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The Veterinary Journal 197 (2013) 3–12
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The Veterinary Journal
journal homepage: www.elsevier .com/ locate/ tv j l
Review
Pathogenesis of epiphyseal osteochondrosis
Sheila Laverty a,⇑, Christiane Girard b
a Comparative Orthopaedic Research Laboratory, Department of Clinical Sciences Faculty of Veterinary Medicine, University of Montreal, Quebec, Canadab Department of Veterinary Pathology and Microbiology, Faculty of Veterinary Medicine, University of Montreal, Quebec, Canada
a r t i c l e i n f o a b s t r a c t
Keywords:Horse
OsteochondrosisJuvenile disordersEtiopathogenesis1090-0233/$ - see front matter � 2013 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.tvjl.2013.03.035
⇑ Corresponding author. Tel.: +1 450 773 8521x826E-mail address: [email protected] (S. La
Osteochondrosis (OC) of the articular epiphyseal cartilage complex (AECC) is a developmental diseasethat is present in the first weeks of life. It is characterized by focal chondronecrosis and retention ofgrowth cartilage due to failure of endochondral ossification. Fissures may extend from the lesion throughthe overlying articular cartilage to create a cartilage flap and an osteochondral fragment. This articularform is known as osteochondritis dissecans (OCD).
There have been many hypotheses about the etiopathogenesis of OC of the AECC including, amongstothers, ischemia of growth cartilage or altered cartilage type II collagen metabolism. The ischemia theoryproposes that necrosis of the vessels in the cartilage canals of the sub-articular growth cartilage leads tonecrosis of chondrocytes and retention of necrotic cartilage. Several studies have measured biomarkers inserum and synovial fluid to demonstrate a consistent increase in type II collagen synthesis in young ani-mals of different species. Although these changes could represent lesion reparative events, there is nocomparable increase in the synthesis of cartilage matrix proteoglycan molecule. It is therefore speculatedthat an altered type II collagen metabolism may be involved in the early changes associated with OC. Fur-ther studies of OC susceptible animals in utero and the first weeks of life are required to elucidate thecause of vessel necrosis and the exact role of type II collagen structure and metabolism in OC.
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Introduction
Osteochondrosis (OC), a common cause of lameness in domesticanimals, occurs in foals, large breed dogs, pigs, cattle, lambs, poul-try (tibial dyschondroplasia) and in humans (Thomson, 2007). Theclinical, pathological, biochemical and molecular aspects of thecondition have been extensively studied in horses, pigs and poul-try, but the majority of these studies have been conducted on ani-mals or specimens with clinical signs, radiographic changes ormacroscopic lesions that represent the chronic stages of the dis-ease trajectory. Interpretation of the results of such studies is dif-ficult as the changes observed may reflect a combination ofsecondary reparative and degenerative processes rather than theprimary events (Ytrehus et al., 2007).
The most appropriate approach to determine the exact se-quence of events in the pathogenesis of OC is to examine the earlysubclinical lesions (Olstad et al., 2007; Ytrehus et al., 2007). How-ever, detecting the early subclinical lesions remains a challenge initself as it is now known that they occur in the very early months oflife and may potentially arise in utero (Lecocq et al., 2008; Olstadet al., 2008a; Ekman et al., 2009). The earliest changes may involve
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7.verty).
molecular events and it will be difficult to differentiate them fromnormal physiological events associated with development. Recentresearch on epiphyseal vascularization and development in fetusesand young foals is shedding new light on this unresolved issue (Le-cocq et al., 2008; Olstad et al., 2008b,c, 2009, 2011).
OC has been defined as a focal failure of endochondral ossifica-tion with subsequent persistence of the growth cartilage in theepiphyseal or in the physeal growth plates (Olsson, 1987). The ear-liest histological lesion of OC in the AECC is a necrotic area ofgrowth cartilage that may extend into the subchondral bone. Inthe advanced stages a fissure may extend from this site throughthe articular cartilage to create a cartilage flap (Fig. 1) or, later,an osteochondral fragment (Fig. 2) (Olsson and Reiland, 1978; Rei-land, 1978; Pool, 1993). The early lesions may heal in young ani-mals or progress to clinical lesions (Ytrehus et al., 2007; vanWeeren, 2012). This articular form of OC is debilitating for domes-tic animals, particularly equine athletes and the porcine industry,and is the theme of this review.
Multiple factors contribute to the phenotypic expression of OCin domestic animals including, amongst others, heredity (Reiland,1978; Carlson et al., 1988; van den Hoogen et al., 1999; van Weerenand Barneveld, 1999; Vervuert et al., 2003; Donabédian et al., 2006;Ytrehus et al., 2007), anatomical conformation, trauma (Kincaid andLidvall, 1983; Nakano et al., 1985; Olsson, 1987) and dietary
Fig. 1. OCD lateral ridge of trochlea of an equine femur of 5 month old foal. (A) A large cartilage flap is visible on the trochlea with a corresponding lesion on the patella to theleft of the image. There is severe synovial membrane inflammation and thickening; (B) the cartilage flap is elevated with a tissue forceps to reveal granulation tissue and somerepair in the lesion bed.
Fig. 2. OCD of the distal intermediate ridge of an equine tibia. A typicalosteochondral fragment (arrows) is attached by fibrous tissue to the distal tibia.This represents the end stage of OCD.
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imbalances (Kronfeld et al., 1990; Savage et al., 1993; Ytrehus et al.,2007). The earliest events on the trajectory to clinical OC have beenthe subject of debate over many years and were recently compre-hensively reviewed (Ytrehus et al., 2007). There are currently twoleading hypotheses on the etiopathological events giving rise toOC lesions and these include the ischemia theory and a role for car-tilage type II collagen. This review will explore, in depth, both ofthese current hypotheses in respect to articular OC.
Epiphyseal development in health
The bones of diarthrodial joints develop from mesenchymalcells that condense and differentiate into chondrocytes to form acartilage template. Following formation of the primary ossificationcenter in the diaphysis, the chondrocytes of the central area of theepiphysis proliferate, hypertrophy and undergo cell death. A com-plex sequence of molecular signals orchestrates these events (Rivasand Shapiro, 2002). The hypertrophic chondrocytes themselves se-crete type X collagen and alkaline phosphatase that contribute tocalcification of the adjacent matrix (Alini et al., 1992). The miner-alized cartilaginous septae, between the cell lacunae, undergo pro-teolytic digestion by septoclasts (Lee et al., 1995). Proteinases, suchas matrix metalloproteinases (MMPs), cathepsins (Fig. 3) and
gelatinases, degrade the mineralized type II-collagen-rich networkto allow blood vessel invasion (Glaser et al., 2003; Lee et al., 2009).The invading vessels are a source of mesenchymal stem cells andbone progenitors that differentiate into osteoblasts, secrete osteoidand promote ossification of the secondary center of ossification inthe epiphysis (Rivas and Shapiro, 2002).
These events occur in the equine distal femoral epiphysis after7 months of gestation (Shingleton et al., 1997; Lecocq et al.,2008) (Fig. 4) and in the pig in the second third of gestation (Con-nolly et al., 2004). The growth cartilage surrounds this ossificationcenter and merges with the articular cartilage under the joint sur-face. The joint shape is determined principally by the growth ofthis subarticular cartilage. The articular cartilage and underlyinggrowth cartilage of the epiphysis have been described as the artic-ular epiphyseal cartilage complex (AECC) (Carlson et al., 1995).
Progressive thinning and involution of the epiphyseal growthcartilage occurs with maturation until the ossification front abutsthe articular cartilage. It is unknown why the ossification endswhen it comes in contact with articular cartilage (Alvarez et al.,2005). Formation of the subchondral plate and calcified cartilagethen occurs (Rivas and Shapiro, 2002). There are differences of tim-ing of maturation of the epiphyses within the same bone and be-tween bones in the same animal (Rivas and Shapiro, 2002;Lecocq et al., 2008; Olstad et al., 2008a, 2009).
