Prediction of the lower lip and chin response consequent to surgical advancement of the mandible

Abstract

It has been evident from the literature that the reports of response of the lower lip and soft tissue chin to the surgical advancement of the deficient lower jaw are strongly discrepant. Therefore, the objective of this study was to improve the understanding of the soft tissue response, with particular reference to the role of the preoperative soft tissue thickness. It has become evident from a review of current studies that tissue thickness may not play as significant a role as was previously thought, thus indicating the role of other factors in the prediction of the lower lip and chin response for the individual. This study assessed the soft tissue changes of the lower lip within a homogenous sample group of 39 patients who had undergone a surgical advancement of the lower jaw. Lateral cephalometric radiographic records at time periods before and after the surgical procedure were used. The radiographs were hand traced and specific landmarks were computer digitized with the Analysis System (Olympus Pty, Ltd) relative to a constructed X-Y axis. The change in the position of these landmarks at the various time periods was calculated according to the mean value of each reading for each landmark. Multiple regression analyses of the data resulted in poor correlations of the lower lip position with the extent of mandibular advancement surgery when the variables of horizontal overjet and the vertical overbite of the incisors, the lower lip thickness (lower incisor tip to labrale inferius as well as lower lip protrusion ahead of the upper incisor teeth) and the pre-operative soft tissue thickness were included (0.27 in the horizontal dimension; 0.51 in the vertical dimension). The addition of the variables tissue thickness and lower lip thickness within the multiple regression equation did not result in a notable improvement in coefficients of correlation either (0.77 to 0.78). Within the stepwise regression equations specific variables have been identified as having an influence on the prediction of the response of the lower lip and chin consequent to the mandibular advancement surgery (R2=0.93-0.97(horizontal); R2=0.89-0.96(vertical)). These points include stomion, upper incisor tip, hard and soft tissue pogonion, lower incisor tip, gnathion and menton. The prediction equations were independently cross-validated against each individual within the sample group, achieving high cross-correlation values (0.85-vertical to 0.90- horizontal) between the prediction equation and the observed values. The findings of this study have identified factors that play a role in the soft tissue response of the lower lip and chin consequent to mandibular advancement surgery, enabling a more accurate prediction for the individual.