Evaluation of Facial Symmetry in Congenital Unilateral Lower Lip Palsy Patients with Depressor Labii Inferioris Muscle Resection

Background: Congenital hypoplasia of the depressor labii inferioris muscle can lead to Asymmetrical crying facies(ACF). The objective of this research was to examine the alterations in both static and dynamic facial symmetry following the resection of the depressor labii inferioris on the healthy side through an intraoral approach.

Methods: Patients exhibiting palsy of the depressor labii inferioris muscles were included in the study. smile modiolus angle, excursion, and exposure of teeth were assessed on both sides before and after resection of the depressor labii inferioris using Emotrics photographic analysis.

Results: Thirty-six patients were included. Before the resection of the depressor labii inferioris, the modiolus excursion of the closed-mouth smile is significantly asymmetric (p = 0.001), and after the operation, it becomes symmetric (p= 0.185). The change of symmetry with modiolus angle and teeth exposure in an open-mouth smile are the most significant. The surgery can turn the obvious skew lip (p<0.0001) into an almost symmetrical (modiolus angle: p = 0.321; teeth exposure: p = 0.368) expression. The teeth exposure significantly reduced on the healthy side (p<0.01).

Conclusions: ACF patients have asymmetry both in closed-mouth and open-mouth smiles. Resection of the depressor labii inferioris muscle offers a treatment that rapidly improves the symmetry of closed-mouth and open-mouth smiles.

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