvement of the cartilage lesions.
Current Issues in Sports and Exercise Medicine
OUTERBRIDGE - description ICRS - description
GRADE 0 normal cartilage normal cartilage
GRADE 1 cartilage with softening and swelling nearly normal:
soft indentation and/or superficial fissures
and cracks
GRADE 2 a partial-thickness defect
(fibrillation or superficial fissures)
less than 0.5-in diameter
a partial-thickness defect:
extending down to <50% of cartilage
GRADE 3 deep fissuring of the cartilage to the level of subhondral
bone without bone exposed
greater than 0.5-in diameter
a partial-thickness defect:
extending down to "/>50% of cartilage
GRADE 4 exposed subchondral bone. severely abnormal (through the
subchondral bone)
Table 6. Classification of articular cartilage lesions: Outerbridge and ICRS classification
The modified ICRS classification describes the defect macroscopically and correlates better
with clinical outcome; grade 1 has good, grade 2-3 intermediate and grade 4 poor clinical
result. However, along with the grade and depth, it is important to record the dimensions
and position of the lesion (Modified ICRS Chondral Injury Classification System), to assess
any bone loss or sclerotic change, the thickness of the surrounding cartilage and surround‐
ing walls. Additionally, overall outcome depends also on patient’s age, BMI index, the level
of physical activity, etc.
4.3. Treatment of articular cartilage lesions
The main goals of surgical management of cartilage defects are to reduce symptoms, restore
cartilage congruence, prevent additional cartilage deterioration, and to maintain the func‐
tion of the joint without the insertion of artificial implants. Surgical treatment options may
be divided upon their expected outcome as palliative, reparative or restorative [15]. Many
procedures lead to the formation of fibrocartilaginous tissue with significantly inferior bio‐
chemical properties compared with those of hyaline cartilage. The newly formed scar tissue
is unable to prevent a progression of a degenerative cartilage disease. The application of a
specific surgical method is based on the patient’s demand and the level of symptoms. For
example, in lower demand patients with fewer symptoms the effective first-line treatments
are palliative such as debridement and lavage. Similarly, reparative techniques are used in
patients with moderate symptoms such as bone marrow stimulating procedures (drilling,
abrasion arthroplasty, or microfracture) in effort to promote fibrocartilage formation. How‐
ever, larger cartilage defects in higher demand patients (e.g. athletes with extreme weight
bearing activity) with significant symptoms may not profit from standard treatment options,
but should be advanced towards reparative treatment options such as autologous chondro‐
cyte implantation (ACI) or osteochondral grafting [100].
The Physiology of Sports Injuries and Repair Processes
4.3.1. Debridement and lavage
The goals of palliative treatment options (debridement and lavage) are the reduction of
the inflammatory response due to mechanical irritation, functional improvement, and
pain relief. Debridement involves the smoothing of cartilage and meniscal surfaces, re‐
moving necrotic tissue, and refreshing edges of cartilage lesions. Likewise, the beneficial
effect of lavage implies the reduction of inflammation; removal of free cartilage frag‐
ments due to an injury and potential calcium phosphate crystals. Although the effective‐
ness of such a method is short-termed since it does not apply the restoration of cartilage
defects, it significantly reduces pain symptomatic and improves the functionality of the
articular joint compared to the conservative therapy. It is primarily recommended for pa‐
tients with lower daily physical load and specifically localized mechanical symptoms
(e.g. meniscal tear). Rehabilitation time after surgery is short and allows immediate load‐
ing activities without restrictions [101, 102].
4.3.2. Marrow stimulating techniques
Articular cartilage is deprived of its own blood supply; therefore traditional wound heal‐
ing and clot formation is not possible. By opening up the subchondral bone plate, which
separates the cartilage layer from the blood supply in bone marrow, hemorrhage can be
induced to stimulate mesenchymal stem cells (MSCs), leukocytes, and growth on the side
of the lesion as well as trigger remodeling and fibrocartilaginous cartilage repair. Bone
marrow stimulating techniques are divided into drilling, microfracture, and abrasion,
and are all based on the infiltration of blood products, fibrin clot formation, and fibrocar‐
tilage tissue repair [103].
