Effect of cyclical forces on the periodontal ligament and alveolar bone remodeling during orthodontic tooth movement

Abstract

Objective: To investigate the effect of externally applied cyclical (vibratory) forces on the rate of tooth movement, the structural integrity of the periodontal ligament, and alveolar bone remodeling.

Methods: Twenty-six female Sprague-Dawley rats (7 weeks old) were divided into four groups: CTRL (unloaded), VBO (molars receiving a vibratory stimulus only), TMO (molars receiving an orthodontic spring only), and TMO+VB (molars receiving an orthodontic spring and the additional vibratory stimulus). In TMO and TMO+VB groups, the rat first molars were moved mesially for 2 weeks using Nickel-Titanium coil spring delivering 25 g of force. In VBO and TMO+VB groups, cyclical forces at 0.4 N and 30 Hz were applied occlusally twice a week for 10 minutes. Microfocus X-ray computed tomography analysis and tooth movement measurements were performed on the dissected rat maxillae. Tartrate-resistant acid phosphatase staining and collagen fiber assessment were performed on histological sections.

Results: Cyclical forces significantly inhibited the amount of tooth movement. Histological analysis showed marked disorganization of the collagen fibril structure of the periodontal ligament during tooth movement. Tooth movement caused a significant increase in osteoclast parameters on the compression side of alveolar bone and a significant decrease in bone volume fraction in the molar region compared to controls.

Conclusions: Tooth movement was significantly inhibited by application of cyclical forces.

Figures

Figure 1.

The model used in this study. (A) Schematic of the OTM model showing the appliance design with the spring being activated from the left maxillary first molar to the incisor. Arrow shows the direction of orthodontic force. (B) Inserted appliance. (C) External application of cyclical force on the left maxillary molar. The controlled feedback loop/electromechanical actuator is used to apply cyclical force (30 Hz at compression force 0.1–0.4 N) vertically on the occlusal surface of the first molar. (D) Rod of electromechanical actuator being placed on the molar of the anesthetized rat. (E) Sagittal two-dimensional micro-CT section used for measuring intermolar (1M-2M) distance. (F) Axial micro-CT section of the left maxillae. The square shows the ROI; the white line represents the center of the analysis.

Figure 2.

Intermolar distances at day 14. Each value represents the mean ± SD (n  =  4–9). # significance compared to the CTRL group; * significance compared to the VBO group; ^ significance compared to the TMO+VB; & significance compared to the TMO group (one-way ANOVA, P

Figure 3.

Histological examination of osteoclast number and surface at day 14 in CTRL (A), VBO (B), TMO (C), and TMO+VB (D) groups. Many TRAP-positive cells were observed on the alveolar bone and within the periodontal ligament in the TMO and TMO+VB groups. Note no osteoclasts were seen within the periodontal ligament in the CTRL and VBO groups. (E) Quantification of active osteoclasts in alveolar bone; # significance compared to the CTRL group; * significance compared to the VBO group (one-way ANOVA, P  =  .0012). (F) Osteoclast surface parameters (one-way ANOVA, P

Figure 4.

Micro-CT analysis of alveolar bone. A) BVF-bone volume divided by total volume (one-way ANOVA, P  =  .0127). # significance compared to the CTRL group; * significance compared to the VBO group. (B) Tissue density (one-way ANOVA, P  =  .0252). Each value represents the mean ± SD (n  =  4–6).

Figure 5.

Examination of PDL collagen fibers on the tension side by two-photon microscopy. In the VBO group (B) there was slight fiber thickening (arrow, white) compared to the CTRL group (A) to accommodate the compressive cyclical force. In the TMO group (C), fibers exhibited thick, uniform, and smooth morphology. In the TMO+VB group (D), fibers were thinner, wavy, and exhibited disrupted morphology when compared to the TMO group.