Effects of placement angle and direction of orthopedic force application on the stability of orthodontic miniscrews

Abstract

Objectives: To evaluate the influence of placement angle and direction of orthopedic force application on the stability of miniscrews.

Materials and methods: Finite element analysis was performed using miniscrews inserted into supporting bone at angles of 90°, 60°, and 30° (P90°, P60°, and P30°). An orthopedic heavy force of 800 gf was applied to the heads of the miniscrews in four upward (U0°, U30°, U60°, U90°) or lateral (L0°, L30°, L60°, L90°) directions. In addition, pull-out strength of the miniscrews was measured with various force directions and cortical bone thicknesses.

Results: Miniscrews with a placement angle of 30° (P30°) and 60° (P60°) showed a significant increase in maximum von Mises stress following the increase in lateral force vectors (U30°, U60°, U90°) compared to those with a placement angle of 90° (P90°). In accordance, the pull-out strength was higher with the axial upward force when compared to the upward force with lateral vectors. Maximum von Mises stress and displacement of the miniscrew increased as the angle of lateral force increased (L30°, L60°, L90°). However, a more dramatic increase in maximum von Mises stress was noted in P30° than in P60° and P90°.

Conclusion: Placement of the miniscrew perpendicular to the cortical bone is advantageous in terms of biomechanical stability. Placement angles of less than 60° can reduce the stability of miniscrews when orthopedic forces are applied in various directions.

Figure 1.

Schematic illustrations of the finite element model with its placement angles and the loading conditions. (A) An FEM was constructed with cortical and cancellous bone, and a combination-type miniscrew was inserted at various angles (α). (B) Loading conditions of the vertical upward forces in an ascending order of 30° to the z-axis (U0°, U30°, U60°, U90°). (C) Loading conditions of the lateral forces in an ascending order of 30° to the x-axis (L0°, L30°, L60°, L90°).

Figure 2.

Specialized jig system for the application of directional pull-out forces.

Figure 3.

Comparisons of stress distribution and displacement according to the placement angle and upward force. (A) Stress distribution and maximum von Mises stress (MPa). (B) Displacement and maximum displacement (µm). Arrows indicate the direction of force application regardless of the number of arrows; ms, maximum von Mises stress; and md, maximum displacement.

Figure 4.

Maximum von Mises stress (MPa) according to placement angle and upward force.

Figure 5.

Comparisons of stress distribution and displacement according to the placement angle and lateral force. (A) Stress distribution and maximum von Mises stresses (MPa). (B) Displacement and maximum displacement (µm). Arrows indicate the direction of force application regardless of the number of arrows; ms, maximum von Mises stress; md, maximum displacement.

Figure 6.

Maximum von Mises stress (MPa) according to placement angle and lateral force.