Medial deviation of the first metatarsal head as a result of flexion forces in hallux valgus.

Several questions with regard to the hallux valgus complex, which includes metatarsus primus varus, give rise to discussion. How do bunions develop? Is disturbed muscle balance at the first metatarsophalangeal joint important in the pathogenesis of the hallux valgus complex? What is the relation between dynamic plantar load distribution and pain in the ball of the foot? What is the cause of recurrences of deformity after surgery? To answer these questions, we started with the biomechanical model of Snijders et al., which states that contraction of flexor muscles of the hallux worsens its valgus angle and causes medial deviation of the first metatarsal head. The present study was designed to validate the model on patients. When pressing the hallux downward, simultaneously the force under the toe and the medial deviation of the first metatarsal head were measured on preoperative patients and on controls (35 subjects in all). We could demonstrate with statistical significance that (1) when the subjects with hallux valgus push the great toe on the ground, the first metatarsal head moved in medial direction; in other words the foot widened. In the controls, as an average, the foot became narrower. (2) The greater the valgus deviation of the hallux, the greater the effect of the toe flexors, and (3) the greater the valgus deviation of the hallux, the less maximal flexion force it can apply. Implications of these findings on conservative and surgical therapy are discussed. The recurrences of deformity after first metatarsal osteotomies are explained by the action of the hallux flexors.(ABSTRACT TRUNCATED AT 250 WORDS)