CAD/CAM technology has made it possible to create restorations out of high-strength ceramics like alumina and zirconia. Previously, ceramic restorations in the posterior region were limited to single units. Now, with the introduction of zirconia as a dental material, clinicians can place all-ceramic restorations in the anterior and posterior regions. Zirconia ceramic materials have almost 2 times higher flexural strength than alumina, as well as high fracture toughness.

Although most manufacturers claim to offer chemically similar zirconia, the way the zirconia has been processed can create significant differences in the final restorations. Because all products are not alike and can perform differently in clinical situations, it is important for clinicians to evaluate the depth of clinical studies and testing that have been conducted. In general, this evaluation process should be applied to all materials and devices launched in the dental market.

Many physicians learned a painful lesson when zirconia hip implants were first introduced. A lack of understanding of material properties and how processing conditions affect quality of the final part, as well as a distinct lack of controlled clinical trials led to unreliable products. Understanding the science behind the materials has allowed some companies to successfully offer zirconia hip implants, today. The several steps in the manufacturing process must be carefully and accurately controlled, and adequate tests must be conducted to substantiate material reliability.

Think about bread. Breads are often "chemically similar" (ie, they have the same ingredients); however, the color, consistency, and taste can be very different. Many other factors outside of chemistry influence the final result, including the order in which ingredients are mixed, the grain size or consistency of the flour, and time and temperature used for incubating the dough. In addition, breads can be baked at different temperatures. Aside from the ingredients and baking process, other differences, such as the skill of the baker, can lead to a substantial difference in the final product.

Although all zirconia ceramic powders are chemically similar, once processed they can exhibit very different mechanical and optical characteristics. The procedural steps, from powder to final dental restoration, are of key importance. The physical properties of the zirconia block can change with each processing step, and the cumulative effect then controls the final properties-strength, translucency, color, and long-term stability. Suppliers use various pressing techniques to form "presintered blocks," which then are milled and sintered. Differences in pressing technique and conditions (eg, isotactic, uniaxial), in milling conditions (wet or dry), and in final sintering (temperature cycle and final high temperature of sintering) can contribute to differences in the final dental restoration.

In addition to the material properties, software and hardware also play an important role. The quality of the digitally scanned model and mill add further complexity to achieving a final restoration with good marginal fit, color, esthetics, and long-term strength and toughness.

 

What Can Be Different?

 

1.       Processing parameters for presintered zirconia that affect performance attributes.

2.       Differences in the zirconia powder that affect the strength/long-term stability and translucency of the restoration.

3.       The pressing condition and pressing method that affects the marginal fit, strength, and translucency of the restoration.

4.       Presintering conditions that affect the strength of the presintered material and its millability.

5.       Coloring of the zirconia that can affect the marginal fit, strength, and translucency of the material.

 

What Should I Be Asking?

Clinicians should be mindful of what zirconia product they are receiving for their final restorations. Settling for a zirconia that is "just like the zirconia you asked for" shouldn't be good enough. Dental professionals who educate themselves about the differences in zirconia materials should be asking their laboratory partners:

1.       What brand of zirconia is being used? Why?

2.       Is there good science and good clinical data behind this product?

As with all dental products, dentists should be cognizant of what material is being used and choose the material that best meets their needs, and those of the patient. Once dental professionals understand the differences in zirconia materials, it becomes much easier for them to choose the best material for their needs.

 

I would like to acknowledge the support of Anke Behrens, PhD Scientific Affairs Manager, Lab and Digital
3M ESPE Dental Products

 

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