Effects Of The Mechanical Properties Of Veneering Porcelain On Stress Distribution Of Dental Zirconia Layered Structure: A Finite Element Model Study

Objective To construct a three-dimensional finite element model of zirconia layered structure of the first maxillary molar, and study effects of the mechanical properties of veneering porcelain on stress distribution of dental zirconia layered structure.Methods After the CT scanning and image processing to the first maxillary molar and periodontal tissue in vivo, the DICOM files were input to Mimics10.0 software, a 3-D geometric model was established, then a tooth preparation was simulated by the Imageware software. A crown was designed to occupy the space between the original tooth form (plus a cement layer) and the tooth preparation. And the crown was divided into three layers: veneer, core layer and middle layer. The mesh model was then built in Abaqus, a finite element analysis software. Each model was meshed by structurally solid elements of tetrahedral bodies. For the control model, elasticity modulus and the thickness of the middle layer is 70GPa and 0.6mm. The thickness of the first test group is 0.6mm, and the elasticity modulus is 100GPa, 150GPa and 175GPa, respectively. For second test group, elasticity modulus of the middle layer is 100GPa, and the thickness of middle layer is 0.3mm, 0.6mm, 0.9mm, respectively. Loads of 200N were applied over a 1 mm diameter circle at the tip of the mesial-distal cusp ridge, simulating typical occlusional contact areas. then maximum principal stress were calculated. Results The three dimensional finite element all-ceramic crown models of the first maxillary molar was constructed in the computer, which had 60160 nodes and 186328 tetrahedrons elements. The crown system contained adhesive layer, core layer, veneer layer and a middle layer. The model also contained cortical bone, cancellous bone, and periodontal ligament. The components of the model can be observed individually or integrally. The maximum principal stress varied throughout the geometry of the full crown configuration. For veneer, one concentration district of maximum principal stress occurred on the occlusal surface closely proximal to the loading position, several sub-maximum principal stress area were observed, such as,margin regions of the mesial face, lingual face, distal faces, buccal face and fossa of the occlusal face. Concentration of maximum principal stress occurred near the mesial-distal cusp of the middle layer. And a small maximum principal stress area was observed at the margin of the mesial face. Concentration of maximum principal stress occurred in the inner-face of the core near the mesial-distal cusp, and a small maximum principal stress area was also observed at the margin of the mesial face. In the first group, high stresses developed in the surface of the veneer were not affected by the vary of the elasticity modulus, but the value and the extent decreased while modulus increased. In the second group, models with 0.3mm or 0.9mm thickness middle layer has obviously low values of highest maximum principal stress compared with 0.6mm model. All test groups decreased highest maximum principal stress in the top face of the middle layer, increased highest maximum principal stress in the bottom face of the middle layer,but contributed little to the core.Conclusion 1.The finite element model constructed has the advantages of intact structure and precise elements,which can simulate the natural condition and facilitate the further biomechanical research. 2. Middle layer with designed higher modulus can disperse the stress concentration in the layered ceramic system, and the effect positively associated with modulus increase. 3. Models with 0.3mm or 0.9mm thickness middle layer decreased values of highest maximum principal stress of the veneer obviously rather than 0.6mm, appropriate thickness of middle layer need further study.