In-Ceram failure behavior and core-veneer interface quality as influenced by residual infiltration glass

Crown and fixed partial denture fabrication using In-Ceram all-ceramic restorative material (Vita Zahnfabrik, Bad Säckingen, Germany) involves the veneering of a glass-infiltrated alumina core with porcelain. The manufacturer instructs that excess infiltration glass be removed from all core surfaces before++ porcelain application; however, meticulous removal may not be practical. This study evaluates the failure behavior of two different types of In-Ceram structures with or without thin layers of excess infiltration glass left on core surfaces.

Two groups of porcelain-veneered incisor crowns were fabricated having either (1) excess infiltration glass removed from the aluminous core (n = 10) or (2) a thin layer (0.1 to 0.3 mm) of excess glass remaining on the core (n = 10). Each crown was loaded on its incisal edge against a flat compression platen at 0.5 mm/min. Two groups of core disks (thickness, 1 mm; radius, 7.5 mm) underwent similar surface treatments; i.e., group A as recommended (n = 15) and group B having excess glass (n = 15). Disks were veneered with porcelain, polished to 1.5 mm, and loaded in biaxial flexure at 0.5 mm/min. All fracture surfaces were analyzed using light microscopy and a select sample were examined using scanning electron microscopy. A simple two-dimensional finite-element model was used to evaluate the stress state at the core-veneer interface of bend samples.

Failure loads were significantly higher for crowns having excess glass (Student's t test, P < .004). Weibull moduli were indistinguishable (m = 6.2) between crown groups, consistent with the microscopic impression that they shared a common failure mode. Microscopic evaluation of cross-sectioned crowns showed core-veneer interfaces with less porosity in the presence of excess infiltration glass. Failure loads for the disk groups were indistinguishable (Student's t test, P > .8). Scanning electron microscopic analysis of disk samples revealed that failures originated either at core-veneer interfaces or at lower disk surfaces, consistent with the finite-element analysis.

Excess infiltration glass on the core surface will not degrade the strength of In-Ceram structures. Central incisor crowns (as tested) were strengthened by excess infiltration glass, but disk samples (stressed differently) were not. Finite-element results suggest that disk failure originates from three possible sources.