BACKGROUND: 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. METHOD: 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. RESULTS: 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). CONCLUSION: 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.
BACKGROUND: 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. METHOD: 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. RESULTS: 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). CONCLUSION: 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.