Effects of premolar extractions on Bolton overall ratios and tooth-size discrepancies in a Japanese orthodontic population.

Introduction The purpose of this study was to investigate the effects of premolar extractions on the Bolton overall ratios and overall tooth-size discrepancies in a Japanese orthodontic population. Methods Mesiodistal tooth widths were measured on 198 pretreatment dental casts of subjects with Class I, Class II, and Class III malocclusions. The overall ratios and tooth-size discrepancies were determined before and after hypothetical premolar extractions. Before and after extractions, the subjects were divided into small, normal, and large overall ratio groups categorized by the Bolton standard deviation definition, and into small, normal, and large correction groups by the actual amount of change calculated for tooth-size correction in millimeters. Extractions were performed in the following combinations: (1) all first premolars, (2) all second premolars, (3) maxillary first and mandibular second premolars, and (4) maxillary second and mandibular first premolars. For statistical evaluations, analysis of variance, Kruskal-Wallis, Friedman, Scheffe, Bonferroni, and Tukey tests were performed. Results The overall ratios decreased in every malocclusion group after extraction of any combination of premolars. The decreases were significantly notable in combinations 2 and 4. Some subjects in the normal overall ratio and maxillary and mandibular correction groups moved into the clinically significant tooth-size discrepancy group after premolar extraction, and the reverse was also true. Conclusions In formulating a treatment plan involving premolar extractions, orthodontists should consider that the overall ratios might decrease, and normal and clinically significant tooth-size discrepancies could change mutually after extractions.