Regional variation and differential response to therapy for knee synovitis adjacent to the cartilage–pannus junction and suprapatellar pouch in inflammatory arthritis: Implications for pathogenesis and treatment

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Abstract
Objective To use magnetic resonance imaging (MRI) to investigate the importance of knee joint synovitis at the cartilage–pannus junction (CPJ) in rheumatoid arthritis (RA) as compared with synovitis at a distant site in the suprapatellar pouch (SPP) and as compared with CPJ synovitis in the spondylarthropathies (SpA), and to assess the relative response of knee joint synovitis to therapy at the CPJ and SPP sites. Methods Dynamic contrast‐enhanced MRI (DEMRI) of actively involved knee joints in 24 patients (13 with RA and 11 with SpA) was undertaken. The area of synovitis was calculated at the CPJ and SPP regions of interest in patients with RA and in patients with SpA. Differences in CPJ and SPP synovitis were determined using calculated DEMRI parameters which included the initial rate of contrast enhancement (IRE) and the maximal enhancement (ME). Changes in the synovial area at the CPJ and SPP were also measured in 10 patients with early RA, following treatment with disease‐modifying antirheumatic drugs (DMARDs) (either methotrexate or leflunomide). Results In patients with RA or SpA, the area of synovitis was significantly larger immediately adjacent to the CPJ compared with a distant site at the SPP (in RA, mean synovitis area 162 mm2 at the CPJ versus 114 mm2 at the SPP [P = 0.010]; in SpA, mean synovitis area 214 mm2 at the CPJ versus 143 mm2 at the SPP [P = 0.002]), but the differences in the areas of synovitis at these sites were not significant between the RA and SpA patients. The IRE and ME values were also higher at the CPJ compared with the SPP, both in the RA patients (IRE P = 0.054, ME P = 0.018) and in the SpA patients (IRE P = 0.002, ME P = 0.001). A larger reduction in the area of synovitis was seen at the SPP compared with the CPJ following DMARD therapy in the RA patients (mean reduction 35% at the SPP [P = 0.023] and 12% at the CPJ [P not significant]). Conclusion The non–disease‐specific variations in synovitis and the differential responses to therapy in RA patients have implications for improving our understanding of CPJ synovitis. The results suggest that the pathophysiologic events at the CPJ reflect common anatomic, immune system, or biomechanical factors that play a role in modulating the severity of arthritis, and these events are not specific to RA since the same process was observed in other arthritides.

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