Force Deficit in Neurovascular Muscle Transfer is Partially Explained by a Denervated Muscle Fiber Population

Neurovascular muscle transfer restores functional loss from injury or disease but a significant muscle force deficit results. This work determined whether atrophy and denervated muscle fibers account for this force deficit. The proximal and distal extensor digitorum longus (EDL) muscle tendons were transected and immediately repaired in a rat neurovascular muscle transfer model. The peroneal nerve was either simply exposed for the SHAM group or divided and repaired. A TOTAL repair group had all the proximal nerve included in an end to end coaptation while a REDUCED group had very few fascicles from the proximal end included in the repair to the entire distal nerve. Whole EDL muscle force was quantified. Muscle cross sections were labeled for neural cell adhesion molecule, a marker for fiber denervation. EDL muscle "corrected" specific force was calculated by normalizing force to the innervated muscle fiber cross sectional area (CSA). The EDL muscle absolute force was significantly decreased and the percentage of CSA occupied by denervated fibers was significantly increased as the number of axons available for muscle reinnervation was reduced. The TOTAL group had a 10.3% deficit in "corrected" specific force (254 ± 49 kN*m) and the REDUCED group had a 34.6% deficit (185 ± 64 kN*m) relative to the SHAM group mean (283 ± 47 kN*m). In both the TOTAL and the REDUCED groups, muscle fiber denervation accounted for approximately 3% of the specific force deficit. Muscle atrophy and fiber denervation do not fully explain the force deficit in neurovascular muscle transfer. Abbreviations: CSA: muscle cross sectional area, "corrected" CSA: innervated physiological muscle cross sectional area, innervated-CSA (%): percentage of fiber area determined to be innervated relative to the total CSA, EDL: extensor digitorum longus muscle; Lf : muscle fiber length, L0: optimal muscle length, SHAM: control surgical exposure of peroneal nerve muscle transfer group, TOTAL : total peroneal nerve repair muscle transfer group, REDUCED : Reduced peroneal nerve repair muscle transfer group, NCAM: neural cell adhesion molecule, P0 : maximal tetanic muscle force, specific P0 : P0 normalized to muscle CSA, "corrected" specific P0 : P0 normalized to innervated muscle fiber CSA.