Discussion
fracture, hardness
CAVITY
This in vitro study investigated the behavior of 3 types of CAD-CAM materials for fabricating inlay restorations regarding the marginal integrity before and after thermocycling. The first null hypothesis was rejected as there was significant difference between marginal gaps in different inlays groups. The second null hypothesis was rejected since the marginal gaps changed in all groups after thermocycling.To possibly simulate the clinical procedures, natural teeth with similar dimensions were selected and the preparation was standardized by using a dental surveyor. Digital impression technique started to replace conventional techniques that allow direct scanning of prepared teeth and eliminate clinical errors. For this purpose, CAD-CAM scanner was used in the present study for precise procedure [Direct evaluation through a SEM was the selected method for marginal fitting evaluation, which allowed obtaining standardized measurements by positioning the restored teeth in a base [Additionally, the measurements of marginal gaps were decided to be done after cementation of inlay restorations to simulate the clinical conditions [Marginal gaps are affected by multiple factors such as the design of preparation [Machinability of blocks differs from one material to another according to the ease of milling of each material, and that depends on the brittleness index, which is affected by the fracture toughness and hardness, microstructure of the material, and chipping factor [Results of the present study revealed that smallest mean marginal gaps values were for group BR (80.3 μm) followed by group EX (111.3 μm) and group SF (133.9 μm), before thermocycling. All marginal gaps were within acceptable clinical marginal gaps as demonstrated in previous studies between 75 and 160 μm [
However, marginal gaps of all groups after thermocycling had been changed. Where the change in marginal gaps was statistically significantly lower in group EX compared to group SF (mean difference=-1.768, 
There were some limitations in this study; firstly, it was an in vitro study, which is different from a clinical study situation, where the scanning of prepared teeth would be affected by saliva, and limited accessability for the scanner inside the oral cavity which make scanning less precise. Secondly, the study didn’t evaluate the internal adaptation of restorations. Lastly, the potential accuracy of milling machine may not be the best.