Simulation of miniscrew-root distance available for molar distalization depending on the miniscrew insertion angle and vertical facial type

Abstract

Buccal interradicular miniscrew has been used for molar distalization without loss of incisor anchorage in nonextraction treatments. In previous studies, it was recommended to increase the miniscrew-root distance by placing the miniscrew obliquely, but specific study was not conducted. The purpose of this study was to evaluate the effects of miniscrew insertion angle and vertical facial type on the interradicular miniscrew–root distance available for molar distalization. Cone-beam computed tomography images of 60 adults with skeletal Class I occlusion exhibiting hyperdivergent (n=20), normodivergent (n=20), and hypodivergent (n=20) facial types were used. Placement of a 6-mm long, 1.5-mm diameter, tapered miniscrew was simulated at a site 4 mm apical to the cementoenamel junction, with insertion angles of 0°, 30°, 45°, and 60° relative to the transverse occlusal plane. The shortest linear distance between the miniscrew and anterior root at four interradicular sites was measured: maxillary second premolar and first molar (Mx 5-6), maxillary first and second molars (Mx 6-7), mandibular second premolar and first molar (Mn 5-6), and mandibular first and second molars (Mn 6-7). Miniscrew–root distance significantly increased as the insertion angle increased from 0° to 60°. In the mandible, the distances significantly differed among vertical facial types, increasing in the following order: hyperdivergent, normodivergent, and hypodivergent. The minimum mean distance was found in the Mx 6-7 (30°; 0.86±0.35 mm), and the maximum mean distance was found in the Mn 5-6 (60°; 2.64±0.56 mm). The rates of miniscrews located buccally outside the root distalization path were up to 70% and 55% when the miniscrews were placed at 60° insertion angles in the Mx 5-6 and Mn 5-6 regions, respectively. Miniscrew–root distance increased significantly with the increased insertion angle, and the amount of increase was affected by the miniscrew placement site and vertical facial type. To ensure adequate distalization of the posterior segment, the miniscrew should be inserted at an angle in the interradicular area between the second premolar and first molar.