Nowadays, microfracture is often used as a primary treatment option, and if not successful,
more invasive cartilage repair methods are performed. The procedure is performed arthro‐
scopically after a careful examination of articular cartilage surface and the quality of the car‐
tilage. First, the focal chondral defects are debrided and the walls of the defect are
smoothened. Any calcified cartilage is removed from the defect zone in order to prepare a
better surface for the adherence of the clot and improved chondral nutrition through sub‐
chondral diffusion. Likewise, the walls of the lesion should be perpendicular to the defect to
provide an area where the clot progenitor cell can form and adhere. After the initial prepara‐
tion, the surgical awl is used to make multiple holes in the exposed subchondral bone. The
holes should be placed 3-4mm from each other and should not connect. Subsequently, blood
clot rich with bone marrow elements is formed which eventually undergoes the phase of re‐
modeling and turns into fibrocartilage tissue [101-103]. Such cartilage resembles the native
cartilage, but it differs significantly in the structural, biochemical, and mechanical properties
and mostly contains type I collagen, which is cartilage non-specific and results in poor me‐
chanical properties and poorly integrates into the adjacent cartilage.
A major concern is therefore the longevity of a fibrocartilage to withstand the stress and me‐
chanical load on an active knee joint [104]. However, in follow-up studies 7-10 years after
the surgery pain release and improved joint functionality was reported [105]. Moreover, mi‐
crofracturing seems to have similar clinical results as ACI (look further chapter). Another
Current Issues in Sports and Exercise Medicine
problem was recently reported with microfracture procedure whether it can decrease the
success of further alternative procedures such as ACI. In the study patients allocated to bone
stimulating technique showed similar results following ACI as those where only debride‐
ment alone was performed [106]. Furthermore, in another study patients who previously
underwent bone stimulating procedure showed a poorer outcome after ACI compared to
those where only ACI alone had been performed [107].
Postoperative rehabilitation plays a key role in overall success of the treatment. Patients
should undergo continuous passive motion physiotherapy for a period of 4 to 6 weeks and
have the protected weight bearing. Following that period, patients are allowed an active
range of motion exercises and progression to full weight bearing. However, no cutting,
jumping or twisting sports are allowed until at least 4 – 6 months after surgery.
4.3.3. Osteochondral autograft transplantation (AOT)
Regeneration of damaged cartilage can be achieved with bone-cartilage transplants called
osteochondral autograft transplantation (AOT). Nowadays, AOT is a well-established tech‐
nique, but since the majority of the cartilage defects found in the knee joint are chondral
rather than subchondral, there is a controversy regarding the overall usage of a osteochon‐
dral grafts and reaming in the healthy subchondral bone. The surgical procedure of AOT in‐
volves the removal of a full thickness hyaline cartilage attached to its underlying bone and
the implantation of the osteochondral graft on the side of the lesion in a press-fit technique.
Osteochondral autografts are usually harvested from non-weight bearing areas in order to
avoid new damage or loss of function on the donor side. The site of the lesion should be pre‐
pared prior to implantation; any remaining cartilage is removed, the walls of the defect are
made smooth and the tunnel of the same size as of the cartilage plug is drilled. However, the
depth on the damage site should be 2 mm less than the plug size in order to achieve a favor‐
able and stable position of the osteochondral graft and maintain an appropriate fit to the
edges of the graft with surrounding intact cartilage. This helps to reduce shear stress on the
border of the graft and ensures long-term success of the transplantation. Cartilage defects
should not range more than 3cm2 due to a limited amount of donor tissue. For larger lesions
several osteochondral plugs are used, therefore the procedure is called »mosaicoplasty«.
The main advantage of osteohondral grafting is that it possesses the normal native hyaline
cartilage and does not include fibrocartilage which develops in the microfrature technique.
However, the disadvantages include donor side morbidity (pain and cartilage defect), tech‐
nical difficulty to match the shape of the plug to the contour of the articular joint, residual
gap between adjacent plugs, and the risk of osteochondral collapse. Postoperative rehabilita‐
tion contains the use of continuous passive motion machine and weight bearing restrictions
for a period up to 6 weeks. Clinical results are satisfactory; they reported good to excellent
results even 10 years after surgery in 79 - 92% patients. The effectiveness of the method de‐
pends on the site of injury and is the most successful in isolated injuries of the femur con‐
dyles [101, 102].
