RESEARCH ARTICLE

https://doi.org/10.5005/jp-journals-10019-1345
International Journal of Prosthodontics & Restorative Dentistry
Volume 11 | Issue 4 | Year 2021

Evaluation of the Effect of Cement Shade and Accelerated Artificial Aging on Color Stability of CAD/CAM Resin-matrix Ceramics: An In Vitro Study

Arife Dogan¹ https://orcid.org/0000-0002-9572-5447, Mustafa Solmazgul²

^1,2Department of Prosthodontics, Gazi University, Faculty of Dentistry, Ankara, Turkey

Corresponding Author: Arife Dogan, Department of Prosthodontics, Gazi University, Faculty of Dentistry, Ankara, Turkey, Phone: +903122034174, e-mail: adogan1956@yahoo.com

ABSTRACT

Aim and objective: This study was conducted to examine the effect of cementation and artificial aging on color stability of three resin-matrix ceramic CAD/CAM materials.

Materials and methods: About 12 × 14 × 1.0 mm rectangular-shaped specimens were prepared from a hybrid ceramic (Vita Enamic), a hybrid resin nano-ceramic (Cerasmart), and a resin nano-ceramic (Lava Ultimate) (n = 30). Specimens of each material were luted with three shades of a resin cement in 0.2 mm thickness (Variolink N; A1, Bleach XL and Transparent), followed by artificial aging step (n = 10). Color coordinates were measured in each step with a colorimeter. Color differences (ΔE₀₀) were calculated from CIEDE2000 formula, and statistically analyzed with one-way ANOVA and paired t-tests.

Results: For each material type, Bleach XL shade luting yielded the highest color change when compared to other two shades (p < 0.05). Artificially aging the specimens resulted in a significant increase in ΔE₀₀ regardless of shade and material type (p < 0.05). No significant color differences due to artificial aging were detected when the three shades were compared for only Vita Enamic samples (p > 0.05). Luting with different shades of resin cement did not result in a statistical difference in ΔE₀₀ between the restorative materials (p < 0.05) except for Cerasmart luted with Bleach XL (p < 0.05); however, artificial aging led to statistically significant differences between the materials when luted with the same shade of the resin cement (p < 0.05).

Conclusion: The final color of resin-matrix ceramics is affected by the resin cement shade and artificial aging.

How to cite this article: Dogan A, Solmazgul M. Evaluation of the Effect of Cement Shade and Accelerated Artificial Aging on Color Stability of CAD/CAM Resin-matrix Ceramics: An In Vitro Study. Int J Prosthodont Restor Dent 2021;11(4):159-167.

Source of support: Nil

Conflict of interest: None

Keywords: Artificial aging, CAD/CAM resin-matrix ceramics, Color stability, Resin cement

INTRODUCTION

One major goal in restorative dentistry is to restore the lost tooth tissue with a biocompatible material whose physical properties resemble those of the native tooth tissue.¹ Ceramics, as a material group, play a vital role in fulfilling this goal thanks to their natural appearance and excellent biocompatibility.² Over the years, ceramic laminate veneers have become an attractive choice for the anterior region mainly due to their ability to provide superior esthetics besides the minimal need for an invasive procedure during preparation of tooth structure.³^-⁵ However, recent studies have revealed higher failure incidences for ceramic restorative materials which is considered to stem from their brittle nature besides the possibility of an abbrasive effect on opposing dentition.⁶

As an alternative material group, dental resin composites are commonly used for restoring the teeth, and they involve a variety of materials with a wide range of properties and indications.⁷ With the recent advances in computer-aided design/computer aided manufacturing (CAD/CAM) technology, manufacturers have introduced a relatively new group of restorative materials, so-called resin-matrix ceramics which inherently acquire the desired properties of both resin composites and glass ceramics for dental restoration purposes.⁸ These new materials result in higher modulus of resistance and flexural strength while displaying lower flexural modulus than ceramics.⁷ The lower resistance of resin-matrix ceramics to wear can be considered as an unfavorable property for dental restoration at a first glance, however, they cause less wear to the opposing dentition and they are not brittle in nature. Additionally, they are easier to repair and polish than glass ceramics.⁷^,⁹^,¹⁰

Depending on the microstructure and industrial polymerizing mode, resin-matrix ceramics can be investigated in two groups as (1) high-temperature polymerized resin-based composite with dispersed fillers and a predominantly organic phase, and (2) high temperature/high pressure (HTHP) polymer-infiltrated ceramic network (PICN) materials with a predominantly inorganic phase.⁷^,¹¹ The latter materials consist of two interconnected phases consisting of infiltrating polymer and a porous sinterized feldspathic ceramic.⁷^,¹⁰ Industrial execution of HTHP polymerization process has led to increased volume fraction of fillers and a higher conversion rate of polymer matrix when compared to manual polymerization of indirect resin composites leading to thus significantly improvements in their mechanical properties.¹⁰^,¹²

Currently, some of the most commonly utilized resin-matrix ceramic restorative materials are Vita Enamic, Cerasmart and Lava Ultimate. Vita Enamic is a well-known PICN material, and is currently advertised as a hybrid ceramic;¹¹^,¹³ Cerasmart is a hybrid nano-ceramic material with an evenly distributed ceramic network¹¹; and Lava Ultimate is a nano-ceramic material, which consists of a highly-cured resin matrix composite in which nanoceramic particles are embedded.⁹^,¹²^,¹⁴ The manufacturers’ data limit the clinical indications of these materials to small-sized restorations, such as veneers, inlays, onlays, single crowns, and implant crowns.