Vascularization of epiphyseal growth cartilage in health
The epiphyseal growth cartilage is highly vascularized in the fe-tus and young animals (Stockwell, 1971; Carlson et al., 1995; Le-cocq et al., 2008; Olstad et al., 2008a,b,c, 2009, 2011) but not theoverlying articular cartilage (Fig. 5). These cartilage vessels nourishchondrocytes as they are distant from synovial fluid nutrients. Theblood vessels extend from perichondral plexuses and are housed incartilage canals (Stockwell, 1971). The cartilage canal area is great-er in the deeper zones of the AECC (Lecocq et al., 2008).
Olstad et al. (2008b,c) reported that equine cartilage canalsmay be separated into different sections. The proximal sectionreceives its arterial source from the perichondrium duringgrowth. The mid-section and distal terminuses derive arterialblood initially from the perichondrium but, with the advanceof the ossifying front, they anastomose with, and receive bloodfrom vessels of the subchondral bone. There is a normal physio-logical regression of these cartilage canals with maturity in pigsand horses (Carlson et al., 1991; Olstad et al., 2007; Lecocq et al.,2008) and the speed of regression in the horse is joint depen-dent (Lecocq et al., 2008). The physiological regression is charac-terized histologically by chondrification: the vessels regress,
Fig. 3. Equine growth plate from 6 day old foal revealing the presence of Cathepsin K. A: Growth plate is immunostained (dark brown) with an antibody that detectsCathepsin K. The image reveals the classical growth plate structure with proliferative chondrocytes in the top part of the image. They proliferate to create columns, enlargeand hypertrophy. The matrix septae are digested by septoclasts containing Cathepsin K (brown stain). Two cartilage canals are visible in the growth cartilage in the upper partof the image. The transition zone between a denser cartilage matrix and the more loosely digested matrix is evident and potentially a zone of weakness; B: The enlargedimage reveals the multinucleated septoclasts digesting the matrix. Scale bar, 0.1 mm. Counter stain is hematoxylin de Gill.
Fig. 4. Development of equine fetal distal femoral epiphysis. The specimen on the left illustrates the cartilaginous distal femoral epiphysis prior to formation of the secondarycenter of ossification in a 6 month equine fetus. The ossified diaphysis is visible in the lower part of the specimen. The image on the right shows the secondary center ofossification with spherical growth plate and the growth plate of the metaphysis in a 10 month old equine fetus. Cartilage canal vessels can be seen in the sub-articular growthcartilage.
Fig. 5. Sagittal section of the lateral ridge of trochlea of equine day old foal withcartilage canal vessels. Many cartilage canals with blood vessels are visible in thegrowth cartilage but not in the overlying articular cartilage on the surface of thespecimen which remains avascular. The growth plate of the metaphysis is visible tothe right of the image.
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disappear and the mesenchymal cells of the canals differentiateinto chondrocytes that eventually fill the canal with matrix(Haines, 1974; Carlson et al., 1991; Olstad et al., 2007). The adja-cent chondrocytes remain viable.
Ischemia and OCD
Early changes in natural epiphyseal OC
Focal areas of chondrocyte necrosis is the earliest histologicalchange observed in the epiphyseal growth cartilage prior to, orcombined with, macroscopic lesions of OC (Fig. 6) (Hill et al.,1985, 1990; Kincaid et al., 1985; Carlson et al., 1986, 1989, 1991,1995; Woodard et al., 1987; Shingleton et al., 1997; Ytrehuset al., 2004a; Olstad et al., 2007, 2008a,b,c, 2009, 2011).
At the ultrastructural level, lipid was observed in chondrocyteswithin and adjacent to these lesions, but not in chondrocytes dis-tant from lesions suggesting that local hypoxia or ischemia pre-ceded matrix changes and chondronecrosis (Carlson et al., 1986).Many investigators have reported pyknotic or karyorrhectic nucleiin endothelial cells in the cartilage canals close to or within theareas of chondronecrosis, which suggested that an initial lesionof the blood vessels subsequently leads to ischemia and chondro-cyte degeneration (Kincaid and Lidvall, 1982; Hill et al., 1984;Kincaid et al., 1985; Woodard et al., 1987; Carlson et al., 1989,
Fig. 6. Early histological OC lesion in a young pig. (A) Articular epiphyseal cartilage complex of gilt with OC. A focus of necrotic chondrocytes surrounds a degeneratedvascular channel. The necrotic cartilage is protruding into the subchondral bone of the epiphysis. Clusters of chondrocytes surround the lesion in the growth cartilagecompatible with attempts to repair the lesion; (B) enlarged view of the necrotic cartilage canal: the lumen of the vessel contains debris and is no longer delineated byendothelial cells. The chondrocytes around the vessel are necrotic; (C) articular epiphyseal cartilage complex of a normal gilt. Normal appearing cartilage canal, with multiplesmall vessels delineated with endothelial cells and containing red blood cells. The chondrocytes surrounding the canal are normal. HPS staining is hematoxylin, phloxin andsafranin. Scale bar, 0.1 mm.
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1991; Ekman et al., 1990; Visco et al., 1991; Olstad et al., 2007,2008b,c, 2011). Woodard et al. (1987) also observed ultrastructuralabnormalities in some normal vessels within or adjacent to non-perfused articular cartilage. These observations were later sup-ported by microangiographic studies of blood vessels supplyingthe AECC. Spontaneous lesions of OC were co-localized to the areasof avascularity in epiphyseal cartilage and were associated withnecrotic blood vessels in the cartilage canals in pigs (Carlsonet al., 1991).
In other species, such as dogs, cattle, lambs and humans, themicroscopic lesions are consistent with those already describedin OC in AECCs of pigs and horses, suggesting a similar etiopatho-genesis in all species (Jensen et al., 1981; Carlson et al., 1991;Wegener and Heje, 1992; Scott et al., 1996; Hill et al., 1998). Chon-drocyte death is followed by degeneration of the cartilage matrixand a response of the surrounding tissues, particularly the growthcartilage that proliferates and increases its production of matrix
Fig. 7. Chronic histologic lesion of OC in pig. The AECC is visible. A focus of necroticgrowth cartilage is present in the upper epiphysis and is partially surrounded bygranulation tissue and bone. Numerous clusters of chondrocytes are present at thejunction of the normal and necrotic cartilage. HPS staining is hematoxylin, phloxinand safranin. Scale bar, 0.1 mm.
(Fig. 7). Cartilage canals from the viable growth cartilage may alsoproliferate toward the necrotic area (Ytrehus et al., 2004a,b, 2007).These microscopic lesions may either heal or progress to the clas-sical macroscopic lesions of OCD described earlier.
Why do cartilage canals become necrotic?
The changes occurring in cartilage canals in early OC lesions arevery different from the physiological process of chondrification,which is not associated with necrosis of the surrounding epiphy-seal growth cartilage (Carlson et al., 1989, 1991). Novel equinestudies now suggest that the sections of the cartilage canals lo-cated close to the ossification front or that are incorporated intothe ossification front are vulnerable to failure (Ytrehus et al.,2004a; Olstad et al., 2008b, 2011). It has been hypothesized thatthe shift of arterial source from the perichondrium to the subchon-dral bone vessels, occurring during normal endochondral ossifica-tion and growth of the epiphysis, may render blood vessels inthe distal termini of cartilage canals considerably more vulnerablethan their proximal counterparts that are supplied by the peri-chondrium (Ytrehus et al., 2004a; Olstad et al., 2008a,b, 2009,2011). As there are no anastomoses between cartilage canals ingrowth cartilage (Visco et al., 1989), the loss of vascularization dur-ing ischemia is permanent.
Factors, such as instability of the newly formed vascular anasto-moses and a weakness of the surrounding matrix resulting in littlemechanical support, may contribute to the vulnerability of thesevessels (Ytrehus et al., 2004a). The ossification front is a metabol-ically active region, undergoing proteolysis of the matrix, and alsoa transitional zone between tissues (growth cartilage and bone) ofdifferent mechanical properties (Fig. 3). The vessels passingthrough this region are potentially subjected to greater biome-chanical stresses. Direct mechanical shearing of cartilage canalsat these sites could account for the distribution of OCD lesions(Pool, 1993).