The Physiology of Sports Injuries and Repair Processes
4.3.4. Autologous cartilage implantation (ACI)
Autologous cartilage implantation represents a promising solution for the treatment of artic‐
ular cartilage and enables permanent replacement of damaged cartilage tissue with its own
native hyaline cartilage. The idea of an ACI is to harvest cartilage cells from the knee and
grow them In vitro under specific laboratory conditions (Figure 14). Once millions of cells
have been grown they are implanted into the area of cartilage defect. The procedure was
first proposed by Brittberg in 1994 [108] and has become more widespread so that it current‐
ly represents the most developed articular cartilage repair technique.
Figure 14. Proliferation of chondrocytes under monolayer culturing condition.
The original technique of ACI is a two-step procedure (Figure 15). The first step of ACI in‐
cludes an arthroscopy to identify and access cartilage damage. Once the lesion is deter‐
mined as suitable to perform the ACI procedure, the cartilage cells are harvested from the
non-weight bearing zone in the knee. The chondrocytes are then isolated and grown in the
tissue culture to allow them to multiply for several weeks. Once a sufficient number of carti‐
lage cells has been obtained in the culture, the second surgery is scheduled. During the sec‐
ond surgery the cell suspension is re-injected into the cartilage defect underneath the
periosteal patch. It is very important that the periosteal patch is carefully sutured in place
and sealed with a fibrin glue in order to prevent any leakage of newly implanted cell sus‐
pension. ACI is usually used in intermediate and high-demand patients who have failed ar‐
throscopic debridement or microfracture. The technique can also be used for larger 2 10
symptomatic lesions. Prior to the surgery, patients must understand and be well pre‐
pared to participate in intensive postoperative rehabilitation and should fit the following
Current Issues in Sports and Exercise Medicine
profile: (1) the cartilage damage is focal and not widespread arthritis, (2)presence of pain or
swelling that limits everyday activities, (4) a stable knee with no associated ligament dam‐
age and (5) normal body mass index (BMI). The postoperative rehabilitation consists of non-
weight bearing in addition to range of motion (ROM) exercises with the use of a CPM
machine for 6 weeks. Due to two surgical procedures and larger open arthrotomy, pain re‐
lief and restoration of function may take as long as 12 to 18 months [109, 110].
Figure 15. Schematic diagram showing the different stages involved in the process of autologous chondrocyte
The effectiveness of ACI varies and different levels of success were reported. Recently, ACI
has been compared to microfracture technique. Both two and five years follow-up results,
after patients were randomized for ACI or microfracture treatment of localized articular le‐
sions of the knee joint, concluded that both methods had acceptable short-term results [111,
112]. There was no significant difference in macroscopic or histological results after two
years. Similarly, after five years both methods provided satisfactory results in about 77 % of
patients with no significant difference in clinical and radiographic results [112]. Currently, it
seems as ACI is as good or a slightly better technique compared to a less invasive, simpler,
and cheaper surgical technique in short-term. On the other hand, the significant superiority
of ACI over mosaicoplasty for the repair of articular defects in the knee was reported in pro‐
spective randomized controlled trails [113, 114].
The Physiology of Sports Injuries and Repair Processes
These results might not be surprising considering the traditional ACI (first generation ACI)
encountered several problems. The most common complication with 10-25% incidence is
periosteal hypertrophy due to scar tissue formation around the edge of the periosteal patch
[115]. In addition, the need of periosteum widens the donor site morbidity and extends the
operation time. The periosteum has to be tightly and waterproof sutured to prevent the po‐
tential leakage of the cell suspension from the defect. Another frequent disorder is patch de‐
lamination due to incomplete bonding of the patch with surrounding tissue. There are
several other disadvantages regarding this method: the growth of cartilage tissue is age-de‐
pendent (lower potential in the elderly), difficulty to harvest and isolate sufficient numbers
of cells from a small amount of tissue removed, fast differentiation of chondrocytes during
In vitro cultivation in monolayers (loss of phenotype, differentiation in fibroblast-like cells),
etc. Reoperation rate as high as 42 % was reported by several authors [116].
