Effect of aging on the translucency of lithium disilicate and zirconia reinforced lithium silicate ceramics: An in vitro study

Abstract

Background

Translucency of dental ceramics have great impact on the esthetic and success of dental restorations. The goal of the study is to investigate the effect of the aging process on the translucency of zirconia-reinforced lithium silicate ceramic material, and lithium disilicate ceramic material made using CAD/CAM and press processes.

Methods

Thirty disc-shaped specimens with 0.5 mm thickness and 10 mm diameter were fabricated of IPS e.max Press (n = 10), IPS e.max CAD (n = 10), and VITA Suprinity (n = 10) (Vita). All ceramic specimens were fabricated with shade that corresponded to A2 with high translucency (HT) and subjected to measurement of relative translucency parameter (RTP) with a spectrophotometer machine prior to and following thermocycling. The results were analyzed using the one-way ANOVA and Bonferroni methods. To identify any statistically significant change in translucency after thermocycling, paired t-tests were utilized.

Results

The mean ΔRTP₀₀ after aging was 3.23 ± 0.36, 0.60 ± 0.34, 3.05 ± 0.90 for IPS e.max CAD, IPS e.max Press, and Suprinity respectively. The mean RTP values in IPS e.max CAD and Suprinity decreased significantly after aging (p˂0.05). However, there was no statistically significant difference between the RTP₀₀ values of the IPS e.max Press before and after aging (p > 0.05).

Conclusion

Prior to aging, there was no difference between the three materials in terms of the translucency parameter. However, the translucency of CAD/CAM lithium disilicate and zirconia reinforced lithium silicate was negatively influenced by aging.

1. Introduction

The ability of ceramics to transmit light through the restoration and the underlying tooth structure improves dental esthetics. The degree to which light is diffused rather than reflected or absorbed is referred to as translucency [1,2]. Translucency resembling that of a natural tooth is made possible by the absence of an opaque metal sub-structure in all-ceramic restorations, which has been cited as one of the key variables in managing esthetics. Since dentin and enamel are naturally translucent, it is important to replicate the optical properties of normal teeth when designing ceramic restorations to look as natural as possible next to adjacent natural teeth. On the other hand, less translucent materials are needed to mask discoloration of teeth [[3], [4], [5]].

One of the commonly used dental restorations for esthetic purposes is laminate veneers. Veneers are thin bonded ceramic restorations that restore the facial surface and part of the proximal surfaces of the teeth requiring esthetic restoration. Their indications include discolored anterior teeth, diastema closure, and morphological modifications. The typical thickness of a ceramic veneer is 0.3–0.7 mm. With such minimal thickness, selection of a ceramic material with adequate translucency is crucial to achieve aesthetically pleasing results [6].

Dental ceramic materials used for laminate veneers come in a variety of translucencies. This variation may have an impact on their ability to match natural teeth or to mask discoloration [[6], [7], [8]]. Because of its exceptional esthetic and mechanical qualities, lithium disilicate ceramic is a reliable ceramic material for anterior and posterior indirect restorations including laminate veneer restorations [9]. Lithium disilicate restorations can be made via a computer-aided design/computer-aided manufacturing (CAD/CAM) fabrication technique or a heat-pressed method [10]. The heat pressed lithium disilicate is essentially made up of a modified lithium disilicate glass ceramic. This glass matrix is made up of micron-sized lithium disilicate crystals interspersed with submicron-sized lithium orthophosphate crystals [11]. The Lithium disilicate blocks for CAD/CAM are made of a metasilicate phase and have a bluish appearance. After milling, the metasilicate phase is transferred to the final restoration by a crystallization firing [12]. Although both pressed and milled lithium disilicate materials have similar crystalline composition, the mechanical and optical properties vary between the two materials [13]. Nevertheless, clinical follow-up showed excellent success rate of restorations made with both materials [14].

Dental CAD/CAM systems have recently become more common due to considerable advancements. They allow faster production of accurate restorations with simple workflow and fewer manual steps. Materials for CAD/CAM production are manufactured and processed under ideal standardized conditions which allowed the production of different substitute dental ceramic materials. One of these materials is zirconia-reinforced lithium silicate ceramic, which has a fine-grained and homogenous microstructure. Zirconia is added to this new glass ceramic in an amount of about 10 % by weight. By interrupting the cracks, the zirconia particles are incorporated to strengthen the ceramic structure. According to the manufacturer, this material has a flexural strength ranging from 370 to 420 MPa and supports a wide range of uses similar to Lithium disilicate reinforced ceramics, including anterior and posterior crowns and laminate veneers [14,15].