It is known that sites in juvenile animals with thicker growthcartilage are susceptible to OCD and this may in part be due tothe vessels present in this thick immature cartilage (Olstad et al.,2008b). As epiphyseal cartilage becomes avascular prior to skeletalmaturity, chondronecrosis caused by ischemia can only developduring skeletal growth as the shift in blood supply from the peri-chondrium to vessels originating from the bone marrow occursin a certain age window (Ytrehus et al., 2007). With advancing
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maturity the AECC becomes thinner and chondrocyte nutrients dif-fuse from the synovial fluid and the chondrocytes are no longerdependent on the vascular supply (Olstad et al., 2007).
Why the lesions are often symmetric and at specific sites
The vascular supply to the growth cartilage is joint- and site-specific, is quite consistent within a species and has bilateral sym-metry (Carlson et al., 1995; Ytrehus et al., 2007). The early sponta-neous lesions always occur in predictable locations of the femoralcondyles in pigs, in an area devoid of or with poor vascular supplyand the most recent lesions correspond exactly with the front ofreceding cartilage canals (Carlson et al., 1991).
Experimental OC in pigs
Experimental investigations have been performed to create le-sions similar to OC in pigs by surgical interruption of the vascularsupply to the medial femoral condyle (Carlson et al., 1991; Ytrehuset al., 2004b). Necrosis of blood vessels within cartilage canals wasobserved, followed by necrosis of surrounding cartilage, lesionssimilar to naturally occurring OC (Carlson et al., 1991; Ytrehuset al., 2004b). These experiments demonstrated that the viabilityof the epiphyseal part of the AECC complex is strongly dependenton an adequate blood supply and support the hypothesis that adefective blood supply is implicated in the pathogenesis of OC(Carlson et al., 1991).
Summary of ischemia theory of OC of the AECC
The histological observations of vessel necrosis and chondro-cyte death in early porcine and equine OC lesions and the creationof these early changes by experimental cartilage ischemia stronglysupport the theory that alterations in cartilage canal vessels play acentral role in the pathogenesis of OC in the AECC (Woodard et al.,1987; Carlson et al., 1991; Ytrehus et al., 2004b, 2007).
Unlike cartilage canal chondrification, a normal physiologicalprocess occurring during epiphyseal bone formation, interruptionof the vascularization in OC causes necrosis of the distal part ofthe affected cartilage canal and subsequent necrosis of adjacentgrowth cartilage. This interruption of the cartilage canal blood sup-ply occurs when anastomoses develop at the junction betweengrowth cartilage and bone and over a short period of time (Ytrehuset al., 2004a). Consequently OC should be considered a sequel topremature cessation of blood supply to the epiphyseal growth car-tilage consecutive to necrosis of cartilage canal blood vessels (Ytre-hus et al., 2004a, 2007). The ensuing focal failure of endochondralossification is the consequence of the presence of necrotic chon-drocytes that cannot proliferate and hypertrophy and respond to,and release cues for, the cascade of events of normal endochondralossification. There is lack of mineralization or vascular penetrationof the necrotic cartilage leaving a characteristic indentation, con-taining necrotic cartilage, at the ossification front.
Cartilage type II collagen in health: adult and fetal cartilage
Adult articular cartilage type II collagen
Type II collagen is the most abundant (80% dry weight) articularcartilage matrix molecule. Type II collagen’s extensively cross-linked fibrillar network confers the biomechanical properties oftensile strength and resistance to shear forces to cartilage (Eyreet al., 2006). In adult cartilage it has a zonal structure created bythe predominant fiber orientation. The type II collagen fibrils areparallel to the articular surface (tangential), lack a predominant
direction (transitional) are aligned perpendicular (radial) to thesubchondral bone to create the classical ‘Benninghoff structure’(Benninghoff, 1925). The number and thickness of collagen fibrilsalso vary with zone (Aspden and Hukins, 1981; Changoor et al.,2011).
When type II collagen is synthesized by chondrocytes, two pro-peptides of type II procollagen, the carboxy (C-) and amino (N-)propeptides are released extracellularly by proteolysis and cannow be detected by immunoassays or biomarkers that permitmonitoring the process both in vivo and in vitro (Nelson et al.,1998; Rousseau and Garnero, 2012). Degradation of the intact tri-ple helix of the mature fibrils of type II collagen by MMP collagen-ases or Cathepsin K can also now be detected by immunoassays(Dejica et al., 2008, 2012; Vinardell et al., 2009; Kraus et al.,2011; Rousseau and Garnero, 2012) that detect neoepitopes onthe cleaved fragments. Once cleaved, the molecule depolymerizesand many additional enzymes can subsequently degrade the frag-ments produced, including, amongst others, MMP collagenases,gelatinases and cathepsins (Fig. 3). Combined, measurement ofboth biomarkers of cartilage type II collagen synthesis and degra-dation in body fluids now permit monitoring type II collagenmetabolism in vivo (Kraus et al., 2011).
Fetal articular cartilage: type II collagen
Articular cartilage has been reported to be homogeneous or iso-tropic at birth, based on its cellular and histologic appearance,becoming anisotropic and developing stratified layers with matu-rity (Hunziker et al., 2007). However, a recent study of equine fetalarticular cartilage suggested that it already possesses an aniso-tropic organization in utero (Lecocq et al., 2008) (Fig. 8). A multi-zonal collagen structure was apparent in utero and birefringenceincreased with gestational age, indicating an increase in collagencontent or alignment, but to a lesser degree to adults (Lecocqet al., 2008).
The collagen network undergoes major biochemical and struc-tural changes with maturation from the fetal stage to attain theadult cartilage zonal structure (Brama et al., 2000, 2002; Hyttinenet al., 2001, 2009; Hughes et al., 2005; Hunziker et al., 2007; Lecocqet al., 2008; Julkunen et al., 2009, 2010; van Turnhout et al., 2010,2011) and the functional properties change as the collagen contentand fibril orientation mature (Rieppo et al., 2009). The biomechan-ical function of this maturing cartilage is highly dependent onthese collagen network changes that include an increase in typeII collagen content and cross-links and a reorientation of the pre-dominant collagen fiber direction from a parallel to perpendicularorientation to the articular surface (Williamson et al., 2003; Julk-unen et al., 2009). This immature tissue is not fully functionallyadapted to external loads and may be prone to injury because ofthis.
Endochondral ossification and type II collagen
In fetal and juvenile epiphyses the sub-articular growth carti-lage undergoes endochondral ossification. A series of characteristicchondrocyte changes and type II collagen metabolic events occur atthe ossification front (Poole et al., 2000). The chondrocytes pro-gress through resting, proliferative and hypertrophic stages to celldeath. Chondrocyte hypertrophy entails an increase in cellular vol-ume, and the subsequent cell death leaves empty lacunae (Fig. 3).Type II collagen synthesis increases in the pre-hypertrophic zonebut, with chondrocyte hypertrophy, MMP-13 expression is in-creased and causes proteolysis of the collagen molecules with aresulting reduction in collagen content (Mwale et al., 2002; Wuet al., 2002). In the later stages of cartilage hypertrophy there is aswitch to type X collagen synthesis combined with degradation
Fig. 8. Collagen structure in the medial femoral condyle of an 11 month equine fetus. (A) Section reveals the collagen structure of the articular epiphyseal complex onpolarized light microscopy examination. The top of the image is the articular surface (dense bi-refringent line with darker band underneath and containing no vascularchannels). Below this, containing cartilage canals and densely packed collagen is the growth cartilage. The bottom 1=3 of the image is the newly formed bone of the secondarycenter of ossification; (B) the osteochondral junction. Bone is in the distal one-third of the image. Various shaped cartilage canals are evident in the growth cartilage (Uppertwo-thirds of the image). The transition zone with densely packed collagen fibers running parallel and with lacunae of resting cells (not visible) lying above an area where thecollagen is remodeled to become bone (lower part of image). Scale bar, 0.1 mm. Stain is picrosirius red.
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of the mineralized collagen by MMPs, gelatinases and cathepsins toallow vascular invasion as described previously.
