fraktur sub kondral
TRANSCRIPT
FRACTURE OF THE SUBCHONDRAL
BONE IN OSTEOARTHRITIS OF THE
KNEE
ADVISER:
Dr. dr. R. Fx. Hendroyono, SpOT, MARS, FAAOS
COMPILED BY:
MUHAMAD REDZUAN BIN JOKIRAM
030.08.281
KEPANITERAAN KLINIK ILMU BEDAH
RUMAH SAKIT UMUM DAERAH KOTA BEKASI
FAKULTAS KEDOKTERAN UNIVERSITAS TRISAKTI
PERIODE 3 SEPTEMBER – 10 NOVEMBER 2012
TABLE OF CONTENTS
CHAPTER I PREFACE
1.1...............................................................................................................................................2
CHAPTER II REVIEW OF THE LITERATURE
2.1 Anatomy...............................................................................................................................3
2.2 Physiological bases of bone regeneration............................................................................8
2.3 General changes in bone in osteoartritis.............................................................................10
2.4 Osteoblasts and osteoarthritis.............................................................................................11
Biomechanical Aspects
2.5 Ranges of physiological forces on joint cartilage..............................................................13
2.6 Supranormal stress and strain can lead to injury................................................................14
2.7 Changes in biomechanical properties with age..................................................................14
2.8 Chondrocyte response to mechanical loading....................................................................15
2.9 Chondrocyte response to pathological forces.....................................................................15
CHAPTER III CONCLUSION
..................................................................................................................................................17
CHAPTER IV REFERENCES
..................................................................................................................................................18
1
CHAPTER I
INTRODUCTION
Osteoarthritis (OA) is a degenerative joint disease characterized by pain, cartilage loss, and
joint stiffness. Although OA has long been considered to be primarily a cartilage disorder
associated with focal articular cartilage degradation, this disease is accompanied by well-
defined changes in the subchondral and periarticular bone, including sclerosis and cyst and
osteophyte formation (1). Osteoarthritis (OA) represents a clinical classification of
pathological conditions involving a progressive degeneration of articular cartilage, a
remodelling of sub-chondral bone and a synovitis which is usually limited.
The condition is variously described as a part of a process of age-related change or a disease.
It is twice as prevalent in women than men and increases in incidence with age, there being a
major rise after 60 years (2). It is believed that the changes that lead to the development of OA
are slow (insidious). That in idiopathic OA clinical presentation may result from changes
over 15-20 years. The disease may involve primarily one or two large joints or may be
generalized. Following joint trauma there is an increased incidence of OA (2). The importance
of the bone changes in the initiation and progression of OA is still being debated. It has been
suggested that increased subchondral bone stiffness reduces the ability to dissipate the load
and distribute the strain generated within the joint. This increases peak dynamic forces in the
overlying articular cartilage and can accelerate its damage over time (3). The functional
integrity of the articular cartilage can therefore depend on the mechanical properties of the
underlying bone. Accordingly, cartilage damage leads to full-thickness cartilage loss only
upon repetitive loading over an already stiffened subchondral bone plate (4).
Recent studies have demonstrated increased subchondral bone turnover accompanied by
specific architectural changes in the subchondral trabecular bone in OA joints (5,6).
Furthermore, epidemiologic studies have clearly documented increased subchondral bone
sclerosis with disease progression (7). These observations suggested a role for subchondral
bone changes in the initiation and progression of OA, raising the possibility that early
intervention that reduces bone sclerosis might retard the progressive loss of articular
cartilage.
2
CHAPTER II
REVIEW OF THE LITERATURE
2.1 ANATOMY
Composition of diarthrodial
joint components
To fully comprehend the effect of
chondrocyte metabolism on joint
integrity and its role in synthesis
of multiple proteins involved in
joint homeostasis, we first discuss
the main components of normal
diarthrodial joints: articular
cartilage, synovial fluid,
subchondral bone and the synovial
membrane.
