A maxillary left central incisor tooth model (A5A-200, Nissin Dental Products Inc, Kyoto, Japan) was selected as the master abutment, and a pre-preparation putty index was fabricated using vinyl polysiloxane impression material (Imflex Putty, Meta Biomed Co, Chungcheongbuk-do, Korea). The preparation for laminate veneer with butt joint design was 1.5 mm incisal reduction, facial reduction of 0.5 mm extended beyond the proximal contact area, and 0.5 mm chamfer margin (Fig. 1A). The preparation was performed using a specific veneer preparation kit (REF 10.801.002, Microdont, Sao Paulo, Brazil) by a single operator. Finally, the preparation of the master abutment was evaluated using the pre-preparation putty index (Fig. 1B).
The prepared tooth model was used to mil sixty resin abutments of the same dimension and color from a polymethyl methacrylate disk (PMMA CAD/CAM Disk-A2 shade, Yamahachi Dental MFG Co., Gamagori, Japan) using CAD-CAM technology. First, the prepared tooth model was scanned using a lab scanner (Freedom UHD Model Scanner, DOF Inc, Seoul, South Korea) and an anti-reflection scan powder (3D Scanning Spray, Bilkim Ltd. Co, İzmir, Turkey) was used. The scanned data was then used to duplicate sixty abutments with the same dimensions using CAD-CAM software (CORiTEC iCAM V5 Smart, Imes-Icore GmbH, Eiterfeld, Germany). Finally, the abutments were milled using the milling machine (CORiTEC 350i Loader pro, Imes-Icore GmbH, Eiterfeld, Germany).
According to the ceramic material and cement type (Table 1), the milled abutments were divided into six groups (n = 10) as follows; Group EV: IPS e.max CAD (Ivoclar Vivadent, Schaan, Liechtenstein) bonded using Variolink Esthetic LC (Ivoclar Vivadent, Schaan, Liechtenstein), Group ER: IPS e.max CAD bonded using RelyX Veneer (3M ESPE, MN, USA), Group CV: Celtra Duo (Dentsply Sirona, NC, USA) bonded using Variolink Esthetic LC, Group CR: Celtra Duo bonded using RelyX Veneer, Group KV: Katana zirconia (Kuraray Noritake Dental Inc, Tokyo, Japan) bonded using Variolink Esthetic LC, and Group KR: Katana Zirconia veneer bonded using RelyX Veneer (Fig. 2).
To fabricate ceramic laminate veneers, all abutments were sprayed with anti-reflection scanning powder spray (3D Scanning Spray, Bilkim Ltd. Co, İzmir, Turkey) and then scanned with the CAD-CAM scanner (Identica hybrid, Medit, Seoul, Korea). Each ceramic laminate veneer restoration was designed using CAD-CAM software (Collab 2017, Exocad, Darmstadt, Germany). The pre-preparation scan was used to support the transfer and to standardize the design for all restorations. To close a 1 mm diastema from both sides, the proximal thickness of the design was gradually increased from cervical to incisal, with a maximum increase in thickness in the contact area (0.5 mm). Each designed restoration had a cement gap thickness of 50 µm, a labial thickness of 0.5 mm, and 1.5 mm incisal thickness. The ceramic laminate veneer restorations were then milled using the milling machine (CORiTEC 350i Loader pro, Imes-Icore GmbH, Eiterfeld, Germany). The milled IPS e.max CAD veneers were finished and then subjected to crystallization and glaze firing in the porcelain furnace (Pogramat P500, Ivoclar Vivadent, Schaan, Liechtenstein) according to the recommendations of the manufacturer. The milled Celtra Duo veneers were cleaned and then subjected to glaze firing following the instructions of the manufacturer. The milled zirconia laminate veneers were sintered at 1550°C with a holding time of 2 hours in a sintering furnace (Tabeo, mihm-VOGT, Stutensee, Germany). The zirconia laminate veneers were then glazed (Cerabien ZR Glaze, Kuraray Noritake Dental Inc, Tokyo, Japan) and subjected to glaze firing following the instructions of the manufactures. Finally, the digital caliper was used to verify the dimensions of each restoration.