Lecocq et al. (2008) recently identified a dramatic transition incollagen structure, from a more dense structure to one with thin-ner fibrils at the ossification front in equine tissues, paralleling cel-lular changes at these sites. The changes in the cartilageextracellular matrix due to these events (increased cell volume,cell death, collagen proteolysis and reduced content) may contrib-ute to a structural weakness and could predispose to micro- ormacro-traumatic injury at this site, blood vessel injury and subse-quent development of OC (Figs. 3 and 8). It has already been re-ported that the osteochondral junction of young people fracturesat significantly lower loads than adult tissues when subjected toimpact injury. This was attributed to structural changes implicat-ing the cell component combined with a softer matrix causingmarked effect on mechanical properties of immature cartilage inshear (Flachsmann et al., 2000; Klein et al., 2007). Pool alsohypothesized that shear forces at the equine osteochondral junc-tion could cause vessel injury and initiate OCD (Pool, 1993).
Cartilage type II collagen and OCD
A link between abnormalities of articular cartilage collagenmetabolism and OC has been posited for some time in many spe-cies, including humans, pigs and horses (Mubarak and Carroll,1981; Carlson et al., 1986; Ekman et al., 1990). Type II collagen fi-bers are initially small and progressively increase in size withmaturity. An increase in the number of small sized collagen fibers,suggesting enhanced synthesis, has been identified in the extracel-lular matrix of porcine cartilage OC lesions compared to normalcartilage (Carlson et al., 1986; Ekman et al., 1990).
The putative role of type II collagen metabolism in equine OCDpathogenesis first arose from the hypothesis that copper deficiencywas a cause of OCD. This was suggested to be due to inhibition oflysyl oxidase, a copper dependent enzyme, with a key role in colla-gen cross-link formation (Bridges et al., 1984). Although coppersupplementation was at first reported to reduce the number ofOCD lesions in foals (Knight et al., 1990; Pearce et al., 1998a,b), la-ter studies concluded that copper was not likely to be a significant
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factor in the etiopathogenesis of OCD in foals (van Weeren et al.,2003; Gee et al., 2005).
Synthesis of type II collagen is enhanced in OCD
Laverty et al. (2000) first identified an increase in cartilagetype II collagen synthesis by measuring the biomarker CPII inthe synovial fluid from joints of foals with OCD lesions. Thishas since been confirmed and extended to reveal a significantpositive relationship between serum C-propeptide of type II col-lagen (CPII) levels and OCD severity in weanlings (Billinghurstet al., 2004). Furthermore, a strong positive correlation betweenserum CPII, with an increased CPII:C2C ratio in the synovial fluidof OCD affected joints, in foals with radiographic OCD was alsoidentified (Donabédian et al., 2008; de Grauw et al., 2011),underpinning a predominant type II collagen synthesis in theseyoung animals.
These independent studies of equine OCD from Canada, USAand Europe confirm that significant rises in the CPII synovialfluid or serum in young foals with OCD reflects cartilage typeII collagen synthesis. It is also further support for the hypothesesthat altered collagen metabolism may have a role in the etio-pathogenesis of OCD, particularly when it is now recognized thatthere is a link between type II collagen synthesis and early OCDin pigs. Frantz et al. (2010) reported that serum CPII levels al-lowed prediction of OCD in 5–6 month old pigs and concludedthat elevated CPII is considered a good indicator of the presenceof OCD in young swine, in agreement with the earlier studies infoals.
A significant increase in type II collagen (Coll-II) gene expres-sion, reflecting up-regulation of collagen synthesis, was observedin equine OCD lesion chondrocytes when compared to normal(Garvican et al., 2008). However, other investigators did not ob-serve this up-regulation in OC (Semevolos et al., 2001, 2004,2005; Mirams et al., 2009) or OCD specimens (Semevolos et al.,2001, 2004, 2005).
It is intriguing to note that the synthesis of proteoglycan, an-other major cartilage structural macromolecule, does not mirrorthe type II collagen molecular events measured in vivo. The bio-marker CS846, which detects synthesis and release of newly syn-thesized proteoglycan molecules (Nelson et al., 1998), wasdecreased in synovial fluid (Laverty et al., 2000; de Grauw et al.,2011) and serum (Billinghurst et al., 2004) of OCD-affected horses.Equally, there was no difference in serum CS846 levels betweenOCD and control pigs (Frantz et al., 2010). Furthermore, serumCS846 levels did not correlate with the total OCD macroscopicscore in young horses, but correlated with the total lesion count(Billinghurst et al., 2004).
A significant decrease in serum glycosaminoglycan (GAG) wasalso present in the youngest 5 month-old OCD-affected foals(Billinghurst et al., 2004). Similarly, a reduced GAG release wasevident from cartilage explants of young horses with OCD, whichindicated reduced proteoglycan degradation (Laverty et al.,2002). A decreased proteoglycan content (Nakano et al., 1985;Lillich et al., 1997; Tomlinson et al., 2001; Bertone et al., 2005)and production (Ekman et al., 1990; van den Hoogen et al.,1999) have been reported for both equine and porcine OCD le-sions. Although Garvican et al. (2008) observed that OCD chon-drocytes synthesized less proteoglycan when compared tonormal, no differences in aggrecan expression were measuredbetween OC-affected and control cartilage (Semevolos et al.,2001; Mirams et al., 2009). Taken together, in contrast to typeII collagen metabolism, both proteoglycan synthesis and degra-dation appeared to be reduced overall in young OCD affectedanimals.
Degradation of type II collagen is increased in OCD
As the earliest lesion of OC involves persistence of growth car-tilage, an imbalance of the enzymes involved in proteolysis ofthe cartilage matrix at the ossification front has been proposed tocontribute to OCD. An increase in the levels of the intracellularlysosomal enzyme Cathepsin B, that can degrade both unwoundtype II collagen and proteoglycan, has been identified in equineOCD lesions associated with chondrocyte clusters (Hernandez-Vi-dal et al., 1998). In addition, a localized increase in MMP gelatinaseactivity (MMP-2 and -9) was identified in the deep zone of equinegrowth cartilage from OCD lesions (Al-Hizab et al., 2002). A distur-bance of type II collagen degradation has also been identified inOCD lesions employing biomarker technology. Laverty et al.(2002) reported degradation of the intact type II collagen triple he-lix was increased in cartilage explants harvested from OCD lesionsof foals (range, 7–12 months; mean, 8.9 months) when comparedto normal cartilage by measuring the biomarker COL2–3/4Cshort
(now called C1,2C) in the media.Significant positive relationships between the concentration of
serum biomarkers of collagen degradation (COL2-3/4Cshort,234CEQ) and radiographic OCD severity scores in 5 month foalsalso suggested substantial collagenase-induced turnover of colla-gen during early OCD (Billinghurst et al., 2004). However therewas a negative correlation with radiographic OCD severity at11 months, in the more advanced stages of disease. Alternatively,De Grauw et al. (2011) reported no difference in synovial fluidC2C (detects cleavage of type II collagen by collagenases) betweenOCD and control joints in young foals. Serum C2C was decreased inpigs with OCD but, in those with OCD lesions, it correlated posi-tively with the severity of the lesions (Frantz et al., 2010). The po-tential for increased type II collagen degradation has also beenobserved at the molecular level as an increase in MMP 13 geneexpression has also been identified in early equine OCD lesions(Garvican et al., 2008; Mirams et al., 2009) and in canine (Kurokiet al., 2005) OCD cartilage.
Collagen content and OCD
Collagen content of cartilage is the sum of chondrocyte synthe-sis and degradation. A reduction of collagen content of OCD carti-lage has been observed in horses and pigs (Wardale and Duance,1994; van de Lest et al., 2004). This observation contrasts to a sub-sequent study of equine OCD lesions where no reduction was ob-served when compared to normal cartilage (Laverty et al., 2002).
Summary of type II collagen and OCD
Enhanced type II collagen synthesis in young animals with OCDappears to be characteristic and, although enhanced degradationand a reduction in collagen content have been reported, the resultsare not as consistent across studies. As the collagen molecule is soimportant for the biomechanical properties of the AECC, structuralor metabolic alterations in its network may render juvenile jointcartilage susceptible to injurious forces or wear and tear, particu-larly in shear.
