Fixation of an orthodontic anchor screw using beta-tricalcium phosphate in a screw-loosening model in rats

Objectives To create an orthodontic anchor screw (OAS)–loosening model and to investigate whether filling the bone hole with beta-tricalcium phosphate (β-TCP) can fix the OAS against orthodontic force. Materials and Methods Bone holes with different diameters (1.6, 2.1, or 2.5 mm) were drilled in the tibias of 11-week-old male Wistar rats, and an OAS (3.0 mm in length and 1.2 mm in diameter) was inserted. After a healing period of 2 or 4 weeks, orthodontic force was applied, and the diameter of the bone hole appropriate for the loosening model was determined. Subsequently, under the loosening model, the bone hole was filled with β-TCP, orthodontic force was applied, and movement of the OAS and surrounding tissue changes were evaluated by micro–computed tomography images and histological specimen analysis. Results The bone hole of 1.6 mm in diameter was employed as the OAS-loosening model. When β-TCP was inserted into the bone hole, the linear distance and mesial tipping angle of the OAS movement decreased markedly. Furthermore, the values of bone morphometry significantly increased with β-TCP filling. Conclusions An OAS-loosening model was established in rats and demonstrated that the loosening OAS was stabilized by β-TCP filling through bone formation. β-TCP may be useful for fixation of a loosening OAS.