The effect of thickness and degree of translucency on the final color of lithium disilicate veneers

Statement of problem: Achieving lighter final shades of A1 and BL1 with lithium disilicate veneers and the impact of different lithium disilicate translucencies and thickness on the final color is unclear. Aim: The aim of this in vitro study was to investigate the effect of different translucencies and thicknesses of lithium disilicate on the final shade of veneers. Materials and methods: Eighty lithium disilicate specimens were fabricated from two translucencies of lithium disilicate blocks(IPS e.max CAD blocks, Ivoclar Vivadent); HT high translucency(HT) and low translucency (LT), in two different shades; A1 and BL1, and with 4 different thicknesses; 0.3mm, 0.5mm, 0.7mm, and 0.9mm. The discs were placed on an A3 shade composite substrate with a 0.1 mm translucent resin cement. ΔE values were recorded to determine color differences between the specimens and the standard A1 and BL1 color parameters. ΔE values were directly recorded with a handheld spectrophotometer (Easyshade Advance) The ΔE values were compared with an acceptability threshold (ΔE=3.7). Two-way ANOVA, and the Bonferroni test were used to analyze data (α=.05). Results: The changes in both thickness and translucency showed high significance. For A1 shade, the 0.9 mm LT showed the least mean ΔE value while the highest mean value was obtained by 0.9 mm HT. For BL1 shade, The 0.9 mm HT samples showed the least mean ΔE value while the highest mean value was obtained by 0.3 mm LT samples. Conclusions: The thickness and the translucency of lithium disilicate affected its final color. A minimum of 0.5 mm thickness of LT lithium disilicate was able to produce a clinically acceptable A1 shade. Achieving BL1 final shade was not possible with both HT lithium disilicate and LT lithium disilicate in thicknesses up to 0.9mm. (2912) Shereen A Nossair E.D. J. Vol. 65, No. 3 all-ceramic systems and adhesive cements allow dentists to use a minimally invasive approach with conservative preparations of thicknesses ranging from 0.5 to 2 mm.4 Laminate veneers may even permit preparations of 0.3–0.7mm thickness to limit preparations within enamel to ensure efficient adhesive cementation.5 However, the thinner ceramic restorations can make masking underlying teeth color difficult due to the translucency of the ceramic and the resultant exposure through the ceramic material of the polymerized resin cement or tooth structure. Furthermore, lighter shades and bleached shades, which are popularly preferred among patients, become more challenging to reach with thin ceramics.It is possible to improve the masking ability of ceramic veneers by increasing their thickness6 although, this would require an increased amount of tooth reduction. Using different shades and opacities of cements to mask underlying tooth color is also possible. Begum et al7 concluded in their study that the color masking ability of a 0.5-mm veneer with an opaque shade of resin cement could be similar to a 1.0-mm veneer with a translucent shade of resin cement, thus being more conservative. Lithium disilicate (IPS eMax CAD Ivoclar Vivadent, Schaan, Liechtenstein) is one of the most commonly used glass ceramics for fabrication of laminate veneers. Owing to its excellent esthetic properties coupled with adequate strength,8,9 and bonding capacities10, enabling them to be used in thin veneers11. By producing a material where the lithium disilicate crystalline phase and the glassy phase possess a similar index of refraction, it is possible to produce the highly translucent formulation of the material12. Although the translucency of lithium disilicate glass ceramic is considered an advantage, the translucency can have a negative effect on the resulting color because of shadowing underlying tooth color. The originally heat pressed ceramic, is available also as partially sintered blocks for CAD/CAM milling. Besides coming in a wide variety of colors, IPS eMax CAD is also available in different levels of translucency or opacity; high translucency (HT), medium translucency (MT), low translucency (LT) and medium opacity (MO). This variation is accomplished via differences in the microstructure of the material. Two levels of translucency may be acquired based on the precrystallization treatment of the CAD/CAM ceramic blocks: high-translucency (HT) and low-translucency (LT) material.13 Both formulations have identical crystal content but differ in crystal sizes, with HT ceramic exhibiting crystals of 1.5 × 0.8 mm dispersed in a glassy matrix, whereas LT ceramic exhibits smaller crystals (0.8 × 0.2 mm) in a higher density matrix.14 IPS e.max CAD is available in the standard A through D shades and also includes a line of bleach shades. Like most dental ceramics, the color of the material is determined by colorant ions dispersed in the matrix15. While conservation of tooth structure is a main goal, a superior esthetic result necessitates either matching adjacent natural dentition or achieving the desired final color, especially lighter shades, as A1 shade, which is the most commonly selected shade by patients.16,17 The final color of ceramic restoration is a result of several interacting factors; ceramic thickness and translucency, substrate color and the thickness and color of the cement.18,19,20,21,22 Since the typical thickness of a veneer is usually much larger than the cement thickness, the degree of veneer translucency was found to be more effective in masking the underlying discolored substrate than the luting agent shade.23 Inspite of the well documented color modifying effect of luting cements, several authors agree that the choice of cement color has less of an effect on the final color of restorations.18,24,25The effect of different thicknesses of lithium disilicate have been previously investigated in thickness ranging from 0.3-2.5 mm. Most reported a significant decrease in ΔE value with increase in thickness. 7,18,26,27 THE EFFECT OF THICKNESS AND DEGREE OF TRANSLUCENCY ON THE FINAL COLOR (2913) Ali et al28 recorded highest ΔE values with HT lithium disilicate in BL1 shade with darker substrates. Czigola et al 19 demonstrated that crowns with greater translucency recorded higher ΔE values resulting in a lower quality shade match. Nonetheless, the results of HT crowns on yellowish substrates showed a decreasing ΔE—despite the decreasing lightness of the shade (ND1-ND6)— though ΔE increase was expected since HT crowns are more translucent. This phenomenon demonstrates the complexity of the optical properties of these materials. There is not enough available information or recommendations for the production of desired lighter and bleached shades by lithium disilicate ceramics. Therefore, the purpose of this in vitro study was to evaluate the color difference (∆E) from standard A1 and BL1 color parameters and corresponding shades of lithium disilicate used in four different thicknesses (0.3 mm, 0.5mm, 0.7mm and 0.9mm) and two translucencies of the lithium disilicate ceramic material, high translucency HT and low translucency LT (IPS e.max CAD). The null hypothesis proposed that: 1. There would be no effect from the use of different translucencies of lithium disilicate on the final color of ceramic veneers, and 2. There would be no effect from the use of different thicknesses of different translucencies of lithium disilicate on the final color of ceramic veneers. MATERIALS AND METHODS A total of 80 ceramic discs were prepared from lithium disilicate CAD blocks (IPS e.max CAD blocks) in shades A1 and BL1 using both high translucency and low translucency blocks. The discs were divided into two main groups according to the shade of the blocks; shade A1 and shade BL1, 40 discs each. Each group was further subdivided into two subgroups according to the translucency of the lithium disilicate: Low translucency LT emax CAD and high translucency HT emax CAD; 20 discs each. Each subgroup was further subdivided into four divisions, 5 discs each, according to disc thickness: 0.3, 0.5, 0.7 and 0.9mm thicknesses. ΔE values were recorded to determine color differences between the specimens and the standard A1 and BL1 color parameters. ΔE values were directly recorded with a handheld spectrophotometer (Easyshade Advance, VITA Zahnfabrik). The ΔE values were compared with an acceptability threshold (ΔE=3.7).