Effect of lip thickness and competency on soft-tissue changes.

INTRODUCTION: Improvement in facial esthetics is a major reason for seeking orthodontic treatment. Soft tissues responsible for esthetics show a variable response to the movement of underlying teeth during orthodontics.

METHODS: The purpose of this cross-sectional study was to analyze the response of soft tissues and compare it among groups presenting with different lip thickness and competence. It was carried out on 37 patients with Class I and Class II Division 1 malocclusion who had undergone extractions of maxillary first premolars and had completed their orthodontic treatment. Data were obtained by corresponding lateral cephalometric radiographs taken before and at the end of orthodontic treatment. Association was analyzed with a Pearson correlation test. Differences among groups exhibiting varying competency and lip thickness were tested with an independent sample t test. A P value of ≤0.05 was considered statistically significant.

RESULTS: Pearson correlation revealed significant associations among incisor tip retraction with lip base retraction (r = 0.68), lip base thinning (r = 0.41) and vermilion retraction (UV) (r = 0.73). Regression analysis showed a moderate increase in lip length (AB) and thickness at vermilion, which were 0.11 mm and 0.15 mm for each mm of incisor retraction at the tip, whereas stronger effects were observed for UV (0.38 mm) and lip base retraction (0.55 mm). There was significantly more lip base thinning (P = 0.03) and UV (P = 0.04) in the incompetent group compared with the competent group. The AB increased significantly in the sample with thicker lips (P = 0.01). Pearson correlation coefficient showed a strong association of lip retraction at the base and vermilion with the incisor movement at the cervical area, for competent and incompetent lips, and thicker and thinner lips.

CONCLUSIONS: Lip retraction at vermilion and lip base thinning was significantly more in patients with incompetent lips, whereas the AB increased significantly more in the group with thicker lips.