Extensor Tendon Transfers for Treatment of Foot Drop in Charcot-Marie-Tooth Disease: A Biomechanical Evaluation

Abstract

Background: In Charcot-Marie-Tooth (CMT) disease, selective weakness of the tibialis anterior muscle often leads to recruitment of the long toe extensors as secondary dorsiflexors, with subsequent clawing of the toes. Extensor hallucis longus (EHL) and extensor digitorum longus (EDL) tendon transfers offer the ability to augment ankle dorsiflexion and minimize claw toe deformity. The preferred site for tendon transfer remains unknown. Our goal was to quantify ankle dorsiflexion in the “intact” native tendon state, compared with tendon transfers to the metatarsal necks or the cuneiforms. We hypothesized that EHL and EDL transfers would improve ankle dorsiflexion as compared with the intact state and would produce similar motion when anchored at the metatarsal necks or cuneiforms. Methods: Eight fresh-frozen cadaveric specimens transected at the midtibia were mounted into a specialized jig with the ankle held in 20 degrees of plantarflexion. The EHL and EDL tendons were isolated and connected to linear actuators with suture. Diodes secured on the first metatarsal, fifth metatarsal, and tibia provided optical data for tibiopedal position in 3 dimensions. After preloading, the tendons were tested at 25%, 50%, 75%, and 100% of maximal physiologic force for the EHL and EDL muscles, individually and combined. Results: Transfers to metatarsal and cuneiform locations significantly improved ankle dorsiflexion compared with the intact state. No difference was observed between these transfer sites. Following transfer, only 25% of maximal force by combined EHL and EDL was required to achieve a neutral foot position. Conclusion: Transfer of the long toe extensors, into either the metatarsals or cuneiforms, significantly increased dorsiflexion of the ankle. Clinical Relevance: The transferred extensors can serve a primary role in treating foot drop in CMT disease, irrespective of the presence of clawed toes. This biomechanical study supports tendon transfers into the cuneiforms, which involves less time, fewer steps, and easier tendon balancing without compromising dorsiflexion power.