Articular cartilage composition
Articular cartilage, or hyaline cartilage, is designed to bear and distribute loads on the bone
surfaces inside joints, and is composed of a solid phase, including the extracellular matrix
(ECM) and chondrocytes, and a fluid phase, the interstitial fluid, containing water and small
electrolytes, which are primarily Na+ and Cl-.(8) The ECM consists of a highly hydrated
collagen network and of proteoglycan aggregates. The interstitial fluid contains water and
ions. Less than 5% of the tissue volume of cartilage consists of chondrocytes, that are
responsible for maintaining the homeostasis with regard to the cartilage components. (9)
Chondrocytes represent the only cell type inside articular cartilage, as cartilage tissue is not
vascularised or innervated.
The collagen network consists primarily of collagen type II fibrils, and also contains minor
amounts of type I, V, IX and XI fibrils.(10) Collagen is an important contributor to the tensile
3
properties of the cartilage, where proteoglycans attract electrolytes in the interstitial fluid to
generate a swelling pressure and to resist compressive loads.(11) Thus, the solid components of
cartilage have a low permeability for water, while there is continuous interaction with water
through covalent, ionic and hydrogen bonding, resulting in a high interstitial fluid
pressurisation, which is essential for adequate distribution of the loads inside the joint.8
Proteoglycans consist of a core protein to which one or more glycosaminoglycan (GAG)
chains are attached. The most abundant proteoglycan in articular cartilage is the large
aggregating aggrecan, having numerous GAGs attached to its long core protein, mainly
chondroitin sulphate (CS) and keratan sulphate (KS).(12) Another GAG in articular cartilage is
hyaluronan (HA), which has its major function in synovial fluid viscosity.(13) Proteoglycan
molecules form aggregates by binding to hyaluronic acid molecules, and together with
collagen they form the dense ECM network.(11) Smaller, nonaggregating proteoglycans in
articular cartilage are for instance fibromodulin, decorin and biglycan.(14,15)
The articular cartilage surface, also called the superficial or tangential zone, encompass 10-
20% of the articular cartilage thickness and has the highest collagen and interstitial fluid
content.(8,9)In this zone, the collagen fibrils are arranged parallel to the articular surface,
creating a low compressive modulus, meaning that this layer is easily deformed. (11)
Chondrocytes in the surface zone produce relatively little proteoglycans, and synthesise more
collagen type II and smaller proteins with lubricating and protective functions, such as the
superficial zone protein (SZP).(16,17)
The middle zone of the articular cartilage accounts for 40-60% of the cartilage thickness.7
Collagen fibrils in this zone are thicker and packed more loosely than in the superficial zone,
and are obliquely oriented to the cartilage surface. The compressive modulus of the tissue is
higher in this layer.(11)
The radial or deep zone fills 30% of the cartilage thickness and contains collagen fibrils with
a large diameter that are oriented perpendicular to the surface.(9)This layer has the highest
compressive modulus and also contains the most proteoglycans, and less water, compared to
the other zones.(8,11) Chondrocytes in this zone are 10- fold more synthetically active than in
the superficial zone.(18)
4
Below the deep zone is a layer of calcified cartilage, which contains rather collagen type X
than collagen type II, and here is also the tide mark, which lays directly on the subchondral
bone.(19)
As already mentioned, chondrocytes synthesise the cartilage compounds with varying ratios
in the distinct zones, and these cell populations are therefore very heterogeneous, also with
regard to size and shape, according to their position in the cartilage. Besides, the behaviour of
chondrocytes is influenced by the age, pathology or mechanical stress of the surrounding
cartilage,(20) which will be further discussed in the section about the effect of joint loading on
chondrocyte metabolism .
Subchondral bone composition
Directly below the calcified cartilage layer and the tide mark is the interface with the
subchondral bone plate, which separates the articular cartilage from the bone marrow.(19)
Below this dense bone plate is a subarticular spongiosa, with its trabecular or plate-like bone
structures enclosing spaces between them. Near the subchondral bone-cartilage interface,
these spaces are very narrow, and deeper in the bone they are considerably enlarged.
