Effect of Biomechanical Properties of Perioral Soft Tissues on Lip Response to Simulated Protraction of Upper Front Teeth

Abstract

Background: Proclination of front teeth in orthodontics and oral rehabilitation may influence lip protrusion and the overall facial profile. We hypothesized that the degree of profile changes is determined by the biomechanical properties of the lips. Objectives: (1) to investigate the upper lip changes induced by a simulated protraction of upper front teeth; and (2) to assess the relationship between biomechanical properties of the lips and their response to tooth protraction. Materials and Methods: Thirty-four participants were recruited at the Faculty of Dentistry, University of Otago, New Zealand. Simulated protraction of upper front teeth was achieved by placing a customised stent covering the buccal surface of the maxillary upper incisors and canines, along with the corresponding buccal mucosa and gingiva. Stereophotogrammetry was used to assess lip changes in three dimensions. Biomechanical properties of the lips, including muscle tone, stiffness and elasticity, were measured using a non-invasive digital palpation device. Results: During the simulated protraction of upper front teeth, the upper lip advanced approximately 50% of the stent thickness, with a considerable inter-individual variation in lip displacement. Soft tissue responses were correlated with the biomechanical properties of the lips with significant correlation coefficients ranging from 0.38 to 0.40. Higher lip tone was associated with increased lip displacement, while greater lip elasticity resulted in reduced lip displacement. Multivariate analyses indicated that upper lip displacement was associated with biomechanical properties and ethnicity, but not with age, sex and body mass index. Conclusions: The response of the lips to protraction of front teeth varies largely between individuals and can be partly explained by variations in the biomechanical properties of perioral muscles.