Anatomy and Biomechanics of the First Ray

Abstract

The medial longitudinal arch serves as the chief load-bearing structure in the foot1–3 and is dependent on the kinematics of the first ray for optimal support during gait.4 The first ray is a single foot segment consisting of the first metatarsal and first cuneiform bones.5 Pronation of the subtalar joint lowers the first ray to the ground in early stance5 and dissipates the shock of heel impact.3 As body weight moves forward, the mechanics of supination stabilize the medial arch, preparing the foot for the propulsive phase of gait. The dichotomous actions of weight acceptance and weight-bearing stability required of the first ray underscore the importance for clinicians who treat the foot to understand the biomechanics of the first ray. The importance of the first ray to the mechanics of the foot is, in part, because of the metatarsocuneiform joint's location, which intersects the transverse and medial longitudinal arches.6 A curved beam and a truss (Fig. 1) are frequently used when modeling the medial arch.7–10 Beams are designed to withstand bending under an applied force. A truss is a triangular framework with 2 rigid supports connected together at its base. Because the ends of the foot are not secure at the beginning of stance, the foot functions like a beam. As the weight of the body transfers forward, the calcaneus and the heads of the metatarsals are pressed to the ground, with the arch functioning as a truss. Truss-and-beam mechanics of the foot rely on the first ray to function as the pillar for the medial arch. The first ray, therefore, is a critical element in controlling the structural integrity of the foot.4 Figure 1. During early stance (A), the medial longitudinal arch functions like a curved beam to support …