All-ceramic materials are susceptible to heat, cold, and humidity in the oral environment. The materials can undergo changes in color, bending strength, and toughness as a result of the aging process. Experimental research has employed the process of aging to replicate the long-term impact of oral environmental factors. This is achieved by an artificial weathering process that encompasses light exposure, temperature variations, and humidity [16]. Artificial aging leads to the leaching of products, reduction in strength, hardness, and changes in color and optical properties of dental materials. Several studies have investigated the effect of artificial aging on the optical properties of dental ceramics. Most of the studies reported a negative effect on the optical properties of glass ceramics after thermocycling compared to before. Artificial aging results in thermal stresses and changes in the microstructures which affect the optical and mechanical properties of glass ceramics [[17], [18], [19]]. This underscores the importance of considering long-term durability and esthetic outcomes when selecting and designing dental ceramic restorations.

The objective of this in vitro investigation is to assess the degree of translucency exhibited by two distinct variants of lithium disilicate ceramic materials, namely e.max Press and e.max CAD, as well as a Zirconia reinforced lithium silicate material, both prior to and subsequent to artificial aging procedure. The null hypothesis posits that there exists no discernible impact of the aging process on the translucency of the materials under investigation.

2. Material and methods

Before commencing with the current study, approval from the institutional review board of Riyadh Elm University was obtained (RC/IRB/2016/420).

Prior to conducting the test, the researchers used G power software to calculate the required sample size. It indicated that with alpha = 0.05, 0.3 effect size, and a power of 0.9, a total sample size of 30 samples was needed (10 for each group).

30 ceramic discs were produced, with each group consisting of 10 discs (n = 10). The three ceramic systems used were Lithium disilicate blocks (LD) (IPS e.max CAD, Ivoclar Viva-dent), IPS Empress lithium disilicate (ELD) (IPS e.max Press, Ivoclar Vivadent), and Zirconia-reinforced lithium silicates (ZLS) (VITA Suprinity, Vita). The shade of all discs was A2 high translucency (HT) (Table 1).

Table 1.

Material characteristics.

Material Brand	Type	Composition	Shade	Manufacturer
IPS e.max empress	Pressable Lithium Disilicate	lithium disilicate crystals (approx. 70 %), Li2Si2O5, embedded in a glassy matrix	HT A2	Ivoclar Vivadent
IPS e.max CAD	Machinable Lithium Disilicate	57-80%Sio211-19%Li2o0-13%K2o0-11%P2o50-8%Zro2-Zno0-5%Al2o30-5%Mgo-colouring oxides	HT A2	Ivoclar Vivadent
Vita Suprinity	Machinable Zirconia-Reinforced Lithium Disilicate	56-64%Sio25-21%Li2o1-4%K2o3-8%P2o58-12%Zro20-4%Ceo20-6%pigments	HT A2	Vita Zahnfabrik

The discs were constructed using 3D builder software (Microsoft 2017) and were planned to have a diameter of 10 mm and a thickness of 0.5 ± 0.05 mm. The design was then converted into a stereolithography (STL) file format (Fig. 1).

Fig. 1.

Stereolithography (STL), 3D builder software for disc-shape design.

The IPS e.max Press discs were manufactured using the lost wax and heat-press procedure, as per the guidelines provided by the manufacturer. The CAD/CAM system was utilized to mill ten disc-shaped wax specimens from a wax block (Aldente, Germany) using the Roland DWX50 plus milling machine (Japan). The spruing, investing, and pressing processes were executed in accordance with the manufacturer's instructions. As soon as the press cycle ended, the investment cylinder was removed from the furnace and placed on a metal grid to cool evenly to room temperature. When the investment cylinder was completely cooled, the sample was divested. The sprues were removed with a thin diamond disc in a laboratory handpiece, and the sample was repeatedly immersed in water to avoid overheating the ceramic. The area where the sprue was placed was smoothed with a diamond bur in a high-speed handpiece with water coolant. The specimens were then finished, polished and the measurements obtained with a digital caliper (Electronic Digital Caliper, Shan, China) to ensure a final thickness of 0.5 mm and 10 mm in diameter. Then glazing procedure was carried out in a porcelain-firing oven (programmat EP 3010; (Ivoclar Vivdent AG, Schaan, Liechtenstein).

The IPS e.max CAD discs were fabricated using a milling machine (VHF CAM 5-S1, Germany) to cut blocks based on the stereolithography (STL) file. Three equal disks, each measuring 10 mm in diameter and 0.5 ± 0.05 mm in thickness, were cut from each IPS e.max CAD block using a diamond bur and continuously irrigated with water.

