# Influence of Cavity Geometry on the Fracture Strength of Dental Restorations: Finite Element Study
## 4 Discussion
Besides the tooth loading level, the restorative procedure has been identified as another parameter influencing the stress and strain levels of restored teeth [19; 20; 35; 38; 41; 43; 51]. All researchers reported that teeth with onlay restorations appear more resistant to fracture than those with inlay restorations. In our study, the von Mises stress distributions for both restorations' dentin and their corresponding statistical descriptions were very similar. The main differences appeared only in the maximum values; the inlay restoration showed a maximum von Mises stress 5% higher than the onlay restoration. Furthermore, the paired samples \(t\)-test performed over the two dentin \"bed\" geometries showed statistically significant differences between the onlay and inlay results for all stress components (i.e., for von Mises (vM), all three principal stresses and the maximum shear stress (MS)). Similar behavior was also observed in the results of the paired samples \(t\)-test for both cohesive zone models of the \"bed\" dentin, with statistically significant differences between the onlay and inlay results for all stress components except for the second principal stresses (\(p=0.683\)). The other tooth component that plays an important role in the longevity of tooth restoration is the enamel. The literature is rich in reports showing that secondary caries and tooth fractures are the two main factors for teeth failures [52]. Because the activation of dental caries starts with enamel demineralization [53; 54], it is important to avoid stress hot spots on the enamel walls that remain in the restoration. Some works have been reporting that enamel is composed of prisms that are often perpendicular to the enamel-dentin junction [12; 55] and can be fractured if the forces are not parallel to this direction. Hence, although the results reported in Section 3.3 showed that the vM stress distributions were quite similar for both enamel geometries, the inlay enamel geometry presented the highest maximum value. This result could be related to the fracture pattern of the pressable glass ceramic inlays group reported by [PERSON] et al. [3], where the complete fracture of specimens was attributed to the alteration of the enamel point where the stress concentration appeared. They also reported that in cyclic fatigue tests of the inlay restorations, the crack initiation started at the cement interface. This behavior can also be associated with the results presented in Figure 5, showing the vM stress distributions and their corresponding histograms. It is possible to conclude that the onlay adhesive is less loaded than the inlay restoration adhesive and, hence, is less prone to crack initiations. The results presented in Figure 4 also show that cusp reduction could be suggested to the lower stress values in the restorative materials. This work addressed some challenges in the definition of restoration-adhesive-dentin interfaces, allowing the inclusion of CZM to perform strength prediction of the adhesive joint. The first challenge is related to the structured mesh necessary to generate bidimensional cohesive interface elements, and the second is the mechanical properties needed to define the traction-separation laws, critical values of toughness in tension and shear, as well as the cohesive strengths in tension and shear. Hence, the definition of values different from those presented here might lead to different CZM results. Moreover, cohesive finite elements should be extended to all surfaces of the adhesive interface to ensure the appearance of possible damage. Another limitation of this work is related to both cavities' dimensions; namely, the palatal cusp of the onlay restoration needs more restorative material to assure the manufacturer's recommendations. Nevertheless, increasing the restorative palatal cusp dimensions contributes to improving the fracture resistance of onlay restorations [43]. Therefore, accounting for that variation can amplify the magnitude of the results presented here.