Zirconia: Polishing and aging, does it destroy enamel?
We know zirconia crowns and bridges are nearly indestructible. But, what is the best way to polish this very hard material so it won’t be highly destructive?
As it turns out, a highly polished zirconia surface, over time, is less abrasive than emax, empress, and feldspathic porcelain. According to research by Grumser, et al, the other materials degrade over time and cause enamel wear, as well as wear to their own surfaces: “Polished zirconia surfaces showed lowest wear for material and antagonist. Wear mechanism of common ceramics was characterized by abrasive wear.”
Burgess also looked into aging of zirconia. In his paper, Enamel Wear Opposing Polished and Aged Zirconia, he explained using artificial aging of zirconia and simulated mastication to determine age related breakdown. They reported: “All zirconia specimens showed less material and opposing enamel wear than the enamel to enamel control or veneering porcelain specimens. The micrographs of the veneering ceramic showed sharp fractured edges and fragments of wear debris. Zirconia may be considered a wear-friendly material for restorations opposing enamel, even after simulated aging.”
Hmaidouch’s research comparing polished zirconia to polished porcelain was the most definitive when considering the method of polishing, as seen in the charts, above:
“Polishing of the specimens: the ground surfaces were polished with a handpiece for 15 s and with 2 N pressure. Polishing was performed using an NTI polishing kit (HP 802104; P341, P3401, P34001): coarse at 15 000 r·min−1, medium at 10 000 r·min−1 and fine at 5 000 r·min−1
No statistically significant differences were found between the roughness of coarse-polished specimens and roughness of fine-ground specimens for both non-veneered (P=0.54) and veneered zirconia (P=0.99), but through medium and fine polishing, the roughness was significantly reduced in both groups.
The ground surfaces of FZ using the coarse red diamond instrument showed grooves that were reduced after using the following two grinding instruments (medium and fine) and after using the coarse polisher.
Porcelain surfaces ground with coarse diamond burs show ridges and grooves; moreover, many voids appear due to incomplete condensation. After polishing, the condensation defects remain but have been smoothed and slightly rounded; the voids from the porosities appear shallower. Traces of ridges and grooves did not disappear after polishing was completed; however, the polished side contained pitted areas with numerous surface irregularities.
Lower roughness of non-veneered zirconia specimens than that of veneered zirconia (VFZ) specimens was observed after each treatment procedure. This difference can be explained by the different compositions. SEM investigation of the treated surfaces showed that defects on the veneered surfaces caused by grinding were deeper than those on the zirconia surfaces, which led to the higher roughness values.
The roughness of the VFZ surfaces was not significantly reduced after either fine grinding or coarse polishing, which can be explained by the deep defects (grooves) caused by coarse grinding. These defects could not be completely flattened or removed, thereby explaining the higher roughness of VFZ specimens compared with the fine-ground and coarse-polished FZ specimens, which did not acquire deep defects after coarse grinding because of their higher strength.”
The key to long-term, safe use of FCZ is, a highly polished surface. This can be obtained with a course polishing wheel. In this case, the NTI polishing kit was used successfully.