Pathologic anatomy of hallux abducto valgus.

HAV is a joint deformity that has some very characteristic anatomic features but that may be developed from a variety of interactive causes, both anatomic and functional. In the normal condition, the first metatarsal bone and hallucal phalanges form an axial array of bones. At the first metatarsophalangeal joint this arrangement is stabilized by a set of muscles whose tendons surround the joint. The pathology is associated with the deviation of the bones from their axial position and the related destabilization of the muscle group around the joint, specifically: (1) The hallux deviates and rotates laterally, while the first metatarsal deviates medially and also rotates laterally to some degree. (2) Deviation of the two main segments of the joint causes tearing and weakening of the medial capsular fibers and pulls the sesamoid bones and metatarsal head away from their normal plantar articulation. (3) Abnormal articulation of the sesamoid bones under the crista and pressure of the medial capsular wall cause erosion of cartilage and subchondral bone on the metatarsal head. (4) Deviation of the hallux redistributes the tendon insertions in relation to the joint axes of motion such that they serve to aggravate the deviation. (5) The deviated position of the hallux and eccentric actions of the tendons create a reaction force on the metatarsal head that serves to further medially displace the first metatarsal bone.