# Comparison of CAD/CAM and Conventional Denture Base Resins: A Systematic Review
## 4 Discussion
### Retention of Denture Bases  
The clinical performance of CDs is partly determined by their retention within the oral cavity. Lack of retention and instability are two of the main complaints from people wearing CDs. Retention refers to the amount of vertical force needed to resist the dislodgement of the prosthesis away from its supporting structures, similar to the force needed to remove a suction cup. As with a suction cup, wettability of the resins influences the prostheses' retention to the supporting mucosa and has to be taken into account in the manufacturing process. Wettability indicates whether saliva is able to spread more or less easily on a surface and reflects whether fluids aid retention of the denture surface to oral mucosa, thus affecting retention. CDs with good wettability therefore optimize retention. The wettability of a material is defined by observing the angle of contact between the material and a drop of water: the greater the angle, the lower the wettability and therefore, the more hydrophobic the material will be. In the studies included in the review, hydrophobicity varied according to the type of resin used and could be explained by several hypotheses. Hydrophobicity could be correlated with the rate of monomer released by the resins during polymerization [31]. Another hypothesis could call into question the additives contained in the resins. Indeed, dentures are not made of pure PMMA. They contain numerous additives such as polymerization initiators, cross-linking agents, fillers and dyes, which can all influence the hydrophobicity of the resins [31]. The origin of these variations in hydrophobicity value in dental resins, as well as the effect of aging on resin hydrophobicity, should be investigated. On the one hand, these results on hydrophobicity do not clearly explain why some studies show that digital CDs are more retentive than conventional CDs. On the other hand, one study has shown through adhesion tests that the retention of digital CDs is \begin{table}
\begin{tabular}{c|c} \hline
**Average Residual Monomer (\(\pm\)SD) of** & **Average Residual Monomer (\(\pm\)SD) of** \\
**Conventional Resin (ppm)** & **CAD/CAM Resin (ppm)** \\ \hline \multirow{3}{*}{[PERSON] et al. (2017) [21]} & At baseline: P: 1.61 (\(\pm\)0.05) *** \\  & At two-day: 16.04 (\(\pm\)0.13) & At two-day: P: 1.03 (\(\pm\)0.06) *** \\  & At seven-day: 13.54 (\(\pm\)0.46) & At seven-day: P: 0.90 (\(\pm\)0.01) *** \\ \hline \multirow{3}{*}{[PERSON] et al. (2017) [14]} & BDS: 0.6 (\(\pm\)0.4) \\  & WIN: 6 (\(\pm\)2.7) ** \\ \hline \end{tabular} CAD/CAM resin discs: BDS: Baltic denture system; P: Polident; WN: Whole You Nexteeth; ** Statistically significant difference compared to conventional resin (\(p<0.01\)); *** Statistically significant difference compared to conventional resin (\(p<0.001\)).  
\end{table} 
Table 6: Residual monomer content test results. \begin{table}
\begin{tabular}{c|c} \hline
**Average Contact Angle (\(\pm\)Standard** & **Average Contact Angle (\(\pm\)Standard** \\
**Deviation) of Conventional Resin (\({}^{\circ}\))** & **Deviation) of CAD/CAM Resin (\({}^{\circ}\))** \\ \hline \multirow{4}{*}{[PERSON] et al.(2018) [30]} & A: 70.35 (\(\pm\)8.99)\({}^{*}\) \\  & BDS: 75 (\(\pm\)5.42) \({}^{*}\) \\  & VV: 74.4 (\(\pm\)2.32)*** \\ \cline{1-1}  & WN: 77.7 (\(\pm\)9.87) \\ \cline{1-1}  & DD: 77.5 (\(\pm\)3.34) \\ \hline \end{tabular} CAD/CAM resin discs: A: Avadent; BDS: Baltic Denture System; DD: Wieland Digital denture; M: M-PM; P: Polident; T: T: Trizan-Schutz; VV: Vita Vionic; WN: Whole You Nexteeth; \({}^{*}\) Statistically significant difference compared to conventional resin (\(p<0.001\)); ** Statistically significant difference compared to conventional resin (\(p<0.001\)); **, b, c, d indicate a statistical difference between CAD/CAM resins.  
\end{table} 
Table 5: _Cont._better [35]. If the resins' physical properties do not play a major role in CAD/CAM denture retention, then the manufacturing process itself could favor the precision of adaptation and the adhesion of digital CDs.