# Assessment of Contact Pressures between a Mandibular Overdenture and the Prostodontic Area
## 3 Results and Discussion  
The stress concentration in gingiva and nonuniform displacements of the overdenture are observed during loading. Maximal/minimal values are found near the mounting area of the implants and in the posterior zone (ramus mandibular side), which indicates the point-like support (Figure 7). This also manifests itself in the movement of the prosthesis, which tends to cradle/swing on the dental beam and torsional modes, especially when posterior teeth are excited (Figure 8). \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Materials (Parts)**} & \multicolumn{2}{c}{**Parameters**} & \multirow{2}{*}{**References**} \\ \cline{2-2}  & **Young Moduli (GPa)** & & **Poisson’s Ratio (–)** & \\ \hline acrylic resin (prosthesis) & 2.8 & 0.28 & [11] \\ \hline Ti6 Al4V (implants) & 103.4 & 0.35 & [11] \\ \hline stainless steel (bar and & & & \\ implant pillar) & \(110.0\) & 0.31 & [10; 23] \\ \hline polypropylene (POM & 1.5 & & \\ matrices) & 2.5 & 0.45 & [11; 24; 25] \\ \hline INOX 316 Ti (housing of the & & & \\ clips) & 200.0 & 0.30 & [26] \\ \hline Mucosa (dentulous & & & \\ mandible) & 0.001 & 0.45 & [9; 27; 28] \\ \hline cortical bone (dententulous & & & \\ mandible) & 13.7 & 0.30 & [11] \\ \hline trabecular bone & & & \\ (dentulous mandible) & 1.37 & 0.30 & [11] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mechanical properties of the materials. Figure 6: Finite element method model, three load cases, and boundary conditions. Forcing the canine causes a torsional move of the denture in the direction of the working side of the mouth. The plane of the mandibular teeth tilts to one side and results in a gap on the other side. Analogical behavior is observed for molar and incisor bites, but rotations and, thus, occlusion misalignment are the largest for canine excitation. An increase in the bite force causes the progressive degradation of occlusion and more concentrated pressure on the mandible system. In addition, the flexibility of the beam prosthesis joint is only slightly sensitive to the retention level of the applied matrices. This leads to a significant concentration of stress at the anterior (alveolar ridge) and posterior mandible (retromolar pad and linea obliquua) due to biting and chewing (see Figure 8). The stress distribution under the prosthesis mostly depends on the region loaded onto it. The biting force applied on the foreteeth causes higher contact pressures on the front of the mandible tissues, and their rear zone remains almost unloaded. Such response results from the fact that the force acts along the lines which cross the support location, and only a little swinging on the beam is possible. This indicates good stabilization for the incisor load case, and forces are carried directly by the implants. On the other hand, stress concentration around the implants may cause bone loss (atrophy) in that area. The maximum contact pressure is noted when loads are associated with canines. The critical case for the lower posterior is chewing performed by the molars. The pressure is distributed over a larger area, and hence it reaches lower stress values. Instead, the stress distribution is still nonuniform, and the existence of a mismatch between the prosthesis and mandible can be seen (Figure 9). The maximum stress occurs near the retromolar pad. This region has a significantly different bone structure and higher density [31]; thus, in order to avoid its atrophy, it is vital to ensure that the prosthesis has been well fitted. Figure 8: Huber–Mises stress (**a**) and contact stress distribution (**b**) on the cortical bone and gingva, respectively (contour plots for the canine load case and matrix of 3 rd level of retention). Figure 7: Displacements of the prosthesis along Y (**a**) and Z axis (**b**) during canine loading and simplified three-point pin support (**c**). Figure 10 and Table 2 shows horizontal forces, which are carried from the prosthesis to the mandible.