Evaluation of the colour and translucency parameter of conventional and Computer-aided design and computer-aided manufacturing (CAD-CAM) feldspathic porcelains after staining and laser-assisted bleaching.

Aims: The present study aimed to evaluate the color and translucency changes of the feldspathic porcelain samples made using the conventional or computer-aided design/computer-aided manufacturing (CAD/CAM) methods after immersion in staining solutions and laser bleaching. Materials and
Methods: A total of 72 samples of feldspathic porcelain were fabricated using the conventional (n = 36) or CAD/CAM (n = 36) methods. Afterward, each group was randomly divided into three subgroups (n = 12), which underwent exposure to coffee, orange juice, or distilled water for 6 weeks. Then, the samples underwent diode laser-assisted bleaching. A spectrophotometer assessed the samples' color and translucency preintervention, after staining, and after bleaching. Statistical Analysis Used: One-way and two-way analysis of variance and independent t-test were used. The significance level was 0.05.
Results: Staining for 6 weeks significantly changed the color of both types of feldspathic porcelains. Furthermore, there were significant differences between the ceramics made using different methods and immersed in different staining solutions (P < 0.001). Furthermore, laser-assisted bleaching changed the color of immersed specimens significantly (P = 0.005). However, there were no significant translucency changes in all the subgroups after staining (P = 0.215) and laser bleaching (P = 0.325). Conclusions: The staining-induced discoloration of all the subgroups was in the clinically acceptable range (ΔΕ<3.3). In addition, bleaching could effectively remove the stains in both types of ceramics while not affecting the primary color. Moreover, the translucency of the ceramics was not affected by staining or laser-assisted bleaching. Copyright:

INTRODUCTION

Optimally, esthetic restorative materials should have the same color and translucency as natural teeth and should preserve these properties in the long term.[1] Therefore, few materials can be used for this purpose. Feldspathic porcelains are among the well-established esthetic dental ceramics used extensively with successful results. These ceramics are especially popular for their eminent glassy phase, high translucency, and excellent biocompatibility[2] and can be fabricated using the conventional or computer-aided design/computer-aided manufacturing (CAD/CAM) methods. In the conventional method, the feldspathic porcelains are incrementally sintered, with their incisal edge translucency and emergence profile thoroughly controlled.[3] This method is quite time consuming and technically sensitive, which makes the outcome less predictable. On the other hand, CAD/CAM systems have surpassed the traditional methods by their shortened chair time and strengthened ceramic blocks, leading to homogenous restorations with minimum imperfections.[4] However, given the high overall cost, challenging learning curve, and mechanical limitations in forming fine margins and sharp edges in the CAD/CAM method, the conventional method is still extensively used for manufacturing feldspathic veneers.[5]

Considering the increasing esthetic demands, applying bleaching materials on a dentition with preexisting laminate veneers has become a topic of interest. However, the studies investigating the application of bleaching agents on the ceramic color have reported controversial results. Several have reported an undesirable discoloration, leading to the replacement of restorations,[678] while others have reported the effectiveness of bleaching in stain removal and lack of significant changes in the primary color of the dental ceramics.[9101112] These controversies are presumably due to these investigations’ different methods and materials.

According to recent studies, heating may increase the effectiveness of the bleaching process, which can be due to the faster release of free radicals and increased penetration of the bleaching agents.[13] Moreover, some studies have suggested the simultaneous use of a low-power laser for activating the bleaching gel, which may result in the release of higher amounts of hydrogen peroxide in a shorter duration, subsequently leading to shortened treatment duration by half compared to the conventional bleaching.[14]

Therefore, the present study aimed to evaluate the color and translucency changes of the feldspathic porcelain samples made using the conventional or CAD/CAM methods after immersion in coffee, orange juice, and distilled water. To the best of the authors’ knowledge, this is the first study comparing the stainability of the feldspathic porcelain samples made using the conventional and CAD/CAM methods. Moreover, a laser-assisted bleaching method was used with 35% hydrogen peroxide and 940 nm diode laser as the safest additional energy source with the slightest adverse effects on dental ceramics.

The null hypotheses of the present study were as follows: (i) the color of the feldspathic porcelains will not change significantly after immersion in coffee and orange juice compared to the samples immersed in distilled water, (ii) the laser-assisted bleaching has no significant effect on the color and translucency of the samples, and (iii) there will be no significant difference between the feldspathic porcelains made using the conventional method and those fabricated using the CAD/CAM method in discoloration and bleaching.

