Effect of Material Combination on Chipping Resistance of Ceramic Crowns

Objectives: To compare the edge chipping resistance (Re) of three material combinations used to produce zirconia/veneering ceramic bilayered crowns
Methods: Bilayered crowns (n=9) made of Y-TZP framework and a veneering ceramic were produced following manufacturers' instructions. The following material combinations were tested: a) leucite/fluorapatite-based glass-ceramic hot-pressed on zirconia (HP), b) CAD-CAM-processed feldspathic porcelain cemented on zirconia (Rapid Layer Technique, RLT); c) lithium disilicate glass-ceramic milled via CAD-CAM and bonded to zirconia by means of a fusion glass-ceramic (CAD-on technique, CO). The zirconia framework was composed of Y-TZP (either IPS e.max ZirCAD/Ivoclar for groups HP and CO, or In-Ceram-2000-YZ-Cubes/Vita for group RLT). The veneering layers were composed of the following materials: IPS e.max ZirPress (HP), Vitablocs Mark II (RLT) or IPS Emax CAD (CO). After glazing and slow cooling of the veneering layers, the edge chipping resistance (Re) was obtained in a universal testing machine (0.1mm/min), using a 120^o sharp conical indenter. Two buccal cusps of each crown were indented at different distances (0.3 to 1.0mm) from the cusp edge. Chipping forces (F/Newtons) were recorded and Re was calculated by averaging F/d. Data were analyzed by one-way ANOVA (alpha=0.05).
Results: The highest Re mean value was obtained by the specimens processed via CAD-on technique (CO, 563.6N/mm); however, this mean value was statistically similar to that obtained for hot-pressed specimens (HP, 491.2N/mm). Group RLT obtained significantly lower Re (374.1N/mm) compared to CO and HP.
Conclusions: The material combination significantly affected the edge chipping behavior of the bilayered ceramic crowns. Specimens processed via CAD-on or hot-pressing techniques resulted in significantly higher edge chipping resistance at the buccal cusps compared to crowns processed via Rapid Layer Technique.