ẢNH HƯỞNG CỦA YẾU TỐ CHIỀU ĐỨNG XƯƠNG HÀM, SỌ MẶT ĐẾN TƯƠNG QUAN R6 HẠNG II: NGHIÊN CỨU DỌC TỪ BỘ RĂNG SỮA ĐẾN BỘ RĂNG VĨNH VIỄN

Abstract

Introduction: According to Angle’s classification, the first permanent molar relationship is divided into three classes, in which Class I is considered normal and Classes II and III are malocclusions. Class II malocclusion has a high prevalence and is associated with dental, arch, jaw, craniofacial, and growth-related factors. Previous studies have indicated that Class II is not only an anteroposterior discrepancy but also involves vertical skeletal and craniofacial characteristics. However, longitudinal evidence on the role of these vertical factors in the development of a Class II first molar (R6) relationship in the permanent dentition is limited, and no such study has been conducted on Vietnamese subjects. This study aimed to evaluate the influence of vertical jaw and craniofacial dimensions on the development of Class II R6 relationship from the primary to the permanent dentition. Methods: A retrospective longitudinal sample of 25 children (50 dental segments) was followed from primary dentition (T1) through mixed dentition (T2) to permanent dentition (T3). Each child had three pairs of dental casts and three corresponding lateral cephalograms at T1, T2, and T3. Twenty variables were assessed, including total anterior facial height (CCM), middle facial height (TMG), lower facial height (TMD), and the ratio of middle to lower facial height (TMG/TMD) at T1, T2, and T3, as well as the growth of these parameters from T1–T2 and T2–T3. The influence of these factors on the establishment of a Class II R6 relationship in the permanent dentition was analyzed by comparison with subjects presenting a Class I R6 relationship. Results: Compared with the Class I group, subjects with Class II malocclusion showed greater middle facial height and smaller lower facial height, resulting in a higher TMG/TMD ratio at T2 and T3. The increase in TMG/TMD from T2 to T3 was also greater in the Class II group than in the Class I group. Discussion: The findings indicate that disproportionate vertical facial growth, characterized by increased middle facial height and reduced lower facial height, is associated with the development of a Class II first molar relationship. The progressively higher middle-to-lower facial height ratio from mixed to permanent dentition in Class II subjects suggests that vertical growth pattern may contribute to the establishment and maintenance of Class II malocclusion and should be considered in diagnosis and treatment planning.