Introduction

Restoration of anterior teeth in the esthetic zone is of critical importance for both dentists and patients. According to patients’ perception, the final color of the restoration is considered to be one main determinant of a successful treatment (1). Accurate color determination remains a challenge in dentistry. Selection of an appropriate shade can be difficult and complicated, considering the various aspects of tooth color determination including hue, value and chroma (2,3).

The facility of choosing the correct color for a ceramist is one of the most important breakthroughs in fixed prosthodontics and restorative dentistry. Sorensen and Torres defined five levels of weakness regarding common color-matching procedures: observer, viewing conditions, inadequate technique, poor communication and available shade guides (4).

The observer error relates to the fact that visual shade selection is a subjective method (5). Therefore, factors such as age and experience of the operator, eye fatigue, optical disease and psychological variables may affect human perception of color (2,6).

          Viewing conditions and external light play a dominant role in color determination. For instance, metamerism is a phenomenon which causes the color of different objects to differ when the light source changes, even if their color appears exactly the same under another light source (7). Another problematic issue is communication between dentist and ceramist. Using shade guides is an intermediate step,  therefore doubling the possibility of error, either by the dentist when selecting the shade or by the ceramist while replicating it (8).

A color shade guide should be fixed and unchangeable. The Vitapan Classical Shade Guide is assumed to be the gold standard and is a very popular system among those used by many dentists (9, 10). Vita 3D Master guides were introduced in 1998 and have a greater number of shade tabs. Color determination is systemically performed according to value, chroma and hue (11). Although advancements in technology have helped modern dentistry in color assessment by introducing spectrophotometers and colorimeters, it is suggested that both visual and instrumental color-matching methods be used (12). There are many different three-dimensional color spaces based on retinal sensitivity, among which L*a*b* and L*C*h* are the most commonly used for tooth color analysis due to color space uniformity and accurate reflection of human senses (13).

          Shade guide errors may arise from different factors. For example, shade guides are fabricated from porcelain or resin. Porcelains used to make shade guides are not the same as porcelains used in restorations; also shade guides from the same manufacturer may not be identical (14-16). Furthermore, shade guides must be disinfected or occasionally sterilized after use, which in turn make them prone to color changes (17). Huang et al. reported significant color change in shade tabs after disinfection. However, ΔE was below the level of clinical importance (16).

The purpose of this study was to evaluate the color change of Vita 3D Master different shade tabs after disinfection and sterilization. The null hypothesis, according to shade guide manufacturer claims, is that no significant color change happens after disinfection and sterilization procedures. 

Material and Methods

Ninety-eight shade tabs were selected from 14 new, unused Vita 3D Master shade guides (Vita Zahnfabrik H. Rauter GmbH & Co., Germany, 2012) including the following shades: 2M1, 3L1.5, 3M1, 3M2, 3M3, 3R1.5 and 4M1. We logically chose shade tabs of different values (2, 3, 4), chroma (1, 2, 3) and hue (L.M,R). The 98 samples were divided randomly into two equal groups of sterilization and disinfection.

Disinfection method

Deconex (lot: 128745, borer chemie, Switzerland) was chosen because of its popularity in dental clinics for disinfection. Dental disinfectants have different compositions. Deconex belongs to the isopropyl alcohol group, which is not harmful to Vita Shade Guides according to the Vident Web site (http://vident.com/products/shade-management/strilizing-and-disinfectingshade-guides/). Deconex is considered to have antibacterial, antifungal and virus-neutralizing effects.

All samples were sprayed with Deconex solution, kept for 3 minutes in a lidded dish, and then wiped by gauze for drying according to the manufacturer's instruction. After 480 cycles of repeating the process (it was assumed that the process of color selection was performed twice a day, 5 days a week by a clinician; therefore 480 repetitions is equal to 1 year performance), the samples were ready for color measurements. This procedure was also repeated for 2 and 3 years of simulated treatment.

Sterilization method

Sterilization was performed on 49 samples, according to the shade guides manufacturer's instructions, at 135 °C under a pressure of 4 PSI for 10 minutes.  The samples were steam autoclaved (Getinge 442SL, Getinge Group) 240 times, which was equivalent to the usual clinical application per year (once a day, 5 days a week). This procedure was also repeated for 2 and 3 years of simulated treatment.

Color measurement

Color measurement was carried out using a Vita Easyshade Advance spectrophotometer (Vita Zahnfabrik, Germany). The middle third of the shade tabs was used to measure the color in Single Tooth Mode on the Easyshade device. To maintain a fixed position for Easyshade at the middle third of the shade tabs, we made a holder from acrylic resin. The holder provided fixed and identical conditions for all shade tabs in the measurement procedure and prevented environmental light shading the tab surfaces (Fig. 1).

An acrylic holder was made from heat-cured acrylic resin (Meliodent, Germany) to mount each shade tab. It had two parts, upper and lower, which were assembled using two metallic pins at 5 mm from the center of the shade tab. Thus, the two pieces of the acrylic holder completely encircled each shade tab and the tip of the spectrophotometer device came from a hole on the upper part to the same area on all tabs (Fig. 2).

For each sample, we recorded color indices including   L *, a * and b *. After sterilization and disinfection  procedures, total color difference (ΔE) of all samples was calculated by the following formula:

The acrylic holder was used for all measurement in order to ensure consistent results. The Easyshade Device provides two more values, C * (chroma) and H * (hue), that were also recorded. For statistical analysis, we used repeated measures analysis of variance (ANOVA) and independent t-test and paired sample t-test.

Results

The effect of sterilization and disinfection on shade tabs is reported in Table 1. The difference between the sterilization and disinfection groups was significant for ΔE in all three years of simulated treatment (p˂0.001) according to Student's t-test.

In order to compare the differences in value, chroma and hue with the baseline, a paired sample t-test was used for each sample. Comparisons were made in disinfected and sterilized groups independently.  L* changes were significant.  The value decreased in both groups, although the reduction in the sterilized group was more significant.

Evaluations showed a significant difference in chroma between the baseline and all simulations (p˂0.001); during these simulations, chroma decreased in the disinfected group and increased in the sterilized group. In the disinfected group, the change in hue was not significant for the first year, but was significant in the second and third years. In the sterilized group, change in hue was significant for all years. Hue increased in the disinfected group and decreased in the sterilized group. 

In addition, we evaluated the effect of disinfection and sterilization on different hues (3R1.5, 3L15, and 3M1). Changes in the first year were not significant in both groups; greater change was observed in the L hue (yellowish) compared with other hues.

In the Chroma group (3M1, 3M2 and 3M3), the 3M2 sample increased in the disinfected group each year and decreased in the 3M3 and 3M1 samples; 3M3 showed greater reduction in chroma.

 Our findings also revealed that changes in value were not significantly different between the disinfected and sterilized groups in 2M1, 3M1, 4M1 samples (p˃0.05).

Table 1.  Differences in ΔE between the two groups

Changes in color value, chroma and hue are presented in Table 2.

Table 2. Recorded variables before and after sterilization and disinfection including mean and standard deviation in the first, second and third years