Comparison of fracture strengths of onlay restorations applied with different materials on molar teeth after occlusal loading

The aim of this in vitro study is to compare the fracture strengths and evaluate the fracture types of onlay restorations prepared either directly with composite resin or indirectly with different ceramic blocks, are applied at different thickness on the core structure reinforced with fiber-reinforced composire resin for the crowns of mandibular molars with excessive coronal substance loss, after a 2-year mechanical aging process. In our study, MOD cavities were prepared on 120 intact mandibular molars to accommode lingual cusps, and 15 teeth were selected as the group and no treatment was applied to these teeth. Eight study groups were formed, each with 15 specimens, including three groups in which short fiber reinforced composite resin (SRFC) core structure was prepared with different thickness (0,1,4,5 mm) on the teeth, and bi-layer biomimetic restorations were created by using composite resin (CR) on top of it. One group was only restored with SFRC and the occlusal surface of the restoration was stained with flowable composite. Two of the four remaining indirect restoration groups were applied with 2 mm thick SFRC core structure, while the other two were not. Onlay restorations to be applied to the teeth were completed using different CAD/CAM materials (Cerasmart 270 (GC, Tokyo, JAPAN) and Initial LiSi Block (GC, Tokyo, JAPAN)), and adhesive cementations were performed. The completed restorations were subjected to mechanical aging to simulate 2-year clinical use under 150 Newton occlusal load, 1.53 Hz frequency, and 500,000 cycles in a chewing simulator. Static fracture testing was performed using a universal testing machine on the specimens that completed the mechanical aging process until the tooth broke. Fracture types were analyzed visually. Fracture strength data were analyzed using one-way analysis of variance ANOVA (p=0.05) and Duncan's multiple comparison tests. According to the results of our study, the group that showed the highest fracture strength was the one restored only with SFRC with a value of 2647 Newton. There was no statistically significant difference in fracture strengths between the direct and indirect restoration groups without core structures. The presence of SFRC core structure increased the fracture strength of direct composite and indirect Cerasmart 270 restorations, but it was concluded that the fracture strength of indirect Initial LiSi Block restoration was lower when the SFRC core structure was applied than when it was not applied.