In‐Ceram Failure Behavior and Core‐Veneer Interface Quality as Influenced by Residual Infiltration Glass

Purpose 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. Materials and Methods 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; ie, 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. Results 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. Conclusion 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.