MATERIALS AND METHODS

As per a previous study by Saba et al.,[15] α = 0.05, β = 0.2, the mean standard deviation of 0.2 for the color changes, and an effective side of 0.86, the minimum sample size required for each subgroup was calculated to be n = 18. Therefore, a total of 72 feldspathic porcelain samples were used, of which 36 samples were made using the conventional method (VITAVMK Master, Vita Zahnfabrik, Bad Sackingen, Germany), and 36 samples were made using the CAD/CAM method (VITABLOCS Mark II, Vita Zahnfabrik, BadSackingen, Germany). The chemical compositions of the materials used in the present study are presented in Table 1.

Table 1

Study materials in detail

Material	Type	Composition	Manufacturer	Lot number
Vita VMK master	Feldspathic porcelain	KAISi3O8, NaAISi3O, SiO2, Metal oxides	VITA Zahnfabrik, Bad Sackingen, Germany	B4803912
Vitablocs Mark II Laserwhite 20	Feldspathic porcelain Tooth whitening gel kit	KAISi3O8, NaAISi3O, SiO2, Metal oxide Activator: Proprietary dye Base: Hydrogen peroxide 45% The mixture of 2 syringes result in 35% hydrogen peroxide	VITA Zahnfabrik, Bad Sackingen, Germany BIOLASE, Irvine, California, USA	EC42M2CI14 BGX86

To prepare the conventionally made samples, an initial block of 12 mm × 14 mm × 2.5 mm using the modeling wax (Cavex Holland BV, Haarlem, Netherlands) was created and duplicated using a putty impression material (Panasil Fast Putty, Ketenbach Dental, Eschenburg, Germany) to yield 36 identical wax samples. Moreover, the refractory material (Nori-Vest, Kuraray Noritake, Hattersheim, Germany) was used to prepare negative molds for the conventional layering technique. The 2M2-shade porcelain powder was applied to the molds and put into the sintering furnace at a temperature of 930°C based on the manufacturer's instructions. The detailed procedure is presented in Figure 1. For the second group of samples, five pieces of 2M2-shade CAD-CAM blocks with the dimensions of 12 mm × 14 mm × 18 mm were sectioned into 36 samples with 2.2 mm thickness using a low-speed microcutting machine (Mecatome T201 A, Grenoble, France) under water cooling. Then, the top surfaces of the samples in both groups were polished by polishing Kit (ZilMaster, Shofu Dental Corp, California, USA) containing coarse and medium discs for finishing and pre-polishing, and fine discs for super polishing (using electric handpiece at 20000 rpm with a total of 1.5 N pressure). Polishing paste (Renfert all in one, Hilzingen, Germany) was also applied using a prophylactic cup on electric handpiece at 15,000 rpm for 10 seconds. A gauge was used to confirm the uniform final thickness of 2 mm. The detailed procedure is presented in Figure 2.

Figure 1

The steps of preparing the conventionally made porcelain samples. (a) The mold was formed using wax molds. (b) The refractory base was poured. (c) Porcelain tablets were prepared using conventional layering

Figure 2

The steps of preparing the CAD/CAM-milled porcelain samples. The CAD/CAM blocks with 2M2 colour shade were mounted in the polyester material. Afterwards, they were sectioned using a low-speed diamond saw (Mecatome T201 A) under water cooling

In the next step, the color changes (ΔE) and translucency parameter (TP) of all the samples were assessed and recorded using a spectrophotometer. Afterward, the samples in each group of 36 were divided into three subgroups of 12, and each subgroup was immersed in 5 ml of specific staining solution at a temperature of 37°C in a water bath for 6 weeks to simulate 4 years of clinical usage. The staining solutions used included coffee (Nescafe RED MUG, Nestle, Tehran, Iran), orange juice (Sunich Nectar, Tehran, Iran), and distilled water, which was the control. The coffee was made using 1 g of coffee powder in 10 mL of distilled water.[15] The solutions were replaced daily to prevent microbial growth.

