Joint Related Tissue

Previously, it was assumed that synovitis in osteoarthritis (OA) was a secondary change associated with cartilage degeneration. However, recent studies have demonstrated that synovitis plays a pivotal role in the pathology of OA. The importance of synovitis has been primarily illuminated by the findings of epidemiological studies. Firstly, there is evidence that synovitis is associated with the symptoms of knee OA, especially pain. Studies using MRI have shown that the presence of effusion-synovitis correlates with the severity of pain in OA knees. Secondly, synovitis may be related to the structural progression of the disease. The results of cohort studies consistently indicate that knees with effusion-synovitis are at an elevated risk of disease progression. These findings strongly suggest that synovitis may play a pivotal role in the pathology of OA by causing both pain and cartilage degeneration. 

  Synovitis in OA knees may also contribute to the complexity of the clinical picture of the disease. The severity of synovitis is known to change over time, and this may explain, at least in part, the fluctuation of OA symptoms along the course of the disease. Furthermore, the progression of knee OA is often phasic, which may be attributed to the temporal change of synovitis.

  Despite its significance, the mechanism(s) underlying synovitis in OA remain unclear, as do the reasons why synovitis is related to pain or structural progression. It is therefore necessary to elucidate the mechanism(s) underlying synovitis in OA, which would be a crucial step in establishing effective treatments for knee OA.