Enhanced concentration of COMP (cartilage oligomeric matrix protein) in osteochondral fractures from racing Thoroughbreds.

The aim of the present study was to correlate the levels of COMP and aggrecan as indicators of tissue damage, in synovial fluid (sf) from carpal joints of acutely lame racehorses, with macroscopical lesions of articular cartilage (OA), osteochondral fractures and ligament tears found at arthroscopy. Sixty-three lame horses [49 Standardbred trotters (STB) and 14 Thoroughbreds (TB)] in conventional training and racing that underwent arthroscopy of their middle carpal or radiocarpal joints were included in the study. Intact as well as fragmented COMP and aggrecan released into the synovial fluid were quantified by western blot analyses and ELISA. The expression of COMP in tissues was estimated by mRNA in situ hybridisation and protein immunolocalisation in cartilage and osteochondral fractures. The concentration of sf-COMP was higher in TB with an osteochondral fracture than in STB with osteochondral fractures and TB and STB with OA. The chondrocytes in middle and deep zones of the articular cartilage of the osteochondral fragments (from a TB) expressed COMP mRNA, in contrast to the cartilage on the opposite side of the fracture where no expression was detected. In the synovial fluid from a joint (TB) with osteochondral fractures only intact COMP was present, whereas, fragmented COMP was more prominent in synovial fluid from a joint with OA. The concentration of sf-aggrecan did not differ between the two breeds, or between different lesions. The increased concentration of sf-COMP in TB with osteochondral fractures, but not in synovial fluid from equine joints with OA, is a novel finding. The results from this study indicate that elevated sf-COMP concentration in the joints of Thoroughbreds may be a useful marker for carpal joint osteochondral fragments.