Chairside CAD/CAM Restorations

Abstract

Dentistry has experienced dramatic transformations in the last 10 years once digital technologies have revolutionized the entire operational flow. From simple crowns and inlays, almost the entire range of fixed and removable prosthetic restorations on natural teeth or implants can now be made using CAD/CAM technology. The evolution of these systems has led to the need for a change in the mentality. Moving from analog to digital for these technologies involves equipment costs, software, and training time. For a dentist, the first step in CAD/CAM technology is to purchase an intraoral scanner and move to the digital impression. Then it will transmit the information (the. STL file) to a laboratory that will take over the design and milling task. However, if he wants to invest more, he will be able to make the final restoration with chairside CAD/CAM systems, without involving the dental technician.

1. Introduction

Digital dentistry is greatly benefited by the present digital revolution, which is bringing new materials, techniques, and treatment ideas. The dentistry profession is undergoing a considerable shift due to the advancement of CAD/CAM (computer-aided design/computer-aided manufacture) technologies and the advent of novel esthetic materials. The emergence of technology has significantly altered operational procedures and workflow [1].

Through the release of intelligent materials on the market that are intended to improve oral health care and, in turn, quality of life, dentistry has improved and will continue to advance [2].

It is now inevitable to switch from traditional treatment methods—which are laborious and prone to errors—to computerized ones. A significant impact of digital technologies has been felt in most dental specialties. The most noticeable advancements in the digital operational process have been in dental prostheses, which focus on the use of CAD/CAM systems and digital impression techniques [3].

In modern times, dental offices are using intraoral scanners more and more frequently. The principal factors contributing to this achievement are the decreased working hours for the physician, enhanced patient comfort, instantaneous viewing of treatment quality, and the ability to make adjustments without having to redo every stage of the process [4, 5].

A successful outcome depends on choosing the right material for each unique situation, which necessitates understanding each material’s characteristics. The objective of the upcoming chapter is to investigate the benefits of novel methods and materials in order to gather data for an acceptable therapeutic outcome from an esthetically pleasing and long-lasting perspective.

2. In office digital impression – intraoral scanning

The impact of intraoral scanners (IOS) in dental offices has advanced to the point where it is impossible to ignore it. These devices have advantages over traditional methods of capturing intraoral structures, including reduced clinical time, improved patient comfort, the possibility to evaluate preparations during surgery, prevention of cross infections and impression distortion, and the ability to store digital models indefinitely [6]. IOSs have a variety of uses when paired with CAD/CAM (Computer-Aided Design/Computer-Aided Manufacturing) software, including single-tooth prosthetic restorations [7], fixed partial dentures [8], and dental implants.

2.1 Factors that can affect the precision of the digital impression

There are still several factors that, through their direct or indirect effects on the scanner and the substrate, cause differences between the digital model and reality, even though IOSs have made significant strides in recording the surface details of the object of interest. Ambient light, the distance between the scanning tip and the object’s surface, the scanning procedure, the surface humidity, and the material of the scanned surface are the most commonly aspects that contribute to the digital impression distortion [9].

2.2 Ambient light

Most IOSs use a laser to record a surface’s features, allowing a photo sensor to record the reflection’s projection onto the surface. Thus, the position of each point of light projection is determined using a variety of techniques (triangulation, confocal microscopy, etc.). Although scanners have optical filters built in, the photo sensor records not only the light from the laser but also nearby light that has the same wavelength as the wave released by the scanner, which makes it a possible source of inaccuracies. High-intensity light sources have the potential to completely impact the surface, causing all the points in the scanned area to become saturated with artifacts.

When determining the position of the 3D object’s points, different light sources produce varying results; the effect is particularly pronounced when using incandescent light sources. High light levels (2500 lux, comparable to the dental unit’s illumination) can lengthen scanning times and increase the amount of information that can be recorded [10].

It was found that the ambient light had less of an impact when scanning a dark surface since light radiation was being absorbed. Additionally, it has been noted that gray structures, on which light variations have little impact on scan quality, get the greatest results [11].

