Method: In one group, treatment was performed before the eruption of second molars; in the other group both first and second maxillary molars were erupted at the beginning of treatment. Presence of third molar was also considered. The inclusion criteria for all 102 patients were: (1) the exclusive use of a XBow appliance in the late-mixed to early permanent stage of dentition with the only exception of possible placement of brackets on maxillary anterior teeth to gain some extra OJ; (2) a Class II molar relationship defined by at least a molar end-to-end relationship; (3) no previous orthodontic treatment. In order to compare the mean horizontal, vertical and angular changes in molar position between the treatment groups and gender, a multivariate analysis of variance (MANCOVA) was performed with the pretreatment Class II severity used as a covariate. Regression analysis was also performed to further explore any possible relationships between the predictor variables and the quantity and quality of distalization.
Result: Measurement error was 0.53 mm and 0.87o. Gender was removed from the model as it was not a factor. The treatment group did not have a main effect on the overall change in position of the maxillary first molar (p = 0.328). Regression analysis also showed that the eruption stage of the second molar did not have a significant effect on the horizontal distalization and angular change. For the vertical change it appeared that the subjects with erupted second molars consistently had more intrusion (approximately 0.2mm).
Conclusion: When distalizing maxillary first molars with a Xbow appliance, there is no significant difference in the quantity or quality of distalization in patients with erupted and unerupted maxillary second molars.