Effect of surface finishing on the colour stability and translucency of dental ceramics

Two of the most popular CAD/CAM ceramics were chosen in this study. The selected HC and RNC materials are listed in Table 1. Using a precision saw machine, 1.2 mm–thick disk specimens with 14 mm diameters were prepared with high-translucence CAD/CAM blocks by slicing them with a water-cooled diamond disk (Micracut 201, Bursa, Turkey) at low speeds (150 rpm). The shades of the Enamic (Vita Enamic, Vita Zahnfabrik, Germany) specimens were characterised with a specially developed, white Vita Enamic Stains Kit (Vita Zahnfabrik, Germany) following the manufacturer’s instructions. The 2M2 colour for Enamic specimens was chosen according to the Vita 3D-Master shade guide (Vita Zahnfabrik, Germany) which corresponds with the Lava Ultimate (Lava Ultimate, 3M Espe, USA) specimens’ A2 colour according to the Vitapan Classical shade guide (Vita Zahnfabrik, Germany).

As listed in Table 2, 24 groups consisting of 10 specimens (240 specimens in total) were created out of HC and RNC , including six groups to be stored in distilled water served as the controls groups (E1a,E2a, E3a, L1a, L2a, L3a) . All of the specimens’ surfaces were roughened on both sides with 600-, 800- and 1200-grit silicon carbide abrasive papers (English Abrasives, London, UK) under running water at 100 cycles per minute. This standardised polishing procedure eliminated external irregular scratches of the cut specimens and equally rounded the chipped corners [9]. The specimens in the E1b, E1c and E1d groups were then polished with the Vita Enamic technical polishing set (Vita Zahnfabrik, Bad Sackingen, Germany) following the manufacturer’s instructions for the. Specimens in groups L1b, L1c and L1d were polished with a low-speed hand piece at 10,000 rpm with medium and fine rubber wheels (Dedeco Red and Green Rubber Wheels, Dedeco, NY, USA). The specimens in the E2b, E2c, E2d, L2b , L2c and L2d groups were polished with 12.7 mm–diameter Sof-Lex polishing discs (3M Espe, MN, USA) mounted on a hand piece set at a speed of 10,000 rpm for coarse and medium discs and 30,000 rpm for fine and superfine discs according to the manufacturers’ recommendations. The specimens were polished in succession with a low-speed hand piece at 10,000 rpm with an abrasive stone(Dura Green Stones; Shofu Inc, Kyoto, Japan), a coarse silicon carbide polisher (Ceramaster Coarse; Shofu Inc), and a silicon carbide polisher (Ceramaster; Shofu Inc) in the groups E3b, E3c, E3d, L3b, L3c and L3d. All polishing instruments were applied on both sides of the specimens with a low-speed rotating hand piece (Kavo Ewl 4990; KaVo Dental Gmbh, Germany) by the same investigator.

Before colour measurements were taken, all of the specimens were cleaned in deionised water for ten minutes in an ultrasonic cleaner (Pro-Sonic 600; Sultan Healthcare, NJ, USA) and then dried with compressed air. The thickness of the specimens were confirmed with a digital calliper (Absolute Digimatic, Mitutoyo, Japan). The baseline colour measurements were performed with a clinical spectrophotometer (Vita Easy Shade Advance, Vita Zahnfabrik, Germany) under standard illuminant D65 using CIE L*a*b* and were recorded before treatment with the staining solution. All measurements were performed on white, black and neutral grey surfaces. After repeating the colour measurements three times for each of the specimens, the mean values of L*, a* and b* were calculated. In the CIE L*a*b* system color differences (∆E*) formula, L* represent lightness, a* represents the chromaticity coordinate for red-green and b* represents the chromaticity coordinate for yellow-blue for color differences (∆E*) formula [8].