A previous study performed in the Department of Prosthodontics from the University of Medicine and Pharmacy ‘Victor Babeș’ from Timișoara aimed to investigate the influence of ambient light on the accuracy of digital impressions. The purpose of this study was to assess the impact of various settings of ambient light intensity inside a dental office on the accuracy (trueness and precision) of an intraoral scanner. A resin molar underwent a full crown preparation before being scanned with a high-resolution extraoral scanner to create a reference model. The workspace’s six most therapeutically appropriate lighting settings were selected, and the preparation was scanned using an intraoral scanner (PlanScan, Planmeca). In accordance with the six light intensity settings, six groups were made: group 1 = 400 lux, group 2 = 1000 lux, group 3 = 3300 lux, group 4 = 3800 lux, group 5 = 10,000 lux, and group 6 = 11,000 lux. Despite some variations in the trueness and precision data collected under the various light intensities, these differences were not clinically significant enough to draw the conclusion that the ambient light had a significant impact on the accuracy of the intraoral scanning. Therefore, it is impossible to provide a preferred ambient light setting for the scanning process in a dental office. Overall, in the various lighting conditions that were simulated in this test, it was unable to attribute any statistical significance to the accuracy of the intraoral scanner that was used. From the clinical point of view, dentists consider it reassuring that ambient light conditions have little influence on the scanning accuracy because it means that there is a lower potential for external error when considering using digital impressions [7].

2.3 Object translucency

Translucency is another property of the substrate that might affect how accurate a digital impression is. This means that some wavelengths that encounter a translucent surface are either reflected or scattered by the object’s structure.

Unlike waves that are reflected straight from the surface, interferences are caused by light that is scattered in the substrate and is reflected at various locations and angles. This results in a direct reflection pollution that the scanner records, decreasing the accuracy [11, 12, 13].

This distortion of the digital impression occurs even in the case of scanners that use the principle of confocal microscopy, which theoretically counteracts the effects of translucency by detecting point-by-point the shape of an object [14, 15].

2.4 Scanning pattern and scanning distance

The operator’s chosen scanning pattern is another element that affects the accuracy of the IOS. Although manufacturers provide broad instructions on how to position the scan tip, these are frequently challenging to adhere to, particularly in clinical settings when a full arch scan is necessary. Consequently, a number of scanning patterns can be found: (A) scanning on half-arch starting from the occlusal surface from the central incisor, then the transition on vestibular and finally on the palatal one, (B) scanning on half-arch starting from the occlusal surface from the lateral incisor, (C) scanning of sextants with the same pattern, and (D) sequential scanning in which each tooth is completely scanned, the scanner having a “S” movement (Figure 1).

Figure 1.

Intraoral scanning patterns.

Most findings suggest that the scanning method plays a significant effect in scanning accuracy despite other research showing that the scanning pattern does not affect the quality of the digital impression when using current scanners [16, 17, 18, 19].

The scanning distance is another factor that might lead to errors. It is crucial to know if specific scanning distances produce more accurate findings than others because it can be challenging for an operator to keep a constant space between the scanning tip and the recorded soft tissues or teeth during the scanning procedure. The objective of this study performed in the Department of Prosthodontics from the University of Medicine and Pharmacy ‘Victor Babeș’ Timișoara was to evaluate the differences in accuracy between digital impressions in the scenario of different scanning distances [20]. A single operator conducted twenty consecutive scans at five specified distances: 5, 10, 15, 20, and 23 mm. A previous study made use of the i700 IOS. The second molar and premolar’s occlusal surfaces were recorded for the mesh alignment, but only the preparation’s occlusal surface was scanned. The overall scanning time was 20 seconds, and the scanning path was the same for all distances. The typodont was moved in the following manner: Beginning at the first molar’s occlusal surface, moving on to the second molar’s occlusal surface, the second premolar, and finally returning to the original starting location. The typodont always moved in a straight line while resting its feet on a flat surface. The following conclusion can be derived from the study’s findings:

• The accuracy of a digital impression can be affected by the distance between the tip of the IOS and the surface being captured.

• The accuracy of the digital model is decreased by close scanning distances (5 mm) or scanning distances more than 15 mm.

• The best accuracy was shown at a distance of 10 mm between the scanning tip and the prepared area [21].