The consistent pattern of increased collagen synthesis on bio-marker analysis in early OCD may reflect a repair process, but itis surprising that similar metabolic findings are not reported forthe other principal cartilage matrix molecule proteoglycan. Theclear disparity between collagen metabolism abnormalities earlyin OCD, not consistently observed with proteoglycan, underpins arole for alterations in type II collagen metabolism in the earlyevents on the trajectory to OCD.
The quality of the type II collagen or its metabolism may bedetermined genetically or by differences in nutrition. It is specu-
10 S. Laverty, C. Girard / The Veterinary Journal 197 (2013) 3–12
lated that excessively rapid growth, induced for example by anoverly rich diet or genetics, could lead to the creation of a collagenmatrix of growth cartilage that is not sufficiently strong to supportthe integrity of cartilage canals. Rapid joint growth could poten-tially outpace collagen maturation, as it takes time for cross-linksto be created. However, there are numerous and contradictory re-ports in the literature concerning the role of rapid growth in theetiopathogenesis of OC (Ytrehus et al., 2007).
An altered growth cartilage type II collagen matrix or structurecould render the vessels contained therein more susceptible to in-jury at focal sites of enhanced biomechanical stresses due to jointgeometry. In the past it has been posited that the collagen struc-ture is important for prevention of collapse of cartilage canals lead-ing to ischemia (Henson et al., 1996). Physiological collagenremodelling, including extensive proteolysis, at the osteochondraljunction may alter the biomechanical properties between boneand cartilage at this site predisposing cartilage canal vessels to in-jury due to shear forces, but this remains to be elucidated.
The up-regulation of type II collagen degradation and reductionof collagen content, observed in some, but not all investigations,may be secondary to degenerative processes in the chondronecrot-ic cartilage of lesions. Traumatic events could also up-regulate col-lagenolytic activity (Stoop et al., 1999) and, whether isolated, ormore probably repetitive, trauma due to joint geometry may playa role in the genesis of OCD lesions needs to be determined.
Future directions
Further investigations of OCD susceptible animals in utero andduring the first weeks of life are required to elucidate the cause ofvessel necrosis and the exact role of the type II collagen networkstructure, quality and metabolism in OC. Investigations of cartilagecanal endothelial cells, such as studies of apoptosis could advanceunderstanding of the early vascular events. Additional imagingstudies employing high resolution MRI and CT to study growth car-tilage vascularity, matrix structure and composition non- destruc-tively are also needed to advance knowledge in this field.Assessment of the biomechanical properties of cartilage in OCDsusceptible sites in young animals and effects of diet and rapidgrowth on collagen quality may also help elucidate the etiopatho-genesis of this complex disease.
Conclusions
Much has been learned in recent years about normal equine andporcine epiphyseal development and the earliest microscopic le-sions of OC. Joints mature at different stages and regions withinsome joints with thicker immature cartilage at birth are more sus-ceptible to OC and OCD, possibly due to persistent cartilage vascu-larity at these sites. Necrosis of vessels in the cartilage canals leadsto chondronecrosis in many species. The exact cause of the vascu-lar necrosis remains to be elucidated, but vessels are vulnerablewhen incorporated into the ossification front.
Type II collagen is the principal cartilage structural moleculeand alterations in its synthesis, degradation or structure influenceits biomechanical properties. Although the enhanced collagen syn-thesis observed in OCD may reflect a repair response of subclinicallesions, it could also reflect a primary problem in the matrix.Extensive collagen remodeling, including proteolysis, is a physio-logical event at the ossification front. Differences in structure andbiomechanical properties at this site may predispose to cartilagecanal vessel shear.
Although the understanding of the etiopathogenesis of OC hasimproved in recent years, many knowledge gaps remain. Specifi-cally, further studies are required to determine the cause of carti-
lage canal vessel necrosis and the role of cartilage type IIcollagen structure and metabolism in OC.
Conflict of interest statement
None of the authors of this paper has a financial or personalrelationship with other people or organizations that could inappro-priately influence or bias the content of the paper.
Acknowledgement
The authors thank Hélène Richard and Marie-Noëlle Lagüe forhelp with manuscript preparation.
References
Al-Hizab, F., Clegg, P.D., Thompson, C.C., Carter, S.D., 2002. Microscopic localizationof active gelatinases in equine osteochondritis dissecans (OCD) cartilage.Osteoarthritis and Cartilage 10, 653–661.
Alini, M., Matsui, Y., Dodge, G.R., Poole, A.R., 1992. The extracellular matrix ofcartilage in the growth plate before and during calcification: Changes incomposition and degradation of type II collagen. Calcified Tissue International50, 327–335.
Alvarez, J., Costales, L., Serra, R., Balbin, M., Lopez, J.M., 2005. Expression patterns ofmatrix metalloproteinases and vascular endothelial growth factor duringepiphyseal ossification. Journal of Bone and Mineral Research 20, 1011–1021.
Aspden, R.M., Hukins, D.W., 1981. Collagen organization in articular cartilage,determined by X-ray diffraction, and its relationship to tissue function.Proceedings of the Royal Society of London. Series B: Biological Sciences 212,299–304.
Benninghoff, A., 1925. Form und Bau der gelenkknorpel in ihren Beziehungen zurFunktion Zweiter teil: Der Aufbau des Gelenkknorpels in seinen Beziehungenzur Funktion. Zsch. Zelforschis Mikroskopic Anatomie 2, 783–862.
Bertone, A.L., Bramlage, L.R., McIlwraith, C.W., Malemud, C.J., 2005. Comparison ofproteoglycan and collagen in articular cartilage of horses with naturallydeveloping osteochondrosis and healing osteochondral fragments ofexperimentally induced fractures. American Journal of Veterinary Research66, 1881–1890.
Billinghurst, R.C., Brama, P.A., van Weeren, P.R., Knowlton, M.S., McIlwraith, C.W.,2004. Evaluation of serum concentrations of biomarkers of skeletal metabolismand results of radiography as indicators of severity of osteochondrosis in foals.American Journal of Veterinary Research 65, 143–150.
Brama, P.A., Tekoppele, J.M., Bank, R.A., Barneveld, A., van Weeren, P.R., 2000.Functional adaptation of equine articular cartilage: The formation of regionalbiochemical characteristics up to age one year. Equine Veterinary Journal 32,217–221.
Brama, P.A., TeKoppele, J.M., Bank, R.A., Barneveld, A., van Weeren, P.R., 2002.Development of biochemical heterogeneity of articular cartilage: Influences ofage and exercise. Equine Veterinary Journal 34, 265–269.
Bridges, C.H., Womack, J.E., Harris, E.D., Scrutchfield, W.L., 1984. Considerations ofcopper metabolism in osteochondrosis of suckling foals. Journal of theAmerican Veterinary Medical Association 185, 173–178.
Carlson, C.S., Hilley, H.D., Henrikson, C.K., Meuten, D.J., 1986. The ultrastructure ofosteochondrosis of the articular-epiphyseal cartilage complex in growingswine. Calcified Tissue International 38, 44–51.
Carlson, C.S., Hilley, H.D., Meuten, D.J., Hagan, J.M., Moser, R.L., 1988. Effect ofreduced growth rate on the prevalence and severity of osteochondrosis in gilts.American Journal of Veterinary Research 49, 396–402.
Carlson, C.S., Hilley, H.D., Meuten, D.J., 1989. Degeneration of cartilage canal vesselsassociated with lesions of osteochondrosis in swine. Veterinary Pathology 26,47–54.
Carlson, C.S., Meuten, D.J., Richardson, D.C., 1991. Ischemic necrosis of cartilage inspontaneous and experimental lesions of osteochondrosis. Journal ofOrthopaedic Research 9, 317–329.
Carlson, C.S., Cullins, L.D., Meuten, D.J., 1995. Osteochondrosis of the articular-epiphyseal cartilage complex in young horses: Evidence for a defect in cartilagecanal blood supply. Veterinary Pathology 32, 641–647.
