# Comparison of CAD/CAM and Conventional Denture Base Resins: A Systematic Review
## 3 Results
### Mechanical Properties  
Seven articles have focused on the surface hardness of the PMMA resins. Hardness is a measure of the resistance of a material to plastic deformation, induced by either abrasion or mechanical indentation. The data are presented in Table 2. Two studies have shown that resins for CAD/CAM prostheses have a higher microhardness than conventional resins [21; 22; 23]. Others studies showed no significant difference in hardness between the two types of resins when tested for nano-hardness [24; 25]. Recently, however, one study showed that the hardness of CAD/CAM resin was significantly lower than that of hot-pressed conventional resin [26]. [PERSON] [27] revealed differences in material-related hardness and microhardness between CAD/CAM and conventional resins  
The resistance to deformation was studied by three-point bending tests in nine studies [21; 23; 24; 26; 27; 29; 31; 33; 34]. Results showed that CAD/CAM resins have superior mechanical properties; higher elastic moduli [21; 24; 26; 29; 33], flexural strength [23; 24; 26; 31; 33; 34], impact strength [34] and yield strength [24; 25] as compared to conventional resins. However, two studies [21; 27] revealed that the mechanical properties of CAD/CAM resins were not always superior to that of conventional resins. [PERSON] et al. showed that the flexural strength of all the resins tested was significantly lower after thermocycling [31]. Most CAD/CAM resins have a higher toughness than conventional resins [24; 29].  
Only one study focused on the analysis of the load at fracture of resins using a 3-point bending test on rectangular resin samples (4 mm-thick). The maximum load at fracture of six CAD/CAM resins was then compared with that of conventional heat-cured and self-curing resins [29]. Table 3 shows the results of the fracture tests. The fracture resistance of CAD/CAM resins was not always superior to that of conventional heat-cured or self-curing resins. Indeed, fracture resistance is highly variable within CAD/CAM resins. The average breaking loads ranged from 40.27 N \(\pm\) 3.40 to 82.49 N \(\pm\) 7.47 for CAD/CAM resins. Of the six CAD/CAM resins tested, only the Wieland digital dentures (WWD) resin had significantly higher values than conventional resins. This study also showed that there was a positive correlation between fracture surface roughness and the maximum load at fracture of the resin. For example, the WDD CAD/CAM resin had the roughest fracture surface with the highest maximum breaking load.  
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline  & \multicolumn{2}{c}{**Average Hardness (\(\pm\)Standard Deviation) of Conventional Resin**} & \multicolumn{2}{c}{**Average Hardness (\(\pm\)Standard Deviation) of CAD/CAM Resin**} \\ \hline [PERSON] et al. (2019) [22] (in mPa) & 18.09 (\(\pm\)0.31) & T: 19.80 (\(\pm\)1.08) *.a \\  & & A: 20.60 (\(\pm\)0.33) *.b \\ \hline [PERSON] et al. (2017) [21] (in mPa) & 13.22 (\(\pm\)0.88) & P: 22.41 (\(\pm\)1.50) *** \\ \hline [PERSON] et al. (2021) [26] (in (HVN) & 23.1 (\(\pm\)1.9) & 18.7 (\(\pm\)1.7) \\ \hline [PERSON] et al. (2019) [25] (in mPa) & 19.46 (\(\pm\)0.4) & 19.31 (\(\pm\)1.48) \\ \hline [PERSON] et al. (2021) [27] (in mPa) & NC & NC \\ \hline [PERSON] et al. (2020) [23] (in mPa) & NC & NC \\ \hline [PERSON] et al. (2018) [24] (in mPa) (nano-hardness) & 232 (\(\pm\)15) & A: 221 (\(\pm\)14) \\ \hline \hline \end{tabular} CAD/CAM resin discs: A: Avadent; P: Polident; T: Tizian-Schütz; NC: Graphic data: precise values not communicated. * Statistically significant difference compared to conventional resin (\(p\) < 0.05); *** Statistically significant difference compared to conventional resin (\(p\) < 0.001); a and b indicate a statistical difference between CAD/CAM resins.  
\end{table}
Table 2: Hardness test results.  
The study by [PERSON] et al. [29] highlighted the differences in mechanical properties between CAD/CAM resins.