Articular cartilage is supported by the underlying bone in both a biomechanical and
biochemical way.(21)Biomechanically, the rigid bone, mainly composed of collagen type I,
gives strength support to the soft and compression-sensitive articular cartilage, and attenuates
the loads to a much greater extent than cartilage.(19) The quality of subchondral bone directly
influences the response of cartilage to load, which is illustrated by the effect of an increased
5
bone density, which often leads to OA, because bone with a higher density is very stiff and
lays more load on the articular cartilage, resulting in cartilage damage.(22)
Subchondral bone is highly vascularised, especially in regions that experience considerable
mechanical load. Cartilage has no blood supply, but exchange of nutrient solutions is possible
between subchondral bone and articular cartilage by crossing of blood vessels into the
calcified cartilage layer through openings in the bone at the subchondral interface. (19,21) If a
region in the calcified cartilage is devoid of blood vessel entry from the subchondral bone
plate, the chondrocytes in this region are dependent on diffusion of nutrients from the
synovial fluid through the cartilage matrix, which is also the case for the superficial, middle
and deep cartilage zones.
Synovial fluid composition
The joint cavity of diarthrodial joints is filled with synovial fluid, which is a dialysate of
blood plasma containing additional proteins that are synthesised in synoviocytes and
chondrocytes. Synovial fluid has various functions, including cartilage lubrication, and
facilitation of transport of nutrients, waste products, enzymes, cytokines, growth factors and
morphogens to maintain joint homeostasis and to allow communication between distinct cell
populations within the joint.
Since synovial fluid is a dialysate of blood plasma, the major protein components are
identical, except for the larger plasma proteins, because the synovial membrane hinders these
from entering the synovial fluid compartment.(23) Albumin, as well as β1, γ, α1 and α2
globulins and transferrins are the major protein components of synovial fluid. (24) There are
also pro- and anti-inflammatory cytokines and growth factors present, which have important
roles in regulation of the local cell populations.(23) Additionally, synovial fluid contains
several lubricant molecules that are synthesised and secreted by synoviocytes or
chondrocytes, including HA and proteoglycan-4 (PRG-4).(23) HA is a GAG that contributes to
the viscosity of the synovial fluid, and thereby prevents fluid outflow to maintain the synovial
volume, and the two variants of PRG-4, SZP and lubricin, are glycoproteins that mediate
boundary lubrication of the articular cartilage.(13,17) SZP is uniquely expressed in chondrocytes
in the superficial zone of cartilage, and lubricin and HA are synthesised by fibroblast-like
synoviocytes at the luminal surface of the synovial membrane.(17,25,26) There are also few
leukocytes, lymphocytes, macrophages and macrophagic synoviocytes in the synovial fluid.(23,27) Macrophagic synoviocytes are of bone marrow origin and can phagocytose cell debris
6
and other wastes, and have an antigenpresenting function.(26,27) Synovial fluid further contains
matrix metalloproteinases (MMPs), amounts of a distintegrin and metalloproteinase with
thrombospondin motifs (ADAMTS) and tissue inhibitors of metalloproteinases (TIMPs), that
are produced by chondrocytes and synoviocytes, and together determine the extent of ECM
maintenance and breakdown in the articular cartilage.(28)
The synovial fluid is in direct contact with the articular surface and with the synovial
membrane, and, in some joints, also with the meniscus and with ligaments. Therefore, in
various arthropathies, major changes occur to the synovial fluid composition, which is
exacerbated by or contributes to the pathology. In most arthropathies, including OA,
rheumatoid arthritis (RA) and posttraumatic arthritis, the protein concentration in the synovial
fluid is increased, and larger proteins are present inside the fluid as well. (24) This points to a
changed permeability of the synovial membrane during disease.(23) Thus, the synovial
membrane plays important roles in maintaining the joint homeostasis as well.
Synovial membrane composition
The synovial membrane is composed of two layers, including an outer vascularised and
innervated fibrous capsule which contains fibroblasts, macrophages, adipocytes and mast
cells, and an inner layer, the synovial intima, that covers the outer layer. (26,27) The intima
contains the earlier mentioned fibroblast-like synoviocytes, which are specialised to
synthesise HA, and the macrophagic synoviocytes, within an ECM composed of collagen,
HA and proteoglycans.(23,27) As described above, the permeability of the synovial membrane is
the main determinant of plasma protein and water entry into the synovial fluid, but this
barrier also retains the larger synovial fluid contents that are synthesised inside the joint,
including lubricin, SZF and HA.(23) Thus, the synovial membrane physically and functionally
lines the joint edge and provides a homeostatic environment to the cartilage, the subchondral
bone and the synovial fluid. In RA patients, the synovial membrane dramatically increases in
mass and metabolic activity due to hyperplasia of the intima cells, leading to a change in
synovial membrane permeability. The entry of larger plasma proteins into the synovial fluid
has been associated with synovial inflammation, which is characteristic for RA. (29) Indeed, in
OA and in other aetiologies of arthritis, there are changes in synovial membrane permeability
as well, but to a much lesser extent than in RA.(30)
2.2 Physiological bases of bone regeneration .
7
1. REMODELING PHASES
Bone remodeling can be divided into the following phases : quiescent, activation, resorption,
formation, mineralization.
