| The finite element method has been widely applied to biomechanical evaluation of dental implants. However, most of study were based on simple isotropic mandible model, seldom based on transversely isotropic or anisotropic model. In fact, the mandible is an inhomogeneous, polyporous and orthotropic composite structure. The accuracy of finite element analysis is obviously influenced by the simulation of biomechanics model. We will construct a three-dimensional finite element model of orthotropic complete mandible based on multi-mode data in this study, including accurate dental crowns and roots, periodontal tissues, temporomandibular joints and masticatory muscles. Screwed implants are embedded in the mandible model and biomechanics analysis of implant-bone interface is performed with masticatory muscles loading. Furthermore, we will compare and analyze the influence of different material quality of mandible on finite element calculation of dental implants. In addition, we will establish three-dimensional finite element mandible model of trabecular structure with dental implants using microscopic CT scan, and analyze the influence of trabecular microstructure modeling on the biomechanical distribution of implant-bone interface. Part1:Establishment of a three-dimensional finite model of orthotropic mandible with full dentition crownsObjectives:To construct a three-dimensional finite element model of orthotropic mandible with full dentition crowns, which will provide a basis for study of the biomechanical characteristics.Materials and Methods:CT and MRI scan datasets of a dentate mandible and temporomandibular joint (TMJ) of a young man were saved in DICOM format. Scanned images were imported into the ad-hoc medical image processing and simulation software USIS (Universal Surgical Integration System). Then, a3D registration procedure based on the fossa and condyle regions was performed to transform the articular disk model of MRI data into the coordinate system of CT data. Afterward, individual tooth models based on CBCT scan data were assembled and a Boolean operation was performed to generate the final solid model. The solid model was imported into the specially designed biomedical modeling program to generate the finite element volumetric mesh model, with adaptive mesh size that is optimal in the significant biomechanical region. In addition, an extended orthotropic material assignment methodology based on harmonic fields was used to detail the alveolar ridge region of the dentate mandible to generate compatible orthotropic axes fields.Results:A high-quality, three-dimensional finite element model of orthotropic mandible was constructed, including elaborate anatomical structures of all tooth enamel and dentin, periodontal ligament, complete temporomandibular joints and chewing muscle attached in the mandible. The FE model with finer meshes and distortionless elements shapes comprised334781tetrahedron elements with497127nodes. In the cortical and cancellous bone area of mandible, three directions of orthotropic material coordinate system in each node were different, in which each unit had a smooth transition.Conclusion:The three-dimensional finite element model of orthotropic mandible built in the study has high biological simulation, and greatly increases the geometric similarity and biomechanical similarity, which should improve the accuracy of the finite element calculation.Part2:Influence of orthotropy on finite element analysis of dental implant in mandible modelObjectives:To analyze the influence of orthotropic, transverse isotropic and isotropic mandibular model on the finite element analysis of dental implant.Materials and Methods:An accurate Straumann implant system consists of a separate fixture and a separate abutment was built by CAD software and optical projector. In the USIS(Universal Surgical Integration System), the dental implant was embedded in the first molar region of the mandible by Boolean operation. Then, three mandible models with different material properties were built to study the influence of orthotropy on the implant-bone interface. With regard to mandibular cortical and cancellous bone properties, the first FE model regards bone as an elastic, isotropic medium. The second considers bone tissues as elastic and transverse isotropic medium. And the third considers bone tissues as elastic and orthotropic continua. The values of stress and strain of implant-bone interface in the mandible with masticatory muscles loading were calculated with the software of ANSYS14.0.Results:The values of stress cortical and cancellous bone, implant, enamel, dentin and periodontal ligament in the orthotropic model alomost increased, compared with isotropic and transverse isotropic model. The simulation results showed that the values of stress and strain on the implant-bone interface almost increased in orthotropic case than in transverse isotropic and isotropic case, especially for the maximum stress and strain of the cancellous bone and most close to several times. However, stress concentration was more obvious in transverse isotropic and isotropic case than that in orthotropic case. The influence of orthotropic simulation on the stress and strain values of implant, abutment and crown was little, as well as periodontal ligament, tooth enamel and dentin.Conclusion:It has significant effect on stress and strain of implant-bone interface in the mandible, compared orthotropy or anisotropy simulation with isotropy simulation. Orthotropic mechanical properties of mandible should be emphasized in biomechanical FE studies of dental implants. Part3:Influence of trabecular microstructure modeling on finite element analysis of dental implantObjectives:To establish a three-dimensional finite element mandible model of trabecular structure with dental implants using microscopic CT scan, and analyze the influence of trabecular microstructure modeling on the biomechanical distribution of implant-bone interface.Materials and Methods:The mandibular premolars of a beagle dog were extracted and3months later2Straumann dental implants were embeded in each quadrant. After3months of the implant installed, the mandibles with dental implants were harvested and were scaned by Micro-CT and CBCT. In the USIS(Universal Surgical Integration System), two three-dimensional finite element mandible models of with trabecular microstructure (precise model) and macrostructure (simplified model) were bulit. The values of stress and strain of implant-bone interface were calculated using the software of ANSYS14.0, with vertical loading of50N.Results:The maximum and average values of quivalent stress at implant-bone interface in precise model increased161%-214%and125%-219%than those of simplified model, respectively. However, the maximum and average values of quivalent strain at implant-bone interface in precise model decreased42%-53%and78%-80%than those of simplified model, respectively. Stress and strain concentrations at implant-bone interface were obvious in simplified model, in which the stress was mainly concentrated at the neck of cortical bone, and the strain was concentrated at the thread area and the bottom of the cancellous bone. Nevertheless, the distributions of stress and strain were uniform in precise model of trabecular microstructure, in which the stress and strain were mainly concentrated at the trabecular bone.Conclusion:It has significant effect the distribution of stress and strain at implant-bone interface, when the trabecular bone microstructure was simulated. These results suggest trabecular structures could disperse stress and strain and serve as load buffers. |
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