Yttria-stabilized tetragonal zirconia polycrystal has been used as a dental biomaterial for several decades. However, its esthetics are compromised by its poor translucency and grayish-white appearance.
Zirconia exhibits a tetragonal-to-monoclinic phase transformation on cooling, which is accompanied by a 3% to 5% volume increase.
Clinical Implications While fully stabilized zirconia (cubic) has been developed for use in the esthetic zone, this polymorph does not exhibit transformation toughening and so does not exhibit the superior mechanical properties of partially stabilized zirconia (tetragonal + cubic).
Several polycrystalline zirconia materials have been developed for dental applications, including zirconia toughened alumina (ZTA), partially stabilized zirconia (PSZ), tetragonal zirconia polycrystal (TZP), and fully cubic stabilized zirconia (CSZ).
CSZ is differentiated from the other 3 by its isotropic character, which can enhance light transmission
Computer-assisted design and computer assisted manufacturing (CAD-CAM) have been used extensively in prosthetic dentistry. Porcelain may be applied as a veneer to partially sintered (or, more rarely, fully sintered) zirconia, which has been machined to sizes 25% to 30% larger than the final size to compensate for shrinkage during sintering
Although nonmetallic materials commonly have band gaps larger thanw3.1 eV and so are transparent to light, they can exhibit limited light absorption due to electronic polarization, giving a translucent appearance and consequent diffuse light
the translucency and opacity of materials depend on how and to what extent incident light is reflected, scattered, refracted, transmitted, and absorbed.
The light propagation through a material depends on 3 parameters: absorption coefficient (ma), scattering coefficient (ms), and scattering anisotropy factor (g).
The scattering anisotropy factor is the mean value of the scattering angle. The scattering anisotropy factor indicates
the amount of light that is forward-scattered (translucency) or back-scattered (opacity) in a single scattering event.
transmitted light is attenuated by both the absorption and scattering coefficients, and the sum of these two coefficients gives the transport coefficient (mt)