Effects of rapid and slow maxillary expansion on root resorption: a micro-computed tomography study

To compare root resorption (RR) after rapid maxillary expansion (RME) and slow maxillary expansion (SME) through micro-computed tomography (micro-CT).

Twenty-six subjects who required maxillary expansion and bilateral upper first premolar extraction were randomly assigned to RME (n = 13, mean age: 13.25 ± 0.88 years) or SME (n = 13, mean age: 13.53±1.28 years) group. A hyrax-type acrylic bonded expansion appliance was used. The Hyrax screw was activated ¼ turn twice a day for 20 days in the RME group and ¼ turn every second day for 80 days in the SME group. One randomly selected upper first premolar was extracted in each patient after active expansion. The appliance was left in situ for a 24-week retention period then the contralateral upper first premolar was extracted. Extracted teeth were scanned with micro-CT and the volume of the resorption craters was analysed with a specialized software. Transversal skeletal and dental widths were measured on posteroanterior radiographs taken before and after expansion and retention periods.

The resorption craters were concentrated mostly on the buccal surface and middle level in all samples. The total RR in the RME group was less post-expansion (P ≤ 0.05) and more post-retention (P > 0.05) than the SME group. During retention, there was a significant decrease in the total RR in the SME group (P > 0.05) and an increase in the RME group. Both RME and SME groups displayed a similar increase in skeletal transverse dimensions, but inter-molar width increased significantly more in the SME group during the whole experimental period.

RME does not have an advantage over SME in terms of skeletal expansion and the amount of RR when a retention period of six months is followed.