The intaglio surfaces of IPS e.max CAD and Celtra Duo veneers were etched using hydrofluoric acid (Porcelain Etch, Ultra Dent, Utah, USA) for 20 seconds, rinsed and air dried. A thin coat of universal primer (Monobond N, Ivoclar Vivadent, Schaan, Liechtenstein) was then applied to the etched surfaces with a disposable brush for 60 seconds and allowed to air dry for 10 seconds. The intaglio surfaces of zirconia veneers were air-borne particle abraded (Basic eco, Renfert, Hilzingen, Germany) with 50 µm alumina (Al2O3 Cobra, Renfert, Hilzingen, Germany) at 2.5 bar for 10 sec/cm2 at a distance of 10 mm and perpendicular to the surface. The prepared surfaces of the abutment were treated with 37% phosphoric acid (N-Etch, Ivoclar Vivadent, Schaan, Liechtenstein) for 45 seconds, washed and air-dried for 10 seconds.50
For EV, CV, KV groups, the bonding agent (Tetric N-Bond Universal, Ivoclar Vivadent, Schaan, Liechtenstein) was applied according to manufacturer's instructions, then cured for 10 seconds with a light curing device (Bluelex LD-105, Monitex Industrial Co., Ltd, New Taipei, Taiwan) with an average light intensity of 1000 mW/cm2. The light intensity of the curing device was measured using a radiometer (Bluephase Meter II, Ivoclar Vivadent, Schaan, Liechtenstein) before each application. During the cementation, the cement paste (Variolink Esthetic LC, neutral, Ivoclar Vivadent, Schaan, Liechtenstein) was applied to the pre-treated intaglio surface of the restoration, seated on the abutment, and retained under a static load of 250 gm.51 An initial light-curing (Bluelex LD-105, Monitex Industrial Co., Ltd, New Taipei City, Taiwan) was performed for 2 seconds, the excess cement was removed. The margins were covered with a glycerin gel (Liquid Strip Glycerin Gel, Ivoclar Vivadent, Schaan, Liechtenstein), then the final curing was performed for 30 seconds per each surface. For ER, CR, KR groups, the bonding agent (Single Bond Universal Adhesive, 3M ESPE, Minnesota, United States) was applied and cured for 10 seconds. The cement paste (RelyX Veneer TR, 3M ESPE, MN, USA) was applied to the pre-treated intaglio surface of the veneer, then seated on its corresponding abutment and cured as described earlier. After cementation, the margins of each restoration were finished and polished with silicone tips (Kenda Dental Polishers, Vaduz, Liechtenstein). All specimens were stored in distilled water at 37°C for 24 hours after cementation.
