MDT Joachim Maier ran an ad hoc test series to analyse the effects of mechanical processing on sintered zirconia surfaces. The test involved using an atomic force microscope to examine the degree of damage caused to zirconia surfaces by grinding them with various diamond instruments (see image gallery above with the results). In the excerpt from his study published here he also provides hints as to how surfaces can nevertheless remain homogeneous.
Mechanical processing of zirconia after sintering is an important step for ensuring the restoration remains stable long-term. Excessively thick crown margins after milling or optimisation of occlusal surfaces in monolithic regions necessitate grinding of the sintered surface. This is carried out with water-cooled diamond instruments at high speed and exerting only minimal pressure.
But under everyday circumstances in the laboratory, the pressure can only be measured and controlled subjectively. In-lab tests showed that under practical conditions surfaces are often subjected to more stress than during scientific studies run with programmable, defined and uniformly operating machines. Our practical tests showed that all types of instrument damaged the surface. Not until polished in three stages with diamond-impregnated silicone polishers was the surface quality such that monolithic areas were virtually non-abrasive to the opposing dentition and, with bridgework for veneering, no fractures could be initiated in that region of the framework subjected to tensile strain.
1. Zirconia must only be grinded where necessary.
2. Grind with water-cooled diamond instruments and exert only minimal pressure.
Red ring diamonds are the most efficient. (see third image in the gallery above)
3. Optimising the surface by polishing in 3 stages with diamond-impregnated silicone polishers.
(see last image in the gallery above)