Influence of Firing Modality on Lithium Disilicate Ceramic Color

Digital technology has revolutionized the practice of clinical dentistry.¹ The responsibility of producing high-quality, esthetically pleasing restorations has moved from dental laboratories into the dental operatory. To predictably produce the same result in-office, clinicians need to be as competent as the dental lab regarding multiple variables encountered such as color and shade matching in computer-aided design and computer-aided manufacturing (CAD/CAM) ceramic restorations. Patients put trust in the dental professional’s skills to provide esthetic restorations, and in turn, dentists place trust in the materials to deliver the intended results.

Lithium disilicate glass ceramic (LS2) is the most popular CAD/CAM dental ceramic in use today.² At Midwestern University College of Dental Medicine in Glendale, Arizona, we had the opportunity to observe thousands of LS2 milled restorations and evaluate their outcomes. Intermittently, slight color variations in the final restorations did not align with the intended results. It was noticed that the final restorations often appeared lighter than the selected shade. The situation was most notable when a faster firing cycle for block crystallization was used, instead of the standard pre-installed manufacturer’s recommended program.

This study’s aim was to explain color variation of ceramic blocks that occurred intermittently when all other factors appeared to be held accountable. A literature review on this phenomenon explained possible causes such as repeated firings, changes in background color, and surface properties.^3-5 Little research has addressed the potential relationship between optical changes when running a single, shorter firing cycle with various shades and translucencies of linkage disequilibrium (LD) blocks compared to the standard firing cycle commonly recommended for office CAD/CAM applications.

This novel study was performed to assist clinicians in selecting the most appropriate shade and translucency of CAD/CAM ceramic blocks by understanding if varying the firing modality may influence the final restoration color.

METHODS/MATERIALS

In the laboratory, seven LS2 blocks, size C14 were selected for testing, which were the most frequently used at the dental school setting according to size, shade, and translucency. The blocks were in high translucency (HT) shades A1, B2, and C2, medium translucency (MT) shades A1 and A2, and low translucency (LT) shades B2 and C2.

Each partially crystallized preformed block was sectioned under water coolant with a diamond cutting saw into eight 2 mm wafers in 10 mm x 2 mm x 4 mm dimensions. One side of each wafer was polished with increasing grit of sandpaper through 600 grit and measured to a uniform thickness of 1.98 mm with digital calipers to approximate intraoral occlusal dimension and the finish of a polished milled crown.

The nonpolished surface was marked with an indentation for orientation. All wafers were cleaned in an ultrasonic bath with distilled water for 10 minutes and dried. The wafers were then divided into two groups. Four wafers of each type of block were placed in the faster firing cycle group (FG) and four in the standard firing cycle group (SDG). The SDG wafers were mounted with the nonpolished side onto a firing pin with stabilizing firing paste on a standard size firing plate. The FG wafers were mounted with the same method but utilizing the manufacturer-recommended smaller firing plate for this cycle.

A crystallization oven was calibrated per the manufacturer’s instruction before firing the samples. The SDG wafers were fired using the standard “crystal/glaze” manufacturer setting for 24 minutes. The FG wafers were fired under the faster post-installation manufacturer’s program option, “speed/spray” at 15:40 minutes.

After firing, two wafers per firing cycle of the same block were mounted next to one another with polyvinyl siloxane elastomer for universal positioning against a black background for consistent shade measurement (Figure 1).

Delta E value color comparisons were measured between the SDG and the FG wafers (Figure 1). The Delta E value quantifies the optical difference between the displayed and the original color and was recorded with the CIELAB color system.^6,7 The CIELAB system represents colors as three values, or L*a*b. L* signifies perceptual lightness while a* and b* represent red, green, blue, and yellow.⁶

This color system was selected because it is not instrument specific and Delta E can be calculated by plotting the three coordinates.^8,9 The Delta E value is traditionally reported on a scale from 0 to 100, where 0 is no color difference noted by the human eye and 100 indicates complete color distortion.⁹

Table 1 demonstrates the lower optical changes utilized within this Delta E range for wafer comparison, from 0, normally invisible, to greater than 6, a very obvious color difference.⁸ Table 1 is based on a scale verified as universally valid Delta E values and their meaning when in this lower optical color range.⁹ A lower Delta E comparison means the test sample colors were closer and more accurate, while a higher Delta E value shows a significant mismatching of colors and are farther apart on the scale.⁹ The color comparison data were obtained using a BYK-Gardner Color-Guide 45/0 spectrophotometer with L*a*b* color measurements at three points per wafer, performed three times by the same researcher.⁶ Delta E average values were calculated and recorded for each wafer tested.

RESULTS

Altering a preset firing program to speed up the firing cycle of CAD/CAM milled restorations may result in an incomplete crystallization conversion. The research team hypothesized that switching between firing cycles to decrease time and increase speed and efficiency would produce restorations that were lighter in color and would have the same effect on all blocks tested. Previous research had reported that this phase change from metasilicate to disilicate may leave a dense microstructure, with a higher level of opaqueness, resulting in the final restorations appearing lighter in color.¹⁰

The overall Delta E value results, Delta E by block shade results, and Delta E by block translucency measurements are summarized in Table 2. Comparisons between shade and translucency groups were done with a one-way ANOVA test.¹¹ Dot1 and Dot2 were paired between crystal glaze (SDG) and speed/spray (FG) for all wafers using the average score (Dot3 was generally excluded as there was nothing to pair it with). The average Delta E across all samples was 0.92 with a standard deviation of 0.60.

Table 2 shows the p-value is significant, demonstrating a difference in Delta E values between the different block shades. The Tukey’s test was used to perform all pairwise comparisons. A2 shade had a significantly higher Delta E than B2 (adj. p = 0.0169) and C2 shade (adj. p = 0.0188).

