Effect Of The Thickness Of Veneer And Core Porcelain On The Strength And Fatigue Life Of Zirconia All-ceramic Fixed Partial Denture

All-ceramic restoration has been widely used in clinic in recent years for its attractive features such as perfect biocompatibility, the maximum simulation of colors of natural teeth, corrosion resistance and good X-ray transmissivity. However, the brittle feature of ceramic materials has greatly limited their application in the all-ceramic fixed partial denture. It is difficult for all-ceramic restorations with one layer to attain both esthetics and high strength. All-ceramic fixed partial denture is supported by core with high strength and sintered veneer porcelain outside it. These two kinds of ceramic materials have different chemical compositions and physical property, according residual stress would be produced at the interface between core and veneer porcelain after sintering. And this interface is a weak place, which lead to the fracture of all-ceramic fixed partial denture under bite force and effect their clinical application. Many efforts have been made recent years to ameliorate compositions and physical property in order to improve their strength and avoid the facture of all-ceramic fixed partial denture. The bond strength between core and veneer porcelain is critical to the long-term success of posterior fixed partial denture. It is significant to study the bond strength between core and veneer porcelain and its effecting factors, which is essential to improve the strength and fatigue life of zirconia all-ceramic fixed partial denture.In this study, the Zirconia posterior all-ceramic fixed partial dentures with different thickness of veneer and core porcelain were made, and the 3-D finite element models corresponding were established based on morphology and structure of the restorations. The bond strength of the core-veneer interface was analyzed using the finite element method. The effect of the ratio of thickness of core to veneer porcelain, their different elastic modulus and thermal expansion coefficient on the strength and fatigue life of the restoration were investigated to provide theoretical basis for the process of Zirconia posterior all- ceramic fixed partial dentures.The research includes the following three parts of experiment:1. Fabricating of Zirconia all-ceramic restoration with different thickness of veneer and core porcelain and establishing of 3D finite element modelA standard model which was missing first molar of the right upper jaw was chosen for this experiment. The second premolar and the second molar were prepared as abutment according to the requirement of Zirconia all-ceramic fixed partial denture. Through the process of die making, core porcelain cutting, veneer porcelain sintering, polishing and glazing, we completed 3 groups of posterior teeth Zirconia all-ceramic fixed partial denture with thickness ratio of 1:2, 1:1 and 2:1 respectively. The occlusal, crown and margin thickness of the three groups of restorations are 2mm, 1.5mm and 1mm respectively. During the process of restoration making, 3D optical scanning was performed on models. Basal and restoration surface were scanned by WIELAND ZENOTEC Scan S100 to form surface morphology of abutment teeth and restoration with different structures. After performing fine trimming for the aforementioned 3 groups of abutment teeth, basal and restoration with different thickness with the software of Geomagic Studio11.0 and turning them into NURBS curves, and 3 groups of 3D entity all-ceramic models of veneer porcelain and core porcelain with different thickness were guided into CATIA software. Then, the entity models were imported into Abaqus software to complete Zirconia all-ceramic finite element model with different thicknesses finally.The research results indicate that: The experiment established finite element model of Zirconia all-ceramic which missing first molar, and taking the second premolar and the second molar as abutment teeth. Under the bite force, the force is mainly distributed at the coronal edge of retainer and the interface of veneer and core porcelain of the all-ceramic fixed partial denture. With the increasing of veneer porcelain thickness, the stress value of interface will increases gradually as well. This result confirmed that the models can simulate force bearing and changing status of all-ceramic fixed partial denture. The models can be used in the studies of static damage and fatigue life in the future.2. Bonding strength of veneer and core porcelain of Zirconia all-ceramic restorationThis research is based on the established finite element model. Abaqus is used in this experiment. In this section, the stress and residual stress at interface between veneer and core porcelain is analyses. The effect of the ratio of thickness of core to veneer porcelain, their different elastic modulus and thermal expansion coefficient on the bond strength were investigated for Zirconia all-ceramic fixed partial denture.Research results indicate that: With the increasing of veneer porcelain elastic modulus, the stress value at the interface between veneer and core porcelain in Zirconia all-ceramic prosthesis will also increase correspondingly. During the cooling process of restoration, when the difference of coefficient thermal expansion between veneer porcelain and core porcelain increased, the residual stress at the junction interface between veneer porcelain and core porcelain Zirconia all-ceramic will also increase correspondingly. The results of restoration static damage experiment indicate that the bonding strength and load-bearing capability of the group with the proportion of 1:2 is the best among the three groups with different thickness of veneer and core porcelain in Zirconia all-ceramic prosthesis.3. Fatigue life analysis of Zirconia all-ceramic fixed partial denture with different thickness of veneer and core porcelainBased on the model established in Experiment II Part I and the corresponding stress analysis results, *.ODB files of finite element calculation results of Zirconia all-ceramicn restorations with different thickness of veneer and core porcelain were imported into Ncode Designlife fatigue analysis software to simulate and analyze the fatigue life of Zirconia all-ceramic prosthesis with different veneer and core porcelain thickness under the bite force.Research results indicate that: The fatigue life of Zirconia all-ceramic fixed partial denture decreased with the increasing of thickness of veneer porcelain. Results suggest that it is very important to keep proper thickness of veneer porcelain in all-ceramic fixed partial denture correspondingly.