Surgical implications of biomechanics of the foot and ankle.

Because of the nature of the alignment the joints of the lower extremity, any significant loss of alignment or function will place increased stress on the remaining joints. An ankle arthrodesis must be carefully aligned into slight valgus and neutral dorsiflexion, plantar flexion and the same degree of external rotation as the opposite leg. A subtalar arthrodesis should be aligned in 5 degrees of valgus. The forefoot should be placed in a plantigrade position when arthrodesis of the subtalar or transverse tarsal joint is contemplated. The stability of the longitudinal arch and forefoot is dependent in part upon the integrity of the metatarsophalangeal joints and as such, neither a resection of the metatarsal head or base of the phalanx should be carried out except under extreme disease states, such as rheumatoid arthritis. If an arthrodesis of the first metatarsophalangeal joint is contemplated, it is important that the joint be aligned in approximately 5 degrees to 10 degrees of valgus and 15 degrees to 25 degrees of dorsiflexion, depending upon the heel height that the patient anticipates using. When considering a tendon transfer about the foot and ankle, the relationship of the tendon to the axes of the subtalar and ankle joint requires special consideration.