Methods: Standardized preparations were carried out on two groups of ten sound, unrestored, maxillary premolar teeth of similar size. Ceramic crowns were constructed to have a midline fissure thickness of 2mm. A standard C crown morphology database was selected and duplicated for the veneering on the L coping. The fit surfaces of the C crowns were treated with hydrofluoric acid and Sil™silane bond enhancer while the L crowns were treated with CoJet™, Sil™ before luting.
Compressive fracture resistance was determined for each restored tooth using a Universal Testing Machine with a crosshead speed of 1 mm/min transmitted via a 4mm diameter steel bar. Failure modes were examined visually and designated according to a specially designed scale.
Results: Mean fracture resistance of 746 (SD:147)N and 1630 (175)N were recorded for the C and L groups respectively. Statistical analysis by t-test: Paired Two Sample for Means indicated statistically significant differences in the mean fracture resistance of the restored teeth between the groups (p<0.001). The failure modes were different, with the C crowns displaying less destructive modes of failure than the L crowns.
Conclusions: The compressive fracture resistance of teeth restored with Cerec® (C) directly- produced CAD/CAM crowns formed from Vita® Mark II Vitablocs® is significantly less than teeth restored with Lava™ indirect CAD/CAM-produced crowns (p<0.001, and their mode of fracture is different.