The biomechanics of the first metatarsal bone in hallux valgus: a preliminary study utilizing a weight bearing extremity CT.

BACKGROUND: Hallux valgus is a common but aetiologically not perfectly understood condition. Imaging in hallux valgus is based on weight bearing plain radiographs or in exceptional cases on non-weight bearing computerized tomography (CT)-studies.
METHODS: A portable extremity CT was used to study the forefoot with focus on first metatarsal bone in ten hallux valgus patients and five asymptomatic controls at rest and at weight bearing. Two-dimensional (2D) or three-dimensional (3D) hallux valgus angles, intermetatarsal angles and various other parameters were measured on CT data and the measurements between study groups were compared. The measured angles were also compared to angles measured on plain radiographs.
RESULTS: 2D or 3D angles from CT data sets can be used to evaluate hallux valgus. In hallux valgus, when compared with normal asymptomatic foot, the first metatarsal bone is medially deviated (intermetatarsal angle is wider), the width of the forefoot is increased and the proximal phalanx pronates. Between the study groups there was a statistically significant difference of the measured 3D hallux valgus angles at weight bearing but not at rest suggesting the importance of weight bearing CT studies when evaluating hallux valgus.
CONCLUSIONS: To our knowledge, this is the first time weight bearing CT data is presented when evaluating hallux valgus, offering a true alternative to plain radiographs. The relationships of bones of the forefoot, including rotational changes, can be reliably measured using this imaging method.