Chin-throat anatomy: Normal relations and changes following orthognathic surgery and growth modification

Abstract

To determine if a new facial line (T), tangent to the throat, intersects the mandibular border in anterior (ANT) and posterior (POST) parts in proportions varying with facial configuration, and to evaluate the association between chin projection and throat inclination and the potential for the T-line to reflect this association. Measurements on profile photographs and cephalograms of 135 adults (aged 18-50 years)-45 each of Class I, II, and III (CI, CII, CIII) malocclusions-included ANT and POST, chin-throat (CTA), and mento-cervical (MCA) angles. Pre- and posttreatment measurements were compared in two subgroups (n = 25 each) of CII and CIII orthognathic surgery patients and in CII, division 1 early-treatment patients (n = 63). Statistics included analysis of variance and t-test for group differences, and Pearson correlation for associations among variables. ANT was nearly equal to POST in CI (50.99%) and CIII (51.86%) subjects and shorter in CII (36.01%) subjects. CTA and MCA were greater in CII profiles and smaller in CIII profiles. Significant differences (P < .0001) were observed for ANT, POST, CTA, and MCA between Classes I/II and II/III and for MCA between Classes II/III (P = .016). High correlations were noted between ANT and CTA in Classes I (r = -0.83), II (r = -0.73), and III (r = -0.68). In surgically treated patients, posttreatment measurements approached CI values. In the early-treatment group, ANT increased but remained smaller than POST; CTA decreased by nearly 13%. Chin-throat relations and chin extension are associated and require routine assessment in terms of diagnosis and treatment outcome. A practical tool to assess chin-throat relationship, the T-line bisects the mandibular body nearly equally in Class I faces.