2.5 Surface geometry

The geometry and type of the preparation in the case of fixed dental prostheses (FDP) is another element that may lower the accuracy of a digital impression. Dental preparations frequently deviate from the ideal occlusal convergence of the abutment parameters, resulting in areas with excessive convergences, parallel surfaces, or divergences toward the apical (negative angles). Guth et al. believe that a convergence of 6–15⁰ is suitable for the majority of FPDs, although over 86% of the preparations under investigation had occlusal convergences of above 25° [21]. There is general agreement that scanners cannot catch these areas with negative angulation due to the limited access space for the scanning tip (in the interproximal areas), which results in lost data and decreased accuracy [22, 23]. Additionally, it has been shown that the accuracy of the digital impression reduces as the angle of occlusal convergence tends toward 0⁰. This phenomenon is explained by the fact that the light wave occasionally cannot reach the base of the preparation [21].

2.6 Intraoral fluids on the scanned area

Another element that affects the accuracy of digital impressions is the presence of fluids on the preparation’s surface. Even with sufficient isolation, maintaining an ideal layer of antireflective powder in scanners that use optical powder has shown to be quite challenging in situations when there are intraoral fluids present. The presence of saliva also affects IS (intraoral scanning) without powder because it alters the preparation’s shape and surface properties, which, in turn, affects the reflection indexes. As a result, the scanner captures an image that is warped, which will negatively affect how well the fixed prosthetic restoration will fit. At the margin of the preparation, in addition to saliva, bleeding is also possible, which further impairs the intraoral scan. The blood’s dark color causes it to absorb the scanner’s radiation, which results in a lack of information at that level [24, 25].

2.7 Rescanning of the interest area

While it is often simpler to perform in vitro scanning by adhering to the suggested scanning protocol, in vivo scanning may be complicated by intraoral structures, such as the buccal mucosa and the tongue. The newest scanners offer rapid surface identification and data collection, although sometimes only one scan of a given area is necessary because the light emitted from the IOS cannot reach all of a tooth’s surfaces, especially the interproximal portions [22]. These mesh holes may also be purposely created by the operator, for example, when the abutments are modified after the first scan, or when is necessary to cut off portions of the scan when overlapping of undesired oral tissues are present. The effects of digital cutting, rescanning, and overlaying of scanned surfaces on the accuracy of a digital impression have been studied in certain experiments [26, 27]; however, the outcomes can vary. Another study conducted in the Department of Prosthodontics aimed to determine which scanning method produced the most precise digital impression of a single-tooth preparation, whether data was obtained from a single continuously scan, a rescan to gather more information, or the omission of an area of interest subsequent to a rescan (Figure 2).

Figure 2.

Single uninterrupted scan (a), rescanning of the area of interest (b), and the deletion of the area of interest followed by a rescan (c).

The following conclusion can be drawn from the study’s findings:

The “SINGLESCAN” group provided the best trueness and the “RESCAN” group provided the best precision in the statistical analysis of the trueness and precision of the IOS scans obtained with the three distinct scanning techniques.

No specific scanning methodology could be suggested among the three tested methods as offering the best overall accuracy.

The investigated in vivo and in vitro instances showed no significant clinical differences in terms of trueness and precision. While the precision results varied slightly but clinically insignificantly, the trueness outcomes were equivalent for both in vivo and in vitro scenarios [28].

3. Digital workflow for chairside smile design rehabilitation

After intraoral scanning, there are two options to send the: STL file via Internet to the dental laboratory, and then the dental technician will create the digital design, mill the restoration, or print it 3D, or the practician can make the design in office, with an adequate software and then finalize the restoration by milling or 3D printing.

There are several designs software designated for chairside restorations — CEREC (Dentsply Sirona), and Plan CAD Easy (Planmeca). The concept of same-day crown or same-day dentistry means that the dentist has the full control and responsibility from the case selection, to preparation, intraoral scanning, digital design, material selection, milling, and postprocessing of the restoration until the final cementation.

In order to make the final digital design, the dentist should make a careful examination and treatment planning for the future restoration. From veneers, inlays, onlays, crowns, and different minimally invasive restorations, such as tabletops, endo-crowns, and a multitude of single-unit restorations, can be designed with an in office software.

The following case reveals the importance of examination and planning when smile design rehabilitation is obtained with chairside CAD/CAM technology. A 42-year-old female patient presented for improving the appearance of her smile (Figure 3). When all the frontal teeth are involved, it is convenient to simulate the future smile design with the help of a digital smile design software (Figure 4). The color of the future restoration can be appreciated by combining traditional methods (tooth shade guide) and digital methods (spectrophotometers and software such as digital shade assistant (Ivoclar) Figure 5).

