Background:
When dealing with a weak smile, nerve transfer is a viable strategy. We evaluated outcomes of masseteric nerve to facial nerve transfers and compared them with direct muscle neurotization (DMN).

Methods:
In a retrospective cohort study of 20 patients (n = 20), we compared nerve transfer versus DMN over a 6-year period (2016–2021). Outcomes were measured using the validated Sunnybrook score, Ackerman Smile Index, and Terzis scores. Statistical analysis was performed using the Wilcoxon sign rank and Mann-Whitney U tests.

Results:
Comparing pre- versus postoperative scores after nerve transfers, there was a significant improvement in median overall Sunnybrook score (24 versus 47, P = 0.043), lip elevation (1 versus 2, P = 0.046), open mouth smile (1 versus 3, P = 0.003), and Terzis scores (1 versus 3, P = 0.005), with no difference in resting symmetry (−15 versus −5; P = 0.496). Compared with DMN, there was no difference in median Terzis score improvement from preoperative to postoperative state (2 versus 1, P = 0.838), median smile improvement (2 versus 2, P = 0.838), resting symmetry (10 versus 5, P = 0.144) or overall Sunnybrook score (23 versus 21, P = 1.000). Lip elevation improvement was in favor of nerve transfers (1 versus 0, P = 0.047).

Conclusions:
This is the first study evaluating nerve transfer neurotization of smile-mimetic muscles and comparing the outcomes with DMN, with masseteric nerve as donor. Nerve transfer leads to improved facial mimetic function, smile excursion and open mouth smiles, as does DMN, with improvement in lip elevation in favor of nerve transfer. Nerve transfer was preferred for more severe smile weakness.