Evidence-based Considerations on Donor Nerve Selection and Ideal Muscle Weight in Long-standing Facial Paralysis – The Freiburg Algorithm

Abstract

Introduction: Free functional muscle transfer (FMT) is the standard for smile reconstruction in long-standing facial paralysis. Using the contralateral NVII for innervation of the transferred muscle frequently produces satisfying results. However, axon counts of facial nerve branches used as donor nerves for cross-face nerve grafts (CFNG) generally decrease with age resulting in insufficient muscle contraction. Also, the volume of the transferred muscle segment necessary for symmetric oral excursion depends on the donor nerve. This study presents our algorithm for donor nerve selection and ideal muscle weight in long-standing facial paralysis. Materials and Methods: In a prospective cohort study, we evaluated facial palsy patients scheduled for dynamic smile reanimation preoperatively via EMG for involuntary masseter muscle activation upon smiling. Six months after noting the first muscle contraction, we assessed smile synchronicity and correlated the results with the EMG results. Furthermore, we correlated oral commissure amplitude with the weight of the muscle segment and the donor nerve used. Results: We recruited 30 patients for the prospective study and included 22 patients in the retrospective analysis. Postoperatively, all patients demonstrated a voluntary smile. The preoperative coactivation of the masseter muscle predicted the outcome regarding synchronicity of the smile with high sensitivity and specificity. Also, we observed a significant increase in oral commissure excursion with increasing muscle weight when using the NV. Conclusion: The lack of masseter coactivation upon smiling predicts the absence of a synchronous, involuntary smile after FMT using the masseteric nerve. The ideal muscle weight depends on the donor nerve and should be smaller for the NV.