Figure 3.

Initial situation of the patient.

Figure 4.

Digital smile design (Medit).

Figure 5.

Shade analysis with digital shade assistant (Programat CS6, Ivoclar).

After tooth preparation for eight veneers (from tooth 1.4. to 2.4.), an intraoral scanning was performed with Medit i700 (Figure 6). Then with a CAD software (exocad), the digital design for the eight veneers was performed (Figure 7), and send to the milling machine (Planmill 40, Planmeca). Eight veneers from lithium disilicate (emax. CAD) were milled and then bonded to the tooth structure with a precise adhesive protocol, using dual-cured resin cement (Variolink Esthetic, Ivoclar, Vivadent). (Figure 8). The final smile of the patient revealed the esthetic and functional improvement (Figure 9).

Figure 6.

Digital impression after teeth preparation.

Figure 7.

Digital design for the eight veneers with EXOCAD software.

Figure 8.

Lithium disilicate (emax.CAD) veneers after cementation.

Figure 9.

The final smile of the patient.

4. CAD/CAM equipment and materials used for prosthetic restorations

The first generation of CAD/CAM equipment was available on the market in the 1980s and could only be used to design and manufacture single ceramic restorations. Dental milling machines, usually integrated into bigger CAD/CAM systems are a real help for dental technicians or dentists design the restoration using CAD software, and CAM software creates the toolpaths that the milling machine needs to use for creating the restoration. Chairside milling machines are small devices intended for use in offices. Comparing these machines to their laboratory counterparts, they are frequently faster and smaller. Advancements in milling machine technology have led to a continuous improvement in the fit quality of digitally created dental restorations [29]. Chairside systems offer so many new options that they will inevitably become a standard feature of dental practices. Today’s leading chairside systems employ a “full-digital workflow” to create a variety of prosthetic devices, including inlays/onlays, veneers, endo-crowns, bridges, crowns, and implant abutments, to mention only a few [4].

The clinicians can now choose from a variety of materials such as feldspathic, leucite ceramics, lithium disilicate glass-ceramics, resin, or hybrid materials. These materials can be classified using a variety of parameters such as resistance, composition, purpose, and manufacturing method [30]. These materials for chairside CAD/CAM systems are presented in blocks, which are delivered in a variety of color ranges and translucencies, exhibit qualities that are homogeneous, dense, and faultless, and provide highly esthetically pleasing restorations. However, surface characterization and color individualization can be completed before the firing for ceramic materials (Figure 10) [31, 32].

Figure 10.

Different ceramic and hybrid blocks for chairside milling machine (Planmill, Planmeca).

4.1 The classification of chairside CAD/CAM ceramic materials

From the perspective of composition and characteristics, CAD/CAM chairside ceramic materials can be categorized into four categories.

4.1.1 Feldspathic ceramics and leucite-reinforced ceramic blocks

The first generation of chairside CAD/CAM materials is represented by feldspathic ceramics (Vitablocs Mark II- Vita Zahnfabrik, CEREC Blocs - Dentsply Sirona,).

Prior to 1990, these materials were initially made available for commercial usage. They remained dominant in the market until the 2000s, and from a lifespan standpoint, they were also the most studied materials.

Feldspathic ceramics have a glassy phase predominance of between 55 and 70%, making them some of the most translucent and esthetically pleasing ceramic materials [33].

4.1.2 Lithium disilicates and zirconia-reinforced lithium silicates blocks

A significant advancement in the realm of fixed prostheses was the invention of glass ceramics with enhanced resistance qualities. In comparison to earlier glass ceramics, lithium disilicate (IPS e.max CAD Ivoclar Vivadent) was released in 2006 with a bending resistance exceeding 350 MPa.

Lithium disilicate is only provided in partially crystallized form (purple color) for the CAD/CAM method, where the block is made up of lithium metasilicate (Li₂SiO₃) and the remaining portion is made up of the crystallized nucleus of lithium disilicate (Li₂Si₂O₅), which provides a “"soft” state (with a bending resistance of 140 MPa). This lessens the wear on the milling cutters and makes it easier to grind the block. The material needs to go through a two-stage fire cycle in a ceramic furnace for 10 minutes after milling to completely crystallize and change the metasilicate into lithium disilicate, which results in an increase in bending resistance over 440 MPa [34, 35].

