Quantitative Analysis of Root-resorption-repair following the Application of Continuous Force

Objective: Root resorption is an inevitable side-effect of orthodontic treatment. However there is no study on quantitative 3D evaluation of root resorption repair. The aim of this study was to evaluate the amount of repair by quantitative assessment of the two and three dimensional changes of the root resorption craters. 

Materials and Methods: 50 g mesial force was applied to move the upper first molars of sixty Wistar male rats (10-week old) by using nickel titanium closed-coil springs for 2 weeks. The interdental space between the upper first and second molars was filled with resin to retain the molars. The molars were extracted after periods of retention varying between 2 and 16 weeks. Upper right molars were used as controls. Mesial and distal roots (disto-buccal and disto-palatal) were examined using scanning electron and 3D scanning laser microscopes. The surface area, depth, volume, and roughness of the root resorption craters were measured.

Results: The examination showed that all the area, depth and volume of the craters decreased gradually and showed similar trends over retention time approaching a plateau at 12^th week. After 16 weeks of retention, resorption volumes of the disto-buccal and disto-palatal roots reached a recovery peak of 69.5% and 66.7%, respectively. Small pits on the mesial root showed a recovery of 62.5% at 12^th week. The healing pattern in the distal roots with severe resorption and mesial roots with shallow resorption did not show significant differences.

Conclusion: The results of this study suggest that the majority of the reparative process occurs after four weeks of passive retention following the application of orthodontic force and that frequent orthodontic re-activations should be avoided so a recovery and repair of the root surface damage can happen.