A Custom-Made Orthodontic Mini-Implant-Effect of Insertion Angle and Cortical Bone Thickness on Stress Distribution with a Complex In Vitro and In Vivo Biosafety Profile

Abstract

Background: Orthodontic mini-implant failure is a debatable subject in clinical practice. However, the most important parameter to evaluate the success rate of mini-implant is the primary stability, which is mainly influenced by cortical bone thickness (CBT) and insertion angle.

Materials and methods: Three-dimensional finite element models of the maxilla were created and a custom-made, self-drilling, tapered mini-implant was designed. For the pull-out test, 12 simulations were performed, sequentially increasing the thickness of the cortical bone (1, 1.5 and 2 mm) and the insertion angle (30°, 60°, 90°, 120°). For the force analysis, 24 simulations were performed using an experimental orthodontic traction force of 2 N both in the horizontal and vertical axis.

Results: Insertion angle and CBT have significant impact on force reaction values (p < 0.05). Cortical bone stress had the lowest value when the mini-implant had a 30° insertion angle and the highest value when the implant had a 120° insertion angle, while the CBT was 1 mm. Cortical bone stress had the lowest value with an insertion angle of 90° and the highest value when the implant was inserted at an angle of 30°, while the CBT was 2 mm independent of the force direction. Regarding the biosafety profile of the mini-implant alloy, the present results reveal that the custom-made mini-implant presents good biocompatibility.

Conclusions: When the CBT is reduced, we recommend inclined insertion while, when the CBT is appropriate, perpendicular insertion is advised.

Keywords: HET-CAM assay; HGF; cortical bone thickness; finite element analysis; in vitro cytotoxicity; insertion angle; orthodontic mini-implants; primary stability.

Figure 1

Three-dimensional rendering of the custom-made mini-implant design.

Figure 2

Insertion angles and force directions.

Figure 3

Mean von Mises stress level variation with the different insertion angles.

Figure 4

(A) Mini-implant von Mises stress; (B) Cortical bone von Mises stress with CBT of 1 mm; (C) Cortical bone stress with CBT of 2 mm.

Figure 5

(A) Morphological aspects of primary human gingival fibroblasts exposed to the sample (metal alloy of the custom-made mini-implant). The scale bar represents 200 µm; (B) (i) Cell viability percentage of primary human gingival fibroblasts (HGF) after treatment with the extraction medium (ii) Cytotoxic rate of primary human gingival fibroblasts (HGF) at 24, 48 and 72 h post-treatment with extraction medium of test alloy intended for orthodontic mini-implant.

Figure 6

Irritation potential of mini-implant alloy extraction medium on the hen’s egg test on chorioallantoic membrane (HET-CAM) assay—before inoculation of sample/controls (t₀) and after five minutes (t₅) of contact with test sample/controls. H₂O was used as negative control and sodium laurylsulphate (SLS) 0.5% was employed as positive control.