Application of Kubelka-Munk model on the optical characterization of translucent dental zirconia

It is well known that the translucency of a material depends on its scattering and absorption coefficients as well as on the thickness of the sample. Though, it is common in dental studies the translucency of the materials to be evaluated only by determination of TP (translucency parameter) and CR (contrast ratio). This study proposes the use of the Kubelka-Munk theory to evaluate the optical properties: scattering power (SX), absorption (K) and transmittance (T) of three different commercial monolithic dental zirconia (S Smart Dent (TM), TTT3 (TM) and Zirkon (TM)) in order to correlate these optical properties with the microstructure developed. The transmittance spectra were obtained indirectly from measurement of the diffuse reflectance of the samples having different thicknesses (1.0 mm and 1.5 mm). Results evidenced different degrees of translucency for the different groups, with the Smart Dent (TM) presenting the highest transmittances. It was found that the light absorption coefficient (K) for Smart Dent (TM) specimens is significantly higher, suggesting the presence of pigmentation in this group. The power light scattering (SX), however, was significantly influenced by the thickness for Smart Dent (TM) and Zircon (TM) groups, but did not suffer significant variation in the TT3 (TM) specimens. For the TT3 (TM) and Zirkon (TM) specimens the low K/S values for all wavelengths, indicated strong prevalence of scattering over absorption. Already for Smart Dent (TM) group the K/S ratio indicated prevalence of absorption over the scattering at low wavelengths. Zirkon (TM) samples presented smaller grain size (between 0.3 and 0.5 mu m) and larger amount of tetragonal phase, providing higher light scattering, consequently resulting in a less translucent zirconia. Smart Dent (TM) and TT3 (TM) groups presented microstructure with larger and homogeneous grain sizes and higher cubic phase contents giving more translucent materials. The Kubelka-Munk parameters had a good agreement with the microstructural characterization, proving to be a more appropriate optical characterization mean for this type of material than the simple TP and CR measures.

Automated summary

This study utilized the Kubelka-Munk theory to evaluate the optical properties of three different commercial monolithic dental zirconia, discovering varying degrees of translucency across different groups. The Smart Dent (TM) group showed significantly higher light absorption coefficient, while the Zirkon (TM) group displayed stronger prevalence of scattering with low K/S values.