Evaluation of Mechanical Properties Associated with the Stability of Orthodontic Mini-implants by Finite Element Analysis

Abstract

Background: The design of an orthodontic mini-implant is a significant factor in determining its primary stability and therefore its clinical success. The purpose of this study was to measure the relative influence of mini-implant design factors including diameter, length, degree and length of taper both in the threaded and non-threaded area, pitch and thread depth/diameter on the primary stability. Methods: Thirty two 3-dimensional assemblies of mini-implant models with their surrounding bone were generated using finite element analysis software. The maximum displacement of each mini-implant model was measured as they were loaded with a 2 N horizontal force. Employing Taguchi’s design of experiments as a statistical method, the contribution of each design factor to primary stability was calculated. As the effect of the upper diameter and length was abundant, to better detect the impact of the remaining design factors, another set of twenty five models with a fixed amount of length and diameter was generated and evaluated. Analysis of variance was conducted to evaluate the statistical significance of the impact of each factor in the both set of experiments (α=0.05). Results: In the first set of experiments the upper diameter and length showed a great impact on the primary stability (53% and 45%, respectively, p=0.000). The effect of the remaining factors was not statistically significant. According to the second set of experiments, threaded taper angle (64%, p=0.005), non-threaded taper angle (16%, p=0.011), Non-threaded taper length (7%, p=0.016), pitch (6%, p=0.017), thread depth/upper diameter (2%, p=0.026) and threaded taper length (2%, p=0.027) significantly affected the primary stability (in a descending order). Conclusion: Upper diameter and length were the most important factors affecting the primary stability, respectively. Excluding the upper diameter and length; threaded taper angle, non-threaded taper angle and non-threaded taper length were the main factors influencing the primary stability, respectively. Tapering is suggested only if the upper diameter increases as well. Addition of a non-threaded tapered part to the mini-implant also contributes to the primary stability.