# Mechanical Assessment of CAD/CAM Fabricated Hybrid Ceramics: An In Vitro Study
## 2 Literature Review  
Currently, glass ceramics and hybrid ceramics are the two primary types of machinable aesthetic materials involved in the manufacturing of CAD/CAM restorations [21]. Advancements in composite technology have greatly enhanced hybrid ceramics over the last ten years [21]. These hybrid ceramics have been extensively used in CAD/CAM systems to fabricate indirect restorations, including inlays, onlays, veneers, crowns, and bridges [21]. Launched in 2000, the Paradigm MZ100 (3M ESPE, USA) was the first resin composite block designed for CAD/CAM systems. The Paradigm MZ100 block is composed of a direct resin composite and includes 85 wt.% ceramic fillers consisting of ultrafine zirconia and silica [22]. The flexural strength, flexural modulus, compressive strength, and diametral tensile strength are, respectively, stated to be 146 MPa, 12.7 MPa, 524 MPa, and 114 MPa [22]. The Lava Ultimate block was subsequently developed by 3M ESPE using a resin nano ceramic substance that consists of about 80 wt.% zirconia-silica nano ceramic fillers [23]. The flexural strength (204 MPa) showed considerable enhancement in comparison to Paradigm MZ100 [24]. Furthermore, GC Corporation, Japan, introduced Cerasmart as a hybrid ceramic block that incorporates 75 wt.% of nano-ceramic particles within a resin matrix that is intensively cross-linked, similar to the Lava Ultimate block which had a noteworthy flexural strength of 231 MPa [24]. These novel composite block materials, despite having variations in composition and curing conditions, are produced using traditional filler-resin mixing composite technology [25]. The Polymer-Infiltrated-Ceramic Network technology was introduced by VITA Zahnfabrik, Germany, to create Vita Enamic, a genuinely hybrid ceramic designed for CAD/CAM restorations [26]. An initial formation of a porous ceramic network (86 wt.%) was followed by its infiltration by polymeric resins, therefore offering a novel approach to enhance the loading capacity of fillers. In comparison to composite block materials produced by combining fillers and polymeric resins, Vita Enamic demonstrated notably greater modulus of elasticity (30 \(\pm\) 2 GPa) and hardness (2.5 GPa) [26]. In addition, determining the mechanical characteristics of hybrid ceramic material flexural strength plays an imperative role which provides information about the ability of load endurance when tensile and compressive stress are combined [10; 12]. Most of the existing CAD/CAM technologies in dentistry are founded on subtractive design. The subtractive method employs a cutting tool to mechanically abrade the material and obtain the intended shape in accordance with computer-guided instructions. Additive manufacturing, or 3D printing, is the method of constructing materials layer by layer directly from 3D digital raw data [3; 27; 28]. The advantages of additive manufacturing over conventional subtractive (milling) processes are numerous. The additive technology enables the production of an object independent of its numerical and dimensional complexity [29]. A 40% reduction in material waste is achievable, and it is possible to produce features that are finer than the size of the milling bur. A growing body of research indicates that additive manufacturing methods are being increasingly used in multiple dental fields, including the production of dental models, surgical guides, and occlusal devices [30; 31; 32]. Nevertheless, the application of this technique in the field of prosthodontics has not been sufficiently addressed. Moreover, the impact of different printing orientations and their consequences on the ultimate quality and mechanical characteristics of the printed restorations has not been well examined. Recently, we reported a novel CAD/CAM resin composite block named Shofu with high mechanical properties intended to evaluate its flexural, compressive, and hardness properties.