## 4 Discussion
The final mechanical behavior of class II DO restorations has been studied using a 3D finite element analysis, simulating the effects of different cavity margin angles and occlusal loading conditions with foods of varying stiffnesses [39; 40]. This study utilized the finite element analysis method to demonstrate the stress distribution of class II DO amalgam restorations under varying resin thicknesses. However, the polymerization shrinkage of the resin layer was not considered. Maximum principal stress was used as the index to evaluate the materials' stress fracture susceptibility [8; 28; 31]. Moreover, this method aids in the determination of the maximum tensile stress generated by different loading conditions in various oral tissues, structures, and materials [17; 30]. The location of stress can vary significantly due to changes in geometry and the materials used at the interface, while the amount of stress is influenced by the remaining tooth structure and the interface between the tooth and the restoration material [9]. While a healthy tooth is functional, dental tissues are mainly under significant pressure and stress. Only a few parts of the tissues are subject to tensile stress [1], which can cause a concentration of stress in dental material properties [8]. In a previous study, it was stated that enamel absorbs most of the occlusive force and, therefore, shows stresses higher than those absorbed in dentin because enamel is harder than dentin [1]. This study found that enamel had higher localized stresses, whereas dentin had lower and more uniformly distributed stresses. Because the elastic modulus of enamel and materials is not equivalent, stress concentrations result [30]. If the repair material has a higher elasticity modulus, destruction is less noticeable [31]. Enamel, which has the highest adhesive force and elasticity modulus among dental tissues, tends to accumulate more stress while present, and its integrity has a direct impact on restoration longevity [41; 42]. A previous study emphasized that the aim of dental treatments should be to restore the function of enamel and dentin tissues. Enamel has an important mechanical role in the grinding of food. Dentin absorbs the mechanical forces from the enamel, so mechanical properties are important [43]. In our study, the highest stress accumulation was found in enamel tissues. The cement thickness used in restorations is controversial. A greater adhesive cement thickness can reduce the support from the tooth and cause ceramic breakage, while an adhesive cement thickness with excessively thin layers can adversely affect the longevity of the restoration [44; 45]. In previous studies, 70 \(\upmu\)m resin cement and 100 \(\upmu\)m resin-modified glass ionomer cement [46], 120 \(\upmu\)m cement [42], 100 \(\upmu\)m resin cement [47], 50 \(\upmu\)m cement [48], and 60 \(\upmu\)m cements [49] were investigated. Long-term performance in resin cements is said to be optimal with an internal gap size of 50-100 \(\upmu\)m [50]. It has been recommended that the interfacial gap size for resin cement should not exceed 100 \(\upmu\)m [51; 52]. Moreover, an increased film thickness of resin cement can have an impact on the fitting passivity of the restoration, leading to the thickness of the cement thickness being greater than ideal. Considering that the cavity boundaries terminate in dentin, it was believed that the thickness of the resin cement could have an effect on the final outcome. Therefore, we conducted a study using resin cements with thicknesses of 50, 100, and 150 \(\upmu\)m. In particular, as the setting process progresses, the Poisson's ratio should decrease significantly as it transforms into a brittle solid [37]. When cement was applied in varying thicknesses and the Poisson's ratio was reduced, the stress accumulation in the enamel and restoration decreased, while the stress in the dentin increased. This indicates that the thickness of the resin cement used may cause differences in stress levels across different structures.