4.4. Future prospective for cartilage repair
Some of the problems have been avoided by using the collagen membrane instead of tradition‐
al periosteum patch. Anyway, the new technique (ACI-C) has still not solved the problem of
watertight sutures and possible leakage. Nevertheless, in randomized control trial the compar‐
ison among the two procedures showed a lower re-operative rate in ACI-C, most probably due
to the lesser extent of periosteum hypertrophy [117]. The new concept of cartilage tissue preser‐
vation was developed using tissue engineering technologies, combining new biomaterials as a
scaffold, and applying growth factors, stem cells, and mechanical stimulation [118]. The recent
development of so-called second regeneration ACI uses a cartilage-like tissue in a 3-dimen‐
sional culture system that is based on the use of biodegradable material which serves as a tem‐
porary scaffold for the In vitro growth and subsequent implantation into the cartilage defect. It
has been shown In vitro that the application of 3-D environment promotes hyaline-like carti‐
lage production and allows for mechanical stimulation [119, 120]. Several reports already de‐
scribed a superior role of the MACI (matrix/membrane autologous chondrocyte implantation)
and CACI (collagen-covered autologous chondrocyte implantation) compared to the standard
ACI procedure [121]. Additionally, the modern concept of tissue engineering uses various
types of growth factors which are the endogenous regulators of chondrogenesis and their logi‐
cal choice of use and relative ease of application have been reported to promote cartilage devel‐
opment [122]. Further studies are attempting to create the ideal scaffold and explore the
synergistic effect of concomitant application of growth factors and mechanical loading [120]
[123]. Finally, for clinical practice, single stage procedures appear attractive to reduce cost and
patient morbidity. These procedures are promising, but there are only a few clinical studies
and the results are in the process of publication and will be presented in the following chapter
as they represent the most advanced and future therapeutic strategies for cartilage repair.
5. Conclusion
Locomotory system injuries are significant public health problems that contribute to a large
burden of disability and suffering worldwide and are the most common injuries encoun‐
Current Issues in Sports and Exercise Medicine
tered in sports. The management of these injuries in athletes is particularly difficult as they
have high demands and expectations. Achieving a fast recovery time and low possibility for
reinjury is an ideal goal of each therapeutic team. Neglecting physiological processes in an
injured tissue can often lead to inappropriate therapeutical interventions followed by un‐
functional regeneration. The importance of keeping in mind the tissue processes at molecu‐
lar level is therefore crucial and the only way to appropriate therapies.
Author details
Kelc Robi, Naranda Jakob, Kuhta Matevz and Vogrin Matjaz
Department of Orthopedic Surgery, University Medical Center Maribor, Slovenia
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... The specimens were then subjected to a preconditioning test to align the tendon fibers, eliminate dead length, and remove fixture clearances. In this stage, a cyclical preload of 10 to 50 N was delivered to the graft for 10 cycles at a frequency of 0.1 Hz [10], [11], [50], [51]to eliminate any tendon loose length [57]. ...
... as long as the drop after the first peak is within 10% of the first peak load. This drop represents a local microfracture that can be healed in the body without affecting the overall structure [57]. All modes of failure were recorded for all the experimental sets along with the loads and displacements at the time of failure. ...
Full-text available
The present study introduced and mechanically tested an alternative method for graft preparation for ACL reconstruction surgery. The designed method aims to offer a replacement for conventional suturing, which reduces the chance of graft rupture and gradually increases the fixation strength. Six prototypes were introduced for the fixation. A Product Design Specification chart was developed to select the most suitable prototype. the characteristics presented in the chart were determined based on mechanical limitations and medical requirements stated by the consulting surgeons and available in the literature. Variants of the selected prototype were designed, modeled, and subjected to in-vitro experiments to validate the study. A set of experiments was performed. Five bovine tendon samples were harvested and trimmed into identical sizes. Samples were passed through two endo buttons and looped in a manner that both ends share a common surface. Two custom straps were later applied around the graft. A three-stage mechanical test was performed, a cyclical preload of 10 to 50 N, 10 cycles, and 0.1 Hz. The major pullout test with a cyclical load of 50 to 200 N, 1 Hz for 200 cycles, and a pullout force with a loading rate of 20 mm/min determining the ultimate strength of the fixation. No failure occurred on the overall structure during the cyclic stages. Also, no failure associated with tendon tissue damage was recorded. All samples experienced a fixation failure during the final pull-out test. The mean values for ultimate strength, cyclic elongation, and Average cyclic Stiffness were 287.66 ± 11.84 N, 2.08 ± 0.15 mm, and 14.52 ± 1.09 kN/mm respectively. The results indicate that using the proposed strapping method not only reduces any chance of tendon tearing but also presents acceptable major mechanical properties in comparison with the conventional suturing method. The study showed that the new design was mechanically sound. Keywords: Orthopedics, ACL Reconstruction, Mechanical Testing, Experimental, Mechanical Design.