Changoor, A., Nelea, M., Methot, S., Tran-Khanh, N., Chevrier, A., Restrepo, A., Shive,M.S., Hoemann, C.D., Buschmann, M.D., 2011. Structural characteristics of thecollagen network in human normal, degraded and repair articular cartilagesobserved in polarized light and scanning electron microscopies. Osteoarthritisand Cartilage 19, 1458–1468.
Connolly, S.A., Jaramillo, D., Hong, J.K., Shapiro, F., 2004. Skeletal development infetal pig specimens: MR imaging of femur with histologic comparison.Radiology 233, 505–514.
de Grauw, J.C., Donabedian, M., van de Lest, C.H., Perona, G., Robert, C., Lepage, O.,Martin-Rosset, W., van Weeren, P.R., 2011. Assessment of synovial fluidbiomarkers in healthy foals and in foals with tarsocrural osteochondrosis. TheVeterinary Journal 190, 390–395.
S. Laverty, C. Girard / The Veterinary Journal 197 (2013) 3–12 11
Dejica, V.M., Mort, J.S., Laverty, S., Percival, M.D., Antoniou, J., Zukor, D.J., Poole, A.R.,2008. Cleavage of type II collagen by cathepsin K in human osteoarthriticcartilage. American Journal of Pathology 173, 161–169.
Dejica, V.M., Mort, J.S., Laverty, S., Antoniou, J., Zukor, D.J., Tanzer, M., Poole, A.R.,2012. Increased type II collagen cleavage by cathepsin K and collagenaseactivities with aging and osteoarthritis in human articular cartilage. ArthritisResearch and Therapy 14, R113.
Donabédian, M., Fleurance, G., Perona, G., Robert, C., Lepage, O., Trillaud-Geyl, C.,Leger, S., Ricard, A., Bergero, D., Martin-Rosset, W., 2006. Effect of fast vs.moderate growth rate related to nutrient intake on developmental orthopaedicdisease in the horse. Animal Research 55, 471–486.
Donabédian, M., van Weeren, P.R., Perona, G., Fleurance, G., Robert, C., Leger, S.,Bergero, D., Lepage, O., Martin-Rosset, W., 2008. Early changes in biomarkers ofskeletal metabolism and their association to the occurrence of osteochondrosis(OC) in the horse. Equine Veterinary Journal 40, 253–259.
Ekman, S., Rodriguez-Martinez, H., Ploen, L., 1990. Morphology of normal andosteochondrotic porcine articular-epiphyseal cartilage. A study in the domesticpig and minipig of wild hog ancestry. Acta Anatomica 139, 239–253.
Ekman, S., Carlson, C.S., van Weeren, P.R., 2009. Workshop report: Thirdinternational workshop on equine osteochondrosis, Stockhom, 29–30th May2008. Equine Veterinary Journal 41, 504–507.
Eyre, D.R., Weis, M.A., Wu, J.J., 2006. Articular cartilage collagen: An irreplaceableframework? European Cells and Materials 12, 57–63.
Flachsmann, R., Broom, N.D., Hardy, A.E., Moltschaniwskyj, G., 2000. Why is theadolescent joint particularly susceptible to osteochondral shear fracture?Clinical and Orthopedic Related Research, 212–221.
Frantz, N.Z., Friesen, K.G., Andrews, G.A., Tokach, M.D., Yamka, R.M., Loughin, T.L.,Nelssen, J.L., Dritz, S.S., 2010. Use of serum biomarkers to predict thedevelopment and severity of osteochondrosis lesions in the distal portion ofthe femur in pigs. American Journal of Veterinary Research 71, 946–952.
Garvican, E.R., Vaughan-Thomas, A., Redmond, C., Clegg, P.D., 2008. Chondrocytesharvested from osteochondritis dissecans cartilage are able to undergo limitedin vitro chondrogenesis despite having perturbations of cell phenotype in vivo.Journal of Orthopaedic Research 26, 1133–1140.
Gee, E.K., Firth, E.C., Morel, P.C., Fennessy, P.F., Grace, N.D., Mogg, T.D., 2005.Articular/epiphyseal osteochondrosis in Thoroughbred foals at 5 months of age:Influences of growth of the foal and prenatal copper supplementation of thedam. New Zealand Veterinary Journal 53, 448–456.
Glaser, K.E., Davies, M.E., Jeffcott, L.B., 2003. Differential distribution of cathepsins Band L in articular cartilage during skeletal development in the horse. EquineVeterinary Journal 35, 42–47.
Haines, R.W., 1974. The pseudoepiphysis of the first metacarpal of man. Journal ofAnatomy 117, 145–158.
Henson, P., Davis, M., Schofield, P., Jeffcott, L., 1996. Expression of types II, VI, and Xcollagen in equine growth cartilage during development. Equine VeterinaryJournal 28, 189–198.
Hernandez-Vidal, G., Jeffcott, L.B., Davies, M.E., 1998. Immunolocalization ofcathepsin B in equine dyschondroplastic articular cartilage. The VeterinaryJournal 156, 193–201.
Hill, M.A., Hilley, H.D., Feeney, D.A., Ruth, G.R., Hansgen, D.C., 1984.Dyschondroplasias, including osteochondrosis, in boars between 25 and 169days of age: Radiologic changes. American Journal of Veterinary Research 45,917–925.
Hill, M.A., Ruth, G.R., Hilley, H.D., Torrison, J.L., Bagent, J.K., Leman, A.D., 1985.Dyschondroplasias of growth cartilages (osteochondrosis) in crossbredcommercial pigs at one and 15 days of age: Radiological, angiomicrographicaland histological findings. Veterinary Record 116, 40–47.
Hill, M.A., Kincaid, S.A., Visco, D.M., 1990. Use of histochemical techniques in thecharacterisation of osteochondroses affecting pigs. Veterinary Record 127, 29–37.
Hill, B.D., Sutton, R.H., Thompson, H., 1998. Investigation of osteochondrosis ingrazing beef cattle. Australian Veterinary Journal 76, 171–175.
Hughes, L.C., Archer, C.W., ap Gwynn, I., 2005. The ultrastructure of mouse articularcartilage: Collagen orientation and implications for tissue functionality. Apolarised light and scanning electron microscope study and review. EuropeanCells and Materials 9, 68–84.
Hunziker, E.B., Kapfinger, E., Geiss, J., 2007. The structural architecture of adultmammalian articular cartilage evolves by a synchronized process of tissueresorption and neoformation during postnatal development. Osteoarthritis andCartilage 15, 403–413.
Hyttinen, M.M., Arokoski, J.P., Parkkinen, J.J., Lammi, M.J., Lapvetelainen, T.,Mauranen, K., Kiraly, K., Tammi, M.I., Helminen, H.J., 2001. Age matters:Collagen birefringence of superficial articular cartilage is increased in youngguinea-pigs but decreased in older animals after identical physiological type ofjoint loading. Osteoarthritis and Cartilage 9, 694–701.
Hyttinen, M.M., Holopainen, J., van Weeren, P.R., Firth, E.C., Helminen, H.J., Brama,P.A., 2009. Changes in collagen fibril network organization and proteoglycandistribution in equine articular cartilage during maturation and growth. Journalof Anatomy 215, 584–591.
Jensen, R., Park, R.D., Lauerman, L.H., Braddy, P.M., Horton, D.P., Flack, D.E., Cox, M.F.,Einertson, N., Miller, G.K., Rehfeld, C.E., 1981. Osteochondrosis in feedlot cattle.Veterinary Pathology 18, 529–535.
Julkunen, P., Harjula, T., Iivarinen, J., Marjanen, J., Seppanen, K., Narhi, T., Arokoski, J.,Lammi, M.J., Brama, P.A., Jurvelin, J.S., Helminen, H.J., 2009. Biomechanical,biochemical and structural correlations in immature and mature rabbitarticular cartilage. Osteoarthritis and Cartilage 17, 1628–1638.
Julkunen, P., Iivarinen, J., Brama, P.A., Arokoski, J., Jurvelin, J.S., Helminen, H.J., 2010.Maturation of collagen fibril network structure in tibial and femoral cartilage ofrabbits. Osteoarthritis and Cartilage 18, 406–415.
