Skin viscoelasticity as a predictor of optimal facial prosthesis design using 4D facial expression modeling

Facial prostheses restore esthetics and improve the quality of life for individuals with complex facial defects associated with tumors, trauma, inflammation, cysts, and congenital abnormalities. Despite advances in silicone materials and 3- and 4-dimensional (3D and 4D) modeling technologies, the stable retention of facial prostheses-especially in mobile facial regions-remains a clinical challenge that compromises both function and user satisfaction. While 4D facial expression-based modeling improves fit, individual factors such as skin viscoelasticity may influence retention, yet which patient-specific traits are most relevant remains unclear.

This study investigated whether facial skin viscoelasticity influenced optimal prosthesis design using 4D models aiming to establish objective criteria for personalized prosthesis fabrication and to enhance functional retention during facial motion.

Thirty healthy adult volunteers (11 men, 19 women) were recruited. Using 3D printing, 3 silicone prostheses reflecting 30%, 50%, and 70% smile expressions (S30, S50, S70) were fabricated using 4D morphing data. Each prosthesis was applied to the participant's right nasal wing, and retention was evaluated through a standardized "falling-off test" during facial movement. The prosthesis with the longest retention determined group classification (F30, F50, F70). To assess skin properties, a Cutometer was used to measure elasticity and viscosity at defined facial landmarks under controlled temperature and humidity. Statistical analyses, including t tests and ANOVA with Bonferroni correction, examined correlations among the F30, F50, and F70 groups (α=.05).

The falling-off test showed a mean ±standard deviation prosthetic retention time of 11.55 ±4.85 seconds. Skin elasticity differed significantly between F30 and F50/F70 (R5; P<.001, R7; P=.002), whereas skin viscosity showed no significant differences (P=.448).

The results indicated that elasticity, rather than viscosity, significantly influenced prosthetic retention. Based on elasticity levels, an optimal 4D facial expression model was proposed: 30% expression change for low elasticity, 50% for medium elasticity, and 70% for high elasticity. These findings provided a framework for personalized prosthesis design, improving stability during facial movement and enhancing patient-specific fabrication approaches. As this study was conducted on healthy individuals, further investigation involving patients with facial defects is essential to validate clinical applicability and optimize prosthesis retention strategies.