The Effect of Lithium Disilicate Glass-Ceramic Core Thickness on Fracture Thickness of All-ceramic Restorations

Statement of the Problem: Fracture strength is fundamental for the long-term success and clinical service of all-ceramic restorations. Core thickness is an important factor affecting fracture strength.

Purpose: The main objective of this study was to assess and compare the fracture strength of 0.4 mm and 0.7 mm core thicknesses.

Materials and Methods: In this in vitro, experimental study, one brass die was prepared with classic chamfer finish line design (0.8 mm depth). An impression was made from the metal die and poured with epoxy resin. The epoxy resin die was scanned and lithium disilicate glass ceramic core was fabricated by the computer-aided design/computer-aided manufacturing (CAD/CAM) technique. IPS e.max cores with 0.4 and 0.7 mm thicknesses were fabricated using CAD/CAM technique. Ten samples were fabricated of each thickness and veneered with IPS e.max. After adhesive bonding onto the die, they were vertically loaded using a universal testing machine until fracture. The data were statistically analyzed using the Kolmogorov-Smirnov test and t-test.

Results: The mean and standard deviation (SD) of fracture strength were 1754±313.47 and 1073±202.81 N, for 0.7 mm and 0.4 mm cores, respectively. The Kolmogorov-Smirnov test showed normal distribution of data; thus, t-test was applied for comparison of the two groups (p<0.001). The fracture strength of 0.7 mm core was significantly greater than that of 0.4 mm core.

Conclusions: Within the limitations of this study, it was concluded that increasing the core thickness improves the fracture strength. However, the mean fracture strength values obtained for 0.4 and 0.7 mm core thicknesses were far greater than the load threshold applied in the oral cavity; thus, both thicknesses can be successfully used in the clinical setting.