OBJECTIVE: To develop the Outcome Measures in Rheumatology (OMERACT) thumb base osteoarthritis (OA) magnetic resonance imaging (MRI) scoring system (TOMS) for the assessment of inflammatory and structural abnormalities in this hand OA subset, and test its cross-sectional reliability.

METHODS: Included features and their scaling were agreed upon by members of the OMERACT MRI Task Force using the Hand OA MRI scoring system as a template. A reliability exercise was performed in which 3 readers participated, using a preliminary atlas with examples to facilitate reading. Each reader independently scored a set of 20 MRI (coronal and axial T1- and T2-weighted fat-suppressed images, of which 5 included T1-weighted fat-suppressed post-Gadolinium images). Intra- and inter-reader reliability were assessed using ICC, percentage exact agreement (PEA), and percentage close agreement (PCA).

RESULTS: The TOMS assessed the first carpometacarpal (CMC-1) and scaphotrapeziotrapezoid (STT) joints for synovitis, subchondral bone defects (including erosions, cysts, and bone attrition), osteophytes, cartilage, and bone marrow lesions on a 0-3 scale (normal to severe). Subluxation was evaluated only in the CMC-1 joint (absent/present). Reliability of scoring for both joints was comparable. Interreader ICC were good for all features (0.77-0.99 and 0.74-0.96 for CMC-1 and STT joints, respectively). Intrareader reliability analyses gave similar results. PCA was ≥ 65% for all features. PEA was low to moderate, with better performance for subchondral bone defects, subluxation, and bone marrow lesions.

CONCLUSION: A thumb base OA MRI scoring system has been developed. The OMERACT TOMS demonstrated good intrareader and interreader reliability. Longitudinal studies are warranted to investigate reliability of change scores and responsiveness.