The success of such CAD/CAM new blocks relies on the longevity of the fabricated restorations. The mechanical and optical properties are the key factors that determine the preferrability of the fabricated material for dental restoration purposes. In earlier studies, the mechanical properties have been well-documented, and it was shown that their mechanical and physical features resembled those of natural dentin and enamel.¹^,⁶^,¹⁵^,¹⁶ In general, performance of polymer-based materials in flexural tests were better than the ceramic materials which supported the view that these materials were less brittle and more flexible.⁶ Besides the mechanical properties, the optical behavior of a material is an important aspect.¹⁷ Some studies regarding ceramic veneer esthetics showed that resin cement shade had a considerable impact on the long-term color of a restoration.¹⁸^,¹⁹ Hence, it is suggested that accurate knowledge of the relationship between color and luting material plays a crucial role for controlling the final color and fulfilling the esthetics expectations.

Initial color match is considered to be crucial factor for the desired esthetic outcome, however it should also be taken into account that color changes will be likely to occur when the restorations are exposed to oral environment which may be due to extrinsic factors such as temperature, humidity, food and smoking habits; or intrinsic factors such as resin matrix composition, filler load, size and nature of the particles, and/or degree of conversion,⁹^,¹⁴ Sustaining long-term color stability of resin-matrix ceramics remains a challenging critical determinant of the success of a restoration.⁹ In this respect, accelerated artificial aging is a suitable method for simulating oral conditions for anticipating color changes in restorative materials over time.¹⁷ However, to date, long-term behavior of these new materials in terms of color change is seldomly investigated under simulation of oral conditions. With the motivation of systematically investigating the key factors affecting color change, the purpose of this study was to examine the effect of different shades of the same dual-cured resin cement underlying three resin-matrix ceramics, and also to determine the impact of artificial aging on color changes of these restorative materials which are luted with the same resin cement. The first null hyphothesis is that there would be no significant effects of choosing different shades of resin luting cement on the final color of resin-matrix ceramic material. The second null hyphothesis is that accelerated artificial aging would not lead to meaningful color changes between the material groups tested.

MATERIALS AND METHODS

The impact of choosing different shades of a resin cement and artificial aging on the final color was tested on three different resin-matrix ceramic CAD/CAM restorative materials: a hybrid ceramic (Vita Enamic, Vita Zahnfabrik, Germany; VE), a hybrid nano-ceramic (Cerasmart, GC, Tokyo, Japan; CS), and a resin nano-ceramic (Lava Ultimate, 3M ESPE, St. Paul, MN, USA; LU). The color of the high translucent CAD/CAM blocks was 1M2 for VE, and A1 for CS and LU, respectively. A dual-cured veneer luting resin cement (Variolink N Professional Set, Ivoclar Vivadent, Schaan, Lichtenstein) at three different shades of A1, Bleach XL (BL1),and transparent (T) was selected. The brands, types, manufacturers, and chemical compositions of the materials used are listed in Table 1.

**Table 1:** Materials used in this study*
Brand/Code	Type	Color	Manufacturer	Composition	Lot number
Vita Enamic (VE)	Hybrid ceramic (polymer-infiltrated ceramic)	1M2-HT	VITA Zahnfabrik, Bad Sackingen, Germany	86 wt% (75 vol%) feldspathic-based ceramic network; 14 wt% (25%vol%) acrylate polymer network (UDMA, TEGDMA); <1% pigments	78,110
Cerasmart (CS)	Hybrid Nano-ceramic	A1-HT	GC, Tokyo, Japan	71 wt% nanoceramic fillers (silica 20nm, barium glass 300 nm); acrylate polymer network (Bis-MEPP, UDMA, DMA)	180,4031
Lava Ultimate (LU)	Resin Nano-ceramic	A1-HT	3M ESPE, St. Paul, MN, USA	80 wt% (65 vol%) nanoceramic fillers (zirconia filler 4–11nm, silica filler 20 nm, aggregated zirconia/silica cluster filler); 10 wt% (35 vol%) acrylate polymer matrix (Bis–GMA, UDMA, Bis–EMA, TEGDMA)	N 899,630
Variolink N Professional Set	Dual-cured resin cement	Shade/ A1 Bleach XL Transparent	Ivoclar Vivadent, Schaan, Lichtenstein	Bis-GMA, UDMA, TEGDMA monomers; inorganic fillers (Ba-Al-flourosilicate glass, and spheroid mixed oxide); initiators, stabilizers and pigments	X 50,668 X 44,559

*Manufacturers’ data; UDMA, urethane dimethacrylate; TEGDMA, triethylene glycol dimethacrylate; bis-MEPP, 2,2-bis(4-methacryloxypolyethoxyphenyl) propane; DMA, dimethacrylate; Bis-GMA, bisphenol A-glycidyl methacrylate; bis-EMA, ethoxylated bisphenol-A dimethacrylate