The connectors were manually removed using a thin diamond disc attached to a laboratory handpiece. To prevent the ceramic from overheating, the sample was sub-merged in water multiple times. The disk connecting portions were smoothed with a diamond bur in a high-speed water-cooled handpiece. The disks were subsequently exposed to a combination firing process involving crystallization and glazing, following the instructions provided by the manufacturer. This process took place in a porcelain furnace (programmat EP 3010, Ivoclar Vivdent AG, Schaan, Liechtenstein) at a temperature of approximately 850 °C for a duration of 8 min. At this specific temperature, the metasilicate dissolves while the lithium disilicate undergoes crystallization. Glaze Spray (IPS e.max CAD Crystal Glaze Spray; Ivoclar Vivadent Inc) was applied to all specimens before firing at one surface only.

Similar to the LD group, ten disc-shaped specimens were created using Vita Suprinity zirconia-reinforced lithium silicate material. The combination firing process was employed for the purposes of both crystallization and glazing, in accordance with the guidelines provided by the manufacturer. Before firing, VITA AKZENT Plus Spray was sprayed equally across one surface on each of the specimen at a distance of 10–15 cm.

During the manufacturing process, the porcelain disks underwent regular assessment using a digital caliper (Electronic Digital Calliper, Shan, China) in order to verify the correct thickness and diameter.

The CIE L∗a∗b∗ values of each specimen were obtained by measuring them against both a white (L∗ = 93.07, a∗ = −1.28, b∗ = 5.25) and black (L∗ = 27.94, a∗ = −0.01, b∗ = 0,03) backgrounds using a spectrophotometer (color-eye; CE 7000 A). The spectrophotometer was calibrated with a calibration plate positioned at a fixed distance from the specimen.

The illumination provided by the light source is equivalent to the normal daylight conditions, namely the D65 standard. The relative translucency parameter (RTP₀₀) was determined by computing the color contrast between the specimen and both the white and black backgrounds, using the formula.

$${\text{TP}}_{00}={\left[{\left(\frac{{\mathrm{L}}_{\mathrm{B}}^{\prime }-{\mathrm{L}}_{\mathrm{W}}^{\prime }}{{\mathrm{K}}_{\mathrm{L}}{\mathrm{S}}_{\mathrm{L}}}\right)}^2+{\left(\frac{{\mathrm{C}}_{\mathrm{B}}^{\prime }-{\mathrm{C}}_{\mathrm{W}}^{\prime }}{{\mathrm{K}}_{\mathrm{C}}{\mathrm{S}}_{\mathrm{C}}}\right)}^2+{\left(\frac{{\mathrm{H}}_{\mathrm{B}}^{\prime }-{\mathrm{H}}_{\mathrm{W}}^{\prime }}{{\mathrm{K}}_{\mathrm{H}}{\mathrm{S}}_{\mathrm{H}}}\right)}^2+{\mathrm{R}}_{\mathrm{T}}\left(\frac{{\mathrm{C}}_{\mathrm{B}}^{\prime }-{\mathrm{C}}_{\mathrm{W}}^{\prime }}{{\mathrm{K}}_{\mathrm{C}}{\mathrm{S}}_{\mathrm{C}}}\right)\left(\frac{{\mathrm{H}}_{\mathrm{B}}^{\prime }-{\mathrm{H}}_{\mathrm{W}}^{\prime }}{{\mathrm{K}}_{\mathrm{H}}{\mathrm{S}}_{\mathrm{H}}}\right)\right]}^{\mathbf{1}/\mathbf{2}}$$

Since the colour parameters were not determined against ideal black and white backgrounds, the derived translucency parameters are only relative to the backgrounds used [20].

The thermocycling technique was performed on all specimens in a thermocycler machine (THE-1100-SD) (Mechatronik GmbH, Feldkirchen-Westerham, Germany). Each specimen was held by a mesh tray implanted in condensation silicon impression putty, exposing just the glazed surface (Fig. 2). The tray was transferred between two water baths of 55 and 5 °C, respectively, with a dwell time of 35s in each water bath and a transfer time of 5s for 3500 cycles [21].

Fig. 2.

Specimens embedded in putty with the glazed surface exposed.

Following the completion of the thermocycling procedure, the specimens underwent a thorough rinsing in running water and subsequent drying using absorbent paper. This was done prior to conducting another measurement of translucency. Each specimen's CIE L∗a∗b∗ values were measured again in the same manner as described before thermocycling, and TP₀₀ was computed using the same equation.

