## 4 Discussion  
Esthetic treatment of anterior teeth continues to be a challenge in clinical practice. At present, porcelain laminate veneers are steadily becoming more popular as a successful technique for restoring discolored, worn, malformed, or fractured teeth [36]. However, the geometry of the preparation for laminate veneers is a controversial topic. Although several studies have examined the influence of prepared and unprepared veneer designs on the longevity and survival of veneer restorations, data concerning the design and thickness of the laminate veneer preparation are limited [7]. The present study attempted to study the effect of the proposed preparation design as a conservative treatment option that aims to preserve both gingival health and sound tooth structure and compared it with previously used techniques. It used the 3D finite element method to investigate the amount and distribution of stresses under different masticatory and thermal loading conditions in order to understand the sites where stresses and damage to the veneer restorations and tooth structures occur in clinical observations. In the mechanical loading assessments, in all of the studied preparation designs, the maximum level of the stress distribution was concentrated on the veneer surface and it progressively decreased toward the cement layer up to the dental tooth structure. This is because zirconia-reinforced lithium silicate is known to have a higher modulus of elasticity than both the resin cement and the tooth structure, indicating that ceramics act as a barrier during functional movements by absorbing most stresses and protecting underlying dental tissues; this finding is in agreement with previous studies [7; 10]. In the evaluation of stress values in relation to the three layers that were analyzed (veneer, resin cement, and tooth structures), the conventional design showed the highest von Mises stresses at the level of the incision region, followed by the non-preparation design, which showed moderate stresses distributed in the cervical areas, while the proposed technique showed the lowest stresses at the interface between the three layers condensed in the incision and cervical areas. The resulting stresses were higher at 125\({}^{\circ}\) than at 60\({}^{\circ}\), which agreed with the findings of previous studies [7; 16], indicating that protrusive movement causes higher stresses than intercuspal movement. On the basis of the above outcomes, all models may result in failure in the form of fracture, chipping, or debonding at the incision margin and cervical area, which agrees with a study performed by [PERSON] et al., who concluded that most veneer failures occurred in the incision and cervical regions [13]. Thus, the probability of failures will increase with an increase in the angulation from 60\({}^{\circ}\) to 125\({}^{\circ}\) due to increases in the tensile stresses, in agreement with other studies [10; 37] in which the fracture potential of ceramic material was shown to fail in areas of tension because of the low tensile strength of the material. Based on the mechanical results, the proposed preparation design can be considered as an acceptable alternative approach for restoring anterior defects with laminate veneer since it shows the lowest von Mises values in veneer and tooth structures, a more uniform stress distribution in cement and tooth tissues, and a highly promising clinical success rate. The findings showing that the preparation design has a significant effect on the treatment success of veneer restorations are in agreement with the results of a study by [PERSON] et al. [18]. Thermal heat transfer in human teeth is a common process in both daily life and clinical dentistry. Accordingly, transient thermal FE simulation was performed to assess the effect of preparation geometries, temperature variations, and the resultant stresses on the structural changes in dental hard tissues and pathological changes in pulpal soft tissue.