Arch Form Changes in Orthodontically Treated Class III Malocclusion

Arch form changes following orthodontic treatment have been widely studied. Few studies have been done on Class III malocclusions. Objectives: To determine tooth-based (TB) and bone-based (BB) arch form changes in non-surgically treated Class III malocclusions using a 3-dimensional surface laser scanning method. Methods: The sample group consisted of pre-treatment and post-treatment study models of twenty patients with Class III malocclusions that were treated with 4 premolar extractions. Eighty study models were scanned with a 3-D surface laser scanner (Minolta Vivid900). A customized computer program was used to generate arch forms according to a fourth-order polynomial. Arch ratio was defined by arch depth divided by arch width. Intercanine and intermolar distances were measured on the digital images. Results: Arch ratios, arch widths and coefficients of the fourth-order polynomial were analysed using ANOVA with Bonferonni correction and paired T-test. Pre- and post-treatment arch ratios had significant differences (p<0.05). Post-treatment TB arch ratios for maxilla were greater than the mandible (p<0.05). No significant difference was noted between TB and BB arch ratio in the post-treatment mandible. Mandibular intercanine width was not changed with treatment (p>0.05). Conclusion: Maxillary TB pre- and post-treatment arch forms remained square in shape. Mandibular TB post-treatment arch form became squarer as compared to the pre-treatment arch form. BB arch shapes were similar pre- and post-treatment. Possible factors influencing arch form changes were the degree of initial crowding and the proclination of the upper incisors, retroclination of the lower incisors and constriction of the mandibular arch during the correction of the Class III malocclusions.