| Anchorage supplies resistance to activation force, and it is one of the most important keys to success in orthodontic treatment. But traditional anchorage can't meet the needs of some intractable cases. For a long time, orthodontists have been looking for a comfortable, economic, safe, stable anchorage, which is also independent of patients. Recently, implant has been used in orthodontics to increase the anchorage, and good results were achieved. The use of implant as an anchorage has a revolutionary to the orthodontics. Now, mini-implant has been successfully used in clinic[1,2]. Mini-implant has the advantage of small shape, easy for operation, little trauma, low expense and short course of treatment. It is popular with orthodontists and it has become a hot spot of both basic and clinical researches. But there are also some problems in the stability of mini-implant, which limit its wider use.At present, biomechanical 3D finite element researches on mini-implant were limited on single variables such as length, diameter. Those researches can't show the interactions of these variables and their biomechanical influences on jaw bones. There is no experiment using 3D finite element method to test the influence of different kind of bones on stress in the mini-implant-bone interface. The development of CAD/CAE provides us new ways to simulate and analyze the shape of mini-implant. This study was based on the self-adaptation and seamlessly transmission of CAD/CAE software, and provided a new platform to multi-variables biomechanical analysis of mini-implant.The objective of this study was to systematically optimize mandibular mini-implant parameters (such as mini-implant diameter, length, height of connector) and different kind of mandibles. At the same time, present study was designed to provide us the detailed parameters that were necessary to develop mini-implant product and provided us the theoretical references for the clinical design and selection of orthodontic mini-implant.In experiment 1, 3D models of mini-implant,cortical bone and cancellous bones were constructed by Pro/E. Then the models were imported to Ansys Workbench by bidirectional parameters transmitting of the two softwares. Self-adapting assembled 3D finite element analysis (FEA) models of orthodontic mini-implant-bone complexes were rebuild and the accuracy of the models was also evaluated. The self-adapting assembled models provide the technical platform for further mini-implant optimum design and analysis.In experiment 2, implant diameter (D) and length (L) were set as design variables. D was from 0.8 to 2.0mm, and L was from 5.0 to 13.0mm. A force of 2N, which was vertical to the long axis of mini-implant, was applied to the connector of mini-implant. The max EQV stresses in mandible and max displacements in mini-implant were set as objective functions. The effect of design variables to objective functions and the sensitivities of the objective functions to design variables were evaluated. The results showed that with the increasing of D and L, the max EQV stresses in cortical and cancellous bones decreased by 58.0% and 28.8% respectively and the max displacement of mini-implant decreased by 18.7%. When D exceeded 1.2mm, the rate of decay of max EQV stresses in cortical bone decreased slowly. L was restricted to D. When D was less than 1.4mm, the max EQV stresses in mandible rose with D increased. While when D exceeded 1.4mm, the max EQV stresses in mandible descended with D increased. When L exceeded 8.6mm, the rate of decay of max EQV stresses in mandible and the max displacements in mini-implant decreased slowly. The objective functions were more sensitive to D than to L. These results indicated that the stresses in mandible and implant stability are more likely affected by D than L. Mini-implant with diameter more than 1.4mm and implant length more than 9.0mm are optimal selection for orthodontists.In experiment 3, the height of orthodontic connector (H) was set as design variables. H ranged from 2.0 to 6.0mm. The length of mini-implant in the mandible was set as 9.0mm and the diameter was set as 1.4mm. The force and objective functions setting and evaluation were the same as experiment 2. The results showed that the max EQV stresses in cortical bone reached the minimum when H was 3.0mm. The max EQV stresses in cancellous bone and the max displacement of mini-implant rose with H increased. The results implied that 3.0mm is optimal selection for mini-implant connector.In experiment 4, the thickness of cortical bone (T) and the Young's modulus of cancellous bone (E) were set as design variables. T ranged from 0.5 to 5.0mm, and E ranged from 500 to 3,000MPa. The force and objective functions setting and evaluation were same as experiment 2. The results showed that the max EQV stresses in cortical and cancellous bones decreased by 46.5% and 98.8% respectively with T and E increasing, and the max displacement of mini-implant decreased by 70.2%. Objective functions decreased significantly with T increasing and the optimal selection for T was more than 2.1mm. The max EQV stresses in cortical bone and the max displacement decreased slightly with E increasing and the optimal selection for E was more than 1692MPa. To the contrary, the max EQV stresses in cancellous bone increased significantly with E increasing. The results implied that there were less stresses in mini-implant-bone interface and more stability in mini-implant in patients with mandibular cortical bone thickness more than 2.1mm and cancellous bone Young's modulus more than 1692MPa.To sum up, diameter exceeding 1.4mm, length exceeding 9.0mm, connector height exceeding 3.0mm are the optimal selections for mini-implant by biomechanical consideration. Mini-implant are better for patients with mandibular cortical bone thickness more than 2.1mm and cancellous bone Young's modulus more than 1692MPa. |
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