# Influence of Cavity Geometry on the Fracture Strength of Dental Restorations: Finite Element Study
## 4 Discussion  
The present study focused on evaluating the stress patterns associated with the cavity design of the first maxillary premolar restored with indirect ceramic only or inlay geometries when subjected to oblique loading. Two models were compared, one featuring  
Figure 8: Smoothed stress distributions for the “bed” adhesive resin cement. (**a**) Inlay restoration (left side) and only restoration (right side). (**b**) Box plot of the distributions for the two “bed” geometries. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & **Aver.** & **SD** & **Min.** & **1 st Quart.** & **Median** & **3 rd Quart.** & **Max.** \\ \hline Von [PERSON] & _Inlay_ & 3.67 & 7.37 & 0.110 & 1.27 & 2.62 & 3.83 & 211.39 \\ (MPa) & _Only_ & 3.69 & 7.30 & 0.106 & 1.37 & 2.64 & 3.85 & 204.71 \\ \hline \((Inlay-Ontay)/Ontay\) (\%) & \(-0.53\) & 1.02 & 3.78 & \(-6.89\) & \(-0.76\) & \(-0.33\) & 3.26 \\ \hline Maximum & _Inlay_ & 2.05 & 4.12 & 0.06 & 0.71 & 1.44 & 2.14 & 122.04 \\ Shear (MPa) & _Only_ & 2.06 & 4.08 & 0.06 & 0.76 & 1.45 & 2.14 & 118.15 \\ \hline \((Inlay-Ontay)/Ontay\) (\%) & \(-0.53\) & 1.01 & 0.00 & \(-6.82\) & \(-0.77\) & \(-0.21\) & 3.29 \\ \hline \hline \end{tabular}
\end{table} 
Table 11: Statistical description of the stress distributions for the “bed” geometry at the adhesive resin cement. an inlay restoration and the other an onlay restoration, with both restorations having the same geometry for the cervical cavity. Though some finite element studies addressing the biomechanical behavior of the cavity design on teeth restored with inlay or onlay ceramics have been published [19; 22; 41; 42; 43; 44], the work presented here is, to our knowledge, the first nonlinear analysis presenting the results of damage analysis. The study's results seem to indicate that the highest fracture resistance of one restored tooth should be achieved with the onlay cavity restorations. This idea is supported by the nonlinear analysis results, which showed a variation in the maximum load supported by the inlay that was about 17.9% smaller than that supported by the onlay restoration. The similarity of these results with those experimentally reported by [PERSON] et al. [45] and [PERSON] et al. [2] contributes to the proximity of the FEM results and those reported in experimental studies. Regarding the experimental results, the assumption that the preparation design influences the fracture strength of ceramic restorations was also valid, and the onlay restorations were significantly stronger than the inlay restorations. Nevertheless, these results contradict other experiments wherein the load cell placement was directly axial to the tooth [3; 46; 47; 48]. In the [PERSON] et al. [45] experiments, the samples were mounted in a metal base at 15\({}^{\circ}\) with the load cell, which was placed at the fossa of the supporting buccal cusp. In the work of [PERSON] et al. [2], the load cell was oriented on the center of the occlusal surface at 30\({}^{\circ}\) relative to the tooth's long axis. An oblique force relative to the tooth axis will produce normal, shear and bending moment efforts. This might explain why the inlay restorations presented lower fracture strengths under oblique solicitations than the onlay restorations. Nevertheless, premolar teeth are subjected to functional and parafunctional forces of different magnitudes and directions [13; 19], and the generality of studies agrees that oblique loads are responsible for higher stress levels than compressive loads. In this numerical study, an oblique load of 500 N was applied to both tooth cusps, mimicking the lateral force that is directed buccolingually during masticatory movements [42]. Even though the magnitude of the force is about twice the biting force at occlusion during chewing [49], this value allows for the identification of higher levels of stress and, therefore, simulates extreme conditions that might lead to tooth damage. Moreover, [PERSON] et al. [50] measured a maximum biting force of 580 N in healthy people's posterior areas.