Translucency was determined by identifying the value of the TP. Measurements were performed with the spectrophotometer under D65 illumination over white and black backgrounds. Measurements were repeated three times, and the mean CIE L*a*b* values were recorded for both backgrounds. The TP was obtained by calculating the difference in colour between the specimen over a white and black background with the following formula [15, 16]:

In this formula, B signifies the colour coordinates over a black background and W signifies the colour coordinates over a white background. The greater the TP value, the higher the translucency of the material. A TP value of 100 indicates the specimen is transparent and a TP value of 0 indicates that the material is opaque.

After the baseline colour measurements were made, the specimens were stored in four different solutions. The tea solution was prepared with 200 ml of boiling water poured over a 2 g tea bag (Lipton, Unilever, Turkey) according to the manufacturer’s recommendations. The coffee solution was made with 3.6 g of coffee (Nescafe Classic; Nestle, Bursa, Turkey) grounds and 300 ml of boiled water according to the manufacturers’ recommendations. The solution was filtered through a filter paper after 10 min of storing. A 48 h storage period was selected with reference to previous studies [17, 18]. The solution was stirred every 8 ± 1 hours. A Coca-Cola solution (The Coca-Cola Company, İstanbul, Turkey) was prepared with a can of 330 ml of Coca-Cola. Finally, distilled water served as the control group’s solution. All specimens were stored in 100 ml solutions at 37°C for 48 h. Then, the specimens were rinsed with distilled water for 5 min and blotted dry with tissue paper (Selpak; Eczacıbaşı, Istanbul, Turkey) before colour measurements.

The second set of colour measurements were made using the spectrophotometer with the same method as the first. The determination of the colour variation, ∆E*, between the two colour measurements was made in the CIE L*a*b* system using the following equation:

To evaluate whether the colour differences in translucent ceramics were acceptable, this study used an average threshold of ∆E* = 2.7 [19].

The one-way ANOVA test results and the means and standard deviations of ∆E* values are shown in Table 3. No significant differences in ∆E* values were detected between groups E1a, E2a, E3a, L1a, L2a and L3a, which were the six control groups (p > 0.05). Also, no significant differences in ∆E* values were observed within the polished HC groups (E1a, E1b, E1c, E1d), the Sof-Lex HC groups (E2a, E2b, E2c, E2d) or the Shofu HC groups (E3a, E3b, E3c, E3d) (p > 0.05 for all 3). The ∆E* values were significantly different among the polished RNC groups (L1a, L1b, L1c, L1d), the Sof-Lex RNC groups (L2a, L2b, L2c, L2d), and the Shofu RNC groups (L3a, L3b, L3c, L3d) (p < 0.05 for all 3). The ∆E* values of HC specimens in the polished group (E1d), Sof-Lex group (E2d) and Shofu group (E3d) that were stored in the coffee solution had statistically significant differences from the HC groups (p < 0.05). The ∆E* values of the RNC specimens in the polishing group (L1c, L1d), Sof-Lex group (L2c, L2d), and Shofu group (L3c, L3d) that were stored in the tea and coffee solutions were significantly different from the specimens in the groups (L1b, L2b, L3b) stored in the cola solution (p < 0.05). A comparison of the ∆E* values of the different groups of the HC and RNC materials are shown in Figs. 1 and 2.

The one-way ANOVA results and the means and standard deviations of the TP values are shown in Table 4. No significant differences in the TP values were detected between the specimens in the groups E1a, E2a, E3a, L1a, L2a and L3a which were stored in distilled water as the control groups (p > 0.05). However, these groups showed the highest TP values out of all HC and RNC groups. The TP values were significantly higher with the HC and RNC polished groups than the HC and RNC Sof-Lex groups and Shofu groups (p < 0.05). Also, the TP values of the HC and RNC specimens stored in the coffee solution were the lowest, which was significantly different than the specimens stored in cola, and tea (p < 0.05). The TP values of both HC and RNC specimens with polishing, Sof-Lex finishing and Shofu finishing that were stored in tea and cola were not significantly different from one another (p > 0.05). The RNC specimens also showed higher TP values than the HC specimens in all groups, and these values were statistically different from each other (p < 0.05). A comparison of the TP values of the different groups of HC and RNC materials are shown in Figs. 3 and 4.