Abstract
Background: Most studies of degenerative flatfoot have focused on the posterior tibial muscle, an extrinsic muscle of the foot. However, there is evidence that the intrinsic muscles, in particular the abductor hallucis (ABH), are active during late stance and toe-off phases of gait. The purpose of this study was to analyze the kinematic effect of a simulated contraction of the abductor hallucis muscle on a cadaver lower limb specimen. Methods: Eight below-knee cadaver specimens were prepared. The abductor hallucis muscle was exposed and the entire muscle-tendon unit excised. A suture secured to the calcaneal origin of the muscle and tendon was passed through a pulley at the ABH sesamoid attachment. The specimen was mounted on an experimental rig in a ‘standing’ position. Motions in the first metatarsal, tibia, and calcaneus were tracked using the ‘Flock of Birds’ motion analysis system (Ascension Technology, Burlington, VT). Muscle contraction was simulated by applying tension on the suture. Results: All eight specimens showed an origin from the posteromedial calcaneus and an insertion at the tibial sesamoid. All specimens also demonstrated a fascial sling in the hindfoot, lifting the abductor hallucis muscle to give it an inverted ‘V’ shaped configuration. Simulated contraction of the abductor hallucis muscle caused flexion and supination of the first metatarsal, inversion of the calcaneus, and external rotation of the tibia, consistent with elevation of the arch. Conclusions and Clinical Relevance: The abductor hallucis muscle acts as a dynamic elevator of the arch. Understanding this mechanism may change the way we understand and treat pes planus, posterior tibial tendon dysfunction, hallux valgus, and Charcot neuroarthropathy.
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