In vitro analysis of the fracture resistance of CAD-CAM monolithic lithium disilicate molar crowns with different occlusal thickness

Aim: To compare the fracture resistance and mode of failure of CAD-CAM monolithic lithium disilicate crowns with different occlusal thickness.  Materials and methods: Thirty CAD-CAM monolithic lithium disilicate crowns with different occlusal thickness were randomly distributed into 3 experimental groups: 0.5 mm (group 1), 1.0 mm (group 2) and 1.5 mm (group 3). The restorations were cemented onto human molars with a self-adhesive resin cement. The specimens were loaded until fracture; the fracture resistance and mode of failure were recorded. The data were statistically analyzed with the one-way ANOVA followed by the Fisher’s Exact test with Bonferroni’s correction (p=0.05). Results: The fracture resistance values of all the specimens exceeded the maximum physiological occlusal loads in molar regions. The highest fracture resistance was noticed in 1.0 mm-thick crowns. Ultrathin restorations (group 1) proved to be statistically less resistant to fracture than those of the other experimental groups (p<0.05). The crowns were mainly interested by unrestorable fractures. Conclusions: The occlusal thickness of CAD-CAM monolithic lithium disilicate crowns influences either the fracture resistance and the mode of failure of the restorations; the occlusal thickness of such restorations can be reduced up to a lower bound of 1.0 mm in order to keep sufficient strength to withstand occlusal loads; CAD-CAM monolithic lithium disilicate crowns showed sufficient fracture resistance to be used in molar regions but not in an ultrathin configuration (0.5 mm).