The Effect of Frontal Plane Position on First Ray Motion: Forefoot Locking Mechanism

Abstract

Several joints in the foot have a locking mechanism that allows the foot to function as a rigid lever. The transverse tarsal joint (talonavicular and calcaneocuboid joints) has a locking mechanism that is well understood. The purpose of the study is to determine if the first ray also has such a locking mechanism. Five cadaver limbs were loaded onto a custom frame. The first metatarsal was attached to a jig that placed a force of 50 N in plantarflexion and dorsiflexion. The motion of the jig was measured with the first ray in three positions: maximally everted, neutral, and maximally inverted. No tendons were loaded to ensure that any change in motion was solely due to osseous position. The average motion of the first ray for the three testing position was as follows: 7 mm in the everted position, 14 mm in the neutral position, and 18 mm in the inverted position. There was a statistically significant increase in range of motion from an everted position to a neutral position (p=0.003). This increase in range of motion continued when the first ray was inverted compared to neutral, but not statistically significance (p=0.07). This study demonstrates that the frontal plane position of the first ray affects the sagittal plane motion. An everted position has the least mobility, and we hypothesize that this represents a closed-packed or locked position.