Conversion in Light-Cured Resin Cement Across Zirconia Shades and Thicknesses

Objectives: This study explores the impact of zirconia thickness and optical properties, encompassing shade, translucency, and transmittance, on the degree of conversion of light-cured resin cement.
Methods: Ceramic specimens derived from Zirconia CAD (Katana UTML) in shades EA1, EA2, and EA3 were employed. Seventy-five square-shaped zirconia specimens were milled from CAD-CAM material discs and polished using an EcoMet1000 (Buehler). Subsequently, the zirconia specimens underwent sintering within a furnace (Sintra Pro; Shenpaz). These specimens were then divided into thicknesses of 1, 1.25, 1.50, 1.75, and 2.00 mm, with five samples in each thickness category. Panavia veneer light-cured resin cement was administered into a mold beneath the ceramics, and the light-cure tip was positioned atop the ceramics to cure the cement. Following a 24-hour storage period, the resin cement was prepared for FTIR analysis by pressing them with KBr to generate pallets. The degree of conversion was measured using peak heights of 1637 cm-1 (C=C) and 1540 cm-1 (N-H) as internal reference peak. To assess the optical properties, a spectrophotometer (CM-2600D; Konica Minolta Sensing Americas, Inc) was utilized to determine the translucency parameter (TP) at the center of each specimen. A spectrophotometer (MARC Resin calibrator; Bluelight Analytics, Inc) and an LED unit (Bluephase) were employed to measure light transmittance.
Results: The degree of conversion ranged from 62% to 36%, with the highest percentage observed for the A1 shade. A significant decrease in the degree of conversion was noted for thicknesses exceeding 1 mm. Absorption coefficients ranging from 0.616 to 0.948 mm-1 were identified in the zirconia group.
Conclusions: A linear relationship between the translucency parameter and the degree of conversion was observed for each shade. This relationship was found to follow a consistent curve, contingent on the shade and thickness. It was concluded that light-cured resin exhibits an acceptable degree of conversion up to a thickness of 1 mm for the A1 shade in zirconia 5-YTZP.