"Smile Spontaneity Following Cross-Facial Nerve Grafting: A Systematic Review and Meta-Analysis".

BACKGROUND: Contemporary facial reanimation techniques increasingly aim to achieve smile spontaneity, defined by most as an unprompted smile triggered by positive emotions. This study aimed to 1) assess the rate of reporting of spontaneity outcomes and commonly used tools to elicit and assess spontaneity and 2) evaluate the published success rate in achieving spontaneity. METHODS: A systematic review of the Ovid MEDLINE, Embase and Cochrane literature from January 1, 2005 to May 13, 2025 was performed. Original studies reporting on clinical outcomes with cross-facial neural input for flaccid facial paralysis were included. Preferred Reporting Items for Systematic Reviews and Meta-Analysis guidelines were followed. RESULTS: 95 articles met inclusion criteria, including a total of 3,874 patients having undergone facial reanimation with cross-facial nerve grafting either to the native facial musculature (N=384) or for neurotization of free muscle transfer (N=3,487). The neural input was either cross-facial nerve grafting alone (N=3,268) or dual innervation with added input from an ipsilateral nerve transfer (masseteric or hypoglossal) (N=599). Less than half of studies reported spontaneity outcomes (39/95, 41.1%), with most articles only describing binary data (presence or absence of spontaneity). The presence of spontaneity was reported in 88.25% (95% CI, 77.49-96.32, I² = 93.7%, τ² = 0.11, p < 0.0001) (1,456 patients, 31 studies). CONCLUSIONS: While quantitative tools assessing smile spontaneity have been described, their use in the facial nerve outcomes literature remains limited. This study highlights the need to address barriers to widespread implementation of spontaneity assessments.