POS0021 INTERMETATARSAL BURSITIS, A NOVEL FEATURE OF JUXTA-ARTICULAR INFLAMMATION IN EARLY RHEUMATOID ARTHRITIS: RESULTS FROM A LONGITUDINAL MRI-STUDY

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Background:Rheumatoid arthritis (RA) is characterised by inflammation of the synovial lining. In addition to synovitis, the tendon sheaths of small hand and foot joints are also frequently inflamed. This results in tenosynovitis, which is often missed at clinical evaluation in early RA but visible on imaging, such as MRI. A third anatomical structure surrounded by a synovial lining is formed by the intermetatarsal bursae in the forefeet. Inflammation of these bursae (intermetatarsal bursitis; IMB) was recently identified at MRI-studies and shown to be specific for early RA.[1] This suggests that IMB is also a feature of early RA.Objectives:We hypothesised that if IMB is indeed an RA-feature, then (1) at diagnosis its presence associates with other measures of local inflammation (synovitis, tenosynovitis and osteitis) and (2) it responds to DMARD therapy similarly as these other local inflammatory measures. These hypotheses were tested in a comprehensive MRI-study.Methods:157 consecutive early RA patients underwent unilateral contrast-enhanced 1.5T MRI of the forefoot at diagnosis. MRIs were evaluated for presence of IMB and for synovitis, tenosynovitis and osteitis in line with the RA MRI scoring system (summed as RAMRIS-inflammation). MRIs at 4, 12 and 24 months were evaluated for presence and size of IMB-lesions in patients who had IMB at baseline and received early DMARD-therapy. Logistic regression was used for analyses at patient-level; generalised estimating equations were used for bursa-level analyses. Stratification for ACPA was performed.Results:69% of RA patients had ≥1 IMB. In multivariable analyses on bursa-level, presence of IMB was independently associated with local presence of synovitis and tenosynovitis (OR 1.69 (95%CI 1.12–2.57) and 2.83 (1.80–4.44), respectively), but not with osteitis. On patient-level, presence of IMB was most strongly associated with tenosynovitis (OR 2.92 (1.62–5.24)). During treatment with DMARDs, the average size of IMB-lesions decreased (Figure 1). This decrease was associated with decrease in RAMRIS-inflammation scores; most strongly with a decrease in synovitis but not in osteitis. Within ACPA-positive and ACPA-negative RA similar results were obtained.Conclusion:IMB particularly accompanies inflammation of the synovial lining of joints and tendon-sheaths, both regarding simultaneous occurrence at diagnosis and simultaneous treatment-response. These findings suggest that IMB represents juxta-articular synovial inflammation and indeed is a hallmark of early RA.