# Assessment of Contact Pressures between a Mandibular Overdenture and the Prostodontic Area
## 3 Results and Discussion
For the canine and molar load cases, these forces have the same order of values in both directions (coronal and sagittal), but for the load on the incisor, these forces are different in terms of directions and values. The evaluation of the prosthetic system is performed for typical types of matrices with a few different characteristic retention levels. The most commonly used matrices have Young's modulus in the range of 2.3-2.8 GPa. The matrix with stiffness equal to 2.5 GPa has a medium retention level, ensuring the average mounting force. Additionally, two extreme matrices with stiffness out of that range are chosen to indicate their effects on how the prosthesis works. The variation of matrix reten \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Horizontal Force** & **Canine** & **Incisor** & **Molar** \\ \hline H1 & +0.095 & \(-\)0.037 & \(-\)0.071 \\ X axis [N] & \(-\)0.079 & +0.111 & \(-\)0.097 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The values of horizontal forces carried from the prosthesis to the implant system and the dental bar. Figure 10: Horizontal forces carried from the prosthesis to the implant system and the dental bar. Figure 9: Contact stress distribution over gingiva after the application of the matrices of 2.5 MPa Young’s modulus and for three load cases: biting force on (**a**) incisor, (**b**) canine, and (**c**) molar. influence, which was expected in simulations. No correlation between retention levels and prosthesis stability was found. The performed analysis shows an insignificant role of matrices in carrying loading from the prosthesis, its movement, and, finally, its stress distribution on the gingiva. Following the use of a stiffer POM-3 matrix, the extreme changes of measurements were small, i.e., the increment of stress = \(-0.0163\) MPa, and the increment of displacements = \(+0.0082\) mm (see Figures 11 and 12). These results confirm that prosthesis load bearing and support are independent of the types of POM material. However, it is worth mentioning that the thickness of the  
towards prosthodontic area tissues, minimizing axial overload [34]. However, excessive ability to deform under strain may make the denture support in other locations when more significant strain is applied. The force with which the matrix is retained is of less considerable importance, as a change in its stiffness does not affect the value of contact pressures between the denture and its bearing area, which was confirmed in the studies. Many authors emphasize a significant loss in overdentures retention due to matrices' wear and the need for their replacement [35]. However, the studies by [PERSON] et al. (2009) show that in many precision systems, an initial drop in overdenture retention is followed by its stabilization [36]. [PERSON]'s assessment of polymer matrices retention is favorable, though he also noted a decline in their retention from 90% initially to 80% after 6 months of wearing. However, in his in vivo studies, [PERSON] did not notice a decreased retention after three months of wearing overdentures, irrespective of a precision attachment applied (bars, ball attachments, magnets) [37]. [PERSON] et al. (2013) extensively covered the issue of mandibular implant prosthesis retention [38]. His studies concerned various bar attachments with single matrices as well as distal cantilever bars and three matrices. Metal and polymer matrices were used, and vertical and anterior-posterior retention was studied. The highest retention was observed for a Dolder bar with distal cantilever bars and three metal matrices; however, a decreased retention was noted in all types of attachments. This research proved that the level of implant prosthesis retention depended on the number, type, and distribution of matrices. [PERSON] et al. (2008) [39] noticed that retention loss is also related to the location of implants. The greater the distance between implants, the smaller the retention loss with yellow or red matrices.