A reflectance spectrophotometer (UV- Shimadzu 3101 PC, Agilent Technologies Inc, CA, USA) was used to measure and calculate the International Commission on Illumination (CIE) tristimulus color values. The spectrophotometer was calibrated according to manufacturer's instructions before and after each group color measurement. The CIE D65 standard illuminant was selected, as well as a 10° standard observer angle. Color measurements were performed by a single investigator experienced with this device at the center of the labial surface of the tooth, with the measuring area having a 3 mm diameter to reduce edge loss. To secure the specimen in a well-adapted precise position, a self-adhesive pad (Patafix yellow, UHU GmbH Co. KG. Bühl, Germany) was used and black plasticine (Jovi, Barcelona, Spain) as a backing with the CIE values: L*=25.87, a*=-1.12, and b*=0.95. The calculations were carried out according to the CIELAB color system.52,53 The CIE coordinates were determined and the measurements were repeated three times for each specimen and the average result was recorded. The color differences following the CIE DE2000 color difference formula (ΔE00) were calculated:
$$\varDelta {E}_{00}=\sqrt{ \left(\frac{\varDelta L}{{k}_{L}{s}_{L}}\right)+\left(\frac{\varDelta c}{{k}_{c}{s}_{c} } \right)+\left(\frac{\varDelta H}{{k}_{H}{s}_{H} } \right)+{R}_{T}\left(\frac{\varDelta c}{{k}_{c}{s}_{c} } \right)+\left(\frac{\varDelta H}{{k}_{H}{s}_{H} } \right) }$$
ΔL, ΔC, and ΔH represent lightness, chroma, and hue differences, respectively. RT and S refer to the rotation factor and the weighting function. KL, KC, and KH represent the parametric factors. ΔE00 value of 1.30 was considered the perceptibility threshold; while, 2.25 was considered the acceptability threshold.54 The CIE coordinates of the specimen were evaluated against black and white settings to obtain the translucency parameter (TP). L*=25.87, a*=-1.12 and b*=0.95 are the CIE values for the black setting, while L*=69.79, a*=0.03 and b*=2.70 are the CIE values for the white setting. The following formula was used to calculate the TP for each specimen:
$$T{P}_{00}= \sqrt{ \left(\frac{{L`}_{B}-{L`}_{w}}{{k}_{L}{S}_{L}}\right)+\left(\frac{{C`}_{B}-{C`}_{w}}{{k}_{C} {S}_{C}}\right)+\left(\frac{{H`}_{B}-{H`}_{w}}{{k}_{H} {S}_{H}}\right)+{R}_{T}\left(\frac{{C`}_{B}-C`w}{{k}_{C} {S}_{C}}\right)+ \left(\frac{{H`}_{B}-{H`}_{w}}{{k}_{H} {S}_{H}}\right) }$$
represents the difference in value among black and white settings; stands for the difference in chroma in black and white settings; represents the difference in hue under black and white settings. The rotation factor and the weighting function are given by RT and S. The parametric factors are indicated by KL, KC, and KH. Each measurement was performed three times, with the average value being recorded. The translucency perceptibility threshold was set at 0.62, while the translucency acceptability threshold was set at 2.62.55 The contrast ratio was calculated using the following formula:
The CIE tristimulus values of the specimen over black background are represented by Y, while the CIE tristimulus values of the specimen over white background are represented by Yw.
Aging: The specimens were subjected to 10,000 thermocycled cycles to simulate one year aging. The dowel time was 20 seconds in the thermocycling machine between 5°C and 55°C (SD Mechatronic Thermocycler; Germany). The specimens were then subjected to load cycling (Model ACH-09075DC-T, AD-Tech Technology CO, Germany). To mimic one year of masticatory forces in the anterior dentition, the specimens were loaded with 49 N at a rate of 1.6 Hz for 150,000 cycles.56 A 5.4-mm steel piston was used to deliver the load to the cingulum of the tooth (3 mm below the incisal edge) at a descending speed of 40 mm/sec. The specimens were examined for cracks after aging. After aging, the specimens were immersed in coffee solution for 18 hours.57 The coffee solution was prepared by combining 15 gm of coffee with 300 mL of water in a filter coffee machine. The specimens were then cleansed by brushing each specimen circumferentially ten times with toothpaste (Sensodyne, GalaxoSmithKline) under running water. Finally, all the specimens were ultrasonically cleaned (Jeken PS-30, Jeken, Guangdong, China) and dried. Color and translucency measurements were performed a second time for all specimens as previously described.
Statistical interpretations were performed utilizing statistical software (SPSS Statistics for Windows v22.0, SPSS Inc, IBM Corp, NY, USA). The Shapiro-Wilk test showed that the data were normally distributed (P < .05). The one-way ANOVA test was used to compare more than two independent groups. The interaction of the two independent variables (material type and cement type) and the effect of each variable on the color changes and translucency were assessed using the two-way ANOVA test. The post-hoc test was used for multiple comparisons. The significance of the results obtained was judged at P < .05.