Depressor labii inferioris resection: an effective treatment for marginal mandibular nerve paralysis

In patients who show their lower teeth during smiling and facial animation, paralysis of the marginal mandibular nerve (MMN) causes a noticeable asymmetry of the lower lip due to the absence of depressor function. This paper presents a balancing technique for this lower lip asymmetry that involves resection of the depressor labii inferioris (DLI) on the nonparalysed side. The anatomy of the muscle, the operative technique, and the effectiveness of the procedure are outlined. A retrospective chart review was performed for 42 adult patients who were treated for MMN palsy with a DLI resection. Seven cases had only the MMN involved, and 35 cases had unilateral facial nerve paralysis. Thirty-six of these patients were available for a follow-up telephone survey. Of the 42 primary DLI resections performed, 36 cases had successful outcomes. Of the six patients who failed to achieve the expected results, five patients had repeat DLI resection and three of these achieved the desired result; the other two patients required a third resection. One patient continued to have DLI action with smiling and subsequently had a Botox injection into the DLI with good results. Of the 36 survey respondents, 21 patients felt their lower lip was asymmetrical at rest prior to DLI resection and 18 of these patients were improved by the procedure (P = 0.0001). Twenty-nine of the 36 patients reported that their lower lip was more symmetrical when they smiled following the DLI resection (P < 0.0001). The bilateral lack of movement in the lower lip when expressing emotions, such as anger and sorrow, was not as important to the patient as the lack of symmetry when expressing these emotions. Patients' speech either improved or showed no change, the amount patients bit their lower lip significantly improved (P = 007) whereas oral continence showed no significant changes (P = 0.147) following the DLI resection. DLI resection is a simple and effective procedure for the treatment of MMN palsy. The results are permanent and predictable. Lower lip symmetry is produced both at rest and with facial animation, without causing a functional deficit. The expected results of surgery can be trialed by local anaesthetic or botulinum toxin to block the activity of the DLI.