Investigation of the Optimal Design of Orthodontic Mini-Implant Based on the Primary Stability: a Finite Element Study

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. 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. The diameter and length showed a great impact on the primary stability in the first set of experiments (p<0.05). The effect of the remaining factors was not statistically significant. According to the second set of experiments, threaded taper angle, non-threaded taper angle and non-threaded taper length were the main factors influencing the primary stability, respectively. An approximate proportion of 20% of threaded taper length to MI length, with minimum taper angle, would be desirable for an average-sized MI.