Variation in the Angle of the First Pedal Ray Relative to the Midline of the Foot by Sex and Parity History

Abstract

Objective: Although pathological variations of the first pedal ray have been well-studied, non-pathologic variations, including those potentially linked to hormonal changes during pregnancy, have yet to be examined. Here we quantify the angle between the longitudinal axes of the first and third metatarsals and investigate variation in this angle between females and males and nulliparous and parous individuals. Materials and Methods: Foot CT scans were accessed from Harborview Medical Center and the New Mexico Decedent Image Database. A 3D model was constructed in Avizo Lite 9.0.1 and landmarks were placed on the proximal and distal ends of the first and third metatarsal. The vectors between the landmark sets were used to represent the first and third metatarsal axes. The angle between the two axes was calculated in MATLAB. Wilcoxon Rank Sum tests were used to test for significant differences in this angle between (1) females and males, (2) nulliparous and parous females, and (3) nulliparous females and males. Results: Females have a statistically larger angle compared to males (average 10.16° vs. 8.64°, p = 0.03). Parous females have a statistically larger angle than nulliparous females (average 10.44° vs. 8.74°, p = 0.04). Nulliparous females do not significantly differ from males (p = 0.70). Discussion: We propose that the relatively larger angle in females is likely due to hormonal exposure, specifically relaxin, throughout pregnancy given that parous females have a relatively larger angle than both males and nulliparous females by 1.5°–1.7°, while nulliparous females share a similar average angle with males.