Optical properties Of CAD-CAM lithium disilicate glass-ceramic in different firing temperatures and thicknesses

Abstract

Background: With the emerging of digital dentistry, IPS e.max® CAD lithium disilicate (LD) glass-ceramic material has become one of the most popular esthetic restorative materials in digital assisted dental esthetic restoration. The mechanical and optical properties of this material have been investigated in several studies. However, there is a lack of information and consensus regarding the optical properties of IPS e.max® LD glass-ceramic materials. Objectives: 1) To investigate the optical properties as translucency parameters (TP), Contrast ratio (CR), light transmissions (Lt) and color changes (CC) between high-translucent (HT) and low-translucent (LT) IPS e.max® CAD LD glass-ceramic materials with different crystalline phases and thickness in different firing stages. 2) To investigate the optical properties as TP, CR, Lt and CC of each translucent (HT and LT) IPS e.max® CAD LD glass-ceramic materials with different crystalline phases and thickness in different firing stages. 3) To determine the mathematical relationships of thicknesses of IPS e.max® CAD LD glass-ceramics materials with TP and Lt. Materials and methods: The total of 120 of shade A2 IPS max CAD samples (HT and LT) were prepared into square shape (15.25 mm X 15.25 mm) and were divided into two main groups according to the material translucency (HT and LT) (n=60). Each main group was further divided into 5 sub-groups according to the thickness (1.00, 1.25, 1.5, 1.75, and 2.00 mm) (n=12). Each thickness group was assigned into three groups based on different crystallization (firing) temperatures (750, 820 °C in single stage heating schedule with 1 second and 10 second holding times, respectively, and 840 °C with two-stage heating schedule (RECOM) (820°C, 840 °C with 10 second and 7 min holding time, respectively) as recommended by manufacturer (n=4). CIELab*, TP, CR, and Lt were measured and calculated for all samples. Statistical analysis: The effects of the test results were evaluated using 3-way ANOVA with factors for Translucency (HT and LT), Firing Temperature (750, 850, and RECOM) and Thickness (1, 1.25, 1.5, 1.75, and 2), as well as all two-way and three-way interactions among the factors. Pair-wise comparisons were made using Least Significant Differences to control the overall significance level at 5%. Results: The mean irradiance and TP for both HT and LT decrease as the thickness of the samples increases from 1 to 2mm with significant difference between the thickness groups within each material translucency groups (HT and LT) and between both HT and LT. The coefficients of absorption (c) of the two materials were calculated. The effective incidence irradiance when material thickness approaches zero (Ie) was also calculated. There is an unexpected spectral peak shift as the thickness of the samples increases. There is no statistically significant difference in Ie at 750˚C and 820 ˚C between the HT and LT. However, there is a statistically significant higher Ie in HT at the recommended firing temperature as expected. Coefficients of translucency parameter (p) of the materials in various firing temperature were defined and the TP of the material as the thickness approaches zero were calculated (TP0). The TP of the materials is directly correlated to the mean irradiance passing through the samples. There is no statistically significant difference in the TP0 and Ie of the HT and LT groups at the recommended firing temperature. Conclusion: In this project we developed modified Beer-Lambert law to describe the parameters governing the effect of thickness on light transmission in dental ceramic material. We also applied the same equation to describe the translucency parameter. The parameters defined in these equations allow us to compare the optical property of dental ceramic material independent of the thickness of the samples.