Objectives: This study aims to evaluate the volume, amount, and localization of root resorption in the upper first premolars after four different rapid maxillary expansion (RME) appliances using microcomputed tomography (micro CT).
Methods: The study consists of 20 patients who required RME and extraction of the upper first premolars. The patients were divided into four groups: miniimplant supported hybrid RME appliance, hyrax RME appliance, acrylic bonded RME appliance, and fullcoverage RME appliance. The same activation protocol (one activation daily) was performed in all the groups. For each group, 10 premolars and totally 40 premolars were scanned with the micro-CT (SkyScan). The reconstructed 3D images of each root sample were divided into six regions. The resorption craters on these six different root surfaces were analyzed by special CTAn (SkyScan) software for direct volumetric measurements. For the statistical assessment, the Kruskal–Wallis one-way analysis of variance and the Mann–Whitney U test with Bonferroni adjustment were used.
Results: The hybrid expansion appliance had the lowest volume of root resorption and number of craters (P < 0.001). No significant difference was found in the other group in terms of total root resorption (P > 0.05). Except the hybrid group, more resorption was observed on the buccal surface than on the lingual surface in all groups (P < 0.05).
Conclusions: All expansion appliances caused root resorption in the upper first premolar teeth. The resorption craters were generally concentrated on the buccal surface. Minimal root resorption was detected with the mini-screw-supported hybrid appliance after RME.

Keywords: Rapid Maxillary Expansion, Micro-CT, Root Resorption