The effect of swan neck and boutonniere deformities on the outcome of silicone metacarpophalangeal joint arthroplasty in rheumatoid arthritis.

Background: Rheumatoid arthritis patients with swan neck deformities are postulated to have greater metacarpophalangeal joint arc of motion because of their need to flex the joint to make a fist, whereas the boutonniere deformity places the fingers into the flexed position, creating less demand on the joint for grip. This study analyzes the effect of these deformities on the joint's arc of motion and hand function.

Methods: The authors measured the metacarpophalangeal joint arc of motion in 73 surgical patients. Data were allocated into groups by finger and hand deformity. Linear regression models were used to analyze the effect of the deformity on the joint's arc of motion. Functional outcomes were measured by the Michigan Hand Outcomes Questionnaire and the Jebson-Taylor Test.

Results: Nineteen fingers had boutonniere deformity, 95 had swan neck deformities, and 178 had no deformity. The no-deformity group had the least arc of motion at baseline (16 degrees) compared with the boutonniere (26 degrees) and swan neck (26 degrees) groups. Mean arc of motion in the no-deformity group compared with the boutonniere group at baseline was statistically significant, but all groups had similar arc of motion at long-term follow-up. Only mean Jebson-Taylor Test scores at baseline between the boutonniere and no-deformity groups were significantly different.

Conclusions: The results did not support the hypothesis that swan neck deformities have better arc of motion compared with boutonniere deformity. Boutonniere deformity has worse function at baseline, but there was no difference in function among groups at long-term follow-up.