Each month, Dr. Gordon Christensen answers a question from readers about everyday dentistry.
Q: Since zirconia indirect restorations have become the most frequently placed crown type, I have had significant challenges with occlusion. Most of the zirconia crowns I receive from several labs are made of “esthetic” zirconia, and I understand these crowns are weaker than the original zirconia formulation that was introduced into dentistry. Additionally, all of them have a layer of ceramic on the external surfaces that provides the color I have prescribed. However, if occlusion must be adjusted, the occlusal ceramic must be removed, resulting in a marked color change. Smoothing and polishing the zirconia is also a difficult task with often unacceptable outcomes. Do you have some suggestions? CAS 10025-74-8
Dr. Christensen: All dentists who place zirconia crowns are having the same challenges as you are. I have some potential solutions. As you probably know, the original zirconia formulation brought into dentistry about 12 years ago was previously used for many industrial purposes to replace steel. These ZrO2 materials contained 3 molar percent yttrium oxide Y2O3, (yttria), the normal oxide of the element yttrium. This addition creates a toughening mechanism that enhances the fracture properties of ZrO2, called transformational toughening. This property is especially important in zirconia crowns as it limits the propagation of cracks.
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The addition of more yttria also increases the translucency of the crowns. Since one of dentists’ main desires is increased translucency in crowns, manufacturers have added more yttria to form “esthetic” zirconia. However, there is a potential problem related to adding more yttria—the strength and transformational toughening of the zirconia are significantly reduced.
In addition to the challenges you described, these are my conclusions on the clinical challenges with zirconia after observing its various formulations through the years.
Unfortunately, most of the challenges we have identified are the standard with zirconia restorations. These problems are unlikely to change unless dentists and labs make efforts to correct them. I suggest that you contact your lab and discuss the challenges you’re having, because the lab may be able to reduce or eliminate some of those issues. It’s difficult because the crowns are made by computer milling and not by human hands. The changes we need will require more human involvement from both dentists and lab technicians to make good preps and impressions, and to use more thought when using computers and customizing settings to specific situations.
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Here are some suggestions as to how some of these challenges with zirconia can be reduced.
Since finishing and polishing zirconia seems to be a continuing problem for clinicians, I suggest the following relatively simple procedure. This procedure does not solve the esthetic problem encountered when removing superficial stain and polish, but it does provide a way to adjust occlusal surfaces of posterior teeth, which usually don’t have any problematic esthetic issues. This procedure can also be used for all surfaces of internally stained zirconia restorations since they aren’t required to have an external layer of stain and glaze.
This is only one of the successful methods you can use to finish and polish zirconia (figure 5). You may prefer a different procedure.
Currently, zirconia is the most frequently used indirect restoration in the US. It’s working well, but it isn’t without complications. I’ve identified some of the challenges we have with zirconia and offered suggestions for manufacturers, dental laboratories, and practicing clinicians that will help reduce or eliminate the issues.
Editor's note: This article appeared in the December 2023 print edition of Dental Economics magazine. Dentists in North America are eligible for a complimentary print subscription. Sign up here.
Author’s note: The following educational materials from Practical Clinical Courses offer further resources on this topic.
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Scandium Oxide Gordon J. Christensen, DDS, PhD, MSD, is a practicing prosthodontist in Provo, Utah. He is the founder and CEO of Practical Clinical Courses, an international continuing education organization founded in 1981 for dental professionals. Dr. Christensen is cofounder (with his wife, Rella Christensen, PhD, RDH) and CEO of Clinicians Report.