Dental CAD/CAM technology has been on the market for 30 years and has been becoming increasingly popular for esthetic restorations. This technology was developed to solve some challenges, such as working with high strength materials in one appointment and creating restorations with the best natural appearance, and it integrates dental clinic and dental laboratory in a single center.1 CAD/CAM blocks, when fabricated under optimal conditions, increases the intrinsic strength of restorations compared with that of laboratory processed restorations.2 Ensuring the best natural appearance could be handled by using all-ceramic materials. 34 However, all-ceramic materials may display some disadvantages. 5 The brittle nature of conventional ceramic materials potentially leads to catastrophic fractures.6 This phenomenon directly affects the restoration's lifetime. Reinforced glass ceramics were introduced to the market with higher fracture resistance of ceramic restorations. These materials still do not show satisfying mechanical properties after fatigue, under cycling loading.7 Recently, resin-ceramic hybrid materials were developed for CAD/CAM systems as a substitute for machinable ceramics and they were intended to eliminate the brittle behavior of all-ceramics under loading. The resin-ceramic hybrid materials existing in the market are: a resin-infused hybrid ceramic (Enamic, VITA), a resin nano ceramic (Lava Ultimate, 3M ESPE), and a hybrid nano ceramic (Cerasmart, GC Dental Products). Indeed, these materials have been made of resin and ceramic together and have properties close to each other.

The staining resistance of a restoration is as crucial as the fracture resistance and other mechanical properties.8 After a long time, inadequate color stability and staining may create a reason for a clinician to renew the restoration. It has been stated that color stability of a material is effected from these factors: 1) intrinsic factors (via aging), 2) extrinsic factors (plaque accumulation and surface staining from diet), and 3) surface degradation and absorption of staining agents.9 After material selection, a suitable surface finishing procedure could be used to minimize the effects of extrinsic factors. Ceramic materials could be glazed or polished with pastes, rubbers, and discs containing diamond micro-particles.10 Rubbers and discs have 2- or 3-step procedure and polishing could be accomplished using pastes.11 Rubbers, discs, and pastes can also be used for polishing composite resins. In addition, there are light cured liquid polishers for composite resins to achieve smoother polished surfaces. All these polishing materials improve surface smoothness,12 which is desirable for bacterial reduction and shiny appearance.13

Glazing is one of the most popular surface finishing procedures for ceramic restorations. It was shown that glazing is the best way for decreasing surface roughness of ceramic materials, compared with mechanical surface polishing systems.10 Moreover, Motro et al.14 found that glazed ceramic materials have shown less staining than mechanically polished materials. However, it is not possible to glaze the resin-ceramic hybrid materials under heat treatment because of their resin content.15 For this reason, other surface finishing procedures gain importance for resin-ceramic hybrid materials to recreate a surface similar to glazed ceramic surface. However, the effects of different finishing procedures applied to CAD/CAM resin-ceramic materials on staining resistance and staining resistance differences between a glazed CAD/CAM ceramic and CAD/CAM resin-ceramic materials finished with other procedures were unclear. Thus, the aim of this study was to evaluate the effect of different polishing procedures on the staining resistance of CAD/CAM resin-ceramic materials immersed into different staining solutions at 1 day, 1 week, and 1 month after polishing. The null hypothesis was that different polishing techniques would not affect the staining resistance of CAD/CAM resin-ceramic materials.

Four different types of CAD/CAM ceramic blocks and a paste-like composite resin (ME-Clearfil Majesty Esthetic, Kuraray, Tokyo, Japan) were used in this study. The ceramics were a feldspathic ceramic (C-Cerec Blocs, Sirona Dental Systems, Bensheim, Germany) and three resin-ceramics (L-Lava Ultimate, 3M ESPE, St. Paul, MN, USA; E-Enamic, VITA Zahnfabrik, Bad Säckingen, Germany; CS-CeraSmart, GC, Tokyo, Japan) (Table 1). The surfaces of the C sample group were glazed and resin-ceramic samples were divided into four groups according to following polishing materials: Eve Diapol ceramic polishing kit (ed), Bisco Biscover liquid polish (bb), Kuraray Clearfil Twist Dia composite polishing kit (kc), and the control group (c) without any surface treatment. Each of the ceramic groups was then divided into subgroups to be stored in different solutions for staining resistance study: tea (St), turkish coffee (Scf), fermented black carrot juice (Sbc), and distilled water (Sdw). The effects of four different polishing techniques and four different solutions on the color changes of four different ceramic materials and a composite resin material were evaluated. Each of the 24 subgroups consisted of eight specimens (n = 8, N = 192).

