Cortical bone strain during the placement of orthodontic microimplant studied by 3D finite element analysis

Can maxilla and mandible bone quality explain differences in orthodontic mini-implant failures?

Purpose: This study aimed to compare the risk of orthodontic mini-implant (OMI) failure between maxilla and mandible. A critical analysis of finite-element studies was used to explain the contradiction of the greatest clinical success for OMIs placed in the maxilla, despite the higher quality bone of mandible. Materials and Methods: Four tridimensional FE models were built, simulating an OMI inserted in a low-dense maxilla, control maxilla, control mandible, and high-dense mandible. A horizontal force was applied to simulate an anterior retraction of 2 N (clinical scenario) and 10 N (overloading condition). The intra-bone OMI displacement and the major principal bone strains were used to evaluate the risk of failure due to insufficient primary stability or peri-implant bone resorption. Results: The OMI displacement was far below the 50–100 µm threshold, suggesting that the primary stability would be sufficient in all models. However, the maxilla was more prone to lose its stability due to overload conditions, especially in the low-dense condition, in which major principal bone strains surpassed the pathologic bone resorption threshold of 3000 µstrain. Conclusions: The differences in orthodontic mini-implant failures cannot be explained by maxilla and mandible bone quality in finite-element analysis that does not incorporate the residual stress due to OMI insertion.

Strain of bone-implant interface and insertion torque regarding different miniscrew thread designs using an artificial bone model

OBJECTIVES

To evaluate the initial stability of dual-thread miniscrews by analyzing the strain at the bone-implant interface and insertion torque during implantation in artificial bone models with different cortical bone thicknesses.

MATERIALS AND METHODS

Insertion torque, and strain, measured with a five-element strain gauge in 1.0, 1.5, and 2.0-mm artificial cortical bone, during insertion of single- (OAS-T1507) and dual-thread (MPlant-U3) type self-drilling miniscrews were assessed.

RESULTS

Both dual- and single-thread miniscrews showed greater than 7790 μstrain for all cortical bone thicknesses, and dual-thread miniscrews reached up to 19580 μstrain in 2.00 m m cortical bone. The strain of dual-thread miniscrews increased with increasing cortical bone thicknesses of 1.0-2.0mm. For single-thread miniscrews, the maximum insertion torque was relatively constant, but maximum insertion torque increased significantly in dual-thread groups with increasing cortical bone thicknesses (P < 0.0001). The maximum insertion torque with all cortical bone thicknesses was significantly lower with single- than dual-thread types (P < 0.0001).

CONCLUSIONS

Self-drilling dual-thread miniscrews provide better initial mechanical stability, but may cause strain over the physiological bone remodelling limit at the bone-implant interface in thick cortical bone layers.

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

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.

Effect of force on alveolar bone surrounding miniscrew implants: a 3-dimensional microcomputed tomography study

INTRODUCTION

The primary aim of this study was to better understand how bone adapts to forces applied to miniscrew implants. A secondary aim was to determine whether the direction of force applied to miniscrew implants has an effect on bone surrounding the miniscrew implants.

METHODS

A randomized split-mouth design, applied to 6 skeletally mature male foxhound dogs, was used to compare miniscrew implants loaded for 9 weeks with 200 or 600 g to unloaded control miniscrew implants. By using microcomputed tomography, with an isotropic resolution of 6 μm, bone volume fractions (bone volume/total volume) were calculated for bone around the entire miniscrew implant surface. Bone volume fractions were calculated for bone 6 to 24, 24 to 42, and 42 to 60 μm from the miniscrew implant surface. For each loaded miniscrew implant, the bone volume fraction was also calculated for 2 compression and 2 noncompression zones.

RESULTS

The 6 to 24-μm layer showed a significantly lower (P <0.05) bone volume fraction than did the 24 to 42-μm and the 42 to 60-μm layers, which were not significantly different. The bone volume fractions of cortical bone surrounding the apical aspects of the unloaded miniscrew implants were significantly greater (P <0.05) than the bone volume fractions of cortical bone surrounding the loaded miniscrew implants. In contrast, the bone volume fractions of noncortical bone surrounding loaded miniscrew implants were significantly greater (P <0.05) than the bone volume fractions of bone surrounding the unloaded miniscrew implants. Miniscrew implants loaded with 200 g showed significantly greater (P <0.05) amounts of noncortical bone volume fractions than did miniscrew implants loaded with 600 g. With both 200 and 600 g, zones under compression had significantly greater bone volume fractions than did the noncompression zones.

CONCLUSIONS

The application of force, the amount of force applied, and the direction of force all have significant effects on the amounts of bone produced around miniscrew implants.