1.1 Quiescent phase: said of the bone when at rest. The factors
that initiate the remodeling process remain unknown.
1.2 Activation phase: the first phenomena that occurs is the activation of the bone surface
prior to resorption, through the retraction of the bone lining cells (elongated mature
osteoblasts existing on the endosteal surface) and the digestion
of the endosteal membrane by collagenase action. Once exposed, the mineralized surface
attracts the circulating osteoclasts coming from the nearby vessels.
1.3 Resorption phase: the osteoclasts then begin to dissolve the mineral matrix and
decompose the osteoid matrix. This process is completed by the macrophages and permits the
release of the growth factors contained within the matrix, fundamentally transforming growth
factor beta (TGF-β), platelet derived growth factor (PDGF), insulin-like growth factor I and
II (IGF-I and II).
1.4 Formation phase: simultaneously in the resorbed areas the preosteoblast grouping
phenomena is produced, attracted
by the growth factors liberated from the matrix which act as chemotactics and in addition
stimulate their proliferation . The preosteoblasts synthesize a cementing
substance upon which the new tissue is attached, and express bone morphogenic proteins
(BMP) responsible for differentiation. A few days later, the already differentiated osteoblasts
synthesize the osteoid material which fills the perforated areas.
1.5 Mineralization phase: mineralization begins thirty days after deposition of the osteoid,
ending at 90 days in the trabecular and at 130 days in the cortical bone.
The quiescent or ‘at rest’ phase then begins again.
8
The balance between bone resorption and formation is influenced by such interrelated factors
as genetic, mechanical, vascular, nutritional, hormonal and local.
2.3 General changes in bone in osteoartritis
9
OA involves not only the degeneration of articular cartilage leading to eburnation of
bone but also a synovitis that is usually limited. There is extensive re-modelling of sub-
chondral bone resulting in the so-called sclerosis of this tissue observed radiographically.
These bone changes are often accompanied by the formation of sub-chondral cysts as a result
of focal resorption.The bone changes may also be systemic in nature. The work of Dequeker
and his colleagues (31) has produced evidence of changes in bone metabolism in sites such as
the iliac crest which are suggestive of systematic changes. Analyses of the molecular
composition of OA bone have provided indications of fundamental changes in bone
metabolism (32). Deoxypyridinoline cross-links, resulting from bone resorption, are elevated in
urine (33) as is osteocalcin elevated in serum (34).
There is evidence from scintigraphy to indicate that changes in bone metabolism are
identifiable several years prior to evidence for clinical onset of the disease (35-37). Whether
these changes precede those in cartilage remains to be determined once comparable analyses
can be made of metabolic changes in cartilage metabolism.
The dynamic interplay between bone and cartilage is reflected in how changes in one
tissue may influence the other and thus determine the development of OA. Bone cells from
OA patients can alter chondrocyte metabolism (38). This is most strikingly observed in
osteoporosis, where bone density is reduced by excessive osteoclastic resorption of bone.
Patients with osteoporosis usually show little or no evidence of OA. Moreover, patients with
OA do not usually develop osteoporosis (39-41). These observations may be explainable in part
at the level of biomechanical interactions, indicating that reduced bone density may protect
against degeneration leading to OA.
A principle anatomical feature of OA is the development of osteophytes. The
osteophytes, which have a cap of articular cartilage, and an actively remodelling bone base,
may serve to reintroduce some stability into an otherwise unstable joint.
These form from an endochondral process in sites at the edges of the damaged
articular cartilage. Addition to periosteum in culture or injection in vivo of transforming
growth factor-beta results in expression of the hypertrophic phenotype by newly formed
chondrocytes in periosteal tissue and subsequent mineralization of extracellular matrix (42, 43).