All of the specimens were prepared in A2 shade and cut from CAD/CAM blocks in 1 mm thickness by a low speed diamond saw (Isomet 1000) and the final thicknesses of the specimens were confirmed with a digital caliper for standardization.

The polishing procedures were applied in the following order:

In ED and KC groups, sample surfaces were polished for 30 seconds at each step. After the surface treatments, specimens were cleaned for 5 minutes with an ultrasonic bath.

In this study, each of the subgroups was stored in four different solutions that could cause discoloration and followed up after 1 day, 1 week, and 1 month. The specimens immersed in solutions were kept in oven at 37℃. The solutions were used without being diluted. The specimens were stored in opaque plastic containers and enumeration was performed in order to prevent confusion of containers. Solutions were renewed in three-day intervals in order to prevent bacterial contamination.

Measurements of surface discoloration before immersion and after immersion in different time intervals were performed by one researcher three times for each specimen in three different surfaces under the same environmental conditions. Each of the sample surfaces was washed under the running tap water before color measurement and dried with a sterile gauze. Color measurements were made using a white background with spectrophotometer (Shade pilot Degudent Inc., Hanau, Germany).

Color change (ΔE) for each sample was evaluated using the formula below:

ΔE = [(ΔL^*)² + (Δa^*)² + (Δb^*)²]^1/2

ΔL = L^*o - L^*t

Δa^* = a^*o - a^*t

Δb^* = b^*o - b^*t

L^*t, a^*t, b^*t: Before storage

L^*o, a^*o, b^*o: After storage

The Commission Internationale de I'Eclaire (CİE) measurements make it possible to evaluate the degree of color change based on three cordinates L^*, and a^*, and b^*. The L^* value measures the lightness of an object; a^*, a measure of redness (positive) or greenness (negative); and b^*, a mesure of yellowness (positive) or blueness (negative).

The Repeated Measures Analysis of Variance (ANOVA) between the time intervals and one-way ANOVA and Tukey's post hoc tests were performed for statistical analysis using IBM SPSS 20.0 software (SPSS Inc., Chicago, IL, USA).

ΔE values of the ceramics polished with different techniques are presented in Fig. 1 and Table 2.

At all time intervals, the highest staining resistance was acquired with composite polishing kit for all the restorations except for Majesty Esthetic. Also, the control group's ΔE values were higher than those of other polishing techniques.

At all time intervals, the repeated measures of ANOVA results showed that there was no difference among CEREC Bloc samples (P > .05). For LAVA Ultimate, there was no difference among composite polishing kit, ceramic polishing kit, and liquid polish (P > .05). ΔE values of these three groups were significantly lower than those of the control group (P < .05). For CeraSmart, there was no difference between control and liquid polish, and between composite polishing kit and ceramic polishing kit (P > .05). ΔE values of control and liquid polish were significantly higher than those of composite polishing kit and ceramic polishing kit (P < .05). For Majesty Esthetic, ΔE values of liquid polish were significantly lower than those of other groups except for composite polishing kit. ΔE values of control group were significantly higher than those of other groups (P < .05). There was no difference between composite polishing kit and ceramic polishing kit (P > .05).

For Enamic, at all time intervals, ΔE values of the control group were significantly higher than those of composite polishing kit (P < .05). There was no difference between composite polishing kit and ceramic polishing kit (P > .05). At 1 day after immersion, ΔE values of composite polishing kit and ceramic polishing kit were significantly lower than those of liquid polish. However, there was no significant difference between 1 week after immersion and 1 month after immersion.

For restorative materials, separate one-way ANOVA and Tukey analysis of the mechanically polished samples showed that there was no difference between Enamic and CeraSmart, and between Lava Ultimate and Majesty Esthetic (P > .05). The staining resistances of Enamic and CeraSmart were significantly higher than those of Lava Ultimate and Majesty Esthetic (P < .05). For polishing kits, the staining resistances of ceramic polishing kit and composite polishing kit were similar with that of glaze (P > .05), and significantly higher than those of liquid polish and control groups (P < .05).

