The relationship of Bolton’s ratios with orthodontic charactristics of anterior teeth and skeletal pattern.

Abstract

Background: The concept of proportional balance of maxillary and mandibular teeth size has emerged since the onset of dental articulation theory. Since then, various methods (analyzes) have been proposed to investigate the intermaxillary teeth size relationships. Bolton's ratios have been used more widely in research and clinical practice, and over the time. Therefore, the values obtained from Bolton's study are known as the standard of diagnosis and treatment planning of orthodontic cases. Objective: The aim of this study was to investigate the ideal Bolton's ratios required to achieve the class I canine relationship and optimal occlusion in patients with class I, II, and III skeletal patterns, and also to evaluate the correlation of skeletal and dental orthodontic factors with Bolton's anterior and overall ratios. Materials and Methods: This study was performed on post-treatment dental models and lateral cephalograms of 210 subjects achieved optimal occlusion. Bolton's anterior and overall ratios were calculated following the measurement of the mesiodistal width of the teeth, the thickness and inclination of incisors, the angulation of maxillary and mandibular canines, and overbite. To assess the intra and inter-examiner errors, dental models and radiographs of 10 subjects were randomly selected and all their measurements were repeated after eight weeks by the researcher and the supervisor. Then, the Intra-class Correlation Coefficient was analyzed by comparing the data sets of each measurement. Then the relationship of each of the studied factors with Bolton's ratios was analyzed using univariate and multivariate regression models. One-way analysis of variance and one sample t-test were used to compare the ideal Bolton's ratios of each group with each other and with reference values, respectively. Results: The Intra-class Correlation Coefficient was ranged between 0.87 and 0.99, which indicates the high accuracy of the measurements. The anterior ratios of the whole 210 subjects and class I group were significantly different from Bolton's reference value (p-value = 0.008 and 0.009, respectively). Also, the standard deviations of anterior and overall ratios were higher than those of Bolton's study. According to the results of regression analyses, there was a significant correlation between U1-OP and the anterior ratio of class II group. Also, overbite, the labiolingual thickness of maxillary incisors, and MP-FH had significant correlations with the anterior ratio of class III group. On the other hand, the angulation of maxillary and mandibular canines, MP-FH, and overbite were significantly correlated with the overall ratio of class II group (p-value < 0.05). Due to the higher correlation coefficient of "overbite and labiolingual thickness of the maxillary incisors" with "the anterior ratio" of class III group, two formulae were designed to predict the anterior ratio of class III group using these variables. Discussion: Higher standard deviations of anterior and overall ratios indicated that a much wider range of Bolton's ratios can lead to optimal occlusion. Therefore, blind adherence to the reference values can impose unnecessary invasive interventions on the patient. This study showed that the correlation of different variables with Bolton's anterior and overall ratios is very complex. So, each of the skeletal and dental factors, in addition to affecting Bolton's ratios, also affects each other. Generally, it seems that any factor that affects the anterior dental arch will influence Bolton's ratios. Conclusion: Orthodontic treatment planning should not be based on blind adherence to Bolton's reference values. Therefore, it is better to decide about stripping, build-up, and tooth extraction, after achieving relative alignment of teeth and appropriate overjet and overbite, by considering the dental and skeletal factors and the type of skeletal malocclusion.