A model for evaluating friction during orthodontic tooth movement

Orthodontic forces for sliding tooth movement during space closure are applied at a distance from the centre of resistance of the teeth. For this reason, the teeth will tip until contacts are established between the archwire and diagonally opposite corners of the bracket wings. They will also rotate until the wire contacts opposite corners of the ligature tie or the buccal shield with self-ligating brackets, and the base of the slot. Frictional forces measured with models that do not enable such movements may therefore not be representative of the clinical condition. To test this hypothesis, a dentoalveolar model that allowed accurate reproduction of the width of a material of similar elastic properties as the periodontal ligament (PDL) was fabricated. In addition, a device was designed that allowed accurate adjustment of the bracket slot in all three planes of space during mounting of the model in an Instron machine. Frictional forces during sliding of ceramic brackets with 0.022 x 0.028-inch bracket slots along 0.019 x 0.025-inch stainless steel wires were tested using models with simulated PDL widths of 0.00, 0.33, 0.67, and 1.00 mm. ANOVA detected a significant effect of PDL width on mean frictional force (P < 0.001). Pairwise comparisons at 0.05 significance level indicated no differences between the models without PDL and those with a width of 0.33 mm, and between models with PDL widths of 0.67 and 1.00 mm. However, the two models with smaller widths produced significantly lower frictional forces.