... As mentioned, this behavior is probably owing to the inactivation of the crystals in the gypsum and the unsaturation of the calcium hydroxide and metakaolin. The amount of porosity in the GM9 specimen determined by the Archimedes method has the highest value compared to the other specimens (Table 6) The TOE region is characterized by a nonlinear stress and strain curve [33]. Figure 4 displays the amount of strain tolerance that the specimens have endured up to this point at the end of the toe. ...
Full-text available
This study investigated the effect of creating a composite of gypsum with metakaolin as well as the physical and mechanical behavior of the produced composites. For this purpose, gypsum composites were prepared with 2.9, 4.8, 6.5, and 9 wt.% metakaolin in 100 g of gypsum and a constant content of water. To determine the mechanical properties of the composites, the compressive strength test was used and the porosity, water absorption percentage, and bulk density of the composites were obtained using the Archimedes method. The results showed that the porosity was reduced by adding up to 7 wt.% metakaolin to the gypsum specimens, it increases the compressive strength by 41% and also raises the Young's modulus of gypsum by 121%. Scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS) was employed for the microstructural evaluations. The EDS-SEM observations showed the presence of Al and Si elements in the fracture zones. The presence of metakaolin elements at one point increases resistance in that area. Metakaolin-reinforced gypsum composites can be used in boards and panels.
... RMS values were normalised based on the results of 1 kg loading to give a universal frame for every exerciser. When a tendon has 8~10% strain, it is highly likely to tear 77,78 . As a result, such a high-possible injured circumstance is depicted as red range in the muscle injury index. ...
Full-text available
Surface electromyography (sEMG) can provide multiplexed information about muscle performance. If current sEMG electrodes are stretchable, arrayed, and able to be used multiple times, they would offer adequate high-quality data for continuous monitoring. The lack of these properties delays the widespread use of sEMG in clinics and in everyday life. Here, we address these constraints by design of an adhesive dry electrode using tannic acid, polyvinyl alcohol, and PEDOT:PSS (TPP). The TPP electrode offers superior stretchability (~200%) and adhesiveness (0.58 N/cm) compared to current electrodes, ensuring stable and long-term contact with the skin for recording (>20 dB; >5 days). In addition, we developed a metal-polymer electrode array patch (MEAP) comprising liquid metal (LM) circuits and TPP electrodes. The MEAP demonstrated better conformability than commercial arrays, resulting in higher signal-to-noise ratio and more stable recordings during muscle movements. Manufactured using scalable screen-printing, these MEAPs feature a completely stretchable material and array architecture, enabling real-time monitoring of muscle stress, fatigue, and tendon displacement. Their potential to reduce muscle and tendon injuries and enhance performance in daily exercise and professional sports holds great promise.
... We believe that selfresolution of the radial dislocation of the fifth extensor tendon may be due to stress relaxation. Stress relaxation is a mechanical property of tendons that refers to their ability to gradually decrease their tension or stress under constant deformation over time (20). In the case of radial-side dislocation of the fifth extensor tendon following sagittal band reconstruction of the third finger, the tension on fifth extensor tendon due to pulling force exerted on the juncturae tendinum may have initially increased due to the altered mechanics and forces in the hand. ...
Full-text available
Bilateral congenital dislocation of the extensor tendon in the metacarpophalangeal joint is an exceedingly rare disease and often involves multiple fingers. Surgical treatment of multiple congenital extensor tendon dislocations in both hands has been reported; however, no report has clearly stated whether all fingers should be surgically treated in patients with multiple finger involvement. We report a case in which we successfully treated bilateral congenital extensor tendon dislocation on multiple digits with only one single-loop reconstruction of the sagittal band instead of operating on all involved fingers.
