Methods: First-molar teeth were prepared and used for crown construction in two groups (n = 18); unilayered IPS e.max CAD (CADLT), fabricated using a staining technique, and bilayered IPS e.max Ceram/ IPS e.max CAD (Ceram/CAD) fabricated using a layering technique, (Ivoclar-Vivadent, Schaan, Lichtenstein). All samples were loaded in compression with a Universal Testing Machine (Z020, Zwick GmbH & Co. KG, Ulm, Germany) via a Co/Cr maxillary first molar tooth at 0.2 mm/min. Released acoustic signals were collected and analysed. Failed samples were examined fractographically with optical and scanning electron microscopes.15 crowns per group were loaded to final failure, and load-values (Newtons) at initial and final fracture were compared, while 3 samples per group were loaded until fracture initiation and were then examined under the optical microscope.
Results: Although initial fracture values of the two groups did not differ significantly (p > 0.05), CADLT: 881.3 N (227.1) and Ceram/CAD: 808.8 N (259.2); the mean final fracture value of CADLT 2425.5 N (367.3) was significantly higher than that of Ceram/CAD 1469.0 N (290.6). All crowns observed at initial fracture had microscopic signs of initial failure. However, fewer acoustic signals were generally detected during fracture propagation of CADLT samples than with Ceram/CAD samples. Fractographic examination of crowns loaded to final fracture revealed different failure patterns between the two groups.
Conclusions: Uni-layered CADLT crowns showed superior performance under loading than bilayered porcelain veneered samples. Analysis of acoustic signals released during fracture testing, and fractographic examination of failed ceramic crowns revealed essential information regarding their fracture behaviour and modes of failure.