Distal ligamentous restraints of the first metatarsal. An in vitro biomechanical study.

Background: Hypermobility or instability of the first metatarsal is associated with increased hallux valgus deformity and greater risk of recurrence after surgery. The objective of this in vitro study was to determine the ranges of movement, under directional loading, of the normal first metatarsal and the effect of the longitudinal plantar aponeurosis and the transverse tie-bar ligamentous system.

Methods: A model has been developed to quantify the influence of the first intermetatarsal ligament and plantar aponeurosis on movements of the first metatarsal. All muscular structures controlling movements of the first ray in cadaver feet were detached and static ligament structures retained for study. Using a specially designed test system, a static load was applied to the metatarsal to produce flexion, extension, abduction or adduction moments. The first intermetatarsal ligament and the plantar aponeurosis were detached sequentially and a movement map of the first metatarsal in the frontal plane was obtained using the Isotrak II magnetic measuring system.

Results: The first intermetatarsal ligament was shown to be an important stabiliser of the first metatarsal in all directions. The plantar aponeurosis was shown to be a secondary stabiliser resisting medial and dorsal rotation of the metatarsal after division of the first intermetatarsal ligament.

Interpretation: Recognition of the importance of the plantar aponeurosis and the first intermetatarsal ligament in the normal foot has implications for our understanding of hallux valgus.