... We selected a 6-month period because we estimated that given the ACL's relatively slow repair rate (Rucklidge et al., 1992), it would not be able to adapt to a substantial increase in ACL-straining athletic activities or maneuvers within this timeframe. Although the exact repair rate of the ACL is unknown, we do know that ligaments heal more slowly than other tissues (Panjabi, 2006;Jung et al., 2009) given that ligaments have lower metabolic activity rates than muscle, bone, or cartilage (Robi et al., 2013;Nyland et al., 2022). In the present study, ACL reconstruction patients completed a questionnaire to retrospectively quantify the frequency and intensity of their athletic activity/maneuver leading up to injury. ...
Full-text available
Introduction: Recent evidence has emerged suggesting that a non-contact anterior cruciate ligament (ACL) tear can result from repetitive submaximal loading of the ligament. In other words, when the intensity of ACL-straining athletic activities is increased too rapidly, microdamage can accumulate in the ligament beyond the rate at which it can be repaired, thereby leading to material fatigue in the ligament and its eventual failure. The objective of this survey-based exploratory study was to retrospectively determine whether the levels of various athletic activities performed by ACL-injured patients significantly changed during the 6 months before injury. Methods: Forty-eight ACL-injured patients completed a survey to characterize their participation in various activities (weightlifting, sport-specific drills, running, jumping, cutting, pivoting/twisting, and decelerating) at three timepoints (1 week, 3 months, 6 months) prior to ACL injury. Activity scores, which summarized the frequency and intensity of each activity, were calculated for each patient at each time interval. A series of linear mixed-effects regression models was used to test whether there was a significant change in levels of the various activities in the 6-month period leading up to ACL injury. Results: Patients who sustained a non-contact ACL injury markedly increased their sport-specific drills activity levels in the time leading up to injury (p = 0.098), while those patients who sustained a contact ACL injury exhibited no change in this activity during the same time period (p = 0.829). Levels of running, jumping, cutting, pivoting/twisting, and decelerating increased for non-contact ACL-injured patients but decreased for contact ACL-injured patients, though not significantly (p values > 0.10). Weightlifting activity significantly decreased leading up to injury among contact ACL-injured patients (p = 0.002). Discussion: We conclude that levels of ACL-straining athletic activities or maneuvers in non-contact ACL-injured patients markedly increased in the 6 months leading up to their injury, providing evidence that changing levels of certain activities or maneuvers may play a role in ACL injury risk. This warrants further investigation of the hypothesis that too rapid an increase in activities or maneuvers known to place large loads on the ACL can cause microdamage to accumulate in the ligament, thereby leading to failure.
... As a result, tendons resist structural failure even under repeated strain. Its resistance to fatigue results from the tendon cells' capacity to repair and remodel the tendon according to its stress-strain characteristics, expressed by the stress-strain curve (Figure 1) [1,2]. This curve determines the tendon's stiffness and toughness and present the maximum force the tendon can withstand before failing. ...
Full-text available
Medium chain length polyhydroxyalkanoate (MCL-PHA), a biodegradable and biocompatible material, has a mechanical characteristic of hyper-elasticity, comparable to elastomeric material with similar properties to human tendon flexibility. These MCL-PHA properties gave rise to applying this material as an artificial tendon or ligament implant. In this study, the material was solution-casted in cylinder and rectangular shapes in the molds with the designated small holes. A portion of the torn human tendon was threaded into the holes as a suture to generate a composite tendon graft. The tensile testing of the three types of MCL-PHA/tendon composite shows that the cylinder material shape with the zigzag threaded three holes has the highest value of maximum tensile strength at 56 MPa, closing to the ultimate tendon tensile stress (50–100 MPa). Fibroblast cells collected from patients were employed as primary tendon cells for growing to attach to the surface of the MCL-PHA material to prove the concept of the composite tendon graft. The cells could attach and proliferate with substantial viability and generate collagen, leading to chondrogenic induction of tendon cells. An in vivo biocompatibility was also conducted in a rat subcutaneous model in comparison with medical-grade silicone. The MCL-PHA material was found to be biocompatible with the surrounding tissues. For surgical application, after the MCL-PHA material is decomposed, tendon cells should develop into an attached tendon and co-generated as a tendon graft.