This is an essential requirement for endochondral ossification. Intra-articular injection of
TGF-beta1 induces osteophyte formation (44) .Thus TGF-beta, and probably other bone
10
morphogenetic proteins, may play an important role in osteophyte formation. These bone
morphogenetic proteins are also excessively produced in joint inflammation.
2.4 Osteoblasts and osteoarthritis
Osteoarthritis (OA) is a chronic degenerative joint disease characterized by loss and
degradation of cartilage, inflammation of the synovium and peri-articular bone alteration
consisting of the formation of osteophytes and subchondral bone sclerosis (45,46).Radin and
Rose (1986) were the first to suggest the involvement of the subchondral bone in the
progression and initiation of cartilage degradation. Successive studies have confirmed this
hypothesis and demonstrated the abnormal behaviour and metabolism of OA osteoblasts (47,47,48,50,51) .
Some investigators have examined the molecular basis of bone OA changes by
comparing microarray gene expression profiling of bone obtained from individuals with no
evidence of joint disease and from individuals with degenerative hip OA(52) . Several genes
that influence osteoblast function, bone remodelling and mineralization exhibit a different
expression in OA. Many of these genes are components of the Wnt and TGF-β/BMP
signalling pathway. Moreover, a subset of genes are differentially expressed between females
and males; this might in part explain the sex disparity in OA.
La Jeunesse’s group has reported elevated alkaline phosphatase activity and increased
osteocalcin levels in primary human OA subchondral osteoblasts (50) and this data has been
confirmed by the results of several clinical ex/in vivo and in vitro studies (53,54,55) . Differences
in the metabolic response to 1,25(OH) Vitamin D3 stimulation, consisting of a significant
increase of osteocalcin after Vitamin D3 treatment, have been found in osteoarthritic
osteoblasts, proportional to the degree of joint damage (53,56,57) ,suggesting that the abnormal
behaviour of OA osteoblasts includes an altered response to systemic or local factors (53) .
Other investigators have distinguished two different groups of OA osteoblasts: low
OA osteoblasts, associated with low levels of prostaglandin E2(PGE2) and IL-6, similar to
normal cells, and high OA osteoblasts associated with high levels of PGE2 and IL-6 (58) .
Recent data have suggested a close relationship between the OPG/RNK/RANKL system and
the subchondral bone changes observed in OA. Studies performed on osteoblasts derived
11
from patients with OA have demonstrated an abnormal expression of OPG and RANKL and
consequently OPG/RANKL ratio (59,60) .Low OA osteoblasts show a marked decrease in OPG
and increased level of RANKL, whereas high OA osteoblasts exhibit a marked increase of
OPG and a reduction of RANKL-t (61) .Moreover, low and high OA subchondral osteoblasts
express membranous and RANKL isoforms differently and are modulated differently by
osteotropic factors (62) .This might explain the different metabolic states of human
subchondral bone osteoblast subpopulations: low OA osteoblasts promote bone resorption,
whereas high OA osteoblasts favour bone formation.
Recently, human osteoblasts derived from subchondral OA bone have been
shown, for the first time, to express ephrin B2 and its receptor EphB4. EphB4 receptor is
expressed in OA osteoblasts and its levels are increased in low OA cells but no differences
have been observed between normal and high OA cells. Moreover, EphB4 activation by the
specific ligand ephrin B2 inhibits the expression of IL-1β, IL-6 and RANKL, but not of OPG (59,60) . These data suggest that the activation of EphB4 by ephrin B2 affects the abnormal
metabolism in OA subchondral bone by inhibiting resorption factors and their activities.
Dequeker’s group (1993) has demonstrated an elevated production of IGF-I, IGF-II
and TGF-β in bone explants from the iliac crest of OA patients. The same results have
subsequently been obtained in vitro (63) .
