Thin-wall ceramic CAD/CAM crown copings: strength and fracture pattern.

The strength and fracture pattern of posterior CAD/CAM-generated crown copings with 0.4 mm wall thickness were evaluated in vitro hypothesizing that fracture resistance of YTZP-zirconia copings might be independent of mode of cementation whether resin-bonded or cemented because of the high strength of YTZP-zirconia. Two sets of copings (n = 15) each were fabricated using CEREC inLab CAD/CAM from (i) lithiumdisilicate glass-ceramic, (ii) infiltration ceramic as controls and (iii) YTZP-zirconia. Copings (n = 15) of ceramics (i), (ii) and (iii) each were (a) zinc-phosphate cemented, (b) adhesively seated on resin-based composite dies and loaded until fracture. Load (N) data was analysed using anova and Scheffé tests. Crack pattern was evaluated on additional three sample cross-sections for each group at fracture-start. Radial cracks originated early at the cementation interfaces and cone cracks were observed finally at the loading sites. Mean load (N) values (+/-s.d.) of A-copings at fracture-start/-end (i) 804 +/- 195/862 +/- 162, (ii) 923 +/- 180/975 +/- 147, (iii) 697 +/- 110/1607 +/- 145, were all significantly (P < 0.01) lower when compared with their B-crown coping analogs (i) 1183 +/- 318/1919 +/- 326, (ii) 1621 +/- 165/1820 +/- 211, (iii) 731 +/- 115/1973 +/- 287 except for A3 and B3 at fracture-start. This confirmed our hypothesis at fracture-start (P > 0.05) but rejected it at fracture-end (P < 0.01). The A3 fracture-end data, even if significantly (P < 0.01) lower, came close to the B3 values by 18%. A3 was significantly (P < 0.001) stronger by 86/74% than A1/A2 at fracture-end. The data indicates that YTZP-zirconia copings have the potential to provide support for all-ceramic core crowns, which may be adequate for non-adhesive cementation.