1. Introduction
One of the tremendous advancements in the fast-evolving dental field is computer-aided design and computer-aided manufacturing (CAD/CAM) technology, which has significantly improved the provided dental care [
1,
2]. Digital technology will help to provide treatment in a more efficient way than conventional methods in terms of time, effort, and cost-effectiveness [
3,
4,
5]. Recent breakthroughs in the CAD–CAM systems have made them more user-friendly and less costly with more precise outcomes [
4,
6]. Clinicians should be aware of the advantages and disadvantages of different dental restorative materials as CAD/CAM technology and digital dentistry have become more widespread in routine dental practice [
1].
Ceramics is one of the materials used in dentistry as a restorative material. Ceramic restorative materials are categorized as glass-matrix, polycrystalline, or resin-modified ceramics, depending on their formulations [
7]. Resin-modified ceramic materials are further subdivided into resin composites comprising glass particles incorporated in a polymer matrix (e.g., Lava Ultimate, LU and Cerasmart, CS) and porous interconnecting feldspathic porcelain infiltrated by a polymer (Vita Enamic, VE) [
7]. A recently introduced resin-modified ceramic for the CAD/CAM technique is Crystal Ultra (CU), which has a higher polymer content than VE. The manufacturer of CU (
https://crystalultra.com, accessed on 18 August 2022) claims that it is the strongest, lightest, and most durable elastic resin-modified ceramic available in the market as of today. Resin-modified ceramics incorporate the advantageous properties of ceramics, mainly the color stability, strength, and elasticity comparable to human enamel, with low abrasiveness, elasticity simulating human dentin, and repairability benefits of resin composite [
8,
9,
10,
11,
12,
13].
During try-in of the restoration, there might be some adjustments required, either in occlusal or interproximal, which are performed with a rotary diamond bur. This process removes the superficial glaze, thereby affecting the surface properties of the restoration [
14]. Consequently, the aesthetics of the restoration, color scale, and color stability are affected [
15,
16]. In addition, decreased flexural strength and increased wear of the opposing tooth might accompany increased surface roughness [
17,
18].To guarantee the long-term success of dental prosthesis, the surface-modified restoration needs to be polished using the best available technique [
19,
20]. Resin-modified ceramics materials’ manufacturers recommend finishing and polishing using diamond-impregnated rubber points and a soft Robinson brush (#9). Another way to restore the smoothness of the surface is with a light-cured glazing product [
6].
The aspect of color is very important in aesthetic dentistry. That said, the important characteristic that could be affected after adjusting the restoration clinically, followed by repolishing or reglazing, is color stability (∆E) [
21]. The restoration adjustment impacts the ∆E because of the direct relationship between color change and surface roughness; a rougher surface will have more staining than a smoother surface [
21]. Accordingly, the most suitable method should be followed to regain the smoothness of the surface and to have less color change. There are many factors that have a role in the ∆E; one of them is the nature of the material itself. The objective color measurement in dentistry is performed either by spectrophotometers or colorimeters. A spectrophotometer, however, offers more accurate readings since it allows wavelength-by-wavelength spectrum measurement of the object’s transmission and reflection characteristics [
22,
23]. Similarly, the clinical adjustment and repolishing or reglazing of the restoration also affects flexural strength. Flexural strength is the limit that can be tolerated by the material without deformation when subjected to bending force [
24]. It is a crucial feature of a material, and the required limit depends on its clinical application and the amount of the applied masticatory force [
25]. It is well-documented that adjusting the restorations followed by repolishing or reglazing might influence the flexural strength of the zirconia restorations negatively [
26,
27]. However, the data regarding the effect of surface finishing of resin-modified ceramics is limited or unavailable. The three-point and the biaxial tests are the most common methods to measure flexural strength; however, the biaxial test is the test of choice of many researchers since it does not cause the problem of edge fracture [
24]. The international standards organization (ISO-6872) recommends the piston-on-three-balls test for determining the flexural strength of dental ceramics [
28].