The altered osteoblast metabolism might also explain the presence of an abnormal
mineralization of subchondral bone in OA. Type I collagen levels are elevated in OA bone
tissue (64) and should lead to excessive mineralization. This might be the reason for the
subchondral bone sclerosis that characterizes OA, even if, in the early stage of disease, this
tissue is hypomineralized. A rapid and aggressive OA has recently been demonstrated to
develop in the Brittle IV (Brtl) mouse model of osteogenesis imperfecta, which is
characterized by a defect in Type I collagen (65) . These data confirm the idea that the
alterations in subchondral bone tissue microarchitecture play a key role in the progressive
destruction of joint cartilage observed in OA. Human OA osteoblasts present increased
collagen type I deposition, but with an altered ratio of α1 and α2 chains, in particular with an
increase of the α1 chain. This abnormal production of type I collagen leads to abnormal
mineralization and can be correlated with the high levels of TGF-β detected in OA
osteoblasts (66) .TGF-β is a potent inducer of osteophytes and acts directly or via the inhibition
of BMP-2-induced mineralization.
12
A human in vitro study has demonstrated the abnormal production of leptin in OA
osteoblasts: leptin expression is increased five-fold in OA osteoblasts compared with normal
osteoblasts (67) .increased production of leptin might be responsible, at least in part, for the
elevated levels of bone markers observed in OA osteoblasts (osteocalcin, alkaline
phosphatase) and confirms the key role of leptin in OA pathophysiology, as previously
demonstrated by the Dumond group (2003).
Biomechanical Aspects
2.5 Ranges of physiological forces on joint cartilage
Human articular cartilage experiences wide ranges of stress and strain during normal joint
loading. Studies using cadavaric limbs loaded in simulated gait have shown that stresses in
the range of 5–10 MPa are normal in the hip, corresponding to loads that are 300–800% body
weight [67-70] . In vivo measurements with an instrumented hip endoprosthesis have indicated
that higher stresses are possible (up to 18 MPa) during other physiological movements,
especially when muscle forces are high [71] .
Accompanying strains during certain regimes of joint loading can also be high. Herberhold et
al. [72] reported decreases of approximately 40% in patella cartilage thickness after 30 min of
static loading of cadaver joints at 3.6 MPa [72] . In loaded areas of knee joints, cartilage
thickness recorded at the end of the day was decreased by up to 0.6 mm compared to the start [73] . This phenomena was attributed to accumulated fluid loss from the matrix of loaded areas
13
2.6 Supranormal stress and strain can lead to injury
In contrast to the above normal ranges of joint forces, stress and strain above the
physiological range have the potential to damage the matrix and chondrocytes. Acute trauma
to the joint is known to increase the risk of osteoarthritis (OA) [74,75] , while destabilization of
the knee joint due to anterior cruciate ligament rupture or meniscal damage causes
radiographic signs of OA in many patients [76,77] . Other mechanical influences that cause
abnormal forces, such as joint laxity, obesity, and muscle weakness, are also linked to the
progression of OA [78] .
In vivo studies have shown that impact trauma can cause osteoarthritic changes. Radin et al. [79-81] impacted patellofemoral joints of rabbits, causing damage to the bone and cartilage and
subsequently leading to OA-like degradation. Even impacts that do not appear to fracture the
bone can result in cartilage degradation [82] .
2.7 Changes in biomechanical properties with age
As age is the most significant risk factor for OA, many studies have examined how the
material properties of cartilage change with age. In individuals without cartilage lesions,
cartilage thickness does not decrease significantly with age in men (–6%, n.s.), but does
decrease in women by approximately 12% (p < 0.05), possibly as a result of the more rapid
decrease in muscle forces with age in females [83] . Femoral head cartilage shows a large
decrease in tensile stiffness and fracture stress with age, whereas talar cartilage (ankle) shows
significantly less degradation in properties [84] . The superficial zone of human condyle
cartilage shows a steady increase in tensile stiffness, peaking in the third decade of life, then
decreasing thereafter [85] . Deep-zone stiffness decreases continuously with age.
As the collagen network has a very low turnover [86] , alterations to it could cause
changes in its mechanical stiffness and fatigue properties, possibly leading to premature
breakdown. The presence of various sugars in the body cause cross-linking of proteins via a
process called nonenzymatic glycation [87] . These cross-links are only significant in tissues
with low turnover where they can build up. In cartilage, the collagen network is affected,
leading to the browning of tissue associated with old age [87] .
