Einfluss des Wärmeausdehnungskoeffizienten und unterschiedlicher Herstellungsverfahren auf die Beständigkeit zweier Verblendkeramiken geschichtet auf Y-TZP-Gerüsten

All-ceramic dental bridges and crowns have drawn considerable attention and interest in recent years due to their improved esthetics and biocompatibility in comparison to the traditional metal–ceramic restorations. The indication has recently been extended to all-ceramic bridges in the molar region by using high-strength ceramic materials such as Tetragonal Zirconia Polycrystals (TZP) as structural framework in combination with a veneer porcelain. The weakest component in a ceramic compound system is the veneering porcelain, characterized by considerably lower strength values compared to the core ceramic. As a result cracks and fractures initially occur in the veneering porcelain. The aim of this study was to compare two different veneer porcelains (VITA VM9 and Lava Ceram), that don´t have the same thermal expansion coefficient difference to the core ceramic VITA In-Ceram YZ. Furthermore the influence of two varying cooling rates after the final sintering process of the veneer porcelain and the influence of prior airborne particle abrasion with corundum of the core ceramic on the reliabilty of the ceramic compound system was investigated. In total, 40 full coverage all-ceramic premolar crowns with identical layer thickness were manufactured using the VITA In-Ceram YZ as the core ceramic which was veneered with either the Lava Ceram or VITA VM9 veneer porcelain. So we received five groups consisting of 8 test crowns each. Three of these groups were veneered with VM9 and the other two groups were veneered with Lava Ceram. One of the groups veneered with VM9 underwent a slow cooling process after the final sintering. In contrast the other two groups veneered with VM9 underwent a fast cooling process and the core ceramics of one of those two groups were sandblasted prior to the veneering. The two groups veneered with Lava Ceram both underwent a fast cooling process after the final firing and the core ceramic of the test crowns of one of those two groups were sandblasted prior to the veneering. The completed crowns were attached on resin stumps and stored in water. Fatigue loading was delivered on the specimens simulating the posterior tooth cusp inclination creating a 0.7mm slide utilizing a tungsten carbide indenter until cracks reached the veneercore interface. After that the results were analyzed statistically and the results of two groups at a time were illustrated comparatively with Weibull distributions (Alta Pro 7, Reliasoft, Tucson, AZ) and the preexisting hypotheses were reviewed. The results show that the groups veneered with Lava Ceram, that have a smaller thermal expansion coefficient difference to the core ceramic VITA In-Ceram YZ, have a significant higher reliability compared to the groups veneered with VM9. The sandblasting of the core ceramics also results in a higher characteristic durability of the all-ceramic crowns. The comparison of the results of the quickly cooled down test crowns with the slowly cooled down test crowns shows that the fast cooling process results in a significant lower reliability.