Zirconia-reinforced lithium silicate (ZLS) is a newer generation of high-strength CAD/CAM ceramics that have been available since 2012. In ZLS, the glass matrix is reinforced with lithium silicate crystals that are 4–8 times smaller than those of lithium disilicate, and it is also given a 10% by weight tetragonal zirconia component to enhance its mechanical properties [36].

4.1.3 Zirconium oxide (zirconia)

Zirconium oxide or zirconia is a heterogeneous polycrystalline ceramic with exceptional mechanical characteristics (flexural strength 500–1200 MPa, elastic modulus 210 GPa); however, it is resistant to conventional acid etching techniques. It exhibits great biocompatibility both in vivo and in vitro, has lower plaque retention than titanium, and among the many integral ceramics, has the lowest rate of wear against the antagonist [37, 38].

The zirconia blocks for chairside CAD/CAM systems allow for simply soft machining processing of pre-sintered zirconia. Zirconia restorations are milled with a 25% oversize of the final volume, and the sinter process that follow will produce the perfect fit. This material can be used to make single crowns, implant abutments, and bridges with up to three parts by using chairside blocks [39, 40].

4.1.4 The hybrid ceramics or resin matrix ceramic materials

Hybrid ceramics are a brand-new class of CAD/CAM chairside materials that were created to combine the distinctive esthetic qualities of ceramic materials with the reduced fragility and greater fracture resistance of composite resins. A ceramic filler (up to 80% by weight) is coupled with a composite resin-like Bis-GMA, UDMA, UTMA, and Bis-EMA) in nanoceramic resin blocks made industrially by high-temperature and high-pressure processes. In the instance of polymer-infiltrated-ceramic network (PICN) blocks, composite resin (14% by bulk) has been industrially infused into the ceramic structure, which accounts for 86% of the block’s bulk.

Hybrid ceramics have shown to be less difficult to mill and require no or fewer heat cycles. Additionally, they have good bending resistance and can still be employed at thinner thicknesses [41].

In the past 10 years, numerous novel components with greater mechanical characteristics have been created for CAD/CAM technology. The norms of dental care have greatly improved with the advent of new nanomaterials. Restorative dental science is thought to benefit greatly from nano-dentistry since it will enable tailored therapy [42, 43].

Dental composites’ mechanical qualities have been enhanced using nanoparticles, which also strengthen bonds and reduce wear. Smaller particles can more effectively penetrate deeper lesions and lower the porosity of dental composite for increased toughness [44].

4.1.5 Acrylic resin

Polymethyl methacrylate (PMMA) based polymers are pre-polymerized without the addition of fillers and kept in storage until needed. Their cross-linked structure and chemical make-up determine their mechanical qualities primarily. The absence of voids and decreased shrinkage due to polymerization during mixing, packing, and setting have an important role in mechanical characteristics. A shortened chairside time can be used to create interim prostheses. Long-term temporary prosthesis could likewise be made using these PMMA CAD/CAM blocks.

4.2 The properties of chairside CAD/CAM materials

Clinical treatment results are directly correlated with the care taken to select the distinctive traits and features of the various types of CAD/CAM materials. Several variables, including material choice, restoration design, occlusion, and cementation, affect how well chairside restorations will succeed [45, 46, 47].

While the functional aspects of CAD/CAM materials have been extensively studied and implemented, there is an equally compelling need to focus on their esthetics. Esthetics play a pivotal role in several industries, including dentistry, where natural-looking restorations and prosthetics are in high demand [48, 49, 50].

4.2.1 Investigations on esthetic properties

The selection of a suitable material needs to match the natural tooth structure in terms of both mechanical properties and visual appearance, considering that oral restorations are exposed to various complex oral conditions during their lifetimes. Visual characteristics, such as color, texture, and translucency, are crucial for achieving a restoration that seamlessly blends with the surrounding natural teeth, providing a natural-looking smile.

One of the oral conditions that require special attention is the gastroesophageal reflux disease, which is characterized by regular regurgitations of gastric juice from the stomach into the oral cavity [51, 52]. A significant need for intraoral use and a deciding factor when selecting the kind of restoration is dental materials’ resistance to chemical deterioration. Acid concentration, immersion time, and temperature, all these parameters affect the in vitro simulation of acid on the surface of dental ceramics [51]. Several studies performed in Department of Prosthodontics; Faculty of Dentistry Timisoara (TADERP Research Center) involved the consequences of oral conditions with this pathology on the properties of these new materials.

