Degree of Conversion in Resin Cements Light Cured Through Varying Thicknesses of Full Contour Zirconia

Objectives: We have characterized the translucency parameters of several full contour zirconia materials in the past. The objective of this experiment is to further investigate the relation between translucency parameters, percent transmittance, and degree of conversion of resin cement beneath these full contour zirconia ceramics.
Methods: Samples from four different full contour zirconias (CAP FZ, Zirlux, BruxZir, KDZ Bruxer), a lithium disilicate glass ceramic (e.max CAD), and a traditional zirconia (CAP QZ) were prepared. 150 square-shaped specimens (12x12mm) were prepared from CAD-CAM material blocks. All specimens were sintered following the manufacturers' instructions to achieve final thicknesses of 1, 1.25, 1.50, 1.75, and 2 mm. Five specimens in each thickness for each of the six types of ceramic were produced. Resin cement with and without cayalyst (Variolink II, Ivoclar Vivadent) was placed between two mylar strips, with glass slabs beneath and on top. An opaque plastic frame was placed around the specimen in order to prevent light transmission and the resin was cured for 40 seconds through the different thicknesses of ceramic, using a halogen light curing unit (DEMI LED, Kerr). The position of the position of the curing tip was verified using a MARC® Resin Calibrator. 24 hours after light curing, the resin cement samples were scanned with FTIR and the degree of conversion in each group was calculated.
Results: When no catalyst is added, the ranking of degree of conversion is e.max CAD > KDZ Bruxer, CAP FZ, Zirlux, BruxZir > CAP QZ. There is a linear correlation between percent light transmission, translucency parameter, and degree of conversion. When catalyst is added, there is no significant difference in degree of conversion found.
Conclusions: The catalyst effectively eliminated the dependence of degree of conversion in the dual cure resin cement on the optical property of the ceramic.