Optical properties and light irradiance of monolithic zirconia at variable thicknesses

• Polishing of monolithic zirconia increases its surface gloss. The degree of surface gloss is brand dependent, and thickness shows no effect on the gloss.
• Different brands of zirconia have different translucencies, which is highly influenced by the thickness of the material. FSZ is relatively more translucent than PSZ.
• Total irradiant energy and irradiance through zirconia is brand and thickness dependent. Certain thickness of zirconia may have an effect on the polymerization of resin-based cements.

ObjectivesThe aims of this study were to: (1) estimate the effect of polishing on the surface gloss of monolithic zirconia, (2) measure and compare the translucency of monolithic zirconia at variable thicknesses, and (3) determine the effect of zirconia thickness on irradiance and total irradiant energy.MethodsFour monolithic partially stabilized zirconia (PSZ) brands; Prettau® (PRT, Zirkonzahn), Bruxzir® (BRX, Glidewell), Zenostar® (ZEN, Wieland), Katana® (KAT, Noritake), and one fully stabilized zirconia (FSZ); Prettau Anterior® (PRTA, Zirkonzahn) were used to fabricate specimens (n = 5/subgroup) with different thicknesses (0.5, 0.7, 1.0, 1.2, 1.5, and 2.0 mm). Zirconia core material ICE® Zircon (ICE, Zirkonzahn) was used as a control. Surface gloss and translucency were evaluated using a reflection spectrophotometer. Irradiance and total irradiant energy transmitted through each specimen was quantified using MARC® Resin Calibrator. All specimens were then subjected to a standardized polishing method and the surface gloss, translucency, irradiance, and total irradiant energy measurements were repeated. Statistical analysis was performed using two-way ANOVA and post-hoc Tukey's tests (p < 0.05).ResultsSurface gloss was significantly affected by polishing (p < 0.05), regardless of brand and thickness. Translucency values ranged from 5.65 to 20.40 before polishing and 5.10 to 19.95 after polishing. The ranking from least to highest translucent (after polish) was: BRX = ICE = PRT < ZEN < KAT < PRTA (p < 0.05). The ranking from least to highest total irradiant energy was: BRX < PRT < ICE = ZEN < KAT = PRTA (p < 0.05). There was an inverse relationship between translucency, irradiant energy, and thickness of zirconia and the amount was brand dependent (p < 0.05).SignificanceBrand selection, thickness, and polishing of monolithic zirconia can affect the ultimate clinical outcome of the optical properties of zirconia restorations. FSZ is relatively more polishable and translucent than PSZ.