The emphasis on esthetics in dentistry has been instrumental in the search for a material with enough strength and longevity to replace metal. Ceramic restorations drastically changed in the 2000s with the introduction of CAD/CAM systems and the utilization of transformation-toughened zirconia.
The name zirconium is derived from the Arabic word “Zargun,” or golden in color, which is further derived from the Persian words “zar” for gold and “gun” for color. Zirconium is a naturally occurring element with an atomic number of 40 and is a transitional metal found in nature as the mineral Zircon. It is purified to Zirconium, a silver-colored metal that is corrosion-resistant and has properties like titanium. When combined with oxygen, it forms zirconia, a strong and highly biocompatible ceramic. Zirconium was initially discovered in the 18th century.
The German chemist, Martin Heinrich Klaproth (1743-1817), successfully extracted zirconium oxide from Zircon using a transparent gemstone called jacinth as the starting material. Jöns Jakob Berzelius (1779-1848), a Swedish chemist, was the first to isolate metallic zirconium. For the next 150 years, zirconia was considered little more than a curiosity and it was only utilized for making heavy-duty bricks and special glass with a high refraction index.
In 1969 Helmer and Driskell published the first scientific study of the biomedical properties of zirconia. In 1972 Garvie and Nicholson found that alloying zirconia with oxides like calcia, yttria, and magnesia could stabilize the tetragonal modification of zirconia, preventing its transition from tetragonal to the monoclinic phase and producing ceramics with previously unseen crack resistance.
Yttria-stabilized zirconia has been used in orthopedics to manufacture hip and knee joints as an alternative to cobalt-chrome alloy. This biomaterial is sometimes called “cement steel” because its crystalline reticulation changes when force is applied to the surface. Force effectively seals any cracks by causing a volumetric change. Additionally, zirconia supports the growth of blood vessels and bone cells.
The increasing emphasis on esthetics in dentistry has been instrumental in the ongoing search for a material with enough strength and longevity to replace metal. The search initially began in the 1960s with the porcelain jacket crown manufactured from feldspathic porcelain, which was later strengthened with an alumina core. This material, however, had a low compressive strength, restricting its use to anterior crowns.
In 1963 the porcelain fused to metal crown (PFM) was developed, and it remained the gold standard for manufacturing crown and bridge restorations for decades. While the PFM is strong, masking the metal substructure has always been a problem; all-ceramic crowns will always look better than a crown with a metal base. One of the main issues with PFMs is that the color of the metal must be masked with opaque before layering the crown with natural-looking ceramic. The opaque layer blocks the passage of light through the body of the crown, bouncing back and affecting the value, or brightness, of the crown at the thinnest part of the restoration around the gingival margin. Often, a PFM appears slightly grayer in the cervical third, especially if the restoration was fabricated without a porcelain margin. All-ceramic crowns have no such problems, radiating the natural color of the tooth, particularly in thinner sections, resulting in a more esthetically pleasing restoration.
By the 1980s, the first range of glass-ceramics was introduced, the earliest of which was Dicor. Beautiful yet fragile, Dicor was the first glass to be manufactured using the lost wax technique. Its use was confined to anterior teeth.
The 1990s saw the introduction of Cerec, a milling machine that duplicated a resin inlay from a solid block of ceramic. Empress ceramics, In-Ceram and Procera alternatives were also introduced during this time. Procera consisted of a pressed aluminum oxide core, which was sintered under pressure at a high temperature to create a more dense and stronger aluminous oxide core. Veneering porcelain could be added to the core. Before the introduction of these materials, sintered alumina oxide was the strongest non-metal material used for crowns.
Ceramic restorations drastically changed in the 2000s with the introduction of CAD/CAM systems and the utilization of transformation-toughened zirconia.
Unlike earlier forms of all-ceramic restorations, zirconia meets the combined requirements for excellent esthetics and superior strength. It also satisfies the need for an all-ceramic material for patients with a metal allergy. I’s a popular alternative to alumina restorations. Dental applications include:
Zirconia dental posts offer a more esthetic solution when restoring anterior teeth and where metal posts could result in gray discoloration of the gingival margin. Their use can also eliminate any complications caused by corrosive reactions with the oral environment and surrounding tissues, such as burning, pain, or a metallic taste.
Monolithic zirconia restorations and layered zirconia frameworks provide high flexural strength and good esthetics.
Zirconia implants and abutments provide a metal-free solution for patients with metal sensitivities or concerns. Abutments are matched to the color of teeth and exhibit good tissue compatibility and low plaque accumulation.
Zirconia has also been used for orthodontic brackets. Zirconia brackets are cheaper than alumina ceramic brackets but are very opaque, which decreases their esthetic appeal. Zirconia brackets exhibit good frictional characteristics, reduced plaque adhesion, and acceptable bond strengths. However, these characteristics do not offer any significant benefits compared with polycrystalline alumina brackets.
Zirconia restorations have been used very successfully for years and provide the patient with excellent biocompatibility and mechanical properties. Originally, the very first zirconia restorations were difficult to successfully veneer due to their bone-white color that was hard to mask without adding too much bulk to the restoration. The introduction of newer, colored, and more translucent zirconia materials has mostly solved this problem, and it is now possible to create highly esthetic restorations suitable for anywhere in the mouth. Monolithic zirconia is especially suitable for posterior restorations, where space is limited, and there is minimal occlusal restoration space. It is also suitable for patients with bruxism as polished zirconia is more effective in reducing antagonistic tooth wear. Using computer-aided design and manufacturing provides the clinician with precision-made, well-fitting restorations requiring minimal chair-side adjustment. Currently, the demand for zirconia far exceeds the demand for PFMs, and these older-style restorations are gradually being confined to dental history.
Please don’t hesitate to contact us if you would like more information. Our skilled technicians have in-depth knowledge about each of these materials and are always willing to discuss specific cases.
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References
https://benthamopen.com/contents/pdf/TOBIOMTJ/TOBIOMTJ-5-1.pdf
https://www.sciencedirect.com/science/article/pii/S1883195813000972
https://application.wiley-vch.de/books/sample/3527337431_c01.pdf
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4854641/
https://www.tandfonline.com/doi/full/10.1080/13102818.2016.1177470
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