Optimal Analyses And Design Of Orthodontic Mini-implant Macrostructure By Biomechanical Consideration

The design, selection and application of anchorage is crucial in the treatment of orthodontics. With the development of technique, mini-implant gradually took the place of traditional anchorage and played a more and more important role. Mini-implants in small diameters have been developed to facilitate the surgical insertion procedure, minimize patients’compliance; allow immediate loading after initial wound healing, and anchor at different positions of the alveolar bone. However, there are also problems need to be solved, such as loosening, local inflammation, expense and age. Of these problems, mini-implant loosening caused by surrounding bone resorption, which is related to the macrostructure of mini-implant, is commonly seen. Optimal macrostructure improve the biomechanical characters of mini-implant and decrease the failure rate.So far, the researches of individualized mini-implant parameters are still in an exploratory stage. The former researches were mainly about dispersed univariate, which were not suitable to mimic the real situation. This study aimed to analyze the effect of macrostructure and select the optimal design in type III bone by using Pro/E and Ansys Workbench. The results can provide references to orthodontists and manufacturers.In experiment 1, 3D finite element models of mini-implant,cortical and cancellous bones were constructed by Pro/E. These models were then imported to Ansys Workbench by bidirectional parameters transmitting. Self-adapting assembled 3D finite element models of mini-implant-bone complexes were rebuilt. The accuracy of the models was evaluated. This finite element model provides a technical platform for further optimum design and analyses of orthodontic mini-implant.In experiment 2, mini-implant diameter (D) and length (L) were set as variables. D ranged from 1.0 to 2.0mm, and L ranged from 6.0 to 16.0mm. A load of 2 N was applied to the head of the mini-implant, in the mesiodistal horizontal direction paralleling to the buccal surface of maxilla. The max EQV stresses in cortical and cancellous bones and max displacements in mini-implant were set as objective functions. The effects of design variables to objective functions, as well as the sensitivities of the objective functions to design variables were evaluated. The results showed that the max EQV stresses in cortical and cancellous bones and the max displacement of mini-implant decreased by 80.94%, 91.84% and 86.11%, respectively with the increasing of D and L. The objective functions decreased significantly with the increasing of D while the decrease with the increasing of L is smaller. When D exceeded 1.5mm and L exceeded 11.0mm, the highest stability and the lowest levels of stress and displacement were obtained. Analysis of sensitivity indicated that D was more effective than L in reducing maxilla and mini-implant stresses and enhancing mini-implant stability.In experiment 3, the thread form of mini-implant was set as variables, including V–shaped design, square design, buttress design and reverse buttress design. The force and objective functions settings were the same as those in experiment 2. The results showed that the minimum max EQV stress in cortical bone was achieved by using buttress design thread mini-implant. The minimum max EQV stress in cancellous bone was achieved by using V-shaped design thread mini-implant. The effect of thread form on the max displacement of mini-implant is not noticeable. V-shaped design thread is the best thread form to mini-implant.In experiment 4, the number of mini-implant thread was set as variables, including single thread, double threads and triple threads. The force and objective functions settings were the same as those in experiment 2. The results showed that the minimum max EQV stress in cortical bone was achieved by using single thread mini-implant. The minimum max EQV stress in cancellous bone was achieved by using single and double thread mini-implants. The effect of thread number on the max displacement of mini-implant is not noticeable. Single thread is the best thread design to mini-implant.In experiment 5, optimized mini-implants were manufactured. Twenty optimized mini-implants and twenty control mini-implants were divided into 4 groups. The axial pull-out test and removing torque test were carried out. The results showed that optimized mini-implant had better biomechanical characters.To sum up, optimization of macrostructure can improve the biomechanical characters of mini-implant. V-shaped design single thread mini-implant with diameter exceeding 1.5mm and length exceeding 11.0mm were the optimal biomechanical choice for the maxillary posterior region in a screwed orthodontic mini-implant.