Zirconia restorations have become the material of choice in restorative dentistry today, out pacing the previous porcelain fused to metal champions and reaching nearly 80% of the current crown and bridge prescriptions filled by laboratories. With this generational change in mind, lets take a look at how zirconia evolved.
There has been a need for esthetic high strength no metal reinforced ceramics in the dental industry since leucite-reinforced feldspathic dental porcelains were developed in the early 1900’s. Their strength was simply never high enough to resist load conditions in the mouth, especially when they were conventionally cemented directly to tooth structure like early porcelain jacket crowns. The only way to ensure their survival was to bond them to a substructure that was usually made of cast alloy (PFM/PBM). Because the esthetics were generally compromised with these restorations, newer stronger ceramics were developed in order to eliminate the need for these substructures.
High Strength Glass Ceramics
Higher strength glass ceramics, with enhanced, natural optical properties, came into fashion when the 4th and 5th generation adhesive bonding materials were developed. These materials exhibited not only enhanced esthetics, but over twice the strength of the feldspathic ceramic varieties, and they were also enhanced by their ability to be bonded. The chemical and physical bonds combined to strengthen not only the new generation pressed or milled restorative material, but they strengthened the very tooth structure to which they were bonded.
The Introduction of Zirconia
The first use of zirconia was in a high strength ceramic family named In-Ceram, by Vita Zahnfabrik. Zirconia was a constituent of the highest strength In-Ceram intended to be used as a posterior restorative. It was combined with alumina to achieve a flexural strength of 700MPa, nearly double that of the glass ceramic materials. The one drawback: though strong, it was not very translucent, and therefore was relegated to posterior crowns and bridges. The introduction of CAD CAM milling put automation within reach and the industry went for it in a big way. When CAD CAM met millable zirconia, a revolution happened. Suddenly, restorative materials could be made easily and had flexural strengths that exceeded 1,000MPa.