Mechanical Performance And Structural Optimization Of Bilayered All-Ceramic Fixed Partial Denture

Fixed partial denture (FPD) is a common way of repairing dentition defect. In recent years bilayer all-ceramic FPD has received widespread attention and become an alternative to traditionally used metal-porcelain FPD, because the strong framework material veneered with porcelain presents a combination of both high strength and excellent esthetics. However, there are still some problems in the application of FPD. For example, the veneer chipping rate is high; large volume of enamel of the healthy abutment teeth is ground off; grinding volume is random due to lack of quantitative indicators. In order to solve the above problems, the mechanical properties and structural optimization of FPD were studied in this paper. First, stress distribution of zirconia/veneer bilayer beam was modeled, and the effect of zirconia/veneer thickness ratio on load-bearing capacity of the beam was studied; secondly, the effect of zirconia surface treatment and cyclic loading on the zirconia/veneer interfacial toughness was researched by interfacial fracture mechanics method, and the mixed-mode fracture mechanics performance of the interface was investigated; thirdly, three-dimensional finite element analysis (FEA) model of FPD was established by reverse engineering method, and static stress and residual stress analyses were performed with the FEA model; Finally, structural optimization were conducted on FPD with response surface methodology.Thickness ratio between layers can significantly affect structural strength, failure mode and failure origin for bilayer FPD, because stress distribution in the FPD was affected by the thickness ratio. The FPD was subject to structural stress due to loading and thermal residual stress due to veneer sintering process. The FPD can be simplified as bilayer beam, which have similar stress patterns to the FPD. Hence, investigating the stress distribution in the beams is paramount to understanding the effect of the thickness ratio on strength and failure mode of the FPD. Both analytical and finite element analysis methods were used to analyze the stress distributions of bilayered beams subjected to three-point bending test and the residual thermal stresses due to coefficient of thermal expansion mismatch. The ideal load-bearing capacity of the beams as a function of core thickness was evaluated based on the mechanical models. And the mechanical models were verified by three-point bending tests. The study demonstrated that the thickness ratio did not significantly affect the load-bearing capacity of bilayer beams when the thickness ratio changed from 1:2 to 2:1. Therefore, the core/veneer thickness ratio of fixed partial denture could be relatively small to about 1:2 to obtain a good appearance.Chipping of the veneering porcelain is one of the most common failure modes of zirconia-based bilayered restorations. It is related to the weak interfacial adhesion and large difference in mechanical properties of the zirconia and veneer materials. As a result, many studies have been carried out in an effort to improve the bond performance of the zirconia-veneer interface with different zirconia surface treatments. However, the clinical veneer chipping rate is still high. This is largely due to defects and cracks tending to initiate at the zirconia/veneer interface, which has low fracture toughness. Interfacial bond strength was usually used to represent interfacial performance, whereas research on the interfacial fracture toughness is neglected. In this paper, the effect of airborne-particle abrasion and liner application, which are two common used surface treatment methods, on the zirconia/veneer interfacial toughness was evaluated by means of a fracture mechanics test. Liner application and airborne-particle abrasion seem to reduce zirconia/veneer interfacial toughness. Therefore, the two surface treatment methods should be applied with caution. Then the effect of cyclic loading on the interfacial fracture toughness was studied experimentally. The experimental results reveal that cyclic loading has no significantly reducing effect on the interfacial toughness. This indicates that the produce of interfacial crack is not because of fatigue, but because of the stress intensity factor or other indicator reaching the critical value of materials under instantaneous loading. An interfacial crack subjected to mixed mode loading either kinks into substrates or propagates in the interface. Consequently, in order to understand the fracture mechanism of bilayered all-ceramic restorations, it is necessary to investigate both fracture criteria of kinking and propagating along the interface. In the present paper, a modified sandwich test configuration was proposed to set up fracture criteria of zirconia/veneer interfacial crack by interfacial fracture tests.The FEA method is a suitable instrument for determining strains and stresses in the FPD. However, there are always some assumptions in the FEA model to simplify the real conditions which are hard to simulate, leading to model distortion. The verifications of models were usually conducted by measuring the load-bearing capacity or fracture pattern of the FPDs, which can not prove the correctness of the model. In this paper, three dimensional FEA model of FPD was developed by reverse engineering. And the FEA model was verified via electronic strain gauge method. Then the verified model was employed to calculate stress distribution of FPD under occlusal force. It shows that the veneer layer on buccal surface of the connector is the weakest area in the FPD, thereby the structure of veneer on this location should be optimized. In addition, transient thermal-structural coupling analysis was performed on the FPD with the FEA model to calculate residual stresses produced during sintering process. The most area of veneer surface was subjected to small compressive stress. The stress distribution of the interior of veneer was related to curvature of the veneer layer. The veneer inside the cusps was subjected to tensile stress, which has the maximum value near the zirconia/veneer interface.Researches about the structure of the FPD have focused on one or a few factors that influence its strength. Different structures were compared by FEA method or strength tests. However, very few studies have performed comprehensive optimal design on the FPD. In the present paper, structural optimization was performed with response surface methodology. Experiments were designed based on Box-Behnken method. And each experiment was conducted with the FEA model. Response surface was constructed based on the simulation results. The optimal solution was obtained after optimization calculation through genetic algorithm. The FPD volume and grinding volume were obviously reduced (19.7% for FPD volume, and 20.2% for grinding volume, respectively) after optimized. And the geometries of grinding volume were quantified. Consequently, the objectives of minimizing grinding volume, maximizing load-bearing capacity, and achieving better apparent were realized.