Evaluation of the Mechanical Axis of the First Ray Before and After First Metatarsal-Phalangeal Joint Reconstructive Surgery.

LaPorta et al published a description of measurement of the mechanical axis of the first ray in an attempt to introduce center-of-rotation angulation principles into forefoot reconstructive surgery. They found significant differences in the measurement of the first intermetatarsal angle between groups of feet with and without hallux abducto valgus deformity, but they found no difference in the measurement of the M1-M2 mechanical axis angle between groups. The objectives of this study were to further investigate the proposed M1-M2 mechanical axis angle and to examine the effect of hallux abducto valgus corrective surgery on its measurement. We retrospectively evaluated weight-bearing dorsal-plantar projection radiographs taken in the angle and base of gait of 56 consecutive feet before and after first metatarsal-phalangeal joint corrective surgery. On each radiograph, we calculated the first intermetatarsal angle, the hallux abductus angle, the tibial sesamoid position, the M1-M2 mechanical axis angle, and the position of the sesamoids relative to the mechanical axis of the medial column. Statistically significant decreases were observed when comparing pre- versus post-operative measurement of the first intermetatarsal angle (-4.83°; p < .001), hallux abductus angle (-11.46°; p < .001), and tibial sesamoid position (-1.99 positional grade; p < .001). Statistically significant differences were also observed for the M1-M2 mechanical axis angle (-0.47°; p = .007) and the position of the sesamoids relative to the mechanical axis of the medial column (0.38 positional grade; p < .001), but it is unlikely that these results would be considered clinically significant given the differences of less than 1° and less than 1 positional grade, respectfully. The results of this investigation add to the body of knowledge and will hopefully lead to future investigations into the progression, evaluation, and treatment of the hallux abducto valgus deformity.