When does subluxation of the first carpometacarpal joint cause swan-neck deformity of the thumb in rheumatoid arthritis: a 20-year follow-up study.

The aim of our study was to assess the incidence of subluxation of the first carpometacarpal joint (CMC I) and to evaluate which degree of subluxation produces swan-neck deformity of the thumb in rheumatoid arthritis (RA) occurring over 20 years. The hands of 83 rheumatoid factor (RF)-positive RA patients with recent (< or = 6 months) arthritis were evaluated radiographically at onset and at 1, 3, 8 and 15 years; 68 patients were evaluated at 20 years from entry. Subluxation was assessed in millimetres and compared with the MCP-I angle measurement to evaluate the thumb deformity. A statistical end-point analysis was performed between two different grades of subluxation. Subluxation of 2-3 mm was non-specific and only one third of these thumbs showed swan-neck deformity. At the end-point, subluxation of > or = 4mm was present in 17% of the thumbs, 81% of which had the swan-neck deformity; only five thumbs did not show this deformity, but presented deformed and unstable MCP I and interphalangeal joints. The frequency of swan-neck deformity was highly significantly (p < 0.0001) increased in the thumbs with severe CMC I subluxation (> or = 4 mm) compared with lesser subluxation (< 4 mm). When subluxation of the CMC I exceeds 4 mm, the swan-neck deformity of the thumb is a common consequence. This deformity is often progressive, and the hand function of such patients should be followed up carefully, both clinically and radiographically.