Specimen Preparations

Blocks of three different CAD/CAM materials were cut into rectangular plates (12 x 14 mm), using a precision cutting machine (Metkon Micracut 201, Bursa, Turkey) under water cooling with an initial thickness of 1.4 mm. Thus, a total of 90 specimens were obtained (n = 30 per material). One side of the specimens was wet-ground using silicon carbide papers in a sequence of 600-, 800-, and 1200-grit on a polishing machine (Metkon Gripo 2V, Bursa, Turkey) to achieve a uniform standardized surface. During grinding, the thickness of each sample was continuously recorded by a digital caliper (Powertectools, Zhejiang, China) until a final thickness of 1 ± 0.04 mm was achieved. After ultrasonically cleaning the specimens under water, in order to obtain high gloss, the grounded side of the specimens was polished with the materials’ own polishing kits as recommended by their manufacturers. Constant stroking motions were performed in the same direction using a polishing instrument (Kavo Ewl 4990; Kavo Dental Gmbh, Germany) by the same person. The silicon carbid pink rubber disc of the Technical Kit (Vita Zahnfabrik, Germany) was used to polish the Vita Enamic specimens by using with 10.000 rpm hand-piece, then the high gloss white disc with smaller particle size of the same kit was used with 8.000 rpm. Cerasmart specimens were polished with Diapolisher Paste (Gradia Diapolisher, GC, Tokyo, Japan) after using of coarse and fine silicon points; and Lava Ultimate specimens were polished using surface finishing kit (Luster for Lava Ultimate, Meisinger, 3M ESPE, St. Paul, MN, USA) at 10.000 rpm, respectively. After ultrasonic cleaning for 10 minutes in distilled water and dried, the specimens of each material (n = 30) were randomly assigned to three subgroups (n = 10) which have different shades of the same luting agent (A1, BL1, and T). At this moment, the baseline color coordinates (L_o*, a_o*, and b_o*) of polishing surfaces of per group were measured with a colorimeter. The details of this process were described under the section of “Color Measurements.”

Cementation Procedure

In order to form a microretentive surface for enabling the mechanical interlocking of the resin cement, the unpolished side of each specimen was sandblasted with 50µm Al₂O₃ particles for 15 seconds from a distance of approximately 10 mm under a pressure of 1 bar as described by the manufacturer. The specimens were then rinsed for 10 seconds with distilled water and wiped air-dried before luting. In order to standardize the cement thickness (0.2 mm), a split mold in 14 mm of length, 12 mm of width, and 1.2 mm of thickness was made with a bioplastic material including polylactic asid (eSUN PLA, Hong Kong, China) via 3-D printer (BIQU-B1, Hong Kong, China) (Fig. 1). Each specimen of each group was placed into the mold, and a primer agent (Monobond-S, Variolink N Professional Set, Ivoclar Vivadent, Schaan, Liechtenstein) was applied with a clean microbrush to the sandblasted surfaces allowing the material to react for 60 seconds, and subsequently dispersing with a strong stream of air (Fig. 2). Dual-cured resin cement was mixed on a mixing pad for 10 seconds in a 1:1 ratio of base (A1, BL1, T, respectively) and catalyst (transparent/low viscosity), as recommended by the manufacturer. The mixture was applied onto the surface to be bonded with a plastic instrument (Fig. 3). After placing a Mylar strip over the cement, a glass slab was placed on top of the strip, and stabilized with finger pressure, and then kept under a static load of 2 kg weight for 2 minutes. Excess material was removed, and cured by use of an LED-unit (BA Optima 10, B.A. Int Ltd, Northampton, England) which had an output intensity of 800 mW/cm². Irradiation was performed from the top surface for 20 seconds. After curing, the specimens were stored in a light-proof container at 37C and in distilled water for 24 hours to ensure complete polymerization. The total thickness of each specimen was again calibrated and confirmed as 1.2 mm (Fig. 4). At this moment, second color measurement was performed and (L₁*, a₁* and b₁*) values were recorded.

Fig. 1: Split mold prepared via 3-D printer

Fig. 2: Monobond application to unpolished surface of a specimen

Fig. 3: Application of cement mixture onto ceramic surface

Fig. 4: Control of final thickness of specimens with a digital caliper

Accelerated Artificial Aging Process

Accelerated artificial aging was made by utilizing a UV aging machine (BGD 856 UV Light Accelerated Weathering Tester, Biuged Laboratory Instruments Co Ltd, Guangzhou, China). Aluminum molds were prepared in the form of slots whose size matched that of the specimens (Fig. 5). The specimens were inserted into the molds and remained in the testing device for 300 hours. In weathering chamber, fluorescent UV light simulates the sunlight, while a condensation and water spray system simulate the rain, dew, and water cleaning The polished surface of each specimens was continuously exposed to the light source from a 10 cm distance and the relative humidity was adjusted as 90%. Each aging cycle was conducted sequentially with UV irradiation for 8 hours at 60^o C ± 3o C (level of irradiance: 1.55 W/m²); a 18 minutes distilled water spraying; and condensation cycle (light off) for 4 hours at 50^o C ± 3o C. The cycles were repeated with a 120 minutes intervals. After the specimens had received 300 hours of UV aging, they were wiped dry and kept in light-proof containers 24 hours at room temperature before third colorimetric values obtained (L₂*, a₂* and b₂*).