Kincaid, S.A., Lidvall, E.R., 1982. Communicating cartilage canals of the physis of thedistal part of the ulna of growing swine and their potential role in healing ofmetaphyseal dysplasia of osteochondrosis. American Journal of VeterinaryResearch 43, 938–944.
Kincaid, S.A., Lidvall, E.R., 1983. Observations on the postnatal morphogenesis of theporcine humeral condyle and the pathogenesis of osteochondrosis. AmericanJournal of Veterinary Research 44, 2095–2103.
Kincaid, S.A., Allhands, R.V., Pijanowski, G.J., 1985. Chondrolysis associated withcartilage canals of the epiphyseal cartilage of the distal humerus of growingpigs. American Journal of Veterinary Research 46, 726–732.
Klein, T.J., Chaudhry, M., Bae, W.C., Sah, R.L., 2007. Depth-dependent biomechanicaland biochemical properties of fetal, newborn, and tissue-engineered articularcartilage. Journal of Biomechanics 40, 182–190.
Knight, D.A., Weisbrode, S.E., Schmall, L.M., Reed, S.M., Gabel, A.A., Bramlage, L.R.,Tyznik, W.I., 1990. The effects of copper supplementation on the prevalence ofcartilage lesions in foals. Equine Veterinary Journal 22, 426–432.
Kraus, V.B., Burnett, B., Coindreau, J., Cottrell, S., Eyre, D., Gendreau, M., Gardiner, J.,Garnero, P., Hardin, J., Henrotin, Y., Heinegard, D., Ko, A., Lohmander, L.S.,Matthews, G., Menetski, J., Moskowitz, R., Persiani, S., Poole, A.R., Rousseau, J.C.,Todman, M., 2011. Application of biomarkers in the development of drugsintended for the treatment of osteoarthritis. Osteoarthritis and Cartilage 19,515–542.
Kronfeld, D.S., Meacham, T.N., Donoghue, S., 1990. Dietary aspects of developmentalorthopedic disease in young horses. Veterinary Clinics of North America: EquinePractice 6, 451–465.
Kuroki, K., Cook, J.L., Stoker, A.M., Turnquist, S.E., Kreeger, J.M., Tomlinson, J.L., 2005.Characterizing osteochondrosis in the dog: potential roles for matrixmetalloproteinases and mechanical load in pathogenesis and diseaseprogression. Osteoarthritis and Cartilage 13, 225–234.
Laverty, S., Ionescu, M., Marcoux, M., Boure, L., Doize, B., Poole, A.R., 2000.Alterations in cartilage type-II procollagen and aggrecan contents in synovialfluid in equine osteochondrosis. Journal of Orthopaedic Research 18, 399–405.
Laverty, S., Okouneff, S., Ionescu, M., Reiner, A., Pidoux, I., Webber, C., Rossier, Y.,Billinghurst, R.C., Poole, A.R., 2002. Excessive degradation of type II collagen inarticular cartilage in equine osteochondrosis. Journal of Orthopaedic Research20, 1282–1289.
Lecocq, M., Girard, C.A., Fogarty, U., Beauchamp, G., Richard, H., Laverty, S., 2008.Cartilage matrix changes in the developing epiphysis: early events on thepathway to equine osteochondrosis? Equine Veterinary Journal 40, 442–454.
Lee, E.R., Lamplugh, L., Shepard, N.L., Mort, J.S., 1995. The septoclast, a cathepsin B-rich cell involved in the resorption of growth plate cartilage. Journal ofHistochemistry and Cytochemistry 43, 525–536.
Lee, E.R., Lamplugh, L., Kluczyk, B., Leblond, C.P., Mort, J.S., 2009. Neoepitopes revealthe features of type II collagen cleavage and the identity of a collagenaseinvolved in the transformation of the epiphyses anlagen in development.Developmental Dynamics 238, 1547–1563.
Lillich, J.D., Bertone, A.L., Malemud, C.J., Weisbrode, S.E., Ruggles, A.J., Stevenson, S.,1997. Biochemical, histochemical, and immunohistochemical characterizationof distal tibial osteochondrosis in horses. American Journal of VeterinaryResearch 58, 89–98.
Mirams, M., Tatarczuch, L., Ahmed, Y.A., Pagel, C.N., Jeffcott, L.B., Davies, H.M.,Mackie, E.J., 2009. Altered gene expression in early osteochondrosis lesions.Journal of Orthopaedic Research 27, 452–457.
Mubarak, S.J., Carroll, N.C., 1981. Juvenile osteochondritis dissecans of the knee:Etiology. Clinical and Orthopedic Related Research, 200–211.
Mwale, F., Tchetina, E., Wu, C.W., Poole, A.R., 2002. The assembly and remodeling ofthe extracellular matrix in the growth plate in relationship to mineraldeposition and cellular hypertrophy: An in situ study of collagens II and IXand proteoglycan. Journal of Bone and Mineral Research 17, 275–283.
Nakano, T., Thompson, J.R., Aherne, F.X., 1985. Cartilage proteoglycans from normaland osteochondrotic porcine joints. Canadian Journal of Comparative Medicine49, 219–226.
Nelson, F., Dahlberg, L., Laverty, S., Reiner, A., Pidoux, I., Ionescu, M., Fraser, G.L.,Brooks, E., Tanzer, M., Rosenberg, L.C., Dieppe, P., Robin Poole, A., 1998. Evidencefor altered synthesis of type II collagen in patients with osteoarthritis. Journal ofClinical Investigation 102, 2115–2125.
Olsson, S.E., 1987. General and local [corrected] aetiologic factors in canineosteochondrosis. Veterinary Quarterly 9, 268–278.
Olsson, S.E., Reiland, S., 1978. The nature of osteochondrosis in animals. Summaryand conclusions with comparative aspects on osteochondritis dissecans in man.Acta Radiologica, Suppl. 358, 299–306.
Olstad, K., Ytrehus, B., Ekman, S., Carlson, C.S., Dolvik, N.I., 2007. Early lesions ofosteochondrosis in the distal tibia of foals. Journal of Orthopaedic Research 25,1094–1105.
Olstad, K., Cnudde, V., Masschaele, B., Thomassen, R., Dolvik, N.I., 2008a. Micro-computed tomography of early lesions of osteochondrosis in the tarsus of foals.Bone 43, 574–583.
Olstad, K., Ytrehus, B., Ekman, S., Carlson, C.S., Dolvik, N.I., 2008b. Epiphysealcartilage canal blood supply to the distal femur of foals. Equine VeterinaryJournal 40, 433–439.
Olstad, K., Ytrehus, B., Ekman, S., Carlson, C.S., Dolvik, N.I., 2008c. Epiphysealcartilage canal blood supply to the tarsus of foals and relationship toosteochondrosis. Equine Veterinary Journal 40, 30–39.
12 S. Laverty, C. Girard / The Veterinary Journal 197 (2013) 3–12
Olstad, K., Ytrehus, B., Ekman, S., Carlson, C.S., Dolvik, N.I., 2009. Epiphyseal cartilagecanal blood supply to the metatarsophalangeal joint of foals. Equine VeterinaryJournal 41, 865–871.
Olstad, K., Ytrehus, B., Ekman, S., Carlson, C.S., Dolvik, N.I., 2011. Early lesions ofarticular osteochondrosis in the distal femur of foals. Veterinary Pathology 48,1165–1175.
Pearce, S.G., Grace, N.D., Wichtel, J.J., Firth, E.C., Fennessy, P.F., 1998a. Effect ofcopper supplementation on copper status of pregnant mares and foals. EquineVeterinary Journal 30, 200–203.
Pearce, S.G., Grace, N.D., Firth, E.C., Wichtel, J.J., Holle, S.A., Fennessy, P.F., 1998b.Effect of copper supplementation on the copper status of pasture-fed youngThoroughbreds. Equine Veterinary Journal 30, 204–210.
Pool, R., 1993. Difficulties in definition of equine osteochondrosis: Differentiation ofdevelopmental and acquired lesions. Equine Veterinary Journal, Suppl. 16, 5–12.
Poole, A.R., Laverty, S., Mwale, F., 2000. Endochondral bone formation anddevelopment in the axial and appendicular skeleton. In: Henderson, J.E.,Goltzman, D. (Eds.), The Osteoporosis Primer. Cambridge University Press,Cambridge, pp. 3–17.
