Stress distribution in obliquely inserted orthodontic miniscrews evaluated by three-dimensional finite-element analysis

Purpose: The purpose of this study was to evaluate the influence of placement angle and force direction on the initial stability of orthodontic miniscrews using a three-dimensional finite element model that approximates the real interface between the screw and surrounding bone. Materials and Methods: Three-dimensional finite element models with 6-mm-long and 1.4-mm-diameter titanium miniscrews were used. Four insertion angles, ranging from 0 degrees (perpendicular to the bone surface) to 45 degrees, were examined. A load of 2 N was applied to the center of the screw head in four directions (upward, downward, and on the right and left sides). Results: At the same insertion angle, the stresses on the miniscrews were highest in downward force applications, while they were the lowest in upward force applications. This means that with upward traction, stresses are more evenly distributed on the surface of the miniscrew. An analysis of the principal stress distribution in surrounding bone showed that compressive and tensile stresses increased with the angle of insertion up to 30 degrees. For larger insertion angles, the increase almost vanished. Conclusion: An obliquely inserted miniscrew and its surrounding tissues generally provide sufficient anchorage for 2 N of orthodontic loading, but care must be taken to avoid screw failure during placement and removal of obliquely placed miniscrews.