Fig. 5: Aluminum mold used for insertion of specimens in aging chamber

Color Measurements

A colorimeter (Konica Minolta CR-321, Minolta, Osaka, Japan) was used to determine CIE L*, a*, and b* color coordinates of the samples relative to CIE D65 standard illuminant against a white background (Fig. 6). The procedure was in compliance with the ISO standards (ISO7491). Each color measurement was the mean of three consecutive measurements performed from different points of the polished surface. Calibration of the colorimeter was performed with its calibration apparatus prior to each measurement. Furthermore, special care was taken to assure that the 3 mm diameter of the measuring tip was positioned in contact with measuring points of the specimens during all measurements. Initially, baseline L_o*, a_o*, and b_o* values were recorded after specimen preparation, and recorded again after cementation and artificial aging as L₁*, a₁*, b₁* and L₂*, a₂*, b₂*, respectively. The CIEDE 2000 (ΔE₀₀) formula (Eq. 1)²⁰ was used to quantify color changes among the tested groups:

Fig. 6: Colorimeter used for color measurements

Where ΔL, ΔC, and ΔH correspond to the differences in lightness, chroma, and hue for a pair of samples in CIEDE2000, and R_T is the rotation function which mathematically describes the interaction between chroma and hue differences in the blue region. S_L, S_C, S_H are the weight functions that are used for adjusting the total color difference in the location of the color difference pair in L*, a*, and b* coordinates; and the parametric factors K_L, K_C and K_H are the correction terms for experimental conditions. For this study, each of K_L, K_C and K_H was set to 1.0.

CIEDE 2000 (ΔE₀₀) and color changes were examined in terms of perceptibility and clinical acceptability thresholds. The perceptibility threshold was taken as ΔE₀₀=1.30; and the clinical acceptability threshold was taken as ΔE₀₀= 2.25. as stated before.²¹

STATISTICAL ANALYSIS

The mean color coordinates and color differences values with standard deviations were calculated by using SPSS 21.0 statistical software (SPSS v20.0; IBM SPSS Inc. Chicago, USA). Normality of the data distribution was ensured with a Shapiro-Wilk test. Then, one-way analysis of variance (ANOVA) test was performed to identify color changes observed after cementation and after aging for different shades of luting agent on the same material, respectively, and also those of three restorative materials luted with the same shade. Paired t-test was used to determine color differences due to artificial aging on the samples luted with the same shade of resin cement for each restorative material. The significance level for each test was set at 0.05.

RESULTS

Table 2 presents the mean values and standard deviations of the color coordinates (L, a, and b) before and after cementation and after artificial aging, and color differences (ΔE₀₀) of the three CAD/CAM resin-matrix ceramic materials. The one-way ANOVA results revealed that the luting of the same resin-matrix ceramic material with each shade of resin cement led to significant differences of the color changes (Table 3). For Vita Enamic samples, BL1 significantly increased ΔE₀₀ values when compared with other shades of cement (T and A1) (p < 0.05), resulting in an unacceptable clinically value (ΔE₀₀ = 4.89; ΔE₀₀ > 2.25). The ΔE₀₀ value of A1 subgroup (ΔE₀₀ = 1.51) was within the visually perceptible limit (ΔE₀₀ > 1.30) and that of T subgroup (ΔE₀₀ = 1.27) was found to be imperceptible. After artificial aging, although there was no statistical difference between the Vita Enamic samples of three shades of resin cement (Table 3), however, this process led to color change when the same shade of luting agent was used statistical differences were noted for the BL1 and T subgroups compared to the values obtained after cementation, respectively (p < 0.05); the ΔE₀₀ value for T subgroup was above perceptible threshold (ΔE₀₀ = 2.20), and for BL1 subgroup, although a decreasing trend was observed with a value of ΔE₀₀ = 2.37, it was found to be clinically unacceptable (Table 4). Statistically, a similar behavior of the color change was noted between the subgroups of the Cerasmart samples after cementation: the ΔE₀₀ values of BL1 was found to be clinically unacceptable (ΔE₀₀ = 5.36), exceptingly T subgroup was perceptible (ΔE₀₀ = 1.59) and A1 subgroup was imperceptible (ΔE₀₀ = 1.25) (Table 3). With the artificial aging process, ΔE₀₀ values of all Cerasmart subgroups significantly increased when compared with the values obtained after cementation (p < 0.05), and all of them was found to be not clinically acceptable (Table 4). On the color change of Lava Ultimate samples, BL1 luting showed larger ΔE₀₀ values ((ΔE₀₀ = 4.18; clinically unacceptable) compared to those of A1 (ΔE₀₀ = 1.52) or T luting ((ΔE₀₀ = 1.53) as being perceptible after cementation (Table 3). After artificial aging, ΔE₀₀ values significantly increased with unacceptable clinically thresholds for all Lava Ultimate subgroups (p < 0.05), respectively (Table 4).

