Digastric transposition for unilateral lower lip weakness after injury to the marginal mandibular nerve

摘要

Lower-lip dysfunction creates functional and cosmetic issues for patients. The loss of the depressor vector is especially noticeable during laughter and open-mouth smiling. Although a variety of options exist for improving this deficit, most patients are not offered any surgical therapy. The digastric muscle transposition, popularized by Conley, remains a valuable procedure for this deficit.The marginal mandibular branch of the facial nerve is the most frequently injured division of cranial nerve VII, being particularly vulnerable to transection and traction during parotidectomy, submandibular gland excision, face lift, and upper neck dissection. The nerve is responsible for the motor function of the lower lip depressors (depressor anguli oris, the depressor labii inferioris, the inferior fibers of the orbicularis oris, and the mentalis muscles). Injury manifests as failure of the lateral lower lip to retract. The hypotonic, elevated, and inverted vermillion results in a characteristic asymmetry of the open-mouth smile (Figure 1). The closed-mouth smile is unaffected (Figure 2).Functional difficulties include biting of the lower lip and oral incompetence. Reanimation requires careful assessment of the patient's deficit and includes analysis of the skin tone and of the position of the central portion of the lower lip The digastric muscle transposition is intended for the isolated defect of the marginal mandibular nerve.