## 4 Discussion
When V2A or V2B were used, the resin cement absorbed more stress and transmitted less stress to the enamel and restoration. However, when V1C was used, the resin cement absorbed more stress and transmitted higher stress to the enamel and restoration. In addition, there was less stress accumulation in the restoration when V2 was used. This indicates that both the resin cement and dental tissues have greater stress when using the resin cement at 150 \(\upmu\)m thickness. Increasing the cement thickness led to a higher amount of stress in the enamel and dentin while reducing the stress accumulation in the restoration. It was also found that the stress accumulation in the cement was lower than that in the other groups with a thickness of 100 \(\upmu\)m. Resin cement, which has a lower elastic modulus than dentin, increases the stress in dentin to support the material under further distortion [53]. At a resin thickness of 100 \(\upmu\)m, the stress accumulation in the dentin was reduced, and the dentin experienced greater stress. However, when the resin thickness increased to 150 \(\upmu\)m, the stress in the resin and dentin increased, while the stress in the restoration decreased. The thickness of the cement can lead to variations in stress levels across different structures under the restoration. When using a rigid material, failure is expected to occur in the adhesive interface layer [54]. It is crucial to avoid premature contact and changes that may lead to the premature fatigue of the restoration when using a thinner cement thickness [55]. However, a greater cement thickness may lead to more defects, a poorer micromechanical compatibility, and a higher water absorption than the use of thinner layers. It can also cause a deterioration in the adhesive strength to the substrate [56; 57]. However, a thick cement layer can provide a relatively flexible, stress-absorbing layer between the restoration material and the dentin, resulting in low interfacial stress [58; 59]. Microleakage remains a concern for class II cavities where the gum edges are in contact with the dentin [17]. A previous study [60] showed that the bonding cement can influence microleakage, with resin cement showing a lower microleakage than other types. Therefore, in this study, resin cement was selected as the resin interface material under the amalgam. We thought that the thickness of the resin cement would affect the result since the cavity boundaries end in the dentin tissue. However, the mechanical properties of the resin cement were considered for simulation within the limitations of the method. Protecting the marginal crest of the first molar tooth is critical in increasing the strength of wrinkle resistance [8]. Furthermore, the oblique crest in the first molar tooth is an essential anatomical structure that plays a crucial role in the chewing process, particularly in resisting diagonal forces [53]. Our observations revealed an increase in the stress concentration in the axial inner walls of the class II cavity and the oblique crest of the molar tooth. This concentration of stress indicates that it may contribute to the appearance of marginal leakage and secondary caries in the tooth. Additionally, high stresses were detected in the combination of enamel and dentin in the class II cavity [8], raising concerns about possible enamel prism deterioration at this location due to stress. The excessive stress induced by the restorative material in the enamel may be explained by the differing mechanical properties of the enamel and the material [8; 9]. A previous study reported reduced stress values in teeth where materials with elastic modulus values closest to those of enamel were used [24]. In the current study, changing the cements' Poisson's ratios and thicknesses led to the highest stress accumulation in the enamel. However, materials with a high elastic modulus were found to increase the stress in the cement layer, thereby increasing the risk of separation.