Spontaneity Versus Symmetry in Facial Reanimation: A PRISMA-Guided Narrative Review

Introduction: Facial reanimation aims to restore dynamic smiling and facial symmetry after paralysis. Ipsilateral masseteric nerve transfer, contralateral facial nerve via cross-facial nerve graft (CFNG), and dual innervation approaches each demonstrate distinct profiles in spontaneous activation and movement symmetry during free functional muscle transfers or direct facial nerve reinnervation. Methods: A systematic review of PubMed, Embase, and the Cochrane Library was conducted (January 2000-March 2025) in accordance with PRISMA 2020 guidelines. Studies comparing masseteric nerve innervation, CFNG, and dual innervation in free muscle transfer or direct fifth-to-seventh nerve coaptation were included. Case reports, series with fewer than 10 patients, non-English studies, and those lacking quantitative smile outcomes were excluded. Results: Twenty-eight studies encompassing 7574 patients met inclusion criteria. Fourteen studies evaluated single innervation techniques, and 14 assessed dual innervation. Masseteric nerve innervation produced greater commissure excursion and improved symmetry, particularly early postoperatively, but demonstrated low spontaneous smile rates, typically under 30%. CFNG achieved higher spontaneity-up to 75% in select studies-but was associated with delayed activation and reduced excursion. Dual innervation achieved spontaneous smile rates up to 98% while maintaining strong, symmetric movement. Pediatric patients showed superior excursion and spontaneity, reflecting greater neural plasticity. Patient-reported outcomes favored masseteric innervation early and CFNG or dual innervation for long-term emotional benefit. Conclusions: CFNG optimizes spontaneity but may compromise strength and speed. Masseteric innervation offers rapid, symmetric smiles with limited emotional spontaneity. Dual innervation provides the most balanced outcomes, particularly in pediatric patients.