Comparison of the three-dimensional bone morphology of the first proximal phalanx and first metatarsal between normal and hallux valgus foot using the homologous model technique.

Bone morphology is one of the factors involved in hallux valgus development. However, previous studies have not evaluated the overall bone shape in three dimensions. This study aimed to compare the overall shape of the first proximal phalanx and first metatarsal in hallux valgus with those of normal feet.Homologous models were created from computed tomography data of the first proximal phalanx and first metatarsal of 75 healthy men and 53 women in the control group and 10 men and 71 women in the hallux valgus group. Principal component analysis was performed to examine the differences in bone morphology between the control and hallux valgus groups. In men and women with hallux valgus, the proximal articular surface of the first proximal phalanx was characterized by a more lateral inclination and torsion of the pronated first metatarsal. Additionally, the first metatarsal head was characterized by a more lateral inclination in male hallux valgus. This study is the first to reveal the morphological characteristics of the first metatarsal and first proximal phalanx in hallux valgus as a whole bone using a homologous model technique. These characteristics are considered possible causes of hallux valgus development. The shapes of the first proximal phalanx and first metatarsal in hallux valgus were different from those in normal feet. This finding should be useful when considering the pathogenesis and treatment development for hallux valgus.