Assessment of lower lip changes following incisor displacement in 92 orthodontically-treated adults

Abstract

A retrospective assessment of the lower lip changes consequent to incisor displacement in correction of malocclusion in a sample of 92 post-adolescent subjects. The study sample comprised two groups, subdivided according to the direction of incisor movement achieved during orthodontic treatment: the retraction group (Group 1), made up of 41 patients in which the maxillary incisor had been moved in a palatal direction (palatal tipping); and the protraction group (Group 2), composed of 51 patients who had undergone labial movement of the upper incisors (labial tipping). In order to evaluate the mean changes in lower lip position, and consequent alterations in the quality of the patients' profiles, between T1 (prior to orthodontic displacement of the incisors) and T2 (following treatment), the following parameters were measured on lateral head film X rays: variation in lower lip vermilion thickness (dLVT); variation in lower lip sulcus depth (dLLSD); variation in lower vermilion height (dLVH); variation in exposure of the upper and lower incisors in relation to the lower stomion (dIs-STOi and dIi-STOi, respectively); variation in upper and lower incisor tipping with respect to the palatal plane (d1/PP and dinf1/PP); and variation in lower facial height (dLFH). Assessment of the changes due to incisor translocation revealed significant differences between Group 1 and Group 2 in all lower lip soft tissue parameters, except for dLVT, dLVH and dIi-STOi. Furthermore, using multiple linear regression analysis to predict the behaviour of the dependent (cutaneous) lower lip variables, a good coefficient of determination (r-square) value was only obtained for the horizontal variation of the labrale inferius point (dxLi), found to be dependent on horizontal movement of the upper and lower incisors (dxIs and dxIi) and thickness of the lower vermilion (LVT). The results of this study highlight great variability among the patients studied, and suggest that the behaviour of the soft tissues (lower lip changes) following displacement of the upper and lower incisors is multifactorial and very difficult to predict.