Following the 6-week duration, the samples were rinsed with distilled water and air dried for 1 min, and their color and translucency were reassessed. Afterward, each group of 12 was divided into four subgroups (n = 3 in each subgroup) to simulate the dental quadrants. According to the manufacturer's instructions, two cycles of diode laser (Epic X BIOLASE, Irvine, California, USA) with a wavelength of 940 ± 10 nm were required to create 6–12 shades of color change in each bleaching session. Therefore, approximately 1 mm of whitening gel (Laserwhite 20 Whitening Gel Kit, Irvine, California, USA) was applied on the surfaces of the samples and exposed to two doses of laser exposure using a special bleaching handpiece with a power of 7 watts for 30 s. The procedure was repeated using a new mix of whitening gel, and the bleaching gel was left on the ceramics for 5 min before removal. Finally, the color and translucency of the samples were assessed for the last time. The chemical composition of the whitening gel is presented in Table 1.

The color assessments were performed using a calibrated reflectance spectrophotometer (SP64Portable Sphere Spectrophotometer, X-Rite, Hong Kong, China) with the reflectance wavelength of 400–700 nm and 10 nm intervals based on D65 illuminant.

Each sample was aligned horizontally and vertically in the center of a standard black and white background until a green signal showed that the sample was ready for measurement for color and translucency assessments. Then, the parameters of L*, a*, and b* were measured. L* represented the lightness and ranged from 0 (black) to 100 (white), a* represented the color of the sample in a red-green scale, and b* indicated the color in a yellow-blue scale. Each of these parameters was measured thrice, and their mean values were calculated and recorded. Finally, the color difference (ΔE) between two separate assessments was calculated using the following formula by the Commission International de l’Eclairage (CIE):[912]

The TP was defined as the difference between the color of a given sample assessed when the background was white and the color of the same sample with a black background. TP was calculated using the following formula:[6]

Eventually, the mean values of ΔE1, ΔE2, ΔEtotal, TP0, TP1, and TP2 were calculated for each subgroup of 12. The ΔE1 was defined as the color difference between the preintervention and poststaining assessments, while ΔE2 was defined as the color difference between the poststaining and post-bleaching assessments. Additionally, ΔEtotal was defined as the total color difference between the preintervention and postbleaching assessments. Furthermore, TP0, TP1, and TP2 were defined as the mean translucency of each subgroup assessed at the baseline, after staining, and after bleaching, respectively.

Statistical analysis was performed using the SPSS version 25 (IBM SPSS Statistics 25, IBM, Armonk, NY, USA). At first, the normal distribution of the study data was investigated using the Kolmogorov–Smirnov test. Moreover, the one-way analysis of variance (ANOVA) and Turkey's post hoc tests were used to compare the ΔE and TP between different staining solutions in each group of ceramics made using the same method, the independent t-test was employed to compare ΔE and TP between the samples made using different methods based on the staining solution. Finally, the two-way ANOVA investigated the ΔE and TP changes between different assessments based on the manufacturing method and staining solution. The P < 0.05 was considered statistically significant, while the P < 0.001 was considered highly significant.

RESULTS

According to the two-way ANOVA results, by considering the main factors of staining solution and manufacturing method, the discoloration after 6 weeks of immersion was highly significant (ΔE1, P < 0.001), with the samples immersed in coffee showing a higher discoloration compared to those immersed in orange juice. In addition, according to the independent t-test results, the conventionally made samples had a significantly higher discoloration after staining in coffee compared to the CAD/CAM-made samples (ΔE1, P = 0.007). Bleaching caused a significant color change in all the samples immersed in coffee and orange juice compared to the poststaining color (ΔE2, P = 0.005). In contrast, the samples immersed in distilled water, whether conventionally or CAD/CAM made, did not show a significant discoloration after staining and bleaching (ΔE1 and ΔE2, P > 0.05). Furthermore, no significant color changes were observe after bleaching processes compared to the baseline color of the same subgroup (ΔΕtotal, P = 0.534). The descriptive data on color changes are presented in Table 2.

