# Three-Dimensional Photogrammetric Assessment of Facial Symmetry Improvement Following Botulinum Toxin Treatment in Patients with Facial Palsy: An Observational Study

## TL;DR

This study shows that 3D imaging can objectively measure facial symmetry improvements after botulinum toxin injections in facial palsy patients.

## Contribution

The study introduces 3D stereophotogrammetry with RMS analysis as an objective tool for assessing botulinum toxin outcomes in facial palsy.

## Key findings

- Whole-face RMS values decreased significantly after botulinum toxin injections.
- Improvements in facial symmetry were most notable in the middle third of the face.
- Symmetry improvements were most pronounced during surprise, Mona Lisa smile, and frown expressions.

## Abstract

Background/Objectives: Facial palsy causes both functional and aesthetic impairments, with asymmetry significantly affecting quality of life. Botulinum toxin injections are increasingly used to restore facial balance by reducing contralateral hyperactivity, but outcome assessments remain largely subjective. The objective of this study was to evaluate the ability of three-dimensional (3D) stereophotogrammetry coupled with root mean square (RMS) surface analysis to objectively quantify improvements in facial symmetry following botulinum toxin treatment and to support clinicians in patient care. Methods: Sixteen adults with long-standing unilateral peripheral facial palsy underwent individualized botulinum toxin injections. Three-dimensional images were acquired using the Vectra H2 system before and 2–3 weeks after injection at peak efficacy. Five facial expressions (neutral, surprise, frown, Mona Lisa smile, and forced smile) were analyzed. RMS values were calculated for the whole face and facial thirds. Clinical assessment included House–Brackmann and Sunnybrook scores. Results: Whole-face RMS values decreased significantly after injection (1.51 ± 0.42 vs. 1.35 ± 0.43, p < 0.001). Improvements were observed across all thirds, most notably in the middle third. During expression, symmetry improved for all movements, with the strongest effects for surprise (d = 1.270), Mona Lisa smile (d = 0.870), and frown (d = 0.832). Conclusions: Three-dimensional stereophotogrammetry with RMS analysis provides an objective and reproducible method to quantify changes in facial symmetry after botulinum toxin treatment. This technique may complement clinical scoring systems and support personalized treatment planning in facial palsy patients.