Biomechanical Basis for Treatment of Pediatric Foot Deformities Part I: Mechanics of the Foot

Abstract

The foot functions as a flexible structure during the initial part of stance phase but changes into a rigid structure in the terminal part of stance to enable a powerful push-off. This illustrated review describes the normal mechanics of the foot during inversion and eversion and explains in some detail how the calcaneum moves under a stationary talus in three planes simultaneously around a single oblique axis. During eversion, the calcaneum dorsiflexes, abducts, and pronates while it plantarflexes, adducts, and supinates during inversion. The talus remains static while the rest of the foot moves as a unit, referred to as the calcaneo-pedal unit (CPU), around the head of the talus. The socket-like hollow in the CPU consisting of the anterior and middle articular facets of the calcaneum, the articular fact of the navicular and the spring ligament constitute the “acetabulum pedis” which rotates around the talus. On occasion, the foot functions like a twisted plate influencing the inter-relationship between the hindfoot and forefoot as a forefoot deformity may cause a secondary compensatory deformity of the hindfoot. Understanding normal foot mechanics will facilitate greater understanding of altered mechanics seen in abnormal foot deformities. Key Concepts: • The foot is divided into the hindfoot and forefoot; the mechanics of movement of the joints of the forefoot and hindfoot are distinctly different. • The subtalar joint moves around a uniquely oriented oblique axis which permits movement of the calcaneum under a static talus in three planes and these movements occur simultaneously. • The foot behaves like a twisted plate such that primary deformities of the forefoot can cause secondary deformities in the hindfoot with concomitant elevation or depression of the longitudinal arch of the foot.