## 4 Discussion  
In full-mouth rehabilitations, the selection of an adequate restorative material is of paramount importance. The choice of restoring worn teeth with additive materials seems to be an ideal option, as this allows for the remaining tooth structure to be maximally preserved [52]. Full-mouth rehabilitations often lead to prosthetic artificial occlusion, with the upper arch opposing the same material on the lower arch [53]. In order to in vitro mimic such a clinical scenario, in this study the restorative materials were tested in a chewing simulator opposing antagonist abraders manufactured from the same corresponding materials. The tested materials were compared in terms of total vertical wear (i.e., the sum of sample vertical wear and antagonist vertical wear) and the total volumetric loss (i.e., the sum of sample volumetric loss and antagonist volumetric loss). The null hypothesis investigated had to be accepted. No statistically significant differences were recorded in terms of total wear among all restorative materials under investigation. The total wear depth and total volumetric loss mean values after 120,000 chewing simulation cycles opposing abraders manufactured from the same material had statistically similar results. On the basis of the obtained findings, all of the CAD/CAM resin-based materials exhibited adequate wear resistance, since their total wear depth and total volume loss results were statistically comparable to the type 3 gold alloy. In previous in vitro studies, Aurocast 8 showed human enamel-like wear rates [47,48]. An excessive tooth wear can compromise the occlusion and tooth-jaw relationships, and this may lead to muscle fatigue and impaired aesthetics and function [3, 4, 5, 6, 7]. In particular, parafunctional patients are those at higher risk of pathologically increased tooth wear (over 140 um per year) [54]. Many attempts have been made to correlate the materials' wear resistance to other material properties [55, 56, 57]. The gold wear behavior is provided by its metal bonds [5] and, as for other metals, seems proportional to its hardness [58]. As far as composites are concerned, hardness seems more related to the material abrasiveness [56,57], as the antagonist wear is produced by hard filler particles emerging from the worn resin matrix [56,57]. The size of fillers, the inter-particle distance, the matrix formulation and the silane agent can also affect composite wear properties [59, 60, 61]. The Ena Cad disc is a CAD/CAM resin-based radiopaque composite material with a high cross-linked polymeric matrix, in which a filler percentage of approximately 70% is embedded. Concerning the total vertical wear and total volumetric loss, as a result of 120,000 chewing simulation cycles against abraders manufactured from the same material, Ena Cad showed a wear resistance close to the type 3 gold alloy. Ena Cad also performed quite similarly to the other two CAD/CAM composite resins investigated herein. Ena Cad was proposed by the manufacturer to fabricate fully anatomical monoliths, inlays, crowns and superstructures for functional and aesthetic rehabilitation, also on implants. Figure 3: Scanning electron microphotographs (original magnification 7000\(\times\)) showing different sizes of the filler particles embedded within the polymeric matrix of the CAD/CAM composite materials tested: Ena Cad (**A**), Brilliant Crios (**B**) and Grandio disc (**C**). The chemical composition and the optimized high-density filler technology might explain the promising wear behavior observed herein. Brilliant Crios is a CAD/CAM resin-based material whose fillers are embedded in a cross-linked methacrylate matrix. According to the manufacturer, the small filler particles of Brilliant Crios can be strongly organized, consequently reducing the polymer matrix exposition during the wear process [62; 63].