Key Factors Contributing to the Anchorage Loss of Maxillary First Molars: A Retrospective Study of 726 Extraction Cases

Abstract

Background: Anchorage control is one of the key components in the treatment of extraction cases. However, why anchorage loss happens is still an unanswered question. The purpose of this study was to investigate the most important factors contributing to the anchorage loss of maxillary first molars in premolar extraction cases. Methods：The study enrolled 726 upper premolar extraction cases, including 214 male patients and 512 female patients, and the average age was 14 (range: 9-45 years old).Factors including physiological characteristics, treatment mechanics and cephalometric variables were collected and their influences on the angulation changes of maxillary first molars were analyzed. Results: The average angulation change of maxillary first molar after treatment was 2.81°, meaning the molar tipped forward during treatment by 2.81°. The change of UM/PP showed statistically significant difference in different sex ( male 3.84°±5.26 ° vs female 2.38°±5.10°), age (adult -0.05°±4.73 ° vs teenager 3.46°±5.07°), and molar relationship(Class II 3.28°±5.15°vs Class I 2.36°±5.19°). There are six variables accounted in the regression analysis (R=0.608, R²=37.0%). Among them, the pre-treatment molar tipping (Standardized Coefficients：-0.65) and the pre-treatment incisor/molar height ratio (Standardized Coefficients：-0.27)were the most important factors influencing anchorage loss during treatment.Conclusion: Compared with treatment-related factors, the patient’s physiological characteristics play a more important role in anchorage loss. The pre-treatment angulation of the maxillary first molar is the most contributing factor of the maxillary molar angulation changes, which are often predisposing anchorage loss.