The difference between the RTP₀₀ values (ΔRTP₀₀) was calculated. The perceptibility (ΔRTP₀₀ = 0.62) and acceptability (ΔRTP₀₀ = 2.62) thresholds were considered [22].

The data obtained in this study were analyzed using the Statistical Package for Social Sciences (SPSS) for Windows, version 25, developed by SPSS Inc. in Chicago, USA. The statistical analysis of the findings involved the utilization of the one-way ANOVA and Bonferroni procedures. For each group, a paired t-test was conducted to evaluate whether there was a statistically significant difference in translucency before and after aging. The presence of a significant difference was postulated between the groups if the likelihood of observing such a difference was determined to be less than 5 % (p < 0.05).

3. Results

The mean Relative Translucency Parameter (RTP₀₀) values of the three ceramic materials before and after aging are shown in Table 2. Prior to aging, the mean RTP₀₀ values for LD, ELD, and ZLS were, respectively, 22.55 ± 1.71, 22.13 ± 0.89, and 22.16 ± 0.96. After aging, the mean RTP₀₀ values for IPS e.max CAD, IPS e.max Press, and Suprinity were, respectively, 19.32 ± 1.78, 22.6 ± 1.45, and 19.11 ± 0.71.

Table 2.

Mean (±SD) translucency parameter (RTP₀₀) values and ΔRTP₀₀ values.

Material	Ageing	Mean RTP00	SD	Max.	Min.	ΔRTP00	SD.
IPS e.max CAD (LD)	Before	22.55	1.71	24.86	18.39	3.23	0.36
	After	19.32	1.78	21.23	15.24
IPS e.max Press (ELD)	Before	22.13	0.89	23.92	20.68	0.60	0.34
	After	22.60	1.45	25.37	20.59
VITA Suprinity (ZLS)	Before	22.16	0.96	23.85	20.70	3.05	0.90
	After	19.11	0.71	20.55	17.86

The mean ΔRTP₀₀ after aging was 3.23 ± 0.36, 0.60 ± 0.34, 3.05 ± 0.90 for groups LD, ELD, and ZLS respectively. One-way ANOVA showed significant difference between the groups (p < 0.5). The difference was significant between the LD/ELD, and ELD/ZLS (p < 0.05). However, no significant difference existed between LD/ZLS (p > 0.05). The mean ΔRTP₀₀ for both LD and ZLS was above the acceptability threshold (2.62), while the mean ΔRTP₀₀ for ELD was lower than the perceptibility threshold (0.62).

Paired t-test showed a statistically significant difference in the mean RTP₀₀ values before and after Aging with IPS e.max CAD (LD) (p < 0.05) and Zirconia-reinforced lithium silicates (ZLS) (p < 0.05). The mean RTP₀₀ was significantly reduced after aging of both materials. On the other hand, the RTP₀₀ values of IPS e.max Press (ELD) showed no statistically significant difference before and after aging (p > 0.05) (Table 3).

Table 3.

Pairwise comparison of the ceramic material RTP₀₀ before and after ageing.

Material	Ageing	Mean RTP00	Standard Error	95 % Confidence Interval		p value
				Lower Bound	Upper Bound
IPS e.max CAD (LD)	Before	22.55	0.33	21.64	23.46	0.005a
	After	19.33	0.33	18.41	20.23
IPS e.max Press (ELD)	Before	22.13	0.33	21.23	23.05	0.445
	After	22.60	0.33	21.70	23.51
VITA Suprinity (ZLS)	Before	22.16	0.33	21.25	23.07	0.005a
	After	19.12	0.33	18.21	20.03

^aStatistically significant.

4. Discussion

At present, there exists a diverse array of all-ceramic materials that can be utilized for the purpose of dental restorations. The quality of CAD/CAM systems has been enhanced due to technological improvements, hence leading to increased popularity of machinable porcelains. The physical, optical, and mechanical properties of a ceramic material can be influenced by various production procedures, even when the same ceramic material is employed [1].

In this study, pressable (Lithium Disilicate) and machinable (Lithium Disilicate and Zirconia Reinforced Lithium Silicate) ceramic materials have been investigated to compare the effect of aging on their translucency.

The consideration of translucency is crucial in the fabrication process of attractive restorations. Glass ceramic materials are commonly preferred when the natural color of the tooth is normal, mostly because of their high level of translucency, which closely resembles the translucency of natural enamel. Conversely, in cases when the natural tooth structure exhibits discoloration, the preference is for ceramic materials with reduced translucency in order to effectively conceal the discoloration [4].

