Topographical mapping of biochemical properties of articular cartilage in the equine fetlock joint

The aim of this study was to evaluate topographical differences in the bioc hemical composition of the extracellular matrix of articular cartilage of t he normal equine fetlock joint. Water content, DNA content, glycosaminoglyc an (GAG) content and a number of characteristics of the collagen network (t otal collagen content, levels of hydroxylysine- (Hyl) and the crosslink hyd roxylysylpyridinoline, (HP) of articular cartilage in the proximal 1st phal anx (P1), distal 3rd metacarpal bone (MC), and proximal sesamoid bones (PSB ) were determined in the left and right fetlock joint of 6 mature horses (a ge 5-9 years). Twenty-eight sites were sampled per joint, which included th e clinically important areas often associated with pathology. Biochemical d ifferences were evaluated between sampling sites and related with the predi sposition for osteochondral injury and type of loading. Significant regional differences in the composition of the extracellular ma trix existed,within the joint. Furthermore, left and right joints exhibited biochemical differences. Typical topographic distribution patterns mere ob served for each parameter. In pi the dorsal and palmar articular margin sho wed a significantly lower GAG content than the more centrally located sites . Collagen content and HP crosslinks were higher at the joint margins than in the central area. Also, in the MC, GAG content was significantly lower a t the (dorsal) articular margin compared,with the central area. Consistent with findings in P1, collagen and HP crosslinks were significantly lower in the central area compared to the (dorsal) articular margin. Biochemical and biomechanical heterogeneity of articular cartilage is suppo sed to reflect the different functional demands made at different sites. In the present study, GAG content was highest in the constantly loaded centra l areas of the joint surfaces. In contrast, collagen content and HP crossli nks were higher in areas intermittently subjected to peak loading which sug gests that the response to a certain type of loading of the various compone nts of the extracellular matrix of articular cartilage are different. The d ifferences in biochemical characteristics between the various sites may hel p to explain the site specificity of osteochondral lesions commonly found i n the equine fetlock joint. Finally, these findings emphasise that the choi ce of sampling sites may profoundly influence the outcome of biochemical st udies of articular cartilage.