The effect of 4 first premolar extractions on the posterior Bolton ratio

Overall and anterior Bolton ratios have been well covered in the orthodontic literature; however, little has been reported on posterior Bolton ratios. Considering the frequency of premolar extractions in the specialty, it would be relevant to know how the posterior occlusion is affected by premolar extractions. This study aimed to investigate how the posterior Bolton ratio is affected by the extraction of the 4 first premolars.

Fifty-five patients with Class I occlusion within 1 standard deviation of ideal anterior and overall Bolton ratios models were selected and digitized. Tooth widths were measured. Virtual extractions of 4 first premolars were performed, and a digital setup of anterior and remaining posterior teeth observing ideal occlusion relationships was executed. When space closure compromised the occlusion, preference was given to the latter. Residual interproximal spacing was digitally measured on the setups. Analysis of variance and linear regression tests were used to identify factors contributing to interproximal spacing.

An average of 1.1 mm of net residual spacing between mandibular second premolars and first molars was observed. In 27% of the sample, at least 1.5 mm of residual space was found. In addition, 16% showed at least 2 mm of residual space. The ratio of the maxillary second premolars to the mandibular second premolars and the width of the maxillary second premolars best explain residual space (r = 0.554; r² = 0.307). A regression equation for predicting residual space is offered.

Ideal anterior, posterior, and overall Bolton ratios treated with extraction of 4 first premolars and ideal occlusion will likely finish with some spacing in the mandible.