Variation In The Angle Of The First Pedal Ray Relative To The Midline Of The Foot

PURPOSE: The first pedal ray has positional variations that are quantified in pathological states, such as hallux valgus, where the proximal phalanx deviates laterally and first metatarsal head deviates medially. Non-pathologic variation, however, exists among humans and has not yet been quantified, despite its potential usefulness in human performance and mobility. Here we investigate variation in the angle of the first ray relative to the midline of the foot among 57 individuals. METHODS: Previously collected, de-identified weight-bearing CT scans from the Harborview Medical Center (Seattle, WA) database (n = 35) and non-weight-bearing CT scans from the New Mexico Decedent Image Database (n = 22) of healthy individuals without foot pathology (25 biological females, 32 biological males) were accessed. Landmarks were placed on the proximal and distal most ends of the first metatarsal and third metatarsal in Avizo Lite 9.0.1. The vectors between these landmarks were used to represent the first ray and midline of the foot, respectively, and the angle between the two axes was calculated in MATLAB. Angle summary statistics (i.e., mean, maximum, minimum, standard deviation) were calculated. Wilcoxon Rank Sum tests were used to test for differences between biological females and males and between weight-bearing and non-weight-bearing CT scans in the angle of the first ray. RESULTS: The angle of the first ray relative to the midline ranged from 4.37 to 14.39 degrees with an average of 9.31 degrees. There are significant differences (p = 0.03) between females and males. Females, on average, have a relatively larger angle of the first ray (10.16 degrees) compared to males (8.64 degrees). There was no significant difference between the two types of CT scans (p = 0.49). CONCLUSIONS: Natural variation in the first pedal ray angle exists in humans. We hypothesize the larger angle in females may be due to laxity in connective tissue and/or differences in footwear, such as wearing high heels.