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Quantifying the biomechanical relationship between labial augmentation prosthesis thickness and upper lip
Shiu-Fong Ou1, Yu-Hsiang Tseng2, Po-Sheng Wu2
1Doctoral student, Institute of Clinical Dentistry, National Taiwan University, Taipei, Taiwan, ROC; and Attending, Department of Dentistry, National Taiwan University Hospital, Taipei, Taiwan, ROC.
Statement Of Problem:
Predicting soft tissue response to hard tissue support is complex. Current methods cannot easily predict the dynamic biomechanical variations between static and active facial states.
Purpose:
The purpose of this clinical study was to quantify the biomechanical relationship between a labial augmentation prosthesis (LAP) thickness and sagittal upper lip displacement under maximum smile and rest positions and to evaluate the influence of participant-related variables on soft tissue displacement.
Material And Methods:
Twenty-three participants (mean ±standard deviation age: 27 ±8.71 years; 16 men and 7 women) were recruited. Digital facial scans were acquired using a 3-dimensional facial scanning system under 2 conditions (rest and maximum smile) with LAPs of 1-, 2-, and 3-mm thickness. Facial landmarks were analyzed using a coordinate-based alignment system with a custom algorithm. Measurement reliability, scanner trueness, and precision were verified. Generalized estimating equations (GEEs) were used to assess the effects of LAP thickness, facial expression, and their interaction on upper lip displacement. The Wilcoxon rank sum test was used to evaluate the influence of sex, while Spearman correlation coefficients (ρ) were calculated to assess the influence of other participant-related variables (α=.05).
Results:
The scanning system demonstrated high trueness (systematic error 0.24 mm) and precision. Upper lip displacement showed a strong linear correlation with LAP thickness (R2>0.98). A significant interaction was found between thickness and facial expression (P=.049); specifically, at the 3-mm thickness, the maximum smile condition resulted in significantly greater displacement (mean ±standard deviation: 3.24 ±0.35 mm) than the rest position (mean ±standard deviation: 2.76 ±0.41 mm). The mean displacement for 1-mm and 2-mm LAP thicknesses were 1.20 and 2.20 mm during maximum smile and 1.08 and 1.92 mm at rest, respectively. Regarding participant-related variables, both age and sex were identified as significant predictors during maximum smile; older participants and women exhibited significantly reduced lip displacement during maximum smile (P<.05), whereas body mass index and other variables showed no significant correlation.
Conclusions:
Within LAP thicknesses up to 3 mm, upper lip displacement was strongly and linearly correlated with prosthesis thickness. This response was expression-dependent, exhibiting a synergistic increase in displacement at the 3-mm thickness during maximum smile. While the LAP thickness was the primary determinant, older age and female sex were significantly associated with reduced lip displacement during maximum smile.
