| Objective: Malocclusion affects oral health and function, facial esthetics, psysical and psychological health seriously. The number of patients requiring orthodontics therapy has undergone a marked increase in recent decades with the standard of living and awareness of oral health. Anchorage control is a fundamental aspect of success treatment in orthodontic biomechanics. The traditional methods for strengthening anchorage such as TPA, face bow can not meet the requiring in orthodontic treatment, while as a form of bone anchorage, mini-implant anchorage provide steadfast anchorage and expand the scope of treatment, especially, knotty malocclusion. As reported the failure rates for mini-implants in clinical application were about 7%-15% currently, and mini-implant stability becames the constraints for its wide dissemination.The stability of implants was influenced by many factors, such as implant surgery, implant shape, loading direction and cortical bone thickness et al. With the development of materials science, scholars found that biomechanics compatibility of implant-bone interface was the main factor in the stability of implants. The stress and strain distribution in implant-bone interface is an important indicator of the implant stability evaluation. The stress distribution around implants affect bone remodeling activity directly. When the implant-bone interface was constrained by too large and too concentrated stress, the bone tissue absorbed and implant stability decreased. FEA methods are the important means to study biomechanics of implant. The studies confirmed that concentrated stress was in the cortical bone area and the cervical configuration of implant had a major impact on the stress distribution. The cervical configuration form of mini-implant is single, only the screw-type. The study of Motoyoshi draw a conclusion that mini-implants with threadless cervical can reduce the peak stress of bone interface, but for the length of threadless cervical needed further study.Therefore, to provide a theoretical basis for ameliorating implant configuration and improving implant stability, this research established several FEA models which two cortical bone thicknesses jaw models with different length of threadless cervical mini-implants, and analysed the effect of threadless cervical length on stress distribution in bone interface.Materials and Methods:1 Equipment Computer: desktop, (Intelp (R) Core (TM) 2DuoCpu, 2.75oHz: 4G memory, windowsXP operating system)Package: ANSYS12.0 (AnsysInc.Houston)2 Establishing the FEA models of the jaw with mini-implant2.1 Establishing simplified jaw models of two cortical bone thickness: two cortical bone thickness were assumed 1mm (CBT1) and 2mm (CBT2). The dimensions of the jaw model were from the longitudinal section of one side of the mandibular CT image between the second premolar and the first molar of an adult male volunteer. The mandibular CT image was obtained for establishing jaw FEA models. The longitudinal section of mandibular was simplified for hexagon. The surface of jaw models was cortical bone, and the rest for cancellous bone. The hexagonal section size were the upper surface width of 10.89mm, the middle width of 18.2mm, the lower surface widthof 11.94mm and height of 39mm. Then the hexagonal section was tensiled by 20mm.2.2 Setting implant shape : Refer to implant size used in clinical, set the bone segment length of 8mm, diameter of 1.6mm. Setting the longthof smooth cylindrical cervical of implant were 0mm, 0.5mm, 1mm, 1.5mm, 2mm, respectively. Five mini-implant models were obtained:Implant 1: conventional thread implant ( commonly used in clinical);Implant 2: 0.5mm threadless cervical, and the remaining 7.5mm for thread ; Implant 3: 1mm threadless cervical ,and the remaining 7mm for thread ;Implant 4: 1.5mm threadless cervical , and the remaining 6.5mm for thread ;Implants 5: 2mm threadless cervical , and the remaining 6mm for thread .2.3 Position and angle for implant insertion : between the the second premolar and the first molar of mandibular, 5mm from the top of the alveolar ridge implants were inserted vertically. 10 implant-jaw models were obtained respectively :Model 1-1: CBT1-implant 1;Model 1-2: CBT1-implant 2;Model 1-3: CBT1-implant 3;Model 1-4: CBT1-implant 4;Model 1-5: CBT1-implant 5;Model 2-1: CBT2-implant 1;Model 2-2: CBT2-implant 2;Model 2-3: CBT2-implant 3;Model 2-4: CBT2-implant 4;Model 2-5: CBT2-implant 5.3 Loading force and direction : 2 N applied in the implant at the top of the mini-implant. Set the occlusal side for Y-axis positive side, the distance sidecfor the X-axis positive side, the implants axis for the Z-axis . loading point is at the implant top. Loading direction is parallel to the X-Y plane (perpendicular to the implant axis), and 30°tilted to the X-axis positive direction (30°tilted occlusally).4 Meshing the models, and analysing the stress and displacement distribution of implant - bone interface .Result:1 10 FEA models of jaw with mini-implant were established, and all models have good biological similarity and geometric similarity:Model 1-1: CBT1-implant 1;Model 1-2: CBT1-implant 2; Model 1-3: CBT1-implant 3;Model 1-4: CBT1-implant 4;Model 1-5: CBT1-implant 5;Model 2-1: CBT2-implant 1;Model 2-2: CBT2-implant 2;Model 2-3: CBT2-implant 3;Model 2-4: CBT2-implant 4;Model 2-5: CBT2-implant 5.2 The stress distribution of implant - bone interface: The results showed that the peak stress and displacement occurred at the cervical bone margin adjacent to the implants on different models. The stress concentration areas are in the cortical bone range; the stress in cortical bone area decay rapidly, the stress in cancellous bone is small.3 Effect of different cervical configurations of mini-implant on the stress distribution in bone interface: The results showed that the stress distribution of threadless cervical implant - bone interface were more uniformity, the stress distribution of screw implant - bone interface is more concentrated. The Von-Mises stress peak value of screw-type implant - bone interface were 6.65MPa (CBT1) and 6.48MPa (CBT2), while the Von-Mises stress peak value of the threadless cervical implant - bone Interface decreased by 42% -51%, (CBT1), 47.8% -55% (CBT2).The displacement peak value of Srew-type implant - bone interface were 0.32μm (CBT1), 0.233μm (CBT2), while the displacement peak value of the threadless cervical implant - bone Interface decreased by 12% -18% (CBT1) , 18.8% -23.6% (CBT2).4 Effect of cortical bone thickness on bone interface stress : The results showed that compared to CBT1 models the stress peak value of bone interface in CBT2 models declined by about 10% ,and the peak displacement by about 20% ;The Peak stress of cancellous bone interface decreased of approximately 50% and the displacement peak value of about 60% ; The stress distribution of bone interface when CBT2 were more uniform;5 Effect of the height of threadless cervical of mini-implant on stress distribution of the bone interface : With the height of thread less cervical increasing , the stress peak value of bone interface decreased, but did not change significantly;6 The stress distribution of mini-implants: The stress peak value of threadless cervical-type mini-implants were slightly lower than the screw implants; with the length of threadless cervical increasing, the stress peak value of mini-implants decreased, but did not change significantly.Conclusion:1 Threadless cervical of mini-implant is conductive to the stress distribution of implant - bone interface. the stress peak value of threadless cervical-type mini-implant decrease compared to screw implant, and the stress distribution of threadless cervical-type implant-bone interface is more uniformity;2 The longth of threadless cervical of mini-implant effects on stress distribution of bone interface slightly ;3 The cortical bone withstand the major orthodontic force. More cortical bone thickness is to a certain extent, more effectively reduce the stress concentration area of implant - bone interface; so the stress distribution is more uniform, beneficial mini-implant stability.
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