## 4 Discussion
In this study, we wanted to evaluate the manufacturer's design of preparation for Vita(r)Enamic crowns and their stress distribution in the posterior region while loading. For a better insight into stresses generated in the crown-tooth complex, it is of utmost importance to consider several other factors such as milling units used, the minimal amount of crown produced without any flaws, the gap interface between the crown margin and the tooth, and the elastic modulus of different types of cement used. In this study, we faced challenges with modeling due to the segmentation of different layers within a very close thresholding range. The cement layer was uneven and did not follow the uniform pattern throughout. That said, a 'growing a threshold region' and 'boolean operation' were not used but could have helped in better-computed models. Moreover, this could be explained in terms of more cement was present at the occlusal interface just due to the fact of creating a 50 um space within the CAD-CAM design. It is also of importance to note that the CAD-CAM does not have the ability to mill the internal anatomy to the desired preparation and due to this there is sufficiently more gap present on the occlusal surface of anatomical design preparations compared to the marginal gap that can be created according to the desired outcomes. Therefore, the CAD-CAM milling burs lack the proper internal adaptation of the occlusal surface, leaving far more space for the cement [7]. The axial walls hardly had any cement attached since milling was done under a close marginal fit of the crowns. This aspect was important to achieve a good fit of the crowns and to avoid thin or weak crowns at the margins. For the prepared tooth we had enough thickness at the margins, but a close fit was chosen to avoid the possibility of bulky crowns. Therefore, due to CAD-CAM milling deficiencies, we could not achieve uniform cementation as would have been expected. Moreover, there might be some preparation discrepancies along with cuspal tip interferences for cementation as it followed the occlusal anatomy. Moreover, a possibility of cement heterogeneity with a thick consistency and in fewer amounts would have led to an uneven spread throughout the margins but more along the occlusal surface. [PERSON] et al. [30] also showed a flat design with a more homogenous cement thickness all around rather than an anatomical design where more cement was found at the occlusal surface. More stress was seen in the crown and has also been reported by [PERSON] et al. [30] and [PERSON] et al. [39] for the anatomic design. Dentine showed stress generation along the coronal 1/3 of the root surface, which is where most stresses are dissipated even in loading/masticatory/chewing conditions in the mouth (also reported by [PERSON] et al.) [40]. Within the cement layer, more stress was generated in the occlusal surface rather than a uniform spread around the areas of the defect since the thick cement layer had occlusal distributed the load. Lower elastic modulus also plays an important role in the dissipation of stresses, but this was not the case in this study probably due to the uneven distribution of the cement. Some limitations exist in this FE study, such as nonlinear simulation of periodontal ligament properties, plastic and viscoelastic behaviors in materials, and tooth-to-tooth contact analyses. However, there are difficulties such as the dynamic behavior of the periodontal ligament, which is an aspect to be considered. Yet due to its complex structure, the exact mechanical properties of PDL are considered to be poorly understood. Importantly, the different aspects like viscoelastic behavior and cyclic loading have not been performed, which can be done to compare results in future studies.