Evaluation of Morton's theory of second metatarsal hypertrophy

Originally introduced by Morton, the concept of hypertrophy of the medial cortex and the entire shaft of the second metatarsal as an objective sign of increased mobility of the first ray has not been subjected to much scrutiny. The goal of the current study was to assess the clinical relevance and reliability of radiographic measures of hypertrophy of the second metatarsal in relation to mobility of the first ray, pes planus, and tightness of the gastrocnemiussoleus in both control subjects and patients with diagnosed disorders of the forefoot.

Four study groups of forty-three subjects each were evaluated. The cohort included an asymptomatic control group as well as three groups made up of patients with symptoms and a diagnosis of hallux valgus, hallux rigidus, or interdigital neuroma. Mobility of the first ray (as measured with a device and method described by Klaue et al.), arch height, and ankle dorsiflexion were measured on physical examination. Plain weight-bearing radiographs and previously established equations were used to determine hypertrophy and the length of the second metatarsal, and the hallux valgus and first-second intermetatarsal angles were measured on plain radiographs as well.

There was no significant difference between the control and symptomatic groups with regard to the values for hypertrophy of the second metatarsal. The patients with hallux valgus deformity had significantly greater mobility of the first ray (p < 0.001) compared with the controls. No correlation was found between values for hypertrophy of the second metatarsal and mobility of the first ray, the length of the first metatarsal, pes planus, or restricted ankle dorsiflexion. No correlation was found between mobility of the first ray and either pes planus or restricted ankle dorsiflexion. A weak correlation (r = 0.4) was noted between increased mobility of the first ray and the hallux valgus angle.

Our findings do not support Morton's concept that medial cortical hypertrophy and increased shaft width of the second metatarsal are associated with increased mobility of the first ray or relative shortness of the first metatarsal. In addition, hypertrophy of the second metatarsal was not associated with either pes planus or restricted ankle dorsiflexion. We found the practice of using hypertrophy of the second metatarsal as an indicator of mobility of the first ray to be unreliable, and thus we consider it to be an inappropriate indication for arthrodesis of the first metatarsocuneiform joint in the treatment of hallux valgus deformity.