**Table 2:** Means and standard deviations of the color coordinates (L*, a*, and b*) and color differences (ΔE00) (n = 10)
		Baseline values			After cementation				After aging
Material/ shade		L_o	a_o	b_o	L₁	a₁	b₁	ΔE₀₀	L₂	a₂	b₂	ΔE₀₀
Vita Enamic	A1	54,80 ± 0,41	-4,34 ± 0,08	6,99 ± 0,14	53,53 ± 0,50	-4,63 ± 0,22	6,19 ± 0,35	1,51 ± 0,45	51,59 ± 0,62	-4,56 ± 0,11	7,55 ± 0,44	2,01 ± 0,58
	Bleach XL	54,26 ± 0,34	-4,27 ± 0,04	7,01 ± 0,083	59,46 ± 0,69	-5,14 ± 0,09	7,14 ± 0,33	4,89 ± 0,67	57,28 ± 0,75	-5,19 ± 0,14	8,84 ± 0,35	2,37 ± 0,45
	Transparent	54,66 ± 0,89	-4,31 ± 0,07	6,86 ± 0,12	54,48 ± 0,48	-4,80 ± 0,09	5,74 ± 0,43	1,27 ± 0,39	52,64 ± 0,33	-4,79 ± 0,16	7,34 ± 0,31	2,20 ± 0,30
GC Cerasmart	A1	52,81 ± 1,06	-4,83 ± 0,10	3,39 ± 0,21	53,15 ± 0,53	-4,66 ± 0,04	2,52 ± 0,08	1,25 ± 0,44	52,44 ± 0,43	-6,46 ± 0,62	10,52 ± 0,37	6,19 ± 0,27
	Bleach XL	52,79 ± 0,70	-4,91 ± 0,07	3,34 ± 0,15	58,50 ± 0,68	-4,82 ± 0,08	3,26 ± 0,19	5,36 ± 0,69	56,92 ± 0,79	-6,85 ± 0,14	11,62 ± 0,42	6,40 ± 0,17
	Transparent	53,32 ± 0,91	-4,91 ± 0,08	3,22 ± 0,10	54,31 ± 0,69	-4,69 ± 0,06	2,26 ± 0,41	1,59 ± 0,48	52,48 ± 0,57	-6,77 ± 0,14	10,68 ± 0,47	6,74 ± 0,39
Lava Ultimate	A1	54,13 ± 0,92	-4,74 ± 0,12	-1,61 ± 0,48	55,36 ± 0,43	-4,72 ± 0,06	-1,81 ± 0,28	1,52 ± 0,56	52,25 ± 0,79	-6,12 ± 0,17	5,31 ± 0,80	6,97 ± 0,34
	Bleach XL	54,65 ± 0,56	-4,79 ± 0,09	-1,22 ± 0,30	59,11 ± 0,79	-5,01 ± 0,09	-0,45 ± 0,25	4,18 ± 0,76	55,79 ± 0,60	-6,41 ± 0,09	7,08 ± 0,35	7.00 ± 0,30
	Transparent	54,49 ± 0,40	-4,70 ± 0,11	-1,54 ± 0,18	55,96 ± 0,54	-4,85 ± 0,08	-2,14 ± 0,27	1,53 ± 0,39	52,64 ± 0,59	-6,48 ± 0,12	6,06 ± 0,36	7,85 ± 0,21

**Table 3:** One-way ANOVA results of color differences (ΔE00) of each restorative material according to process
		A1	Bleach XL	Transparent
Material	Process	Mean ± SD	Mean ± SD	Mean ± SD	F	P
Vita Enamic	ac	1.51 ± 0.45^b	4.89 ± 0.67^a	1.27 ± 0.39^b	151,9	0.0001
Vita Enamic	aaa	2.01 ± 0.58^a	2.37 ± 0.45^a	2.20 ± 0.30^a	1,55	0,229
Cerasmart	ac	1.25 ± 0.44^b	5.36 ± 0.69^a	1.59 ± 0.48^b	173,6	0.0001
Cerasmart	aaa	6.19 ± 0.27^a	6.40 ± 0.17^b	6.74 ± 0.39^b	8,9	0.001
Lava Ultimate	ac	1.52 ± 0.56^b	4.18 ± 0.76^a	1.53 ± 0.39^b	68,9	0.0001
Lava Ultimate	aaa	6.97 ± 0.34^b	7.00 ± 0.30^b	7.85 ± 0.21^a	29,5	0.0001

*n = 10 specimens per experimental conditionMeans labelled with the identical letters in each row are not statistically different p > 0.05; others are differed statistically by the one-way ANOVA at p < 0.005. Superscript lowercase letters show effect of different shades of resin cement within the same material after cementation, and after aging processes, ,respectively; ; ac, after cementation; aaa, after artificial aging

**Table 4:** Results of color differences (ΔE00) of different shades between cementation and artificial aging for the same restorative material*
After Cementation			After Artificial Aging
Material	Shade	Mean ± SD	Mean ± SD	T	p
Vita Enamic	A1	1,51 ± 0,45^a	2,01 ± 0,58^a	-2,06	0,069
	Bleach XL	4,89 ± 0,67^a	2,37 ± 0,45^b	12,7	0,001
	Transparent	1,27 ± 0,39^b	2,20 ± 0,30^a	-6,76	0,001
Cerasmart	A1	1,25 ± 0,44^b	6,19 ± 0,27^a	-26,7	0,0001
	Bleach XL	5,36 ± 0,69^b	6,40 ± 0,17^a	-4,27	0,002
	Transparent	1,59 ± 0,48^b	6,74 ± 0,39^a	-34,3	0,001
Lava Ultimate	A1	1,52 ± 0,56^b	6,97 ± 0,34^a	-27,8	0,0001
	Bleach XL	4,18 ± 0,76^b	7,00 ± 0,30^a	-11,3	0,0001
	Transparent	1,53 ± 0,39^b	7,85 ± 0,21^a	-57,4	0,0001

*n = 10 specimens per experimental condition; ac, after cementation; aaa, after artificial aging; Values with same superscript letters in each row are not significantly different at p > 0.05; unidentical letters show differences of different shades of resin cement for the same restorative material after artificial aging process by paired t-test at p < 0.05, respectively.

