# Comparison of CAD/CAM and Conventional Denture Base Resins: A Systematic Review
## 4 Discussion
### Resistance to Deformation and Breakage  
When worn, complete dentures are subjected to high stresses during chewing. This creates cyclic deformation of the denture polymer, which in turn can lead to crack formation and possible fatigue fracture of the denture [36]. Therefore, good resistance of the resins to deformation is very important. The results of this literature review showed that CAD/CAM resins have superior mechanical properties to conventional resins.  
First of all, conventional resins had a lower modulus of elasticity than CAD/CAM resins. The elastic modulus is an important property in characterizing the material's rigidity. Resins with a higher modulus of elasticity will be more resistant to elastic deformation. A high elastic modulus may make it possible to fabricate prostheses with reduced thickness without compromising the mechanical properties, an important attribute for patient comfort and acceptance of the prosthesis. In addition, a deformation-resistant prosthesis will have a more stable occlusion. These differences in rigidity between CAD/CAM and conventional resins are probably due to the manufacturing process. The modulus of elasticity would be related to the residual monomer content. It has been shown that a high monomer content reduces the glass transition temperature, making the resin more flexible. The high-temperature, high-pressure polymerization of CAD/CAM resins leads to higher monomer conversion with lower residual monomer values, thus resulting in a more rigid resin. High-pressure and high-temperature polymerization also allows a different arrangement of the polymer chains. Previous studies had shown that the use of high pressure during polymerization increased crosslinks between polymer chains [15; 37].  
Secondly, fracture resistance characterizes the maximum stress a material can withstand before breaking. Results have shown that fracture resistance varied within CAD/CAM resins. For example, in the study by [PERSON] et al. [29], the Wieland digital denture (WDD) resin exhibited higher loads to fracture but had a lower elastic modulus than the others. Thus, it had lower rigidity and more susceptible to plastic deformation. It would therefore be preferable not to reduce the bases thickness too much to compensate for the risk of plastic deformation. Of the six CAD/CAM resins tested, only the WWD resin had significantly higher yield strength values compared to conventional resins. WDD also exhibited the roughest fracture surface. It was the only resin with fine grains on the fracture surface. This difference in resin surfaces is thought to be due to the geometry, size and distribution of the polymer powders used to make the resins. In the study by [PERSON] et al. [29] the measured maximum breaking loads were all greater than 40 N. However, the maximum bite force was reduced to 40 N in complete prosthesis wearers with high resorption of the mandibular alveolar bone [38]. Based on these results, some manufacturers encourage the fabrication of full dentures with reduced resin thickness. It is imperative to remain skeptical regarding this commercial argument, especially since in the study, the tests were performed on 4 mm thick resin samples, which is much thicker than a prosthetic base. Furthermore, CDs are subjected to much more complex stresses in the mouth, involving different types of forces in different directions (such as chewing). For a valid prediction of clinical fatigue resistance, further studies on cyclic loading behavior and fracture toughness are necessary.  
The results highlighted differences in mechanical properties between CAD/CAM resins. More information on the composition and manufacturing processes of CAD/CAM resins would help to better understand some of the results.