The usage of resin-modified ceramics for clinical use (crowns, bridges, and veneers) has significantly increased in recent years, and at the same time, many new products have entered the dental market. However, there is little to no information available on the impact of various surface finishing protocols on the color changes and flexural strength of these newer ceramics. This study compared four resin-modified CAD/CAM ceramic materials’ color changes and flexural strength following various surface finishing protocols. The first null hypothesis is that the studied CAD/CAM materials do not significantly differ in color change after various surface finishing protocols. The second null hypothesis is that the studied CAD/CAM materials do not significantly differ in flexural strength after various surface finishing protocols.
4. Discussion
Resin-modified ceramics are considered a viable substitute for glass ceramics which perform clinically satisfactorily for prosthodontic management in areas with moderate chewing loads. Despite having similar mechanical qualities to conventional CAD–CAM ceramics, resin-modified ceramics are deemed aesthetically pleasing [
36]. This study aimed to determine the influence of various surface finish protocols on the color change and BFS of resin-modified ceramics. According to the current study’s findings, the investigated resin-modified ceramic materials and surface finish protocols significantly affected the color difference (
p < 0.05). Therefore, the stated first null hypothesis had to be rejected.
As it is one of the most consumed drinks worldwide and is more chromogenic than other beverages, coffee was used as a staining agent in this study. Furthermore, coffee’s pH, which ranges from 4.9 to 5.2, will expedite the discoloration process. Discoloration brought by coffee has been related to ingredients, including tannin and chlorogenic acid [
35]. Color is influenced by surface finishing techniques and roughness, as previously described [
21]. A human observer can detect a color change that is perceptible as a color difference. A 50% of spectators can detect a color difference if the perceptibility threshold (PT) is set at 50%. Similarly, a 50% acceptance threshold (AT) refers to a color change that 50% of spectators deem acceptable [
37]. Accordingly, the reference values of the PT and AT are ΔE ≤ 1.2 and ΔE = 2.7, respectively. Any ΔE > 2.7 is considered clinically unacceptable [
37].
In the current study, none of the surface-finished discs demonstrated ΔE values below the PT; however, the values were within the AT limit (ΔE = 2.7). The color changes in the control specimens of nanoceramics (LU and CS) were above the AT limit. The color change in the specimens after surface finishing with MP was 1.96 ± 0.43, and after using polishing paste MP+PP, it was reduced to 1.85 ± 0.35. The two values were statistically insignificant, and the color change was above the PT but well below the AT. This outcome demonstrates that both surface finishing was comparable in terms of color changes in the materials. In a previous study, Acar et al. [
38] studied the influence of various polishing techniques on the Ra and ΔE of CS, LU, and VE. They found that the diamond paste polishing alone produced an acceptable Ra and ΔE, while the two-stage diamond-impregnated polishers resulted in increased color change, and hence they were deemed unsuitable for the tested materials.
The current study findings confirm that glazing (OG) makes the surface more stain resistant. The influence of OG, a light-polymerizing clear resin covering on ceramics, has received little attention in the literature. One study explored the ΔE of 3D-printed interim restorations following surface finishing with aluminum oxide polishers and OG [
39]. After six months of immersion in different staining solutions, they discovered that OG caused the least color change [
39]. This could be explained by OG’s ability to infiltrate the surface, which reduces the restoration’s surface permeability, fills in nanopores, and reduces leakage into the restoration surface [
40]. Kursoglu et al. [
21] investigated the relationship between the surface texture obtained by different polishing procedures and the resultant stainability of IPS ceramic discs after 12 days of immersion in a coffee. They found that glazing resulted in the smoothest and the most stable surface.
The polishability is not the sole determinant of color change, but the composition of the different materials, as well as the variations in polymer-to-filler ratio, have a significant impact on polishability and, hence, the color change. The LU and CS exhibited the highest color change, which can be attributed to both resin nanoceramic clusters and the presence of Bis-GMA, hydrophobic UDMA, and hydrophilic TEGDMA in their composition [
41]. On the other hand, CU and LU are polymer-infiltrated ceramic networks that are created by first producing the ceramic network and then crosslinking the polymer with the existing ceramic network by capillary action [
9]. The heat treatment to form a dense polymer network [
9] could possibly explain the least color change in VE and CU materials.