A comparison of three resin-matrix ceramic materials luted with the same shade of the resin cement was presented in Table 5. Any statistical difference on color change of each restorative material after cementation was not noted with the same shade of resin cement, except to Cerasmart material luted with BL1, which yielded highest ΔE₀₀ value compared to other two restorative materials (p < 0.05). However, artificial aging led to statistical differences of the final color between the resin-matrix ceramic materials for each shade used (p < 0.05). These values were higher for Lava Ultimate compared with other two materials, following Cerasmart, with the lowest for Vita Enamic regardless of the shade of luting agent.

**Table 5:** One-way ANOVA comparison of color differences (ΔE00) between the restorative materials according to processes*
		Vita Enamic	Cerasmart	Lava Ultimate
Cement shade	Process	Mean ± SD	Mean ± SD	Mean ± SD	F	p
A1	ac aaa	1.51 ± 0.45^a 2.01 ± 0.58^b	1.25 ± 0.44^a 6.19 ± 0.77^a	1.53 ± 0.56^a 6.97 ± 0.34^a	1.01 406.3	0.379 0.0001
Bleach XL	ac aaa	4.89 ± 0.67^a 2.37 ± 0.45^c	5.36 ± 0.69^b 6.40 ± 0.17^b	4.18 ± 0.76^a 7.00 ± 0.30^a	7.08 585.8	0.003 0.0001
Transparent	ac aaa	1.27 ± 0.39^a 2.20 ± 0.36^c	1.59 ± 0.48^a 6.74 ± 0.39^b	1.53 ± 0.39^a 7.85 ± 0.21^a	1.6 933.3	0.214 0.0001

*n = 10 specimens per experimental condition; Values with same superscript letters in each row are not significantly different at p > 0.05; nonidentical letters show differences between three restorative materials after luting with the same shade of resin cement and artificial aging processes by one-way ANOVA at p < 0.05, respectively; ac, after cementation; aaa, after artificial aging

DISCUSSION

Generally, the final color of a ceramic restoration depends on a combination of various factors among which the thickness of the ceramic layer, color of the underlying tooth tissue, and the color of the luting agent are the major determinants.³ For the resin-matrix ceramics, material-based factors such as chemical composition, type of monomer, inorganic filler size and/or arrangement of the material could be also effective in optical properties.⁹ In present study, three resin-matrix ceramic materials with different chemical compositions were chosen to see the effect of three different shade of the same resin luting agent on final color, and also to predict their clinical performance on color stability by accelerated artificial aging simulating oral environment.

During cementation, the translucency level of the restorative materials plays a critical role for effective curing of the underlying resin-based cement matrix. The curing light can traverse translucent materials to a greater extent when compared to more opaque materials. Higher transmittance of curing light through a restorative material ensures a greater degree of conversion, which in turn, leads to a more stable color after polymerization.³ Within this context, the thickness of ceramic restoration also influences the amount of light transmitted through the material with thinner materials allowing more light transmission.²² Therefore, in the present study, high translucent and lighter shades of the tested materials were chosen. Some studies found that composite cements resulted in perceptible color differences depending on die material, cement, and ceramic crown,²³ and the shade change and thickness of luting agent had a significant impact on the final color of thinner specimens.³ Meanwhile, some studies reported that the cement color may have significant impact on the final color if the thickness of ceramic restoration is less than 1.5 mm, or if the restoration was placed on to tooth structure with dark color, or abutment to mask the color and prevent undesirable results.⁶^,²⁴ Since, the thickness of ceramic materials influence the degree of polymerization of the resin cements, and optimal polymerization is an important aspect for the long-term color stability, the ceramic specimen thickness was set at 1.0 mm to identify detectable color changes stem from different shades of the resin cement with a 0.2 mm thickness.

During color measurements, both the actual color of the surface and the lighting conditions of the measurement set-up have impact on the final readings.⁹ Different backgrounds have been reported to influence color of glass-infiltrated ceramic veneers.²⁵ However, the effect of background lightning and background color on the color perception still remains a controversial issue,⁹ color discrepancies may be more prominent in cases where the restorations are surrounded by tooth walls.²⁶ White background has widely been used as a standard background,²⁷ and in the current study, a white background was chosen for a more realistic simulation of the clinical conditions where tooth walls are simulated like anterior veneers as pointed out by Ardu et al.²⁶ The colorimeter used in the present study had a limited measurement area and allowed for collecting data from 3 mm diameter areas of the surface being measured. Hence, the color coordinates were determined from different points of the polished surfaces and their average values were taken in order to obtain a more reliable evaluation of color change (average of total of three measurements).

