Radiologic Patterning of Hallux Deformity in Rheumatoid Arthritis and Its Relationship to Flatfoot.

Hallux deformities other than hallux valgus, especially those in the sagittal plane, have not yet been elucidated in the feet of patients with rheumatoid arthritis. The objectives of the present study were to classify rheumatoid arthritis hallux deformity in both the horizontal and the sagittal planes and investigate its relationship with flatfoot. Using a cross-sectional study design, we assessed patients with rheumatoid arthritis (527 feet in 274 patients) using radiographs and classified the deformity patterns of the great toes using cluster analysis. Of the 274 patients, the range of motion in the metatarsophalangeal joint was clinically investigated in 44 (16.1%) patients. The great toes could be divided into 5 clusters according to the characteristic configuration as follows: cluster I (normal type), cluster II (hallux valgus type), cluster III (boutonniere type), cluster IV (boutonniere with hallux valgus type), and cluster V (swan-neck type). Radiographic measurements revealed the characteristic deformities of each cluster, including splayed foot for cluster II; flat foot, metatarsal primus elevatus, and plantar displacement of the proximal phalanx for cluster III; and a mixture of these characteristics for cluster IV. Plantar displacement of the proximal phalanx, which was a specific characteristic of the boutonniere deformity, correlated significantly with the decreased dorsiflexion in the metatarsophalangeal joint. Our classification method revealed the relationship of hallux deformity in the sagittal plane to flatfoot and also demonstrated the usefulness of measuring basal phalanx displacement in predicting the range of motion of the metatarsophalangeal joint.