Statement of problem
Severely discolored substrates have been shown to limit the use of computer-aided design and computer-aided manufacturing (CAD-CAM) ceramic blocks because they provide insufficient color masking.
The purpose of the in vitro study was to evaluate the effect of a layer of high-value opaque composite resin over discolored substrates to determine its masking ability with CAD-CAM ceramics.
Material and methods
Six ceramic groups (n=10) were tested. A bilayer group of zirconia and porcelain served as the control. The CAD-CAM monolithic groups were translucent zirconia, zirconia-reinforced lithium silicate, lithium disilicate, leucite-reinforced glass-ceramic, and feldspathic ceramic. Five substrates were used: A1 (used as reference), A3.5, C4, and coppery and silvery metals. The substrates were separated as nonlayered or layered (with flowable or restorative opaque composite resins). The tested luting agents were white, opaque, and A1. Color differences (ΔE 00 ) were assessed with the CIEDE2000 formula. A 2-way ANOVA (α=.05) was used to detect significant differences in ΔE 00 among the groups for each substrate. The results were compared with acceptability (1.77) and perceptibility (0.81) thresholds.
The flowable composite resin layer associated with A1 luting agent ensured ΔE 00 lesser the than perceptibility thresholdwith the use of CAD-CAM monolithic ceramics, with the lowest values for zirconia-reinforced lithium silicate in substrates A3.5 (0.53) and C4 (0.32) and for leucite-reinforced glass-ceramic for coppery (0.49) and silvery (0.81) substrates ( P <.001). The same benefit was observed when zirconia and porcelain was tested over the silvery substrate. The absence of substrate treatment only provided ΔE 00 lesser than the acceptability threshold with CAD-CAM ceramics for the A3.5 background.
The application of a flowable opaque composite resin and the use of a shaded luting agent ensure masking with CAD-CAM monolithic ceramics.
When managing the restoration of teeth with severely discolored substrates, clinicians should consider layering with an opaque flowable composite resin and the use of a shaded luting agent to obtain adequate and predictable masking with CAD-CAM monolithic ceramics.
Dental ceramics present advantages that include esthetics, adequate translucency, color stability, durability, adequate strength and chemical stability, and high biocompatibility and provide an attractive option for patients with severe loss of dental structure or color alteration. , However, the use of translucent ceramics over dark tooth structure or metal post-and-cores can be challenging because they may not mask these dark substrates.
Zirconia copings have been popular as an alternative to metals because they provide increased translucency, but a coping thicknesses up to 0.8 mm is necessary to ensure the adequate masking of discolored substrates. However, monolithic glass-ceramics and translucent zirconia restorations are increasingly used because of the reduced production time and cost and the risk of chipping with bilayer systems. Computer-aided design and computer-aided manufacturing (CAD-CAM) systems offer a standardized manufacturing process, resulting in a reliable, predictable, and economic workflow for implant- and tooth-supported restorations ; moreover, they offer the possibility of single-visit restorations. However, monolithic CAD-CAM ceramic crowns may not be able to mask dark tooth structure or metal substrates.
Lithium disilicate is a popular glass-ceramic used for CAD-CAM applications. Its composition with small and uniformly distributed crystals provides adequate mechanical properties and excellent esthetics for use as bilayered or monolithic restorations. Recently, alternative materials have been introduced that include high-translucency zirconia and zirconia-reinforced lithium silicate ceramics. , Both materials were developed to provide high-strength restorations with adequate clinical appearance, , but studies on the masking ability of these materials are sparse.
A recent study evaluated the substrate masking ability of different ceramic systems and the influence of their association with an opaque resin-based luting agent. The substrates evaluated were composite resins in shades A3.5 and C4, and copper- and silver-colored metals. The authors reported that all discolored substrates were adequately masked only with a medium-opacity zirconia coping veneered with glass-ceramics. The CAD-CAM monolithic restorations associated with a white opaque resin-based luting agent were only able to mask the substrate A3.5 to within the acceptability threshold. Therefore, additional techniques are necessary for the satisfactory use of CAD-CAM restorations over discolored substrates.
