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.

Evaluation and Management of Adult Footdrop

“…[19][20][21][22] Treatment success in the setting of neurologic etiologies such as cerebral palsy and Charcot-Marie-Tooth disease are more guarded with higher proportions of patients failing to walk without assistive braces and higher rates of additional surgical treatment. 6,20,27 This is likely because of the more complex nature of deformity correction and soft-tissue and muscle power balancing in these patients versus an otherwise normal patient who experienced an acute nerve injury. Gait analysis after PT tendon transfer surgeries does show notable strength deficits, with lost dorsiflexion strength (approximately ⅓ that of control subjects), loss of plantar flexion ROM because of some tenodesis effect, and increased medial foot contact pressures.…”

“…For example, the EDL tendons may be transferred to the dorsum of the foot to provide more advantageous dorsiflexion force while also removing a deforming force for claw toe deformities. 27 Long-term functional success after PT transfer is defined as the ability to walk without an AFO or brace. Most case series have shown that nearly 100% of patients were successful when treated with tendon transfer after a CPN injury.…”

“…For example, the EDL tendons may be transferred to the dorsum of the foot to provide more advantageous dorsiflexion force while also removing a deforming force for claw toe deformities. 27…”

Abnormal Bone Morphology in Charcot-Marie-Tooth Disease

“…Clinical review of CMT patients with WBCT scans yielded a total of 54 scans in 45 patients. Twenty-seven scans in 24 patients were excluded for the following reasons: previous bony surgery (15), arthritic changes (7), open physes (4), motion artifact (1). After exclusion, 27 scans in 21 patients were included in the study cohort; 6 patients had bilateral WBCTs.…”

“…6,11 Biomechanical evaluation of tendon transfers, 3-dimensional (3D) modeling of osteotomies, and a consensus statement detailing the evaluation and treatment of foot and ankle manifestations of CMT highlight the unique challenges presented to surgeons treating these patients. [15][16][17] Despite the increased focus on this disease process, the etiology of the deformity remains relatively unknown. No previous studies have ever demonstrated whether the etiology of these deformities is chronic soft tissue contractures, pathologic bony architecture, or both.…”