Dependence of Translucency on Thickness for Various Ceramic Frameworks

Objectives: To determine the dependence of translucency on ceramic thickness for zirconia (Y-TZP) and other ceramic framework materials, including alumina and glass-ceramic (lithium disilicate).

Methods: Commercial and in-house prepared circular Y-TZP discs (n = 3 each, 20 mm in diameter) were machined to various thicknesses: 1.00, 0.40, 0.25, 0.21, 0.17, and 0.10 mm. The circular surfaces of these discs were then polished to 1 µm finish. Commercial alumina and lithium disilicate (Empress II) plates (n = 3 each, 12 mm in diameter, 1.00 and 0.40 mm thick) were also polished to 1 µm finish. The relative translucency (black-white image transmission) of each sample was measured in an optical microscope under x32 magnification with transmitted light and a CCD camera with commercial translucent porcelain control (10x10x1 mm³ in size with polished surfaces).

Results: For 1.00 mm thick specimens, the relative translucencies of ceramic cores in reference to porcelain control were 40.0±3.9% (lithium disilicate), 37.1±1.6% (alumina), 26.0±1.8% (commercial Y-TZP), and 26.0±1.3% (in-house Y-TZP). For 0.40 mm specimens, alumina and Empress II exhibited a comparable translucency to that of 1 mm thick porcelain. However, the translucencies of the two 0.4 mm thick Y-TZPs were 25.2±1.4% (commercial) and 22.8±1.2% (in-house) of that of porcelain control (1 mm thick). More strikingly, the translucencies of the two Y-TZPs only showed marginal improvements as their thicknesses decreased from 0.40 to 0.17 mm; significant change in translucency observed when the sample thickness was reduced from 0.17 to 0.10 mm.

Conclusion: Alumina and lithium disilicate experienced a significant change in translucency when the sample thickness changed from 1.00 to 0.40 mm. However, the only significant increase in translucency of zirconia occurred in the thickness interval between 0.17 and 0.10 mm. A balance needs to be reached between strength and aesthetics. Supported by NIH/NIDCR 1R01 DE017925.