Effects On Torsional Vibration Behavior Of Dental Implants: A Numerical Approach

Background: In 21 century, people who understand deeply about the theory of osseointegration proposed by Br(a|°)nemark, begin to look after methods to accelerate the healing process and enhance BIC, including implants design, implant surfaces and clinical surgical techniques, etc. Finally, they want to reduce the initial healing period, increase the stiffness of integration, and improve implant stability and the capable of loading function by these methods. So evaluating implant stability becomes an important component of researching osseointegration. It is, therefore, a main aim to be able to find a noninvasive and objective measurement which shows biomechanical properties of the implant-bone interface and the level of osseointegration, and projects a long-term prognosis. With the development of technology, varieties of methods have been used to evaluate osseointegration: histologic analysis, percussion test, radiographic evaluation, cutting torque resistance analysis, reverse torque test, Periotest. Due to the limitation of some methods, including empiricism, invasiveness, poor sensitivity, inaccuracy, and susceptibility to operator variables, unsuitability for long-term assessing, currently resonance frequency analysis(RFA) is frequently used to measure dental implant stability in clinic. However, RFA also has various effects, such as"stiffness"of an implant, the stiffness of the implant-tissue interface, the stiffness of the surrounding tissue, effective implant length (EIL), design of the resonance frequency analysis transducer, etc. In recent years, researches on dental implant RFA and RFA equipment (Osstell^TM) are based on bending vibration mode, of which resonance frequency shows the implant stability in bending vibration. Although it is easier to discriminate by vibration mode contours whether implant or mandible vibration is induced, its resonance frequency isn't sensitive to new bone formation and remodeling in implant-bone interface, as various influences above. The flexural vibrational resonance frequency may not reflect the healing period and the level of osseointegration. Another vibration mode --- torsional vibration mode may reveal the shear stiffness of implant-bone connection and the level of osseointegration.Objectives: The aim of this study was to make torsional vibration mode change to main vibration mode by redesign the resonance frequency analysis transducer and investigate the effects of effective vibration length, bone quality and the osseointegration level on torsional vibration mode and resonance frequency of dental implants using the three-dimensional finite element analysis.Methods: Three-dimensional finite element models of the bone–implant– transducer system were created by UGS NX and ANSYS software. The influences of the thickness of transducer, effective vibration length, bone quality and the osseointegration level on torsional vibration modes, resonance frequencies and the deformation of the system were computed using modal analysis.Results:①Three-dimensional finite element models of the new transducer–bone–implant system were successfully created. Different thicknesses of transducer were stimulated to investigate the effects on vibrational mode and resonance frequency. The results showed that main vibration mode of the system in 1.0mm thickness was torsional vibration mode. And the model of the 1.0mm thickness transducer had a good mode separation from bending vibration.②Three-dimensional finite element bone–implant–transducer system models of four bone qualities were generated. The decreasing rates of resonance frequency of torsional vibration mode inⅡ,Ⅲ,Ⅳmodels were 0.9%, 1.5%, 4%, respectively, comparing withⅠmodels. Three-dimensional finite element bone–implant–transducer system models of three effective vibration length were produced. The decreasing rates of resonance frequency of torsional vibration mode in 3.8mm and 4.8mm effective vibration length ranged from 2 to 3.9% comparing with 2.8mm effective vibration length.③Three-dimensional finite element bone–implant–transducer system models of different osseointegration levels with 0.2mm implant-bone interfacial area were stimulated. The elastic modulus of the cortical bone in the interfacial layer was varied in the range 0.1-10GPa while the elastic modulus of the trabecular bone in the interfacial layer was chosen in the range 0.05-0.25GPa. The increasing rates of resonance frequency of torsional vibration mode inⅠ,Ⅱ,Ⅲ,Ⅳmodels were 21.9%, 35.4%, 36.9%, 36.3%, respectively.Conclusions: The new design of the resonance frequency analysis transducer may be a foundation of three-dimensional finite element bone–implant– transducer system models with torsional vibration mode. The moderate thickness of transducer would be a satisfactory compromise between torsional vibrational sensitivity and mode separation. The effective vibration length and bone quality have slightly effects on resonance frequency of torsional vibration mode in bone–implant–transducer system models. Moreover, the resonance frequency of torsional vibration mode would suggest the biomechanical properties of the implant-bone interface during the bone healing process and be more sensitive to the change of the implant-bone interface`s stiffness.