Visual thresholds for perceptibility and acceptability should be specified by combining visual and instrumental color measurement methods. This matter is of primary importance in clinical dentistry and dental research for accurate and objective interpretation of color differences. The introduction of new color difference formula (CIEDE 2000) has enabled development of a systematic approach and standardization of methods for accurate assessment of color change.²⁵ A 50:50% perceptibility threshold corresponds to cases where the color difference between compared objects can be observed by 50% of the participants where remaining participants will not notice any difference. On the other hand, a 50:50% acceptability threshold corresponds to a situation where the color difference is reported as acceptable by 50% of the participants while remaining 50% of the participants would consider the color difference as unacceptable.²⁵ There is no any agreement on a specific value on this respect; the perceptibility threshold value as ΔE₀₀ = 0.81; 1.28 and 1.30 units, and clinically acceptable threshold value as ΔE₀₀ = 1.77; 2.24 and 2.25 units have been reported in respective studies.²¹^,²⁸^,²⁹ In the current study, the baseline color coordinates (L*, a*, and b*) for each specimen prepared with three resin-matrix ceramic restorative material were determined by 3D color measurement system, and then repeated after cementation and artificial aging, respectively. CIEDE2000 formula was applied to obtain color changes, in which the perceptibility limit as ΔE₀₀ = 1.30 unit and clinically acceptable limit as ΔE₀₀ = 2.25 unit was accepted as stated by Ghinea et al.²¹

For each material type, one-way ANOVA results indicated a significant main effect of resin shade and also of artificial aging process on the final color of the same ceramic material, leading to the rejection of the first null hyphothesis, which stated that there would be no significant effect of applying of different shades of resin luting cement on the final color of each resin-matrix ceramic material (Table 3). Second null hyphothesis was also rejected, because artificial aging led to meaningful color changes on the material groups tested (Table 4). Data showed that different shades of the resin luting agent influenced each material differently. The luting with Bleach XL led to clinically unacceptable color change (ΔE₀₀ > 2.25) for each resin-matrix ceramic material; A1 shade luting yielded perceptible color change (ΔE₀₀ > 1.30) on the materials of Vita Enamic and Lava Ultimate, whereas luting with transparent shade affected the color of Lava Ultimate and Cerasmart with a perceptible change (ΔE₀₀ > 1.30) (Table 3). On the other hand, a comparison of three resin-matrix ceramic materials luted with the same shade of the resin cement did not present any statistical difference on color change after cementation (p > 0.05), except to Bleach XL subgroup of Cerasmart (Table 4). The color differences observed between different shades of resin cements on the same material might be due to the varying amounts of opacity ‘ingredients’ present in the cement structure.³ The inorganic fillers within the resin cement represent a phase with a different refractive index from the bulk of the resin-matrix ceramic materials, with subsequent scattering of light.³^,¹²^,²⁴ Moreover, degree of chroma may also influence the final color resulting in meaningful differences.

Failure of esthetic restorations is mainly associated with their unpredictable color stainability and stability during clinical service. Several conditions, such as humidity, nutritional habits, and temperature are influental on restorations in the oral cavity.¹⁴ Noticeability of the color change is an indicator of failure of the and result in an urge for replacement.³⁰ In this study, in order to explore the potential impact of long-term exposure to these environmental variables within the oral cavity, the specimens were exposed to different temperatures, humidity and UV irradiation in a weathering chamber for 300 hours. While the clinical relevance of this method is not clear,¹⁹ it has been stated that a total of 300 hours of artificial aging process is accepted to simulate a duration of 1 year after the clinical implementation of a restoration.³¹^,³² Based on the results, all resin-matrix ceramic specimens luted with each shade of resin cement yielded significantly more color changes on all tested materials after artificial aging compared to data obtained after cementation (p < 0.05) (Table 3). The ΔE₀₀ values more increased above clinical unacceptable limits for all Cerasmart and Lava Ultimate specimens. Although color differences of Vita Enamic specimens luted with A1 and transparent had increased, they remained below clinical acceptable limits; for Bleach XL luted specimens the values decreased, but was still above clinical unacceptable threshold. Variolink N used in this study is a dual-cured resin cement and its monomer matrix is composed of bisphenol A-glycidyl methacrylate (Bis-GMA), urethane dimethacrylate (UDMA) and triethylene glycol dimethacrylate (TEGDMA). As this material ages, the water sorption of resin monomer resulted in hydrolytic degradation might contribute to differences of color changes regardless the shade used.¹⁹ Another explanation may be related to chemical alterations in the initiator system, activator, and the resin itself induced by UV irradiation.¹⁹ The oxidation of reactive groups with amine accelerators and inhibitors during the activation of dual-cured resin cements might be responsible for color changes; ¹⁹ this point needs to be confirmed by further studies.

