Objectives Baseline erosions are characteristic for rheumatoid arthritis (RA) and predictive for a severe disease course. The mechanisms leading to baseline erosions being a strong predictor for radiological progression are unknown. We aimed to increase this understanding by mediation analyses in an observational cohort and a cross-sectional MRI study.

Methods 3256 hands and feet radiographs of 653 early RA patients assessed during 7 years of disease were scored using the Sharp–van der Heijde method. Mediation models and multivariate regression analyses were used to explore the association between baseline erosions, other predictors and radiological damage over time. 603 joints (MCP2-5 and MTP1-5) of 67 RA patients underwent 1.5 T MRI at baseline. Data on MRI inflammation were compared with clinical inflammation and baseline radiological erosions.

Results Patients with baseline erosions had, at any point in time during 7 years, 3.45 times more joint damage than patients without erosions (p<0.001, 95% CI 3.00 to 3.98). Baseline erosions were an independent predictor and not a mediator between symptom duration, systemic or local clinical inflammation (erythrocyte sedimentation rate (ESR), swollen joint count (SJC)) or autoantibodies (anti-citrullinated-peptide antibodies, rheumatoid factor) and radiological damage. Subclinical MRI inflammation was studied in relation to erosions, revealing that 83% of the non-swollen joints with baseline erosions had subclinical MRI inflammation compared with 25% of the non-swollen joints without baseline erosions (OR 15.2 95% CI 3.1 to 102.1). The association between MRI inflammation and baseline erosions was independent of symptom duration, ESR, SJC and autoantibodies.

Conclusions Baseline erosions are a predictor for future joint damage, independent of known predictors as time, autoantibodies or clinical measurable inflammation. Subclinical inflammation is suggested as an underlying mechanism.