The BFS test is among the common methods for determining fracture strength and predicting the forces a material can withstand [
24]. The current study’s findings indicate a substantial variation in the BFS of the investigated materials following different surface finish protocols (
p < 0.001). The second null hypothesis that the studied CAD/CAM materials do not significantly differ in BFS after various surface finishing protocols was rejected. The OG protocol resulted in the highest BFS. There is no study that the authors are aware of that evaluates the effect of OG surface finish on the resin-modified ceramic’s flexural strength. However, Thompson et al. [
42] in their study assessed the effect of surface sealing on polymer-based provisional materials. They found that surface sealing improves the mechanical properties, including the flexural strength of the tested materials. On the contrary, Çakmak et al. [
43] demonstrated that surface sealing does not affect the flexural strength of polymer-based provisional materials. Their contradicting conclusions about using the same glaze (Palaseal glaze) can be attributed to the different brands of the materials tested.
The OG (154.75 ± 3.05 MPa) and MP+PP (153.33 ± 2.78 MPa) demonstrated the highest BFS values, and there was an insignificant difference between the two protocols among the tested resin-modified ceramics, except for VE (
p > 0.005). Earlier studies have demonstrated an inverse association between BFS and Ra, which implies that a smoother surface has a higher BFS [
44,
45,
46]. Mohammadibassir et al. [
27] explained the effect of polishing paste in reducing Ra and stated its ability to round the profile, making the surface more uniform and smoother. The composition of the polished material has an impact on polishability and, accordingly, the BFS. This fact can partially explain why VE demonstrated a significant difference (
p < 0.005). VE has the lowest percentage of polymer (14% of its composition) compared to the other tested materials (20–30% of their composition) [
41], and manual polishing using polishing paste may not smoothen the higher percentage of incorporated fillers.
When compared to OG and MP+PP, the MP resulted in a lower BFS and a higher color difference. Based on these findings, it is apparent that the MP protocol is not effective at smoothening the surface compared to other protocols. Because of the heterogenicity in the composition of the resin ceramics, using MP without subsequent polishing paste might be insufficient. This is particularly evident in the CU material, as there is no statistically significant difference between MP and control (p > 0.005).
Fractography, a useful technique for failure analysis in dentistry, is based on the evaluation of microscopic fracture surface characteristics that reveal the cause of failure and analyze the extent of the crack [
47]. Descriptive fractography in the current study shows many features, including compression curls, crack arrest lines, hackles, and delamination defects. Compression curls are always present on the opposite side of the fracture origin, and it is a sign that the specimen has a bending characteristic after loading [
48]. A crack arrest line is another feature that is defined by a sharp line as a result of a change in the primary tension, indicating crack propagation in a different direction [
48]. There is also another feature that can help in locating the origin of the fracture, which is the hackle because it points toward it [
47].
The present study had a few shortcomings, which could limit generalizing the outcome of the study. The specimens were stained on both surfaces of the disc, which does not reflect the real clinical scenario in which the restoration is exposed to solutions or beverages only on the exterior. Secondly, the intra-oral conditions such as the patient’s saliva, diet, acidic beverages [
49], oral hygiene protocols [
50], and dentifrices [
51] that could influence the ΔE or the BFS were not considered in this study. Thirdly, various specimen thicknesses may have different effects on the ΔE or the BFS compared to a single uniform thickness used in this study.
Future research should consider patient-related aspects such as smoking, saliva, and abrasive dentifrice brushing effect on ΔE or the BFS. It is vital to investigate the color change in resin-modified ceramics in different beverages, especially acidic beverages. Furthermore, different thicknesses of the tested CAD/CAM materials should be tested with regard to ΔE and BFS.