The effect of hydrothermal aging on the color stability of gradient zirconia : In vitro study /

Abstract

Color stability of ceramics is an important factor for the long- term success of esthetic restorations. So, the aim of the current study is the evaluation of the effect of hydrothermal aging on the color stability and the microstructure of gradient zirconia. For standardization of gradient zirconia discs, a resin disc with a 10 mm diameter and 1.5 mm thickness was constructed using 3D printing technology. Then the 3D resin disc was scanned by using an extraoral desktop scanner. Ten Gradient zirconia disc specimens were milled from IPS e.max ZirCAD Prime blank using 5-axis dental milling machine. Then the specimens were sintered, polished and glazed according to the manufacturer recommendations. Hydrothermal aging was carried out at 134oC temperature, 2 bar pressure for 5 hours in an autoclave to be equivalent to 3 to 4 years of aging in vivo. Color stability and microstructural changes of all disc specimens were tested two times; before and after accelerated hydrothermal aging. Color difference measurements (ΔE) were carried out using a spectrophotometer. The total difference in color, according to L*, a*, b* coordinates, is calculated with the following formula. ΔE= [(Δ L*)2 + (Δ a*)2 +(Δ b*)2 ] ½ ΔL*, Δa*, and Δb* represent the difference in the color parameters before and after hydrothermal aging. Where delta E (ΔΕ) of 3.33 or less was considered clinically acceptable . X-ray diffraction analysis was used to determine the crystalline phases inside the gradient Zirconia disc specimens before and after aging. The percentage of t-m phase transformation was calculated using Gravie Nicholson equation. Xm = Im (-1,1,1) + Im (1,1,1) / Im (-1,1,1) + Im (1,1,1) + It (1,0,1)