## 4 Discussion
[14] also confirmed that MOD cavities preparations and endodontic treatment increased the stress concentration within the dental structure, mainly due to the greater tissue removal. To extrapolate the data obtained in this in vitro study with those clinically observed in the oral environment, the samples were exposed to cyclical masticatory loads with thermocycling to analyze their fracture resistance. [PERSON] et al. [29] observed that direct restorations can distribute the functional stress through the interface of the restorative material and the tooth in a better way and also have the potential to support the weaker. However, these types of restorations tend to suffer shrinkage during polymerization causing cusp deflections [4; 9; 10; 12; 29]. To overcome these inconveniences, glass fibers were added to the resins changing their behavior by altering the elastic modulus of the material and therefore modifying the distribution of tensions to the walls of the tooth cavity. FRC has been considered as a new alternative for the restoration of endodontically treated tooth. In addition, placing composites with fibers in the cavity, allows a better distribution and dissipation of tension in the structure, which decreases and homogenizes the transmission of tension to the support teeth [29; 30]. [PERSON] et al. [30] indicated that fiber reinforced composites do not significantly increase the fracture resistance of endodontically treated premolar teeth. This statement is not in line with our findings since; in the present study the use of FRC resin core showed significant results in terms of resistance of the endodontically treated tooth with MOD cavities, when used as a final restoration or combined with a fiber post build-up restoration. It obtained better mean fracture resistance values (3040 N) of the endodontically treated premolar teeth directly restored with CR (2560 N). [PERSON] et al. [30] also stated in their study that the orientation of the glass fibers placed within the reinforced composites in MOD cavities showed greater fracture toughness. This statement is proven in our study where FRC resin core has a better resistance due to the intrinsic placement of its fibers and its orientation. In endodontic teeth with substantial loss of tissue, intraradicular posts are advisable to provide enough retention of restorative materials [19]. [PERSON] et al. [31] in their study found that resistance to fracture was significantly reduced in upper jaw premolars treated endodontically with direct restorations without fiber posts. This may be because they did not previously fatigue the samples and used a higher crosshead speed (1 mm/min). The thermal and mechanical cycling fatigue device and the universal static load testing machine used in this in vitro study tried to simulate the oral environment [32] and have been described as the most effective procedure of evaluating the fracture resistance of dental restorations [33]. It also fulfils with the requirements and recommended established in ISO 6872:2015 [34] and have been management with the cycling fatigue parameters established in other similar trials [35]. The highest mean fracture resistance value and best biomechanical behavior was observed at the FRC post-FRC resin core study group (\(3620\pm 470\)). Weibull distribution analyzes the material's variable failure rate. Weibull statistical analysis expresses the probability of failure of restore materials and allow a greater understanding of a material's biomechanical behavior [36]. Some potential limitations of the present research was that is a destructive method and many studies have shown that the use of finite element stress analysis to evaluate stress in endodontically treated teeth is the ideal method for assessing post-core application, compared to several other methods of stress analysis [16; 17; 37; 38; 39]. Nonetheless, further in vitro non-destructive studies should be conducted in conjunction with clinical studies.