Rat Molar Mesial Drift Using Removable NiTi Coiled-spring Orthodontic Appliance

Objectives: We aimed (1) to developing a reliable experimental model for directing the mesial displacement of rat second molar after extraction of the first molar and (2) to promote visualization and quantification of tooth displacement, alveolar bone modeling and root resorption as well as influence of force application on surrounding craniofacial structures. Materials and Methods: Sprague-Dawley rats, 5 week-old (male, 200 g b.w.) were used. Maxillary incisors were pinned and utilized as anchorage for a titanium-nickel alloy (NiTi) closed-coil spring appliance. At the same time, upper first molars were extracted. After 5 days interval for wound healing, the activated orthodontic appliance was placed between the fixed incisor and the second molar for 7-14 days. The magnitude of force applied to the molar was 10 g, 25 or 40 g by use of three NiTi appliances. Influence of the orthodontic treatment on alveolar bone remodeling and craniofacial morphology was assessed using μCT and histology in conjunction with 3D reconstruction. Results: Based on the 3D μCT views, types of tooth displacement were grouped into horizontal or tilted drift. The majority of the 10 g-applied molars yielded the horizontal drift across the interceptum alveolar bone and into the extraction socket, whereas tooth tilting and exfoliation became prominent with 25 g and 40 g application, respectively. Remarkable findings were that the continuous force application even at 10 g level induced the mesial drift of the third molar, in accordance with the displacement of the directly force-applied second molar, the occurrence of root resorption on compression sides of both second and third molars, and the inflection of the maxillo-incisor bone due to suture compression. Conclusion: The results prove that the continuous application of light forces brings about horizontal tooth drifts in a reproducible manner. Supported by Grants-in-Aid from MEXT/JSPS of Japan (#15390640).