Lithium disilicate is a popular ceramic material for single-tooth restorations, such as porcelain laminate veneers, because it can be used in sections as thin as 0.3 mm without fracturing or cracking. The material is offered in two forms: pressable ceramic and milled CAD/CAM blocks [13]. Zirconia-reinforced Lithium Silicate is a comparatively novel ceramic material that shares comparable indications with Lithium Disilicate.

Thickness is known to affect the translucency of dental ceramics. In order to ensure accurate and valid comparisons between different systems, it is imperative that specimens are manufactured at thicknesses that are clinically applicable [7]. In order to replicate the clinical scenario of laminate veneers, discs with a thickness of 0.5 mm were manufactured in the present investigation.

The findings of the present study support the rejection of the null hypothesis. There was a considerable variation in the relative translucency parameter before and after aging for CAD/CAM Lithium Disilicate and Zirconia Reinforced Lithium Silicate.

The present investigation observed that the average RTP₀₀ of Lithium Disilicate ceramic materials were similar to the values published in the existing literature [8,17,18]. The study observed a range of mean RTP₀₀ values, specifically between 22.16 and 22.55 prior to aging, and between 19.11 and 22.6 after aging.

It is imperative for the dentist to select materials which have long-term color stability. Change in the translucency of veneers over time would have a negative influence on the esthetic appearance and color. The findings of the present study demonstrated a significant disparity in the translucency between Lithium Disilicate veneers produced through heat pressing and those produced through CAD/CAM techniques following a period of aging. This suggests that the reaction of materials to the aging process may be influenced by diverse compositions characterized by varying crystalline contents, as well as different fabrication methods. After aging, the pressable Lithium Disilicate RTP₀₀ mean values (22.6) were significantly higher than the CAD/CAM Lithium Disilicate RTP₀₀ mean values (19.32) and the Zirconia-reinforced lithium silicates (ZLS) RTP₀₀ mean values (19.11).

The e.max Press variant exhibits an elongated microcrystal structure, surpassing the crystal structure observed in the e.max CAD version. The production of longer crystal structures results in a final restoration with enhanced strength [13]. The CAD version of the crystallization process has a distinct stage where the attainment of longer crystal structures is hindered by the requirement of grinding prior to the final crystallization. In addition, it has been seen that e.max Press exhibits a higher level of surface quality, hence yielding more favorable outcomes in comparison to the much coarser e.max CAD [12,13]. The translucency of ZLS was also significantly reduced with thermocycling and this could be contributed to structural and compositional changes that affect light transmission through the material. Aging can induce microstructural changes in the ceramic material, increase in microcracks and changes in the zirconia crystal phase. ZLS also contains less silica particles which may cause more degradation in the glassy matrix upon aging and a more pronounced scattering of light, diminishing translucency over time [14]. The observed distinctions between materials produced through pressing and those manufactured using CAD/CAM machining may potentially account for the variations in their optical properties, particularly during the process of aging.

Like any other in vitro research, this work has limitations that necessitate careful consideration. The study employed disc-shaped specimens that were not affixed to actual teeth using resin cement, deviating from an accurate representation of the clinical scenario. However, this methodology was followed in order to standardize the samples while eliminating other variables [1,8]. Moreover, the specimens in the current study were glazed and this could affect the translucency of ceramic materials. However, in our study as many others, glazing was done by single calibrated technician according to the manufacturer's instructions to mimic a real clinical situation when ceramic veneers are used [1,4,7].

5. Conclusion

Within the limitations of this study, the following conclusions were drawn:

• 1No difference in the relative translucency parameter existed between pressed lithium disilicate, machined lithium disilicate, and Zirconia reinforced lithium silicate ceramic materials at 0.5 mm thickness before aging.
• 2Relative translucency parameter of pressed lithium disilicate was not affected by aging and the change was below the perceptibility threshold.
• 3The translucency of machined lithium disilicate, and Zirconia reinforced lithium silicate decreased significantly after aging and the change of relative translucency parameter in both materials was above the clinical acceptability threshold.

CRediT authorship contribution statement

Mohammed Ali Alasmari: Writing – review & editing, Writing – original draft, Visualization, Validation, Software, Resources, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Mohammad Ramadan Rayyan: Writing – review & editing, Writing – original draft, Visualization, Supervision, Resources, Project administration, Methodology, Investigation, Formal analysis, Conceptualization.

Informed consent

N/A.

Ethical approval

The study protocol was approved by the Institutional Review Board of ∗∗∗∗∗ (RC/IRB/2016/420).

Data availability

Data will be made available on request.

Funding

No funding was obtained for this study.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.