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Analyzing the Role of the Mentalis Muscles on the Lower Lip in Paretic Peri-Oral Synkinesis
Cristina Sanchez, BSA1, Federico Facchin, MD2, Benjamin Rail, BS1, Natalie A. Gault, BS2, Rozen M. Shai, MD1
1Department of Plastic Surgery, University of Texas Southwestern Medical Center, Dallas, TX, 2University of Texas Southwestern Medical Center, Dallas, TX

In peri-oral synkinesis, the involuntary activation of muscles and imbalance between hypo- and hyper-toned muscle groups results in the inability to produce an effective smile. While selective depressor anguli oris (DAO) myectomies, platysma muscle myectomies, and selective neurectomies have shown to improve some smile parameters in patients with synkinesis, the role of the mentalis muscle, which is responsible for lower lip elevation, eversion, protrusion, and chin dimpling, is not fully understood. The aim of our study was to compare lower lip height deviation, symmetry, and teeth exposure of patients undergoing selective neurectomies targeting the mentalis muscle to patients who underwent selective neurectomies that did not target the mentalis muscle.

This retrospective study included patients that underwent targeted selective neurectomies including to the mentalis muscle (mentalis group) and those that underwent selective neurectomies without targeting the mentalis muscle (general group). Both groups underwent additional selective myectomies of the DAO and Buccinator muscles. Pre- and post-operative standardized images of patients in open smile were analyzed using ImageJ (ImageJ (Rasband, W.S., ImageJ, U.S, National Institutes of Health, Bethesda, MD) to obtain lower lip height deviation at the dry vermillion border (lower border) and wet-dry vermillion border (upper border), dental show, and lower lip inclination angle at the wet-dry vermillion border.

Sixteen patients were included in the mentalis group and 12 patients were included in the general group. In the mentalis group, statistical analysis showed significant pre- to post-operative improvements in dental show (p=.007), lower lip upper border deviation (p=.004), lower lip lower border deviation (p=.023), and inclination angle (p=.011). In the general group, no pre- to post-operative differences were found in dental show (p=.484), lower lip lower border deviation (p=.239), lower lip upper border deviation (p=.363), or inclination angle (p=.530). Post-operative comparisons between the mentalis group and general group showed superior improvements in the mentalis group across all measures— significantly improved dental show (mean difference: 17.41 (95% confidence interval (CI) (-41.20 to 6.37)); p=.004), significantly decreased upper border deviation (mean difference: -3.77 (95% CI (1.31 to 6.22)); p=.004), significantly decreased lower border deviation (mean difference: -2.99 (95% CI (-0.22 to 5.76)); p=.035), and significantly decreased inclination angle (mean difference: -4.57 (95% CI (0.80 to 0.834)); p=.019).

In post-paretic patients who underwent selective myectomies and neurectomies, the addition of targeted selective neurectomies to the mentalis muscle improved lower lip depression, lower dental show, and lower lip symmetry.
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