## 2 Materials and Methods  
The mechanical behavior of materials and applications in Dentistry have been extensively investigated by means of CAD-FEM (Computer Aided Design and Finite Element Method) methods [10; 27; 28]. Therefore, the present study has been carried out using previously reported models with properly known elastic response.  
Starting from these 3D CAD models, six models simulating six posterior restorations consisting in class I (occlusal) and class II MO (mesio-occlusal) lower molar cavities (Figure 1) were exported to the analysis software to investigate the influence on the stress distribution of resin composite (shrinking) and glass ionomer cement (no shrinking) material combinations in a bi-layer or bulk technique to substitute enamel and dentine [10; 27; 28]. These six models were nominated as A, B, C, D, E and F and were indicated in the Table 1. Their geometric features are exposed in Figure 1.  
According to the modelling process from previous studies [27,28], the sound tooth lower molar model was got by means of reverse engineering techniques. The shapes and volumes of dentin and enamel were digitized using a micro-CT scanner (Bruker microCT, Kontich, Belgium). Images and data were processed via InVesalius 3.1.1 software and 3-D tessellated surfaces were elaborated. Cross-section curves were generated, and the parametric 3D CAD model was created using loft surfaces in (Rhinoceros(r)4.0 Robert McNeel & Associates, Seattle, WA, USA) starting from 3-D tessellated surfaces. Boolean seperations guaranteed the interfacial dentin and enamel boundaries congruence. The tooth model was cut 2.5 mm below the cervical area to obtain the final model. The buccal-palatal and mesio-distal sizes were 10.60 mm and 12.36 mm respectively and the enamel thickness was of about 1.5 mm [10,26,27].  
A class I cavity 4.0 mm deep was modelled for the restored models A, B and C obtained via Boolean operations between the cavity, enamel and dentin surfaces [27]. Class I pulp and lateral walls had rounded modelled angles. A class II MO cavity 4.0 mm deep in the central and 5.5 mm in the box. A bevel of 0.5 mm was shaped all along the enamel [8]. The margins of class II MO were modelled, and the restored models D, E and F were obtained  
\begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**Models**} & **Bonding Layer** & **Material Lower Layer** & **Material Upper Layer** & **Single Material Layer** \\ \hline \multirow{3}{*}{A and D} & \multirow{3}{*}{10 μm thick} & & & Bulk fill composite (4 mm in-between cavity and 5.5 mm at the box) \\  & & & & the box) \\ \hline \multirow{3}{*}{B and E} & \multirow{3}{*}{10 μm thick} & Flowable composite (1.5 and 3 mm thick in-between cavity and box) & Bulk fill composite (2.5 mm thick) & \multirow{3}{*}{-} \\  & & & & \\ \hline \multirow{3}{*}{C and F} & \multirow{3}{*}{10 μm thick} & Glass ionomer (GIC) (1.5 and 3 mm thick in-between cavity and box) & Bulk fill composite (2.5 mm thick) & \multirow{3}{*}{-} \\  & & & & \\ \hline \end{tabular}
\end{table}
Table 1: The groupings restorative material.  
Figure 1: The geometric features of the analyzed model-cavity according to the filling.  
via Boolean actions between the cavity, dentine and enamel surfaces. The class II MO lateral and internal surfaces had rounded modelled angles.  
Masticatory function has been reported as depending on the contact between tooth surface and food bolus. In this study it was simulated by modelling food bolus geometry on the occlusal surface [28]. The dental model was positioned in a coordinate system, where X- and Y-axes indicated the bucco-lingual and meso-distal directions, respectively. The Z-axis was vertically oriented (Figure 2).