The use of a thin layer (around 50 μm) of a high-value opaque luting agent has been reported to show potential for masking. Therefore, a thicker composite resin layer (0.2 to 0.3 mm) might provide adequate masking with minimal alteration in the tooth preparation. Therefore, the present study question was, could using a layer of high-value opaque composite resin over discolored substrates, alone or in association with a white opaque luting agent, ensure masking for monolithic CAD-CAM ceramics?
The aim of this study was to test the masking ability of a layer of high-value opaque composite resin over the discolored substrates in association with luting agents (white opaque and A1 shaded) for CAD-CAM ceramic restorations. The tested hypotheses were that the masking strategy (association of composite resin layer and luting agent) and the ceramic type would influence the masking of discolored substrates.
Material and methods
Table 1 summarizes the ceramic restorative assemblies tested in this study. Ceramic slices were obtained from CAD-CAM blocks with a diamond blade (15 LC diamond; Buehler) and a cutting machine (ISOMET 1000; Buehler). The ceramic surfaces were polished with diamond disks (Dia-Grid Diamond Discs #120 – average grit size 160 mm; Allied High Tech Products) and silicon carbide papers (#600-2000 grit).
|Materials Used||Type/Thickness of Restoration||Commercial Name/Manufacturer||Batch Number||Group Abbreviation|
|Zirconia and porcelain|| Bilayer, 0.8-mm zirconia core
1.0-mm porcelain veneering
| IPS ZirCAD, MO
IPS e.max ceram, shade A1 (0.5 mm enamel, 0.5 mm dentin)/Ivoclar Vivadent AG
|Translucent zirconia||1.8-mm monolithic structure||Prettau anterior, Shade A1/Zirkonzahn.||ZB6111D||TZ|
|Zirconia-reinforced lithium silicate glass-ceramic||1.8-mm monolithic structure||Suprinity T, Shade A1/Vita Zahnfabrik.||49 144||ZRLS|
|Lithium disilicate reinforced glass-ceramic||1.8-mm monolithic structure||IPS e.max CAD, LT, shade A1/Ivoclar Vivadent AG||V13882||LD|
|Leucite-reinforced glass-ceramic||1.8-mm monolithic structure||Empress CAD, Shade A1/Ivoclar Vivadent AG||T50414||LGC|
|Feldspathic ceramic||1.8-mm monolithic structure||CEREC Block, shade A1/Vita Zahnfabrik||42 740||FC|
The manufacturer instruction was followed for ceramic processing. A sintering furnace (Programat EP 5000-G2; Ivoclar Vivadent AG) was used to crystalize monolithic structures of lithium disilicate and zirconia-reinforced lithium silicate at 840 °C. The zirconia infrastructures were sintered in a specific furnace (Vita Zyrcomat T; Vita Zahnfabrik) – IPS ZirCAD at 1500 °C and Prettau Anterior at 1450 °C. To veneer the zirconia and porcelain group, the modeling liquid and porcelain powder were mixed, applied over the zirconia core, and sintered (Programat EP 5000-G2; Ivoclar Vivadent AG). Glaze was applied to the top surface of the glass-ceramics. The specimens of monolithic zirconia were stained to A1 shade with the specified liquid (Prettau Anterior Aquarell; Zirkonzahn Inc). Ten Ø8×1.8-mm disk specimens were assigned to each group, based on a previous analysis. A random number generator program was used for specimen randomization.
The shaded substrates A1 (reference), A3.5, and C4 were made with composite resin (Filtek Z350XT Dentin; 3M ESPE) increments introduced in layers into a silicone mold and light activated with 1200 mW/cm 2 (Radii-Cal; SDI) for 20 seconds. Acrylic resin patterns were produced from silicone molds to cast the metal backgrounds. Copper-aluminum alloy (Goldent L.A; Alloy Tech) was used to obtain the copper-colored substrate, and cobalt-chromium alloy (Duceralloy; Dentsply Sirona) to obtain the silver-colored one. The substrates were polished with silicon carbide papers (#600–2000 grit) and had a final dimension of Ø8×3.0 mm.