Table 2

The mean (standard deviation) of the colour changes (∆E) of all the feldspathic porcelain subgroups at the baseline, poststaining, and postbleaching assessments

Solutions	Materials	ΔΕ1	ΔΕ2	ΔΕtotal
Water	VMK master	0.28a (0.19)	0.83e (0.24)	0.80i (0.32)
	Mark II	0.20c (0.10)	0.22g (0.09)	0.32j (0.11)
Orange juice	VMK master	0.52a (0.37)	1.67e (0.98)	1.58i (1.15)
	Mark II	0.64d (0.17)	0.42g (0.27)	0.87k (0.09)
Coffee	VMK master	2.08b (0.65)	3.18f (1.12)	1.36i (0.65)
	Mark II	0.98d (0.33)	0.73h (0.25)	0.36j (0.08)

†Different superscript letters (a,b,c,d, etc.) in the columns indicate the significance of the differences in a group of dental ceramics made using the same method and immersed in different staining solutions. ΔΕ1: Color change after staining compared to the baseline color, ΔΕ2: Color change after bleaching compared to the poststaining color, ΔΕ total: Total colour change after bleaching compared to the baseline color

Regarding the TP changes investigated using the two-way ANOVA, immersion in staining solutions (P = 0.215) and bleaching (P = 0.325) did not significantly change the samples’ translucency. The descriptive data on the translucency of the samples are presented in Table 3.

Table 3

The mean (standard deviation) of the translucency parameter values of all the feldspathic porcelain subgroups at the baseline, poststaining, and postbleaching assessments

Solutions	Materials	TP0	TP1	TP2
Water	VMK master	2.02a (0.62)	2.08c (0.42)	2.14f (0.75)
	Mark II	9.62b (1.46)	9.67d (1.65)	9.71g (1.56)
Orange juice	VMK master	2.72a (0.54)	2.78c (0.63)	2.56f (0.73)
	Mark II	9.50b (0.79)	9.24d (0.69)	9.32g (0.81)
Coffee	VMK master	2.56a (0.59)	2.19c (0.45)	2.77f (0.50)
	Mark II	9.35b (0.67)	8.74e (0.62)	9.23g (0.76)

†Different superscript letters (a,b,c,d, etc.) in the columns indicate the significance of the differences in a group of dental ceramics made using the same method and immersed in different staining solutions. TP0: Preintervention translucency parameter, TP1: Poststaining translucency parameter, TP2: Postbleaching translucency parameter, TP: Translucency parameter

DISCUSSION

Feldspathic porcelain is one of the most prevalent ceramic options used for esthetic indications. This ceramic is fabricated by conventional layering or CAD/CAM milling and can be polished or glazed for final esthetic outcomes. The present study investigated this highly esthetic dental ceramic's ability to preserve its color and translucency in the long term and its vulnerability against staining or chemical materials prevalently found in the oral cavity. In addition, the current study selected the polishing of the ceramics rather than glazing them to decrease the mirror reflection, which can interfere with the accurate color assessment.[16] According to Heffernan et al., diverse degree of glazing in the same group can affect translucency.[17] From a clinical perspective, many clinicians call for in-office polishing of the restorations to perform the occlusal or profile adjustment at the delivery session. Hence, the stainability of the polished ceramics was investigated instead of glazed ones.

According to the present study results, the null hypotheses were rejected because the color changes of the conventionally layered (VITA VMK Master) and CAD/CAM-made (VITABlocs Mark II) samples after immersion in the staining solutions for 6 weeks were highly significant (P < 0.001). According to previous studies, the ΔE <1 is not visible to the naked eye, while the ΔE >3.3 is clinically unacceptable.[9] In the present study, conventionally, layered samples immersed in coffee showed a noticeable color change (ΔΕ>1). However, the poststaining discoloration was clinically acceptable for all the subgroups (ΔΕ<3.3). In addition, the color change of the conventionally layered samples was significantly greater than the CAD/CAM-made samples after immersing in the coffee solution (P = 0.007). This finding can be explained by the fact that CAD/CAM-made blocks have a more homogenous structure with minimum structural defects than the handmade porcelain samples, and the porosity caused by the conventional method may lead to a higher discoloration in the samples made using this method.[415]

Furthermore, Ghahramanloo et al. showed the minimal discoloration of the Vita VMK 95 ceramics (ΔE: 0.21–2.51) after immersion in the staining solutions at a temperature of 50°C for 30 days, which was in the range of clinically acceptable.[18] Another study by Lauvahutanon et al. revealed that VITABlocs Mark II had the lowest water absorption ability among all the ceramics and composite blocks.[19] The phenomenon of water absorption by dental materials leads to their expansion and formation of microcracks, resulting in pigment absorption and discoloration.[20]

According to the present study's results, bleaching caused a significant color change in stained samples (ΔE2, P = 0.005), which was clinically noticeable for the conventionally layered samples immersed in orange juice (ΔΕ2 = 1.67) and coffee (ΔΕ2 = 3.18). By contrast, these changes were neither significant nor detectable by the human eyes for the CAD/CAM-made samples and those immersed in distilled water (ΔΕ2 <1).

