Translucency evaluation of an ultra-translucent monolithic zirconia and a super translucent multilayer zirconia: a pilot study on the thickness effect

Abstract

Introduction: The increasing application of zirconia as an aesthetic material has led to an evolution with improvement on its optical properties [1]. Nowadays, zirconia presents a compatible translucency with natural teeth. Depending on the clinical situation, you can choose a more translucent or more opaque zirconia. Due to its excellent mechanical properties, it is possible to return the function and aesthetics through a restoration with thicknesses lower than those required by other materials [2]. Materials and methods: Twenty specimens of pre-sintered ultra-translucent (UT) monolithic zirconia (Bloomden W00098014UT) and twenty specimens of pre-sintered super translucent (ST) multilayer zirconia (Bloomden W00098016STML-A2) were cut through the computer-aided design/computer-aided manufacturing system (Wieland Dental). For each type of zirconia, four subgroups (n=5) were defined according to its thickness: 0.5, 1.0, 1.5 and 2.0mm. All monolithic specimens were coloured with colouring liquid (BloomZir® UT Coping Crown A2) for 2 min. Finally, all zirconia specimens were sintered in the furnace (IMES-Wieland Zeno® Fire) at a temperature range of 1500-1550 °C and subsequently submitted to an ultrasonic bath (VGT-2120QTD 20 L) [3]. The values of L∗, a∗ and b∗ were measured, under natural light (D65), using the spectrophotometer SpectroShade Micro (MHT S.p.A., Arbizzano di Negrar, Itália) at six different locations on a white background (Comission Internationale de l' éclairage (CIE) L∗=95.6 a∗= 0.8, b∗= 0.1) and on a black background ((CIE) L ∗= 13.2, a ∗= 0.8, b ∗= ∗0.7). Translucency was assessed through the contrast ratio (CR) [4] and the translucency parameter (TP) [5]. Data was submitted to descriptive statistical analysis. Results: Mean CR values ranged from 0.7 (±0.0) to 0.9 (±0.0) in UT monolithic zirconia and 0.8 (±0.0) to 0.9 (±0.0) in the ST multilayer zirconia. Mean TP values ranged from 14.1 (±0.3) to 26.7 (±0.4) in UT monolithic zirconia and 9.9 (±0.4) to 18.6 (±0.3) in ST multilayer zirconia. As thickness increases, translucency values decreased, in both materials. Discussion and conclusions: Overall, ultra-translucent monolithic zirconia was found to be more translucent than the super translucent multilayer zirconia. The translucency showed to decrease when zirconia thickness increases.