Archform comparisons between skeletal class II and III malocclusions.

Wei Zou,Tianmin Xu,Cuiying Li,Jiaqi Wu,Jiuhui Jiang,Roberto Macchiarelli

doi:10.1371/journal.pone.0100655

Abstract

The purpose of this cross-sectional research was to explore the relationship of the mandibular dental and basal bone archforms between severe Skeletal Class II (SC2) and Skeletal Class III (SC3) malocclusions. We also compared intercanine and intermolar widths in these two malocclusion types. Thirty-three virtual pretreatment mandibular models (Skeletal Class III group) and Thirty-five Skeletal Class II group pretreatment models were created with a laser scanning system. FA (the midpoint of the facial axis of the clinical crown)and WALA points (the most prominent point on the soft-tissue ridge)were employed to produce dental and basal bone archforms, respectively. Gained scatter diagrams of the samples were processed by nonlinear regression analysis via SPSS 17.0. The mandibular dental and basal bone intercanine and intermolar widths were significantly greater in the Skeletal Class III group compared to the Skeletal Class II group. In both groups, a moderate correlation existed between dental and basal bone arch widths in the canine region, and a high correlation existed between dental and basal bone arch widths in the molar region. The coefficient of correlation of the Skeletal Class III group was greater than the Skeletal Class II group. Fourth degree, even order power functions were used as best-fit functions to fit the scatter plots. The radius of curvature was larger in Skeletal Class III malocclusions compared to Skeletal Class II malocclusions (rWALA3>rWALA2>rFA3>rFA2). In conclusion, mandibular dental and basal intercanine and intermolar widths were significantly different between the two groups. Compared with Skeletal Class II subjects, the mandibular archform was more flat for Skeletal Class III subjects.

Highlights

Archforms play an important role in orthodontic diagnoses and treatment plans
In 2000, Andrews reported ‘‘six elements to normal occlusion’’ theory, Indicating that FA points(the most prominent part of the center of the clinical crown where an orthodontic bracket would be placed in an appliance system)represent the dental arch form and the WALA ridge reflects the basal bone shape at the same vertical level (WALA points: the most prominent point on the soft-tissue ridge at the mucogingival junction; WALA ridge: the band of keratinized soft tissue directly adjacent to the mucogingival line)[5]
The mandibular intercanine and intermolar widths of FA and WALA points are significantly larger in the SC3 group compared with the SC2 group (P,0.05)

Summary

Introduction

Archforms play an important role in orthodontic diagnoses and treatment plans. Limitations for tooth movement, especially for incisor retractions, arise from the basal bone where teeth are rooted [1]. How to define basal bone is not unanimous: Lundstrom put forth the apical base theory to explain basal bone boundaries located at the apical root level [3]. He proposed that the basal bone were not changed by orthodontic tooth movement and that their expansion was limited by the apical base bone. In 2010, Ball used to evaluate the correlation of dental and basal bone of normal occlusion and Class II malocclusion [6] They concluded that the WALA ridge estimates the basal bone archform. In other several studies[7,8], the WALA points have been used to refer the basal bone arch form,proven reliable to represent the basal bone archform

Methods

Results

Discussion

Conclusion