In addition, all the feldspathic porcelain samples showed an insignificant difference after laser bleaching compared to their baseline color assessments (ΔΕtotal), which can be explained by the hydrophobic properties of the feldspathic ceramics, leading to the stain deposition on a more external surface that can be effectively removed by hydrogen peroxide.[9] These findings are compatible with the previous studies on the same topic. For example, Alharbi et al. reported that hydrogen peroxide (40%) could effectively remove the pigments from VITABlocs Mark II ceramics that were stained for 120 days, leading to a clinically acceptable residual color.[9] In addition, Rodrigues et al. showed that brushing cycles combined with bleaching agents could lead to a shade reduction of up to 6° in the glazed VMK Master porcelain samples. However, bleaching materials could not affect the samples’ color without simulated toothbrushing.[21] Furthermore, Juntavee et al. reported that LED-assisted bleaching with 35% hydrogen peroxide significantly changed the color of the CAD/CAM-made ceramics of Vita Enamic, IPS Emax, and two types of zirconia blocks. However, it did not affect the Lava Ultimate ceramics significantly. This finding was because the crystalline structure of the lithium disilicate found in the IPS Emax ceramics tends to be dissolved by hydrogen peroxide compared to the feldspathic porcelain.[22]

According to our findings, the staining and bleaching processes did not significantly affect the samples’ translucency. Karci et al. reported that the effectiveness of bleaching using 16% carbamide peroxide was significantly correlated to the depth of the free radical penetrations. Accordingly, the Empress CAD ceramics’ translucency showed significantly lower changes than the Emax CAD ceramics because of their larger crystal sizes (1–5 μm) than the Emax CAD (0.21 μm). Therefore, the translucency of the Mark II and VMK Master ceramics investigated in the present study was not affected by the bleaching agents because they had large crystalline particles (4 μm and 19 μm, respectively).[6]

A recent study by Al-Maliky reported that the bleaching gel pigments increased the temperature of the gel by captivating more light and simultaneously reduced the peroxide leaching to the tooth pulp.[14] The present study used diode laser-assisted bleaching to limit peroxide penetration by shortening the contact duration between the bleaching gel and ceramic surface. Furthermore, investigations have shown that the thermal effects of Er: YAG, Nd: YAG, and CO2 on materials tend to separate ceramic veneer bonds. Therefore, we used the laser with a 940 nm wavelength diode to minimize the detrimental impact on the veneer bonding.[23]

The current study has several limitations. Similar to other in vitro studies, the conditions of the oral environment could not be fully simulated by the in vitro conditions. Therefore, lack of saliva, heat cycles, and dynamic environment limited the generalization of the results to clinical situations. Also, further investigations are needed to compare the bleaching effects in the presence and absence of power sources and investigate the related effects on different ceramic systems. Moreover, the impacts of laser-bleaching on other ceramic characteristics, such as surface roughness and microhardness, should be investigated to ensure the procedure's safety.

CONCLUSIONS

Both conventionally layered and CAD/CAM-milled feldspathic porcelain samples underwent a significant color change after staining. However, CAD/CAM-made samples showed a significantly greater color stability than their conventional counterparts. In addition, bleaching could effectively resolve the discoloration in all the ceramics regardless of the fabrication method. However, the baseline color and translucency of both types of ceramics were resistant to the effects of laser bleaching. Therefore, there was no need to protect the feldspathic porcelain samples during this procedure.

Within the limitations of the present study, it was concluded that laser-assisted bleaching using 35% hydrogen peroxide and 940-nm diode laser was clinically acceptable and could even be used for stain removal from the feldspathic porcelain samples.

Financial support and sponsorship

This study was financially supported by the vice-chancellery of Tehran University of Medical Sciences and Health Services, Tehran, Iran (Grant No: 98-02-69-42548).

Conflicts of interest

There are no conflicts of interest.