14
2.8 Chondrocyte response to mechanical loading
Compression of cartilage causes deformation of cells and matrix, gradients in hydrostatic
pressure, intratissue fluid flow, and associated electrokinetic effects (e. g., flow-induced
streaming potentials). Since the compressive stiffness of chondrocytes is about three orders of
magnitude less stiff than of the surrounding ECM, the cells will deform with the matrix [88] .
The deformation of the charged ECM will change ionic concentrations, osmolarity, and pH of
the cellular environment according to Donnan equilibrium theory [89,90] . Tissue fluid flow
during loading can also dramatically enhance transport of nutrients and macromolecules (e.g.,
growth factors and cytokines [91] . Therefore, mechanical and chemical changes during
loading can alter chondrocyte behavior, and hence matrix synthesis and turnover.
Areas on joints that are more highly loaded during locomotion generally have a
higher proteoglycan content compared to adjacent cartilage experiencing lower stress [92–94] .
However, these highly loaded chondrocytes synthesize less total proteoglycan (although the
synthesis of certain small proteoglycans, especially decorin, is elevated) [90, 95, 96] . Together,
these findings suggest that less proteoglycans are degraded and lost from the cartilage in
these regions. In young and neonatal cartilages, these trends between areas have not been
observed, suggesting that the loading itself is responsible for the change in cartilage matrix
composition and also the zonal variation in chondrocyte phenotype [94, 97] .
2.9 Chondrocyte response to pathological forces
As mentioned previously, traumatic joint injury has been linked to an increased
risk of developing OA in later life. Until recently, little was known about the state of the
chondrocytes or ECM macromolecules in the time between injury to human joints and the
development of disease. Lohmander et al. [98] removed synovial fluid from patients
immediately after an articular cartilage or meniscal tear and up to 15 years postinjury. The
synovial fluid samples were analyzed for the presence of degradative enzymes and fragments
of enzymatically cleaved or intact matrix molecules. The matrix metalloprotease stromelysin-
1 (MMP-3) is thought to be one of the major proteolytic enzymes responsible for the normal
turnover of ECM molecules and the enhanced turnover during disease. In the days following
injury, the level of MMP-3 in the synovial fluid (measured as the latent or proenzyme form)
was elevated by 50–100 times the level in healthy athletes. These levels decreased with time
15
after injury, but remained almost tenfold higher than in uninjured controls even by 10–15
years after injury. Interestingly, the levels of tissue inhibitor of metalloprotease were also
elevated after injury.
Collagen degradation also occurred soon after injury, as indicated by a 15-fold
increase in the amount of MMP up to 15 years after injury, leaved collagen molecules in the
synovial fluid [98] . Cleavage at this site in the C-telopeptide cross-linking domain indicated
that mature, rather than newly synthesized, collagen molecules were being cleaved and
leaving the tissue. Again, these levels remained higher.
CHAPTER III
CONCLUSION
The development of cartilage degeneration is concomitant with subchondral
bone thickness in osteoarthritis, whereas it is related to higher subchondral bone activity and
16
dysregulation in the synthesis of bone proteins. As an immediate consequence, homotrimers
of type 1 collagen are formed that could lead to undermineralization of this tissue. This
dysregulation also leads to abnormal production of different factors by osteoblasts such as
prostaglandins, leukotrienes, and growth factors. Because microcracks or neovascularization
provide a link between the subchondral bone tissue and articular cartilage, these factors could
contribute to the abnormal remodeling of osteoarthritic cartilage.
Lastly, it is clear that a prophylactic response may be required immediately after a
biomechanical insult, rather than when OA symptoms present themselves. Levels of catabolic
cytokines and enzmyes are upregulated almost immediately after injury and stay increased for
many subsequent years. Chondrocytes lose their ability to increase biosynthesis of matrix
components in response to dynamic compression and die through necrosis and apoptosis. One
of the pathways for decreasing the incidence of OA may be ensuring that chondrocyte
response to both physiological and pathological mechanical loads is optimized for longterm
survival of the cartilage.
Recent research has contributed to furthering our knowledge that OA can no longer
be considered a disease of a single tissue but is rather a whole joint Borgan failure.We still
need to acquire a better understanding of these changes and the biochemical signals and
biomechanical aspects between bone and articular cartilage. This in turn will help us to
devise better therapeutics aimed at treating not only the consequences but the causes of OA.
CHAPTER IV
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