Rella Christensen, PhD, discusses new findings critical to successful use of zirconias.
Q: Other than better aesthetics, is translucent zirconia different from non-translucent zirconia?
A: Yes! It is important to realize that translucent zirconia has a different formulation which results in entirely different physical properties. It is not as tough and strong as low-translucent zirconia. In our newest clinical study, including several brands of both the original low-translucent and the new higher translucent formulations, we have found that the new higher translucent zirconias are more likely to fracture if substantial chairside adjustment or endodontic access become necessary and when tooth reduction is too minimal. Changes necessary to gain translucence have caused a loss of half the flexural strength and half the fracture toughness, plus nearly complete loss of the unique characteristic called transformation toughening—the slight expansion of zirconia as a crack develops which stops the crack propagation. Translucent zirconia is related to, but different from, the earlier zirconia materials used for substructures and the original BruxZir (Glidewell Laboratories).
Q: What should the clinician be mindful of when handling the new translucent zirconias?
A: Translucent zirconias can serve well for anterior restorations. Just realize you cannot handle the new translucent zirconias as aggressively as you may have handled the original BruxZir. “More gentle chairside handling” means the use of new diamonds, low cutting pressure, and plenty of water when occlusal adjustments or endodontic access preparations are necessary. Also be aware that often the colors of the new translucent zirconias are not yet perfected.
Q: Will translucent zirconia serve better clinically than lithium disilicate?
A: Currently, no one knows the answer to this question. Although the translucent zirconias have flexural strength that is about twice that of lithium disilicate (650+ MPa versus ~350 MPa for e.max CAD, and ~400 MPa for e.max Press [Ivoclar Vivadent]), the clinical significance of this difference in terms of less breakage and greater longevity is unknown. Both materials have the potential for appealing aesthetics. However, ultimate aesthetics depend on the skills of the dentist and the laboratory technician to first evaluate properly and then reproduce translucence, color, form, and function. Both materials need careful handling during chairside adjustments and neither tolerate endodontic access preparations well. Both materials gain strength from tooth reduction that allows greater material thickness. Lithium disilicate has established an excellent clinical history for anterior and posterior single-unit restorations in both its milled and pressed forms. The challenge for the translucent zirconias is to equal and exceed this history by withstanding multiunit and high-stress clinical challenges, where lithium disilicate has sometimes fallen short.
Q: There are now many different brands of translucent zirconia. How does the clinician know which one is best?
A: Unfortunately, you have no way to know which zirconia brands are best. Right now, we are in the midst of a brand name and strength numbers “war” as various entities vie for position in the marketplace. We are seeing unrealistic claims and confusion. Private labeling by the dental laboratories is the big contributor to the ever-expanding number of zirconia brand names. In many cases, the name is different, but the products are identical. This makes it difficult to locate the source if restorations do not serve as expected, or if systemic hypersensitivity or legal problems exist. Today, there is no traceability of zirconia restorations delivered to patients, with zero information on chemical constituents and origin. In situations like this, it is wise to use products from known dental companies, even though costs may be higher. Ideally, the dentist and laboratory technician communicate to decide which brands to use. However, dentists can ensure their choice by listing the brand name they desire on their laboratory prescription. All must realize that all zirconia formulations do not perform equally well clinically. In addition, strength numbers in promotional materials can vary according to test methods, procedures, and ethics.
Q: How can zirconia formulations from various sources differ?
A: There are a number of ways zirconia formulations can differ, such as oxides added, binders used, particle size and particle size distribution, radioactivity, purity, porosity, mixing zirconia powders from different sources and in different ratios, methods used to form disks and blocks, and firing cycles used. All of these can affect clinical performance. In addition, there are different grades of zirconia produced for uses outside dentistry for electronics, abrasives, and other industry applications. These other grades are not appropriate for dental restorations, but they can find their way into dentistry if/when profit concerns override good business ethics.
Q: Is there still a place for a low-translucent zirconia, such as the original BruxZir?
A: Absolutely, yes! Right now the original BruxZir is the only zirconia formulation on which we have clinical data from a 6-year controlled comparative study. It was the first and only formulation on the market in 2009 for full-contour restorations in the United States when we started our study. It has shown excellent clinical performance in molar full-contour crowns, anterior and posterior multiunit restorations, bruxing/clenching patients, minimal preparations similar to cast gold preps, and where maximum longevity all-ceramic restorations are needed. This product has proven itself to be tough, strong, affordable, and reasonably aesthetic for posterior full-contour restorations. It, and now other formulations similar to it, are your dependable choices when ultimate aesthetics are not the primary concern.
Q: What is the future for translucent zirconia?
A: Very promising. Even as you read this, people are moving fast to produce a zirconia formulation that has the aesthetic properties desired, without sacrificing the strength, toughness, and transformation toughening that make zirconia famous. When achieved, this will be a valuable product for all concerned—patients, dentists, laboratory technicians, third parties, and manufacturers.