Premolar extraction influence on third molar angulation and retromolar length

Introduction: Previous studies evaluating premolar extraction influence on third molar angulation have always evaluated the changes of the 3rd molar by examining the long axis of the tooth. However many orthodontic patients begin treatment when root formation has not begun on 3rd molars. This study will examine the angulation changes in 3rd molars using a horizontal axis in the occlusal plane. Retromolar length has been found as a contributing factor to impaction versus eruption. This study will examine the change in retromolar length before and after orthodontic treatment in extraction and non-extraction cases. Methods: In this retrospective study, 90 patients were divided into 3 groups of 30 patients each. Groups were compared using one-way ANOVA by extraction pattern (nonextraction, first premolar extraction, second premolar extraction). The horizontal axes of the second and third molars were traced on initial and final panoramic radiographs. The angulation change between the molars was determined between initial and final angulations. The retromolar length was determined by comparing the width of the 3rd molar to the space available from the distal of the 2nd molar to the ramus. The initial and final proportions were compared using chi-square analysis to calculate the proportional change over the treatment period. Results: The mean third molar angulation change in the non-extraction group was 3.1±11.6°, first premolar extraction group was 10.0±16.7°, and second premolar extraction group was 6.3±13.1°. The final retromolar length to third molar width proportion among the three groups was: non-extraction 1.88±1.38, first premolar extraction group 1.26±0.53, and second premolar extraction group 1.19±0.37. Discussion: There was not a significant difference in 3rd molar angulation changes among the three groups. Extraction orthodontic treatment did not help improve eruption angles of third molars. There was a highly significant difference (p value< 0.01) between the extraction groups and non-extraction group in the proportion of third molar width to retromolar length. Extraction orthodontic treatment results in mesial movement of the mandibular dentition and more space distal to second molars for eruption of third molars. Conclusion: Further studies could be performed using CBCT images pre- and post-treatment to calculate accurately the positional changes in mandibular third molars and retromolar length.