Osteoarthritis (OA) of the fetlock and carpal joints may arise from numerous inciting causes, including subchondral bone and articular cartilage injury. Synovial changes are an important factor in the pathophysiology of OA but it is unclear whether joints affected by direct osteochondral injury show any synovial inflammation prior to the development of OA. The aims of this study were to characterise and grade the histologic changes in the synovium of the fetlock, carpal and tarsal joints of Thoroughbred racehorses; and to examine their association with gross osteochondral lesions.
The fetlock, carpal and tarsal joints of 54 Thoroughbred racehorses were examined postmortem and gross changes including the presence of palmar/plantar osteochondral disease (POD), wear lines, cartilage loss/ulceration, marginal remodelling, linear fissures, and dorsal impact injury were graded. Synovial membranes of the same joints were examined histologically for the presence of oedema, congestion, angiogenesis, inflammation and intimal hyperplasia, and a semi-quantitative grading system was developed.
The most prevalent gross lesion in the forelimb fetlock joints was palmar osteochondral disease (54%), including cartilage loss and cartilage wear lines, while in the hindlimb fetlock joints the presence of cartilage linear fissures and cartilage loss were most prevalent (50%). The total histopathological score of the synovium from carpal joints (6.4±2.5) was significantly higher than the total score of the forelimb fetlock joints (4.9±3.7; p<0.05). The histological scores for intimal cell hyperplasia, inflammation and angiogenesis were not correlated with the total gross scores for POD.
These findings suggest that while synovitis may play an important role in OA, and osteochondral disease may eventually lead to OA, synovitis is not an inherent feature of osteochondral disease, including POD. This may have implications for the treatment and management of these conditions, as well as understanding the causes of pain and lameness.