The null hypotheses tested in this study were partially rejected; the stain resistances of the samples polished with composite polishing kit and ceramic polishing kit were significantly higher than those of the samples polished with liquid polish. Also, glazed ceramic samples showed higher stain resistance than the samples finished with liquid polish (P < .05), and no difference was found between composite polishing kit and ceramic polishing kit (P > .05).

There are varying factors causing color changes in restorations: plaque accumulation, pigmentation effect of staining solutions, dehydration, water absorption, surface roughness, and chemical degradation. In the present study, the efficacy of different polishing techniques against staining was evaluated and compared with that of glaze.

Several studies reported that glaze was superior to other polishing techniques in terms of surface properties and staining resistance.1416171819 Since it is not possible to glaze hybrid resin-ceramics, the main objective of this study was to evaluate some commercially available materials on their effectiveness with comparison to glaze. Due to mixed compositions of resin-ceramics that are neither pure ceramic nor pure composite resin, there is no conventional polishing material existing in the market. Thus, it is aimed to test effectiveness of a composite polishing kit, a ceramic polishing kit, and a liquid polish. Recently, the staining resistance of resinceramics polished with silicon carbide abrasive papers was evaluated in a few studies.202122 However, this technique is not useful in clinical practice. The polishing materials used in this study were applicable to all restoration surfaces and gave immediate results, which gave them real clinical usability.

In this study, staining resistances were evaluated by immersion of samples to solutions that are consumed frequently in daily life. Distilled water was chosen for control. Throughout the study, the samples were immersed in different solutions for 1 month, similar with the study of Ertas et al., which should be equivalent to about 2.5 years of clinical ageing.

Based on the previous studies, ΔE values greater than 3.3 are considered to be clinically unacceptable.232425 A month after the immersion, unpolished Lava Ultimate and Majesty Esthetic groups showed dramatically higher ΔE values than 3.3. ΔE values were close to 4 in unpolished Enamic and CeraSmart groups. These results could be attributed to water absorption properties of monomers. Monomers may also have absorbed the pigments from the staining solutions. Both Lava Ultimate and Majesty Esthetic contain Bis-GMA, distinct from Enamic and CeraSmart. Water absorption potential of Bis-GMA was shown to be higher than those of UDMA, TEGDMA, and Bis-EMA.232627 According to previous studies,2128 ceramic materials exhibited better color stability than composite resins. Also, Acar et al.21 found that discoloration of Lava Ultimate was more than that of Enamic, which is in agreement withthe results of the present study. However, they claimed that the color change of Lava Ultimate was clinically unacceptable, which is different from our results. This may be caused by the polishing techniques (silicon carbide papers 600, 800, 1200 grit) different from those used by the other researchers.

Further, for the results of the unpolished control groups, the highest staining resistance was found for glazed ceramic group. Also, all of the finishing procedures for Lava Ultimate and Enamic were acceptable. Liquid polish for CeraSmart, and composite polishing kit and ceramic polishing kit for Majesty Esthetic showed higher discoloration than the clinically acceptable value. Unlike CeraSmart, liquid polish performed well with Majesty Esthetic. Also, the highest staining resistance values were obtained with composite polishing kit and ceramic polishing kit for CeraSmart groups. The staining resistance of the control group was better than that of liquid polish for CeraSmart. This may be related to porosities on the surface of the liquid polish caused by insufficient coherence with the underlying ceramic. This can be explained by steric hindrance: The central oxygen atoms, existing in the organic matrix of both liquid polish and CeraSmart, push each other and may affect the molecule's preferred shape. Unlike CeraSmart, dipole-dipole force between central carbon atoms in PMMA (organic matrix of Enamic) and central oxygen atoms in the organic matrix of liquid polish may strengthen the bonding. Further studies with different liquid polish materials are required to better understand the staining resistance of resin-ceramics.

In this study, it can be concluded that, polishing is crucial for all materials as dramatical changes were observed between polished and unpolished groups. Ceramic polishing kit and composite polishing kit ensure satisfactory results in resin-ceramics. Liquid polish best performs when used on composite resin. Also, liquid polish may have unsatisfactory results on some resin-ceramics; as unexpected color changes occurred in CeraSmart.