Basic properties and types of zirconia: An overview

Abstract

This paper describes types and characteristics of zirconia materials in relation to their applications in dentistry. The zirconia material typically used today by most manufacturers is a tetragonal polycrystalline zirconia, partially stabilized with yttrium oxide. The mechanical properties of zirconia have been extensively investigated in the scientific literature and zirconia clearly measures up to any other equivalent manufactured material. The biocompatibility of zirconia has also been extensively evaluated and no local or systemic adverse reactions or cytotoxic effects have been found in relation to it. However, ceramic bonding, ageing, light transmission and manufacturing processes are all factors that need to be further evaluated in order to guide the successful use of zirconia as a prosthetic restorative material. Milling zirconia to full-contour might be an alternative to traditionally veneered restorations. A potential adhesion mechanism appears to be the combination of air abrasion with aluminum oxide particles (silanated or not), followed by sintering with materials containing special reactive monomers. Changes in zirconia properties before and after the sintering process have also been investigated. It was found that after sintering, surface roughness was greater, and micro hardness was slightly reduced; however, accurate precision of fit was not affected by the sintering process. Currently, zirconia restorations are manufactured by either soft or hard-milling processes, with the manufacturer of each claiming advantages over the other. Chipping of the veneering porcelain is reported as a common problem and has been labeled as its main clinical setback. As zirconia has demonstrated good mechanical and biological performance, future technology is attempting to improve esthetics and minimize veneer fracture, aiming to create confidence in the dental community towards this all-ceramic system. Milling zirconia to full-contour might be an alternative to traditionally veneered restorations. Finally, implications are drawn for manufacturing, machining, and widespread use of these materials.

Keywords: Zirconia; Biocompatibility; Porcelain chipping; Mechanical properties

Core tip: Although all zirconia is chemically similar, the ultimate product can vary from manufacturer to manufacturer, with materials of varying density, uniformity homogeneity and crystalline transformation. This can be due to varying grain sizes of the powdered material ultimately affecting strength, with variations producing porosity. One type of restoration will not fit every clinical condition but today we have a range of options in zirconia ceramics, including monolithic full-contour type and conventional veneered type zirconia.