Reiland, S., 1978. Morphology of osteochondrosis and sequelae in pigs. ActaRadiologica, Suppl. 358, 45–90.
Rieppo, J., Hyttinen, M.M., Halmesmaki, E., Ruotsalainen, H., Vasara, A., Kiviranta, I.,Jurvelin, J.S., Helminen, H.J., 2009. Changes in spatial collagen content andcollagen network architecture in porcine articular cartilage during growth andmaturation. Osteoarthritis and Cartilage 17, 448–455.
Rivas, R., Shapiro, F., 2002. Structural stages in the development of the long bonesand epiphyses: A study in the New Zealand white rabbit. Journal of Bone andJoint Surgery 84-A, 85–100.
Rousseau, J., Garnero, P., 2012. Biological markers in osteoarthritis. Bone 51, 265–277.
Savage, C., McCarthy, R.N., Jeffcott, L.B., 1993. Effects of dietary phosphorus andcalcium on induction of dyschondroplasia in foals. Equine Veterinary Journal,Suppl. 16, 80–83.
Scott, C.A., Gibbs, H.A., Thompson, H., 1996. Osteochondrosis as a cause of lamenessin purebred Suffolk lambs. Veterinary Record 139, 165–167.
Semevolos, S.A., Nixon, A.J., Brower-Toland, B.D., 2001. Changes in molecularexpression of aggrecan and collagen types I, II, and X, insulin-like growth factor-I, and transforming growth factor-beta1 in articular cartilage obtained fromhorses with naturally acquired osteochondrosis. American Journal of VeterinaryResearch 62, 1088–1094.
Semevolos, S.A., Nixon, A.J., Strassheim, M.L., 2004. Expression of bonemorphogenetic protein-6 and -2 and a bone morphogenetic proteinantagonist in horses with naturally acquired osteochondrosis. AmericanJournal of Veterinary Research 65, 110–115.
Semevolos, S.A., Strassheim, M.L., Haupt, J.L., Nixon, A.J., 2005. Expression patternsof hedgehog signaling peptides in naturally acquired equine osteochondrosis.Journal of Orthopaedic Research 23, 1152–1159.
Shingleton, W.D., Mackie, E.J., Cawston, T.E., Jeffcott, L.B., 1997. Cartilage canals inequine articular/epiphyseal growth cartilage and a possible association withdyschondroplasia. Equine Veterinary Journal 29, 360–364.
Stockwell, R.A., 1971. The ultrastructure of cartilage canals and the surroundingcartilage in the sheep fetus. Journal of Anatomy 109, 397–410.
Stoop, R., van der Kraan, P.M., Buma, P., Hollander, A.P., Poole, A.R., van den Berg,W.B., 1999. Denaturation of type II collagen in articular cartilage inexperimental murine arthritis. Evidence for collagen degradation in bothreversible and irreversible cartilage damage. Journal of Pathology 188, 329–337.
Thomson, K., 2007. Bones and joints. In: Jubb, Kennedy, Palmer (Eds.), Pathology ofDomestic Animals, Fifth Ed. Philadelphia, PA, USA, pp. 101–184.
Tomlinson, J.L., Cook, J.L., Kuroki, K., Kreeger, J.M., Anderson, M.A., 2001.Biochemical characterization of cartilage affected by osteochondritisdissecans in the humeral head of dogs. American Journal of VeterinaryResearch 62, 876–881.
van de Lest, C.H., Brama, P.A., van El, B., DeGroot, J., van Weeren, P.R., 2004.Extracellular matrix changes in early osteochondrotic defects in foals: A keyrole for collagen? Biochimica et Biophysica Acta 1690, 54–62.
van den Hoogen, B.M., van de Lest, C.H., van Weeren, P.R., van Golde, L.M.,Barneveld, A., 1999. Changes in proteoglycan metabolism in osteochondroticarticular cartilage of growing foals. Equine Veterinary Journal, Suppl. 31,38–44.
van Turnhout, M.C., Schipper, H., van Lagen, B., Zuilhof, H., Kranenbarg, S., vanLeeuwen, J.L., 2010. Postnatal development of depth-dependent collagendensity in ovine articular cartilage. BMC Developmental Biology 10, 108.
van Turnhout, M.C., Kranenbarg, S., van Leeuwen, J.L., 2011. Contribution ofpostnatal collagen reorientation to depth-dependent mechanical properties ofarticular cartilage. Biomechanics and Modeling in Mechanobiology 10, 269–279.
van Weeren, P.R., 2012. Osteochondrosis. In: Auer, J.A., Stick, J.A. (Eds.), EquineSurgery, Fourth Ed. St. Louis, MO, USA, pp. 1239–1254.
van Weeren, P.R., Barneveld, A., 1999. The effect of exercise on the distribution andmanifestation of osteochondrotic lesions in the Warmblood foal. EquineVeterinary Journal, Suppl. 31, 16–25.
van Weeren, P.R., Knaap, J., Firth, E.C., 2003. Influence of liver copper status of mareand newborn foal on the development of osteochondrotic lesions. EquineVeterinary Journal 35, 67–71.
Vervuert, I., Coenen, M., Bothe, C., 2003. Effects of oat processing on the glycaemicand insulin responses in horses. Journal of Animal Physiology and AnimalNutrition 87, 96–104.
Vinardell, T., Dejica, V., Poole, A.R., Mort, J.S., Richard, H., Laverty, S., 2009. Evidenceto suggest that cathepsin K degrades articular cartilage in naturally occurringequine osteoarthritis. Osteoarthritis and Cartilage 17, 375–383.
Visco, D.M., Van Sickle, D.C., Hill, M.A., Kincaid, S.A., 1989. The vascular supply of thechondro-epiphyses of the elbow joint in young swine. Journal of Anatomy 163,215–229.
Visco, D.M., Hill, M.A., Van Sickle, D.C., Kincaid, S.A., 1991. Cartilage canals andlesions typical of osteochondrosis in growth cartilages from the distal part ofthe humerus of newborn pigs. Veterinary Record 128, 221–228.
Wardale, R.J., Duance, V.C., 1994. Characterisation of articular and growth platecartilage collagens in porcine osteochondrosis. Journal of Cell Science 107, 47–59.
Wegener, K.M., Heje, N.I., 1992. Dyschondroplasia (osteochondrosis) in articular-epiphyseal cartilage complexes of three calves from 24 to 103 days of age.Veterinary Pathology 29, 562–563.
Williamson, A.K., Chen, A.C., Masuda, K., Thonar, E.J., Sah, R.L., 2003. Tensilemechanical properties of bovine articular cartilage: Variations with growth andrelationships to collagen network components. Journal of Orthopaedic Research21, 872–880.
Woodard, J.C., Becker, H.N., Poulos Jr., P.W., 1987. Articular cartilage blood vessels inswine osteochondrosis. Veterinary Pathology 24, 118–123.
Wu, W., Billinghurst, R.C., Pidoux, I., Antoniou, J., Zukor, D., Tanzer, M., Poole, A.R.,2002. Sites of collagenase cleavage and denaturation of type II collagen in agingand osteoarthritic articular cartilage and their relationship to the distribution ofmatrix metalloproteinase 1 and matrix metalloproteinase 13. Arthritis andRheumatism 46, 2087–2094.
Ytrehus, B., Ekman, S., Carlson, C.S., Teige, J., Reinholt, F.P., 2004a. Focal changes inblood supply during normal epiphyseal growth are central in the pathogenesisof osteochondrosis in pigs. Bone 35, 1294–1306.
Ytrehus, B., Andreas Haga, H., Mellum, C.N., Mathisen, L., Carlson, C.S., Ekman, S.,Teige, J., Reinholt, F.P., 2004b. Experimental ischemia of porcine growthcartilage produces lesions of osteochondrosis. Journal of OrthopaedicResearch 22, 1201–1209.
Ytrehus, B., Carlson, C.S., Ekman, S., 2007. Etiology and pathogenesis ofosteochondrosis. Veterinary Pathology 44, 429–448.
