Anatomic determinants of first metatarsophalangeal flexion moments in hallux valgus.

A laboratory cadaver model was used to assess the effects of the major anatomic components of hallux valgus deformity on first metatarsophalangeal flexion moments. The individual components of the flexor hallucis longus' effective tendon moment arm were evaluated as a function of imposed hallux valgus, of hallux pronation, and of combined hallux valgus and pronation. To simulate the effects of distal first metatarsal surgery, the components' changes after metatarsal head lateral translation or supination were assessed. Significant changes in effective tendon moment arm occurred with increases in the hallux valgus angle, with changes in the hallux pronation angle, and with combined increases in the hallux valgus angle and hallux pronation angles. The changes involved primarily were a redirection of the moment vector's projection within the superior and inferior plane or medial and lateral plane, rather than an appreciable alteration of the magnitude of the resultant. By contrast, neither translational nor rotational displacements of the metatarsal head influenced any of the individual components of the effective tendon moment arm.