Data showed that artificial aging led to statistical differences in the final color between the restorative materials for each shade used (p < 0.05). In general, Vita Enamic, followed by Cerasmart showed the lowest color change compared to Lava Ultimate, regardless of the shade of the cement. Bleach XL luting showed larger ΔE_oo values compared to other two shades after aging for tested materials (Table 4). Polymer-based materials are known to absorb water molecules on their surfaces to different extents and as water molecules are absorbed into deeper layers, they contribute to the sorption and solubility of the material.³³ The staining susceptibility or color stability of resin-based materials is positively correlated with the water absorption capability which, in turn is influenced by the hydrophilic/hydrophobic nature of the resin matrix.³⁴ Water sorption takes place exclusively in the matrix of the polymers, especially at the –OH groups.³³ For instance, monomers containing UDMA, which had no hydroxyl side groups¹⁴ have been proven to have a better stain resistance than Bis-GMA due to lower water sorption.⁹ Despite its comparable molecular weight with that of Bis-GMA, UDMA displays lower viscosity. Its main difference from the Bis-GMA relies in its flexibility, as the ether bonds in UDMA allow easy rotation when compared to the two bulky aromatic rings in Bis-GMA.³⁵ On the other hand, TEGDMA is usually in conjunction with Bis-GMA or UDMA. The higher flexibility of TEGDMA compensates for the rigidity of Bis-GMA, and admixture results in resins with higher conversion rate.³⁵ However, TEGDMA exhibits higher water absorption, and hence, it permits any hydrophilic colorant to penetrate into the resin matrix.¹⁴

Although all resin-matrix ceramic materials may be susceptible to some extent of discoloration because of their monomer matrix,³³ the color changes of the tested materials were not similar in this study, and Lava Ultimate discolored more than the Vita Enamic and Cerasmart. When the chemical compositions of tested material are taken into consideration, it is not surprising that Lava Ultimate had higher ΔE₀₀ values after aging. Vita Enamic is composed of a porous presintered ceramic matrix (86% by wt), the pores of which are filled with a polymer, mainly consisted of hydrophilic TEGDMA, and hydrophobic UDMA monomer with no hydroxyl side groups. Lava Ultimate contains approximately 79% (by wt) nanoceramic particles bound in the resin matrix which is consisted of Bis-GMA and its ethoxylated version Bis-EMA in addition to UDMA and TEGDMA. On the contrary, Cerasmart contains ultrafine glass particles (71% by wt) in a highly cross-linked resin matrix, which is mainly consisted of UDMA, dimethacrylate (DMA) and 2,2-bis (4-methacryloxypolyethoxyphenyl) propane (Bis-MEPP). Gajewski et al.³⁶ stated that Bis-GMA caused the highest water absorption compared with UDMA, TEGDMA and ethoxylated bisphenol-A dimethacrylate (Bis-EMA). Based on chemical composition of resin matrix, the more discoloration of Lava Ultimate could be attributed to presence of Bis-GMA monomer. Although, a direct comparison of data with those of studies made on this field is not possible because of differences in test methodologies and optical parameters investigated, however, the results are in good aggrement with those testing color stability of similar materials.⁹^,¹²^,³⁰^,³²^,³³

Another factor affecting the results also lies in the different size and distribution of inorganic fillers within resin matrix. Vita Enamic is composed of a dual network which consists of a feldspathic ceramic network (which had Al₂O₃ ceramic content approximately 23% by weight) and a polymer network.¹³ Lava Ultimate is a composite filled with discrete silica nanoparticles (20 nm diameter), zirconia nanoparticles (4 to 11 nm diameter), and zirconia silica nanoclusters (bound aggregates of nanoparticles).⁷^,¹³ Cerasmart is novel resin-based ceramic with a homogenous and evenly distributed ceramic network,⁷ containing silica particles (20 nm diameter) and barium glass (300 nm diameter).¹³ The nanoceramic particles are treated with a silane coupling agent in order to bond chemically the nanoceramic surface and the resin matrix,¹² however zirconium silicate was not effectively silanated due to the high crystalline content.³⁷ Artificial aging may lead to degradation of the matrix/silane/filler, as the composite absorbed water by resin matrix penetrates to filler/resin interface.³⁴^,³⁸ This leads to hydrolysis of the interfacial silane coupling agent, and may reduce the retention of the filler particles.³⁸ Although the materials with ceramic network structures did not absorb water,³⁹ storage caused water penetration into the resin matrix to some extent.⁴⁰ The reason why Vita Enamic has the lowest color change than other two tested materials might be related to its high Al₂O₃ ceramic content.

Based on the results presented in this study, it seems that both resin-matrix ceramic type and different shades of the same luting agent affected final color of the specimens, which is crucial in long-term clinical serviceability of restorations. As a limitation, this in vitro study tested the impact of shading and artificial aging while thickness of the restorative materials and the resin cement were keep constant. Future studies which investigate the impact of different thicknesses of the CAD/CAM restorative materials and different brand of the luting agents will provide a deeper insight for predicting the clinical behavior in oral conditions. Clinical follow up studies should also be performed for future work.

CONCLUSION

Although this in vitro study was not without limitations, data recorded from colorimetric color measurements revealed that the shade of a dual-cured resin cement and accelerated artificial aging had significantly effect on the final color of resin-matrix ceramic CAD/CAM restorative materials being tested. The luting with Bleach XL resulted in clinically unacceptable color changes (ΔE₀₀ > 2.25) for each resin-matrix ceramic material. When aged, all materials luted with different shades of cement showed an increase in color change, and significant differences were noted between the materials luted with the same shade of cement. Generally, hybrid ceramic material yielded less color change than resin nano-ceramic and hybrid nano-ceramic materials. The verification of these results needs to be investigated in clinical trials.

ACKNOWLEDGMENTS

The present study is based on a thesis submitted to the Dentistry Faculty, Gazi University for PhD degree and was supported by Scientific Research Projects Committee of Gazi University with a grant number of 03/2018–20.

ORCID

Arife Dogan https://orcid.org/0000-0002-9572-5447

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