# Comparison of CAD/CAM and Conventional Denture Base Resins: A Systematic Review
## 4 Discussion
### Adhesion of microorganisms  
Preventing the adhesion of microorganisms to dentures helps preserve the health of tissues in the oral cavity. The prevalence of denture stomatitis among denture wearers has been shown average around 28% [39]. Microorganisms can cause pathology in the mucosa supporting the prosthesis, such as candidiasis (induced mainly by _Candida albicans_). Microorganisms accumulate more easily on prostheses than on natural tissue. The surfaces of removable maxillary prostheses are the main reservoir of _Candida albicans_[40, 41]. The physical properties of the prosthetic resin influence this adhesion, including surface roughness and hardness. The results of this literature review showed that CAD/CAM machined resins had globally more advantageous physical properties than conventional resins. Surface roughness is the most important factor for _Candida albicans_' adhesion to the prosthesis surface. Optical and electron microscopy studies revealed that initial adhesion of microorganisms usually begins in the microscopic pits and cracks of rough surfaces. This surface roughness provides protection against removal (shear) forces and gives time for microbes to irreversibly adhere to the surface [42, 43, 44, 45]. A smoother surface might reduce _Candida albicans_ adhesion, which ultimately lowers the risk of prosthetic stomatitis. It has been demonstrated that CAD/CAM resins have significantly lower surface roughness than conventional resins [22, 28, 29, 32]. The polishing technique and the operator's ability to polish both affect the roughness of the resins' cameo (external) surface [46]. However, the prosthetic intaglio (internal surface in contact with the supporting mucosa) is never polished, and only the polymerization of the resins would determine surface roughness. Therefore, irregularities in conventionally fabricated dentures would be due to the evaporation of monomers during polymerization. This can be avoided by applying high-pressure polymerization, as seen when utilizing proprietary CAD/CAM resins [32]. In case of conventional resin polymerization, the amount of pressure applied is limited to avoid fracturing the gypsum mold in which resins are polymerized. Conversely, machined resins for CAD/CAM prosthetics are made from resin discs that are polymerized at very high pressure, which could explain their lower roughness [30]. Furthermore, in order to avoid the accumulation and colonization of microorganisms on the prostheses' surface, roughness of the resins should not exceed 0.2 \(\upmu\)m [47]. In this review, most of the resins had roughness above 0.2 \(\upmu\)m. This characteristic should be improved in the future, taking into consideration that the surface profile of the prosthetic intaglio surface always exhibits striations due to milling process used for CAD/CAM resins. In addition to the biofilm adhesion onto the resins, surface roughness also tends to induce halitosis and mucosal inflammation [48], and reduces patient comfort. Thus, milling resins could reduce these nuisances and facilitate denture maintenance. Hardness is another factor influencing bacterial adhesion to the prostheses over time. Surface hardness is defined as the ability of the material to resist penetration or permanent indentation related to the wear and scratches appearing as the dentures are worn and cleaned. A prosthesis should have a hard surface to reduce denture deterioration and therefore limit the adhesion of microorganisms [37, 49]. Included studies focused on the microhardness as well as the nanohardness of resins. For the most part, surface microhardness was higher in CAD/CAM prosthetic resin blocks and no significant difference was found between nanohardness values. These results could be related to the polymerization process because the polymerization conditions of the CAD/CAM resin discs (high temperature and pressure) and the addition of inorganic substances promote the formation of longer polymer chains and limit the dimensional shrinkage of polymerization [50].