| As a remarkable invention in orthodontics, mini-implant anchorage has revealed advantages to conventional anchorages. Nowadays, orthodontic mini-implant has been widely used in clinical treatment. While orthodontists and patients are enjoying the convenient of mini-implant, they often suffer from the failure of mini-implant, especially in osteoporosis patients. Studies on the improvement of mini-implant were focused on the structure optimization and surface treatment. The modified mini-implants showed superior stability and success rates. However, there is still no breakthrough in osteoporosis.Recently, expandable design has been introduced to medical implant. Compared with conventional implant, expandable implant can compress surrounding bone. When expandable implant is inserted in bone, compressive force between implant and condensed surrounding bone increases, as well as friction at the bone-implant interface. As a result, the initial stability of implant is improved, which will guarantee the process of osseointegration. Meanwhile, bone tissue gradually grows into the expansive gap. Therefore, both short- and long-term stability of implant is reinforced by expandable design. So far, there is no report on expandable mini-implant anchorage (EMIA).In this study, we applied expandable design to mini-implant as an EMIA. The biomechanical properties of EMIA were analyzed by 3D finite element analysis, in vitro and in vivo experiment.Part 1: Design and optimization of EMIAMethods: A two component EMIA was designed. A maxillary bone segment, a conventional mini-implant anchorage (CMIA) and an EMIA were constructed in Pro/E. CMIA and EMIA were assembled with maxillary bone and defined as model 1 and model 2, respectively. A point load of 2 N was applied to the head of the implant. The equivalent stresses (EQV stresses) in cortical, cancellous bone and implants, as well as the displacements in implants were used to evaluate the reliability of the two models.In Ansys workbench, the max EQV stresses in cortical, cancellous bone and implants, as well as the max displacements in implants were set as output variables. CMIA was set as a control to evaluate the biomechanical property of EMIA.In Ansys DesignXplorer, the expansive angle (α) and expansive gap length (L) were set as input variables. Output variables were used to optimize these two parameters. Results: The distributions of output variables in the models were similar to previous studies.The max EQV stress in cortical bone of model 1 was higher than that in model 2, while the max EQV stress in cancellous bone of mode 1 was lower than that in model 2. In model 1, EQV stress in cancellous bone was concentrated in the bone surrounding the neck and tip of CMIA, while in model2, EQV stress in cancellous bone was distributed in the bone along EMIA. The magnitude and distribution of EQV stress and displacement in CMIA and EMIA were similar. With the changing ofα, the max EQV stress in cortical, cancellous bone and EMIA decreased by 49.02%, 66.85% and 42.50%. The max displacement in EMIA decreased by 18.59%. With the changing of L, the max EQV stress in cortical, cancellous bone and EMIA decreased by 5.15%, 75.34% and 29.61%. The max displacement in EMIA decreased by 10.95%. Whenα=6°and L=4 mm, EMIA could achieve the best biomechanical property.αplayed a more important role in influencing bone stress and implant displacement, while L was more important in influencing implant stress.Conclusion: EMIA can increase cancellous bone stress magnitude and distribution by compressing surrounding bone. Consequently, cortical bone stress decreases. Therefore, EMIA can achieve better biomechanical property in the early period after insertion.αis more important than L in influencing the biomechanical property of EMIA. Theoretically, whenα=6°and L=4 mm, EMIA can achieve the goal of reliable stability with limited expansion.Part 2: In vitro biomechanical analysis of EMIAMethods: Eighteen implants (9 CMIAs and 9 EMIAs) were inserted in the interradicular aeres of a fresh mandible from an old woman cadaver. Resonance frequency test and axial pull-out test were conducted in the implants.Results: The ISQ of CMIA and EMIA were 31.11±3.28 and 39.15±8.45, respectively. The max axial pull-out strength of CMIA and EMIA were 384.94±95.79 N and 477.39±123.80 N, respectively. Both the ISQ and pull-out strength of EMIA were significantly higher than those of CMIA (P<0.05).Conclusion: The initial stability of EMIA is superior to CMIA.Part 3: In vivo biomechanical analysis of EMIAMethods: Twelve sheep were used in this study. Osteoporosis was induced by ovariectomy, hormone injection and low calcium diet. A CMIA and an EMIA were inserted in each gonial angle area of the sheep. A Ni-Ti coil spring was applied between the two implants with the force of 2N. The 12 sheep were averagely divided into two groups, randomly, and sacrificed after 3 and 6 months, respectively. Micro-CT and histological section were used to observe the implant-bone interface. Resonance frequency test and axial pull-out test were conducted to evaluate the biomechanical property of EMIA.Results: The bone mineral density (BMD) of sheep mandibles were decreased from 1.60±0.16 g/cm2 to 1.17±0.13 g/cm2 after 12 months. The BMD decreased by 2.7 standard deviation (P<0.05).The 3D reconstruction of Micro-CT data showed EMIA and surrounding bone were integrated as a whole body. Histological sections showed newborn trabeculae in expansive gap after 3 months. Six months later, the newborn trabeculae were thicker.The immediate resonance frequency test showed that the ISQ of CMIA and EMIA were 30.25±2.95 and 35.08±3.71, respectively. Three months later, the ISQ were 29.83±4.51 and 41.42±5.16. Six months later, the ISQ were 25.75±4.65 and 43.83±6.10. In the three time points, the ISQ of EMIA were all higher than those of CMIA (P<0.05).The max axial pull-out strength of CMIA and EMIA in the third month were 315.50±123.00 N and 450.42±157.89 N. In the sixth month, the max axial pull-out strength of CMIA and EMIA were 302.42±97.97 N and 489.42±105.61 N. The max axial pull-out strength of EMIA in the third and sixth month were both higher than those of CMIA (P<0.05).Conclusion: EMIA is a partially osseointegrated implant with better biomechanical property than CMIA.
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