Lip closing force of Class III patients with mandibular prognathism: a case control study.

INTRODUCTION: To compare the lip closing force of patients with mandibular prognathism to that of patients without dentofacial anomalies. METHODS: The subject group included 62 female patients of Class III relationship with mandibular prognathism. The control group been comprised of 71 patients of Class I relationships without skeletal deformities. Maximum lip closing force and average lip closing force were measured using a Y-meter. Student's t-test was carried out to analyse the differences between the groups. Correlation and stepwise multiple linear regression analyses were performed to analyse the relationship between lip closing force and craniofacial morphology. RESULTS: The lower lip closing force of subjects with mandibular prognathism was significantly greater than that of patients in the control group (P < 0.001), while the upper lip closing force showed no difference (P > 0.05). The lower lip closing force of patients with mandibular prognathism was strongly correlated with IMPA (Lower Incisor - Mandibular Plane angle, P < 0.001) and FMA (Frankfort Plane-Mandibular Plane angle, P < 0.001). Multiple regression equations: (MaxLL) = 12.192 - 0.125 * (IMPA) + 0.082 (FMA); (AveLL) = 9.112 - 0.091 * (IMPA) + 0.054 (FMA). CONCLUSIONS: The lower lip closing force was markedly increased in Class III patients with mandibular prognathism and was strongly correlated with lower incisor position and mandibular plane angle.