A significant need for intraoral use and a deciding factor when selecting the kind of restoration is dental materials’ resistance to chemical deterioration. Acid concentration, immersion time, and temperature, all these parameters affect the in vitro simulation of acid on the surface of dental ceramics [52, 53, 54, 55]. It can be inferred from other study publications in the literature that there is not a lot of agreement on how to simulate stomach acid and how long it takes to replicate it in an in vivo model. The usage of 4% acetic acid and an exposure length of 16 hours at 8°C is equivalent to 2 years of clinical exposure, according to ISO standard 6872, which deals with the solubility test for dental materials [48].

Several studies performed in Department of Prosthodontics; Faculty of Dentistry Timisoara (TADERP Research Center) involved the consequences of oral conditions with this pathology on the properties of these new materials. In one study, it was shown that for all tested monolithic materials, scanning electron microscopy revealed noticeable alterations to the material’s topography after simulated gastric acid exposure for feldspathic ceramic, nanoceramic resin, hybrid ceramic, and leucite-reinforced glass ceramic. Triluxe Forte (VITA Zahnfabrik- Bad Säckingen, Germany), Cerasmart (GC Europe), Enamic (VITA Zahnfabrik- Bad Säckingen, Germany), and Empress CAD (Ivoclar, Schaan, Liechtenstein) were the tested materials. Using scanning electron microscopy, their microhardness, surface roughness, translucency, and surface morphology were examined both before and after exposure to simulated stomach acid liquid. The results of this investigation revealed that Triluxe Forte was the CAD-CAM monolithic restorative material that underwent the most significant modifications following exposure to a gastric acid simulation. The Cerasmart monolithic restorative material, however, was found to be least impacted following a simulation of exposure to gastric acid [56].

Another in vitro investigation into the color stability of chairside CAD/CAM ceramic blocks made of leucitic, feldspathic, and disilicate following exposure to common liquids was performed. According to the study’s findings, all materials showed color changes after being submerged in red wine that was just barely above the thresholds for perceptibility and acceptability. The feldspathic CAD/CAM ceramic blocks showed the most notable color changes after being submerged in coffee. Within the bounds of this study’s limitations, it may be inferred that typical beverages may have an impact on the CAD/CAM ceramic blocks’ color stability, which may jeopardize the esthetics of the restorations [57].

4.2.2 Investigations on mechanical properties

The mechanical characteristics of CAD/CAM materials require a special consideration. For researchers and physicians understanding a restorative material’s mechanical characteristics is crucial because significant fracturing of these materials has been identified as the primary reason of failure. One in vitro study evaluated the fracture resistance and surface of full contour Vita Enamic CAD/CAM crowns with different thicknesses [58]. According to the study’s findings, crowns with a 1.5 mm thickness had a larger compression load than crowns with a 0.5 or 1.0 mm thickness. The fracture loads in the groups with occlusal thicknesses of 0.5 and 1.0 mm did not differ significantly.

In contrast to stiffer materials such as high translucency zirconia and zirconia-reinforced glass ceramic, PINC distributes more stress to the abutment [59, 60]. A thicker restorative material will increase the restoration’s fracture resistance, according to some researchers who observed that prolonged tooth preparation will damage the remaining tooth structure and cause permanent failure [61, 62, 63, 64]. The findings of this study show that, regardless of the thickness of the crown, restored teeth using PICN CAD/CAM crowns can attain compression load values between 700 and 2500 N. Even in bruxism patients who are developing masticatory forces of 780 to 1120 N during mastication, these values are higher than the human masticatory forces (600–800 N). The conclusion was that the force generated by the physiologic masticatory process was less than the load placed on polymer-infiltrated ceramic network restorations. For posterior region restorations, CAD/CAM hybrid ceramic materials can offer enough fracture strength and load capacity.

