First ray kinematics in women with rheumatoid arthritis and bunion deformity: a gait simulation imaging study.

OBJECTIVE: The first ray plays a role in the development of bunion foot deformity, but for unknown reasons. This prospective, cross-sectional study investigated first ray kinematics in women with rheumatoid arthritis (RA) and bunion.

METHODS: Nine participants having RA-bunion were analyzed in comparison to a control group (n = 10). Data were acquired using a magnetic resonance scanner. Conditions were standardized to simulate gait midstance, heel off, and terminal stance. Foot posture (hallux angle, intermetatarsal angle, arch angle, and calcaneus angle) and relative first ray position angles/helical axis parameters registered across gait conditions were measured. An analysis of variance model compared data between groups and across conditions, and correlation assessed the relationship between selected variables.

RESULTS: Eversion of the calcaneus averaged 9°, and adduction of the first ray was increased (F = 6.29, P = 0.02) by ≥4.6° across conditions in the RA-bunion group. There was an interaction (F = 7.73, P = 0.01) for the first ray axis. Followup comparisons identified increased inclination of the first ray axis over middle stance compared to late stance in the group with RA and bunion. There was moderate correlation (r = -0.42) between the calcaneus angle of eversion and inclination of the first ray axis.

CONCLUSION: Optimal treatment for bunion has not been defined. This research identified calcaneus eversion and first ray adduction, as well as inclination of the first ray axis as risk factors of bunion. This result may inform the evaluation and treatment of bunion in women with RA.