Comparison of the response to experimentally induced short-term inflammation in the temporomandibular and metacarpophalangeal joints of horses

Objective-To investigate the relationship between inflammatory responses of the temporomandibular joint (TMJ) and the metacarpophalangeal (MCP) joint in clinically normal horses. Animals-7 mature horses. Procedures-In each horse, 1 TMJ and 1 MCP joint were injected with lipopolysaccharide (LPS; 0.0025 mu g). The contralateral TMJ and MCP joint were injected with saline (0.9% NaCl) solution. Synovial fluid samples were collected from all 4 joints over 24 hours after injection. Concentrations of interleukin-6, tumor necrosis factor-alpha, transforming growth factor-beta, and total protein were measured via immunoassay. Horses were assessed for clinical signs of joint inflammation at each time point. Results-Concentrations of interleukin-6 were not significantly different between LPS-injected MCP joints and TMJs at any time point. Transforming growth factor-beta concentrations were significantly increased in MCP joints, compared with concentrations in TMJs, at 12 and 24 hours after injection. Tumor necrosis factor-alpha concentrations were significantly higher in LPS-injected TMJs than in LPS-injected MCP joints at 1 and 6 hours after injection. Total protein concentration did not differ significantly between LPS-injected MCP joints and TMJs. Injection of LPS induced clinical inflammation at all time points; additionally, 2 MCP joints (but no TMJs) had an inflammatory response to injection of saline solution. Conclusions and Clinical Relevance-The inflammatory response to LPS appeared to be attenuated more quickly in TMJs than in MCP joints of horses. The difference in response suggested that a lack of clinical osteoarthritis in the TMJ of horses could be attributable to a difference in cytokine response. (Am J Vet Res 2011;72:1586-1591)