The obliquity of the first metatarsal base

The degree of obliquity of the base of the first metatarsal is thought to predispose to metatarsus primus varus deformity. Radiographs currently are used to interpret the obliquity of the proximal first metatarsal but are subject to error. A 'normal' obliquity of the first metatarsal base has not yet been established.

Direct measurements of 77 human first metatarsals were used to establish mean values for the proximal first metatarsal obliquity angle with respect to age, gender, and ethnicity in this sample population. The overall length of the first metatarsal and the width of the base were measured. The measured values were compared in relation to age, gender, and ethnicity of the specimens.

The overall mean obliquity angle was 3.42 degrees (range, -3 to 8 degrees, SD 2.54). The mean obliquity angle in females was 3.67 degrees (range, -3 to 8 degrees, SD 2.91). The mean obliquity angle in males was 3.30 degrees (range, -2 to 7 degrees, SD 2.24). Mean base obliquity angle in the African-American specimens was 3.0 degrees (range, 3 to 7 degrees, SD 2.59), while the average first obliquity angle in the Caucasian specimens was 3.83 degrees (range, -2 to 8 degrees, SD 2.34). The obliquity of the first metatarsal base increased with age from a mean of 3.5 degrees in the youngest group to 5.13 degrees in the oldest.

Exaggerated obliquity of the first metatarsal base is a proposed cause for metatarsus primus varus. No average values had been established for the proximal first metatarsal obliquity. These mean values are useful in determining if pathologic obliquity is present. In this study group, the mean medial obliquity angle was 3.42 degrees.