Objectives It is unknown whether joint inflammation precedes other articular tissue damage in osteoarthritis. Therefore, this study aims to determine if synovitis precedes the development of radiographic knee osteoarthritis (ROA).

Methods The participants in this nested case–control study were selected from persons in the Osteoarthritis Initiative with knees that had a Kellgren Lawrence grading (KLG)=0 at baseline (BL). These knees were evaluated annually with radiography and non-contrast-enhanced MRI over 4 years. MRIs were assessed for effusion-synovitis and Hoffa-synovitis. Case knees were defined by ROA (KLG≥2) on the postero-anterior knee radiographs at any assessment after BL. Radiographs were assessed at P0 (time of onset of ROA), 1 year prior to P0 (P-1) and at BL. Controls were participants who did not develop incident ROA (iROA) from BL to 48 months).

Results 133 knees of 120 persons with ROA (83 women) were matched to 133 control knees (83 women). ORs for occurrence of iROA associated with the presence of effusion-synovitis at BL, P-1 and P0 were 1.56 (95% CI 0.86 to 2.81), 3.23 (1.72 to 6.06) and 4.7 (1.10 to 2.95), respectively. The ORs for the occurrence of iROA associated with the presence of Hoffa-synovitis at BL, P-1 and P0 were 1.80 (1.1 to 2.95), 2.47 (1.45 to 4.23) and 2.40 (1.43 to 4.04), respectively.

Conclusions Effusion-synovitis and Hoffa-synovitis strongly predicted the development of iROA.