A high-value opaque restorative composite resin (Filtek Z350 XT WD; 3M ESPE) and a flowable opaque composite resin (IPS Empress direct Opaque; Ivoclar Vivadent AG) were used to test the effect of applying a 0.25 ±0.05-mm layer over the discolored substrates. Before application, the surfaces of the composite resin backgrounds were cleaned with 37% phosphoric acid, treated with a layer of hydrophobic bonding agent (Adper Scotchbond Multipurpose; 3M ESPE), and light activated. Opaque composite resins were applied over the backgrounds and light activated with 1200 mW/cm 2 for 20 seconds. The surfaces of the metal backgrounds were cleaned and treated with a 10-methacryloyloxydecyl dihydrogen phosphate (MDP)-based primer layer (MZ Primer; Ivoclar Vivadent AG), followed by composite resin application and light activation.
A white opaque and an A1-shaded resin-based luting agent were tested (Allcem Try-in; FGM Dental Products). The composition of this paste was polyethylene glycol (PEG), water, glycerin, pigments, and silica. The tested masking strategies adopted in the study are presented in Table 2 .
|Group Abbreviation||Masking Strategy|
|N-A1 (control)||Ceramic + nonlayered substrate + A1 luting agent|
|RC-A1||Ceramic + substrate layered with restorative composite resin + A1 luting agent|
|RC-WO||Ceramic + substrate layered with restorative composite resin + white opaque luting agent|
|FL-A1||Ceramic + substrate layered with flowable composite resin + A1 luting agent|
|FL-WO||Ceramic + substrate layered with flowable composite resin + white opaque luting agent|
A spectrophotometer (SP60; X-Rite) was used to measure the spectral data in diffuse reflectance mode. The device was calibrated before measurements by using the manufacturer’s reference. A single trained operator (P.S.) performed the measurements in triplicate. The specimen was introduced inside the spectrophotometer chamber and centralized, and the measurements were made under the following parameters: aperture setting 8 mm; illumination 12 mm; and measuring time of 2 seconds. The wavelength range was of λ=400-700 nm/intervals of 10 nm. Commission Internationale de l’Eclairage (CIE)Lab color coordinates were calculated as per the CIE D65 Standard Illuminant and CIE 2-degree standard observer. Color parameters were obtained in 3 coordinate dimensions of L∗ (from 0 [black] to 100 [white]), a∗ (green-red [−a∗=green; +a∗=red]), and b∗ (blue-yellow [−b∗=blue; +b∗=yellow]). The CIELab coordinates of backgrounds tested in the study are presented in Table 3 .
|CIELab of Backgrounds|
The CIEDE2000 equation was used to measure the color difference (ΔE 00 ) with coordinates L∗, a∗, and b∗ of each specimen on a discolored background in relation to the background A1 :
where ΔE 00 is the color difference, the subscripts R and D refer to the lightness (L′), chroma (C′), and hue (H′) of the specimens over the reference and the discolored backgrounds, respectively. R T is a rotational function that accounts for the interaction between chroma and hue differences in the blue region. S L , S C , and S H are weighting functions, and K L , K C , K H are correction terms to be adjusted as per the experimental conditions. The parametric factors K L , K C , K H were set to 1. The color differences obtained were compared with reference values from the literature, which specified values of ΔE 00 =0.81 (perceptibility threshold [PT]) and that the color difference is considered acceptable up to ΔE 00 =1.77 (acceptability threshold [AT]).
The Kolmogorov-Smirnov test showed a normal distribution of data in all groups ( P >.05). The Levene test showed homogeneity of variance ( P >.05). A 2-way ANOVA (α=.05) was used to detect significant differences in ΔE 00 among the groups considering the factors “ceramic material” and “masking strategy–composite resin layer/cement type” and the association between factors. Multiple comparison data were obtained using the Tukey test (α=.05). Data were analyzed with a statistical software program (IBM SPSS Statistics for MacIntosh, v21; IBM Corp).