Effect of the Variation of Posterior Dentoalveolar Height on Anterior Chin Projection in the treatment of Class II Division 1 Malocclusion

Abstract

Introduction In a preliminary study (Ghafari et al 2014), the cant of the anterior contour of the symphysis was shown to be a good predictor of forward chin projection following Class II orthopedic treatment. Given the importance of facial esthetics in determining treatment plan the relevant components should help in improving diagnosis and prediction of treatment outcome, thus projecting more reasonable treatment plans. Aims 1- To assess if the anterior projection of the chin is associated with the control of the posterior occlusion during growth and treatment of Class II division 1 malocclusion

2- To develop predictive models of treatment response (favorable vs unfavorable) based on objective cephalometric classification Design This is a retrospective study conducted on the pre and post-treatment lateral cephalograms taken on orthodontically- treated patients diagnosed with Class II division 1 malocclusion, where linear and angular measurements were performed to gauge relations among cranial base, maxilla and mandible. Each component potentially contributing to treatment outcome will be quantified by its corresponding cephalometric measurement. Methods A total of 285 patients recruited under strict inclusion criteria of Class II/1, and treated in the Division of Orthodontics and Dentofacial Orthopedics at AUBMC, were classified into 2 age groups (195 growing and 90 adults) based on superimposition of T1 and T2 lateral cephalograms. The growing group was further divided into 96 pre-pubertal and 99 post-pubertal subjects based on the time of initiation of treatment (T1). All patients must have reached Class I occlusion at the end of treatment (T2). Linear and angular measurements gauging relations among cranial base and both jaws were taken on pre- and post-treatment lateral cephalograms. Treatment response was defined as “favorable” or “unfavorable” following this approach: Specific pre and post-treatment cephalometric classifier related to the chin projection (Pog-N perpendicular). Various appropriate statistical analyses were applied, including multivariate analyses to the cephalometric data to determine pretreatment predictors of a favorable response in Pog-N measurement.

Results The treatment effects had different responses on the various age categories. In the sagittal discrepancy it was shown that there is more decrease in the anterior chin projection relative to N perpendicular (Pog-N) in the pre- pubertal (1.42 mm, p<0.001) than the post- pubertal (1.107 mm, p<0.001) groups compared to the adult group where there was less of change in this measurement (0.479 mm, p<0.001). On the other hand, the maxillary incisors were significantly retroclined (-6.71°, p<0.001) among the adult patients, more than in the post- pubertal group (-1°, p<0.001), while almost no retroclination occurred in the pre-pubertal group (-0.80°, p=0.56). The initially proclined mandibular incisors remained relatively stable after treatment in all age groups indicating the awareness of the orthodontists to avoid tooth proclination for periodontal and facial esthetics reasons. Pearson correlation coefficient indicates a high correlation between the angle formed between the occlusal plane and palatal plane (OP/PP) and the anterior chin projection (Pog-N) with an r = -.824, p<0.001 in the growing group and a coefficient of r = - .736, p<0.001 in adult group. Based on the multivariate analyses in which one major cephalometric classifier was used (response in Pog-N), the significant predictors of treatment outcome were found in both age groups. More specifically, in the growing group predictors were Co-Go, AFH, SNB, OP/PP and U6-PP. In the adult group, predictors were Co-Go, AFH, MP/SN, and L6-MP.

Conclusion In conclusion, the occlusal plane also translated by the vertical dentoalveolar heights of posterior teeth has a direct relation with the anterior chin projection. Vertical control of posterior teeth, resulting in a decrease of the angle OP/PP will project the anterior chin projection forward. Various predictive equations pointed to several cephalometric measurements as predictors of outcome. Among these predictors, OP/PP. U6-PP and L6-MP play a significant role in predicting the response of the anterior chin projection after treatment. Improvement of the chin projection is better seen within the pre-pubertal group followed by the post-pubertal and least seen within the adult group where growth has ceased. Future research should expand the boundaries of the methodology used in this study, by including larger samples, and panels to judge facial characteristics in relation to the underlying skeletal structures.