The Study Of Bond Strength Between Core And Veneer Porcelain Of All -ceramic Restorations

All-ceramic has become a kind of ideal dental materials and its restorations have been used in clinic widely recent years because of their attractive features such as perfect biocompatibility, the maximum simulation of colors of natural teeth. The past several years witnessed various kinds of all-ceramic materials as well as their fabricated methods, which suggested that all-ceramic restorations would turn into the main material for restorations in prosthodontics.However, the brittle feature of all-ceramic materials has greatly limited their application. Great efforts have been made to overcome the brittleness of ceramic materials, enhance their strength and avoid the facture of all- ceramic restorations. In the past years, some new all-ceramic materials were developed with more excellent features, which resulted in wider application of all-ceramic restorations and better performance and survival rates were achieved. It is difficult for all-ceramic restorations with one layer to attain both esthetics and high strength. Given that, most of all-ceramic restorations are supported by core porcelain with high strength and sintered veneer porcelain outside it. Now that all-ceramic restorations are consisted of two kinds of ceramic materials which have different chemical compositions and physical property, the interface of the core-veneer porcelain is a weak place, where the fracture of this kind of all-ceramic restorations easily take place in their clinical application. Therefore, the bond strength between core and veneer porcelain is critical to the long-term success of all-ceramic restorations. And it is urgent to study the bond strength between core and veneer porcelain, which is essential to improve the overall strength of all ceramic restorations and increase their survival rate in clinical use, in order to provide more rational guideline for their application.In this study, the bond strength on the interface between the core-veneer porcelain were analyzed using the finite element methods based on the models of Zirconia ceramic restoration with different proportion of the thickness of core to the veneer porcelain and with different elastic modulus and thermal expansion coefficient. All these results were compared with the shear tests of normal sample with different proportion of the thickness of core to the veneer porcelain.Results1. In the shear tests, the breaking loads were decreasing(157.40N,141.79 N,98.92N) with the increasing proportion of the thickness of veneer porcelain to core (1:2,1:1,2:1). The proportion of 2:1 was significantly different from the other two groups. In the FEM study, the Mises stress on the core-veneer interface of these three proportions were analyzed, which showed 172.81MPa in the 1:2 group, 166.58 MPa in 1:1 group and 86.52MPa in 2:1 group. The damage load was decreasing with the increment of the veneer porcelain thickness.2. The Mises stress on the core-veneer interface of Zirconia all-ceramic samples and all-ceramic crowns increased with the increase of elastic modulus of veneer porcelain (E=32GPa～160GPa). The amplitude of all-ceramic crown was greater than the specimens, but the specimens got the larger value.3. The residual stress on the core-veneer interface of Zirconia all-ceramic samples and all-ceramic crowns increased with the increase of difference of CTE between the veneer and core porcelain (a_core-a_veneer=0～1.2×10^-6). Followed, the residual stress on the core-veneer interface increased under the occlusal loading.Conclusions1. There are no significant differences in the failure loads and the stress on veneer-core interface, when the ratio of the thickness of the veneer porcelain to core porcelain is 1:2 or 1:1. Therefore, without influence of the bond strength on the interface, the thickness of the veneer porcelain can be increasing properly in order to enhance the esthetics of all-ceramic restorations. But it must not be obviously thicker than that of the core , which would have adverse effects on the bond strength of veneer-core interface.2. The stress both on the veneer-core interface of all-ceramic samples and on the all-ceramic crown are increasing with the increment of the elastic modulus of veneer porcelain, and the amplitude of the all-ceramic crown is greater than all-ceramic specimens. The results suggested that the differences of elastic modulus between veneer and core porcelain should be taken into account in the fabrication of all-ceramic restorations.3. The residual stress on the core-veneer interface of all-ceramic samples and the all-ceramic crown rises considerably with the difference of CTE between veneer porcelains and core porcelains. And the maximal residual stress is mainly distributed around the margin of all-ceramic crown on the veneer-core interface, where veneer porcelain is the thickest. The results suggest that the residual stress on the margin of all-ceramic crown may cause detach of the veneer porcelains from the core or fracture of the veneer porcelain.4. The equivalent stress distributed on the cusps of the crown is larger than that distributed on the margin of all-ceramic crown, after applying the occlusal loading. It is suggested that the thickness of the veneer porcelain and the occlusal load should be controlled on this area, in order to avoid the failure of the cusps.