Effects of different types of tooth movement and force magnitudes on the amount of tooth movement and root resorption in rats

Abstract

Objective: To investigate differences in the amount of tooth movement and root resorption that occurred after tipping and bodily movement of the maxillary first molar in rats.

Materials and methods: Ten-week-old female Wistar rats were divided into two groups according to type of tooth movement and subdivided into four subgroups according to the magnitude of applied force. Nickel-titanium closed-coil springs exerting forces of 10, 25, 50, or 100 g were applied to the maxillary left first molars to induce mesial tooth movement. We designed a novel orthodontic appliance for bodily tooth movement. Tooth movement distance and root resorption were measured using microcomputed tomography and scanning electron and scanning laser microscopy.

Results: The amount of tooth movement in the bodily tooth movement group was less than half that in the tipping tooth movement group. The greatest amount of tooth movement occurred in the 10-g tipping and 50-g bodily tooth movement subgroups, and the amount of tooth movement decreased with the application of an excessive magnitude of force. Conversely, root resorption increased when the heavier orthodontic force was applied in both groups. Root resorption in the tipping tooth movement group was approximately twice that in the bodily tooth movement group.

Conclusions: Root resorption in the tipping tooth movement group was more pronounced than that in the bodily tooth movement group. Although the amount of tooth movement decreased when extremely heavy forces were applied, root resorption increased in both the tipping and bodily tooth movement groups in rats.

Keywords: Root resorption; Tooth movement; Type of tooth movement.

Figures

Figure 1.

Experimental design. The tipping and bodily tooth movement groups were further subdivided according to the orthodontic force magnitude applied.

Figure 2.

Appliance design. The schemes represent the orthodontic appliances used to move the maxillary left first molars mesially. (A to C) The tipping tooth movement group. (D to F) The bodily tooth movement group. (A,D) Buccal views of the appliances. (B,E) Occlusal views of the appliances. The sliding tube constructed from the self-curing resin was bonded to the occlusal surface of the maxillary first molar in the appliance for bodily tooth movement (D,E). (C,F) Intraoral images of rats. Arrows indicate the direction of the applied orthodontic force (F).

Figure 3.

Micro-CT images. (A) Tipping tooth movement group. (B) Bodily tooth movement group. Superimposition of micro-CT images on days 0 and 28; sagittal views are presented on the left, and two different axial views are presented in the middle. The upper axial image displays a slice of the apical region of the tooth, whereas the lower image displays a slice of the cervical region of the tooth. Reconstructed 3D micro-CT images on day 28 are shown on the right.

Figure 4.

Images of root resorption visualized using scanning electron microscopy. Roots of the untreated control (no force), tipping, and bodily tooth movement groups on day 28 are shown. The mesial roots are shown in the upper panel, and the distal roots are shown in the lower panel. The cervical half of each root was measured for root resorption.

Figure 5.

Box plots. (A) The amount of tooth movement with each orthodontic force magnitude applied. White box indicates tipping tooth movement; gray box, bodily tooth movement. (B) Root resorption with each orthodontic force magnitude applied. White box indicates tipping tooth movement; gray box, bodily tooth movement. * P