Additionally, Table 2 demonstrates the p-value is significant, indicating there is a difference in Delta E values by block translucency. The Tukey’s test was used to perform all pairwise comparisons. MT translucency had a significantly higher Delta E than both HT (adj. p < .0001) and LT (adj. p < .0001).

Furthermore, the Delta E values for combined shade and translucency (Table 2) indicated that blocks A1MT (1.67) and A2MT (1.89) had small but obvious optical difference to a trained eye on the Visibility Delta E scale (Table 1).⁸ The remaining shade and translucency samples fell between Delta E of 0.0-1, indicating they were not visibly different. In this study’s results, optical color changes occurred only with medium translucency blocks (Figure 1) when fired on a fast-firing cycle.

DISCUSSION

Variations in a CAD/CAM dental restoration’s final shade can occur for various reasons including differences in material thickness, a tooth’s stump shade, the color and type of luting agent, and surface treatments of stains and glazes.¹² In addition, the perception of color depends on vision, light, and individual interpretation, which all influence the outcome.³ As teeth are evaluated against a shade guide, hue, saturation, and value are assessed to determine the correct CAD/CAM block shade and translucency that will produce the best results. Our study indicated an additional factor that can influence the final restoration by modulating the crystallization firing time of certain blocks with medium translucency.

The use of chairside CAD/CAM acquisition scanning and milling units started in 1985.¹³ The evolution of ceramic materials followed the success of in-office digital dentistry. Due to the convenience of completing highly esthetic restorations on the same day, more dentists began placing CAD/CAM systems in their offices instead of sending restorations to laboratories. Dental ceramics are made from a nonmetallic inorganic material processed by oven firing at elevated temperatures to make the structure of the ceramics more stable.¹⁴

The most popular dental ceramic in CAD/CAM chairside dentistry today is LS2 ceramic blocks.² For a clinician to anticipate the highest esthetic results with CAD/CAM in-office restorations, it is important to understand the crystalline changes that occur in LS2 blocks and the effects of firing cycles, such as are reported by this study. Due to its similarity to dental enamel, LS2 allows restorations to have the appearance and function most closely related to human teeth.¹⁵ LS2 blocks come in two forms:¹⁶

1. Heat-pressed precrystallized ceramic ingots with 70% disilicates, which are used in laboratory applications
2. Partially precrystallized CAD blocks with 40% metasilicates, which are for chairside milling

When restorations are sent to the dental laboratory, a heat-press technique melts LS2 ingots into the forms and are pressed into place.¹⁵ These ingots, at this point, are entirely crystallized into their final shade and translucency and do not require a second firing cycle. Since ingots are precrystallized, the organization of their matrices is more uniformly distributed and smaller in size, therefore, the final shade may have less variation.¹⁵

In contrast, chairside LS2 blocks are partially precrystallized and sold in a purple block form. According to Kaur,¹⁵ the CAD block contains disilicate crystal nuclei and varying levels of oxides to convert the purple block to its final shade. This precrystallized form is necessary to reduce bur wear and breakage and speed up in-office milling times. The purple block requires an additional heat treatment from an in-office oven to fully crystalize the matrix and convert the block to its final shade and translucency. This crystalline change from metasilicates to disilicates under certain pressure and temperature is required to achieve conversion at their preset manufacture firing settings.

During manufacturing, the oxides embedded into chairside block matrices have varied amounts of oxide content.¹⁵ Research has shown that as oxide agents increase, a dense microstructure is formed. The microstructure is not allowed to form fully, thus mechanical properties decrease while opaqueness increases.¹⁷

Fonzer et al¹⁶ looked at the flexural resistance of heat-pressed lab ingots and CAD-CAM LS2 blocks in different translucencies. Manufacturers declared a crystalline volume of approximately 70% for both pressed and CAD forms, no matter the translucency.¹⁶ But they reported that MT blocks showed significantly higher flexural strength compared to other types of translucencies. Therefore, it must be assumed that different translucencies reflect modifications in their mechanical properties, even though the manufacturer does not provide information to this effect. In contrast, Zhang¹⁸ and Oh¹⁹ both reported variations in LS2 where the pressed form had more uniform and specific-size crystal structure compared to the CAD block, which had smaller and randomly oriented crystals. Lupu and Giordano²⁰ also reported that precrystallized LS2 in high translucency blocks had fewer and larger crystals. Therefore, the difference reported in crystalline size and arrangement may contribute to shade variations that may occur more in chairside CAD/CAM blocks, such as in our study, and less so in laboratory ingots.

The initial hypothesis of this study was that all blocks on a fast-firing cycle would have optical color changes, which proved to be incorrect. A limitation of this study was that not all shades and translucencies of LS2 blocks were tested. More research is needed to understand why only certain A shades and only the medium translucency LS2 blocks have the most color variation and if this deviation may impact restoration strength and longevity.

CONCLUSION

Dental offices often must utilize a faster firing cycle for speed and efficiency when producing same-day CAD/CAM restorations. This study showed an observable difference that occurs in medium translucency ceramic blocks when they are subjected to a firing program that is less than the standard firing program preset by the manufacturers.

When CAD/CAM blocks are selected for final restoration in chairside applications, especially medium translucency blocks, according to our study, the clinician should consider that a change in the thermal regimen may influence the shade and may not result in optimum esthetics and expected results. The knowledge that this may occur will allow the practitioner to adjust for shade selection for better esthetic outcomes. In addition, this research may provide economic efficiency in dental practice due to a lack of remakes, lost patient appointments, and wasted clinical chair time.

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From Decisions in Dentistry. June/July 2024; 10(4):36; 39-41