Comparison between synovial fluid cytology and joint capsule histopathology in horses with chronic osteochondritis dissecans

Introduction and objective - The osteochondrosis dissecans (OCD) is a disease that affects humans and animals and its aetiology and pathogenesis have been investigated for long time in human and veterinary medicine. OCD can cause slight changes in viscosity, mild increase in total protein and mild to severe increase in cell count, depending on the stage of the disease, in the synovial fluid of affected joint. Histological examination of the articular cartilage of horses with OCD showed areas of disorganization with reduction of glycosaminoglycans, including chondroitin sulphate, compared to normal horses. The purpose of the present work was to study the relationship between synovial fluid cytology and histopathologic examination of the articular capsule of horses with OCD. Materials and methods - Fifteen horses of different breed, age and sex with OCD were included in the study and all of them underwent arthroscopy. Synovial fluid samples and osteochondral, or synovial capsule, samples were collected from 22 joints affected by OCD during arthroscopic surgery. For each synovial fluid sample was assessed the following parameters: synovial fluid turbidity, viscosity, mucin clot, total protein (TP), total nucleated cell count (TNCC) and differential cell count. Furthermore, osteochondral or synovial capsule samples were evaluated on histologic examination. The samples were retrospectively divided into two groups according to the results of the differential cell count of synovial fluid: a) group 1: hypocellular synovial fluid (few lymphocytes); b) group 2: cellular synovial fluid. Data were expressed as average and standard deviation. T-test was applied to verify differences between group 1vs 2 for TNNC and TP, while chi-square test was used for turbidity, viscosity and mucin clot test. Results - Ten/22 joint samples were classified into the group 1 and 12/22 into the group 2. No significant differences were found between the 2 groups for turbidity, viscosity and mucin clot, while differences were obtained for TNCC and TP. Histological examination was normal in 16/22 joints and abnormal in 6/22. Conclusions - The lower viscosity, abnormal mucin clot test and increasing in turbidity might be related to a dysregulation of pathways involving inflammation, and matrix damage, similar to those found in osteoarthritis. Comparing the two groups, TP were higher in the group 2 vs group 1, supporting the hypothesis of different grade of inflammation in the two group (low vs mild). Synovial TNCC results and the cytological assessment are suggestive of degenerative inflammatory changes in both groups. The prevalences of histological alterations support the hypothesis that the histological evaluation of the cartilages is not useful in the diagnosis of degenerative joint diseases, in line with previous studies. In conclusion, the assessment of synovial fluid cytology seems to be more sensitive than the histological exam in the diagnosis of degenerative joint diseases, as reported by others(4).