Influence of thermomechanical fatigue on the fracture strength of CAD-CAM-fabricated occlusal veneers

Statement of problem. With the development of new computer-aided design and computer-aided manufacturing (CAD-CAM) restorative dental materials, limited data regarding their survival rate and fracture strength are available when they are used as occlusal veneers. Therefore, these materials should be evaluated under conditions similar to those of the oral environment before being recommended for clinical use. Purpose. To evaluate the influence of thermomechanical fatigue loading on the fracture strength of minimally invasive occlusal veneer restorations fabricated from different CAD-CAM materials and bonded to human maxillary premolars using self-etchnig bonding technique. Material and methods. Sixty-four CAD-CAM occlusal veneer restorations were fabricated from group LD (lithium disilicate [e.max CAD]), LS (zirconia-reinforced lithium silicate [Vita Suprinity]), PI (polymer-infiltrated ceramic [Vita Enamic]), and PM (polymethylmethacrylate [Telio CAD]). The occlusal veneers were luted to enamel (n=16) using a self-etching primer (Multilink Primer A/B) and a luting composite resin (Multilink Automix). Half of the specimens of each group (n=8) were randomly selected and subjected to thermomechanical fatigue loading in a masticatory simulator (1.2 million cycles at 98 N with 5 degrees C-55 degrees C thermocycling). All specimens were quasistatically loaded until fracture. The statistical analysis was made using the Kruskal-Wallis and Mann-Whitney U tests (alpha=.05). Results. According to the Kaplan-Meier analysis after the thermomechanical fatigue of the 4 groups, the cumulative survival rate was as follows: group LD, 50% group LS, 62.5% group PI, 37.5%; and group PM, 50%. Although some of the surviving specimens exhibited microcracking, their integrity or bonding to teeth was not affected. Thermomechanical fatigue significantly reduced the fracture strength of group PI (P=.047) and group PM (P=.025). Without thermomechanical fatigue, group PM showed significantly higher fracture strength than group LS (P=.015). Conclusions. In general, thermomechanical fatigue decreased the survival rate and fracture strength in all test groups.