Optical Properties of Novel Graded Glass-Zirconia Crowns

Objectives: Zirconia all-ceramic restorations are more aesthetically pleasing than metal-ceramic retainers. However, zirconia restorations are more susceptible to fracture. Thus, a new material, graded glass/zirconia/glass (GZG), has been developed for better fracture resistance. GZG’s optical properties have yet to be studied. This study focuses on the translucency properties of GZG relative to commercial ceramic framework materials, including zirconia (Y-TZP), alumina (Rondo alumina), and lithium disilicate glass-ceramics (Empress II and e.max CAD and Press).

Methods: Commercial and in-house prepared ceramic discs were machined and polished (1 μm finish) to various thicknesses, ranging from 0.17 - 2.0 mm. For quantitative translucency values, the differences in luminous reflectance (DE) of the specimens were calculated from the Lab values (CIE) of a white background compared to a black background using a spectrophotometer (SpectroShade micro, MHT S.P.A., VIA MILANO, 12; 37024 ARBIZZANO DI NEGRAR (VR), ITALY). 

Results: For 1.0 mm thick specimens, the relative translucencies (DE) of ceramic cores in reference to the transparent soda-lime glass control were 25.41 ± 0.15% (e.max CAD), 23.07 ± 0.19% (e.max Press), 18.91 ± 0.19% (alumina), 16.66 ± 0.53% (Empress II), 7.72 ± 0.27% (GZG), and 6.18 ± 0.21% (zirconia). Various porcelain veneers, however, exhibited transparencies between 43.61% - 51.07% of that of soda-lime glass. For 0.40 mm specimens, e.max Press, e.max CAD, Empress II and alumina possessed a translucency of 42.38%, 38.84%, 30.10% and 26.98% relative to that of 1.0 mm thick glass, while GZG and zirconia displayed a relative translucency of 16.63% and 11.82%, respectively. 

Conclusion: Various materials exhibited a wide range of opacity. Specimen thickness also resulted in a large variation in opacity. Our findings suggest that a material’s composition and microstructure, as well as specimen thickness all play an important role in the optical properties of dental ceramics. Supported by NIH/NIDCR-R01DE017925 and NSF/CMMI-0758530.