The ideal dental restoration should present perfect marginal adaptation, biocompatibility with oral environment, esthetics, and long-term mechanical strength. Another study regarding the compressive strength evaluation of different CAD/CAM materials takes into consideration the oral conditions previously mentioned as crucial when choosing the appropriate material for a long-term rehabilitation case. Thin occlusal veneers processed from various CAD/CAM blocks can be an ideal alternative to restore tooth wear because of the development of new adhesive materials and procedures [63]. The current study’s goal was to evaluate the compressive strength of thin occlusal veneers manufactured of three distinct CAD/CAM supplies (Cerasmart, Straumann Nice, and Tetric CAD) before and after they were immersed in acidic artificial saliva. The results of the current study demonstrated that all three CAD/CAM restorative material types—nanoceramic, glass ceramic, and resin composite—are appropriate alternatives for patients who have tooth wear even when they have a 0.5 mm thin thickness and follow the proper cementation technique. When compared to occlusal veneers that were immersed in artificial saliva that was acidic and/or subjected to temperature cycling, the studied 0.5 mm thick occlusal veneers manufactured from CAD/CAM restorative materials showed higher compressive loads. Even the specimens that had been subjected to artificial saliva that was acidic had values that were higher than both normal and parafunctional bite forces. The composition of a restorative material generally determines its mechanical strength, although endogenous and/or exogenous variables (such as acidic foods or beverages, stomach acid, water sorption, cariogenic biofilm, or salivary enzymes) may also have an impact through material deterioration [64, 65, 66]. Due to the low pH level, endogenous acids deteriorate both dental structure and restoration [67]. The most recent composites and ceramic hybrid materials allow milling surfaces to be created even at a thin thickness, preserving the residual tooth structure. Comparable to reinforced ceramics, occlusal veneers (tabletops) made of composite resin blocks have superior fatigue resistance [68].

The findings of various investigations on the partial coverage of ceramic restorations are inconclusive. Even though most manufacturers advise posterior ceramic restorations to have a minimum thickness of 1.5 mm, several research studies have examined ceramic restorations with a thickness of 1.0 mm or even less that have excellent long-term clinical results [68, 69, 70, 71, 72, 73, 74].

The micro-shear bond strength of glass-ceramic materials, such as lithium disilicate ceramic, leucite-reinforced ceramic, and a hybrid ceramic, was examined in a different study [75]. The vitreous component, a crucial element in adhesive cementation, is a characteristic of both ceramic materials and hybrid ceramics. Adhesive cementation needs both chemical and micromechanical retention [76, 77, 78]. Resin cement have a better strength than self-adhesive cement, and they are typically dual-cured to ensure appropriate polymerization [79, 80]. Ceramic restorations and dentine need to have their surfaces pre-treated before cementation to obtain optimal adhesion and chemical and micromechanical retention [81, 82, 83]. When HF acid gets applied to a ceramic surface, the silica matrix reacts, causing the surface layer of the glassy matrix including silica, silicates, and leucite crystals to dissolve and be removed. According to the composition of the ceramics and the distribution of their crystalline and vitreous phases, each ceramic material exhibits a unique, original etching design. With an increased geometrical pattern in the ceramic structure, HF acid increases the bond strength between the restoration and adhesive, as well as the wettability and surface energy of the substrate. It also leaves behind a surface that is ready for the luting material to penetrate and diffuse into [84, 85, 86]. The results of our study demonstrated that a longer HF acid etching time caused more ceramic to dissolve, which produced a stronger link to resin cement. Still, there was not much of variation between the 30 and 60 s values, thus it was decided that for lithium disilicate, it was best to use either a higher acid concentration with the manufacturer-recommended period or a longer time [75].

4.2.3 Research on the biocompatibility of CAD/CAM materials

Biocompatibility is an important aspect of ceramic materials. According to literature, ceramic materials have greater biocompatibility and cell response compared to polymers in terms of their biocompatibility. In the past 10 years, numerous enhanced materials with superior mechanical properties have been created for CAD/CAM technology [45, 86, 87, 88, 89, 90, 91, 92, 93].

A recent study was focused on the biocompatibility and sustainability of human fibroblasts and keratinocyte cells on ceramic and composite CAD/CAM materials [81]. For this purpose, three ceramic and composite CAD/CAM materials (Cerasmart (CS)—nanoceramic resin; Straumann Nice (SN)—glass ceramic; and Tetric CAD (TC)—composite resin) from various manufacturers were subjected to testing.

