Maxillary Posterior Teeth Distalization with Miniscrew Anchorage Relative to Force Vectors Applied to Different Lengths of Retraction Hook, Analyzed Using the Finite Element Method

Abstract

The purposes of this study were to investigate and compare the stress distributions and displacement pattern for maxillary posterior segment distalization. When the force magnitude 250 g was applied from the miniscew anchorage to the vertical positions of the retraction hook of 0, 2, 4, 6 and 8 mm, analyzed using the finite element method.

A 3-D finite element model was constructed to simulate the maxillary first and second premolars and molars, periodontal ligament, and alveolar process. Distalizing forces were applied through a miniscrew at modified infrazygoma area for maxillary posterior segment distalization. The displacement of each tooth was evaluated on x, y, and z-axis, and the von Mises stress distribution was visualized using color-coded scales.

When the force vector acted at the lowest vertical position of the retraction hook (0 mm), distal crown tipping occurred in every tooth, and intrusion occurred in the second premolars, first molars and second molars. But first premolars showed slightly extrusion. Furthermore, buccal out-rotation occurred in every tooth. When the force vector acted at the highest level of the retraction hook (8 mm), greater distal crown movement occurred in the second premolars, first molars and second molars than at any other level. But the first premolars showed a progressive decrease in distal crown tipping. Moreover, extrusion occurred in every tooth. Lingual in-rotation occurred in the second premolars, first molars and second molars. But the first premolars showed slight buccal out-rotation. However, the retraction hook levels of 0 mm and 2 mm showed distal movement in every tooth, with minimal movement in the vertical direction, but the retraction hook levels of 2 mm and 4 mm showed distal movement in every tooth, with minimal movement in the transverse direction than at the other hook
levels.

These results suggest that the low retraction hook levels of 0 mm, 2 mm and 4 mm showed greater distal movement with minimal vertical and transverse
movement than at the other hook levels. The high retraction hook levels of 6 mm and 8 mm showed greater vertical and transverse movement in every tooth except the second molar in the vertical direction and the first premolar in the transverse direction, which showed the least amount of movement. It was concluded that the vertical position of the retraction hook is an important factor in achieving controlled maxillary posterior teeth distalization because the type of tooth movement depends on the relationship between the line of force and the location of the center of resistance of the force system.