There are not many studies that have been documented in the scientific literature about the biocompatibility of these kinds of CAD/CAM restorative materials. For instance, a recent study provided information on the biocompatibility and sustainability of three types of ceramics: Vita Enamic (EN), Cerasmart (CS), and Brilliant Crios (BC). Transmission electron microscopy (TEM) measurements of surface roughness, biofilm formation, cytotoxicity, genotoxicity, and cellular alterations led the scientists to the conclusion that there was no appreciable variation in surface roughness between the examined CAD/CAM blocks. Furthermore, there is no link between surface roughness and the development of biofilms. When cytotoxicity was considered, BC displayed the highest values, followed by CS and EN. As a result, EN was determined to be the evaluated materials’ most biocompatible substance [94]. A unique lithium silicate glass ceramic was created and developed by Daguano and colleagues, who also tested its biocompatibility in vitro. In contrast to other glass ceramics that already exist in the same family, the new lithia-silica glass ceramic is bioactive. This is the most significant discovery. It encouraged the formation of a bone-like matrix in MG-63 cells, which may be crucial for bone regeneration in and dental applications. Additionally, it promoted cell adhesion and growth [95]. As pointed in the preciouses’ studies, long-term oral exposure to the tooth-restoration hybrid necessitates consideration of the impact of intrinsic and extrinsic substances on these surfaces. Because acid from the stomach commonly refluxes into the or mouth cavity, patients with gastroesophageal reflux are the most vulnerable in this circumstance [51].

Considering the aforementioned information, the objective of our study was to examine the early response and biocompatibility of three dental materials when they encountered human keratinocyte and gingival fibroblast cells. To determine whether acidic artificial saliva could affect the biocompatibility of restorative materials when in contact with living organisms, it was intended to look at how the three different CAD/CAM restorative materials would differ in composition and structure in an acidic environment. Regarding biological activity, it was discovered how cytotoxic two different types of typical human cell lines—fibroblasts (BJ) and keratinocytes (HaCaT)—to three restorative materials—Cerasmart—CS, Straumann Nice—SN, and Tetric CAD—TC—were [93]. The results indicated that the examined samples can be classified as slightly cytotoxic and noncytotoxic in terms of the proliferation of human keratinocytes, except for SN_B, which recorded a reduced mitochondrial activity (about 50%). In terms of NO production, keratinocytes produced somewhat more NO than fibroblasts, while fibroblasts produced NO at a decreased rate that was cut to half. Determining cell oxidative stress on the nitric route is, therefore, not possible with the examined materials. When the fibroblast cells were set up on each compact dental material (CS, SN, and TC), they became attached and spread out, preferring the TC material. Yet when the fibroblast cells were exposed to the acidic environment of each material and tested, they ought to suffer the most [96].

5. Conclusion

• Chairside CAD/CAM restorative materials have many advantages such as their ease of preparation, polishing, reparability, and future studies, examining the substances released from CAD/CAM restorative materials and their effects will be significant.

• Even if there are many advantages of using the digital approach, there are certain factors that can influence the accuracy of a digital impression. The ambient light, the fluid isolation, scanning pattern and distance, or the rescanning of the same surface, all these variables can lead to a less accurate digital impression.

• Regarding the ambient light, there is no consensus regarding the ideal lighting conditions since different studies presented different results, but it is safe to assume that high illumination sources may oversaturate the scanned areas leading to artifacts in the digital model.

• Rescanning of an area is to be avoided. However, there are mixed reports regarding its influence in the accuracy of a scan.

• Furthermore, no single study has produced conclusive results about the best scanning pattern, and the majority of research indicates that the scanning technique has a major impact on scanning accuracy, even though other studies indicate that, with the scanners available today, the scanning pattern has no effect on the quality of the digital impression.

• The accuracy of a digital impression can be affected by the distance between the tip of the IOS and the recorded surface; maintaining approximately 5–10 mm can generate accurate digital models.

• The materials used in the digital workflow also play a key role in the success of the prosthetic treatments.

• Knowing the mechanical strength and esthetical properties of each class is essential for predictable treatments.

• While feldspathic ceramics are used mainly in the anterior arch due to their inferior strength and high esthetics, lithium disilicate and zirconia are well suited to withstand the occlusal loads of the posterior area.

• Hybrid ceramics are also an option presenting good flexural resistance and allow for more conservative treatments.