Zirconia Crowns: The Good and the Bad from Dr. Raymond L. Bertolotti, DDS, PhD

By Raymond L. Bertolotti DDS, PhD

Current Zirconia Status

The use of zirconia crowns has dramatically increased in the last few years. This change may be due to the availability of the "cheap" crown. Furthermore, they are made of unknown sourced zirconia and used in a marketplace where corporate dentistry is expanding, practice profits are shrinking, and new grads don't have the necessary skills or perhaps the time to provide more desirable tooth-conserving restorations.

I am generally opposed to the routine placement of crowns where a previous crown did not exist. Most of the time, there are better alternatives. Undesirable subgingival margins are often placed in attempting to hide the margin. To counter this trend, Jose-Luis Ruiz, DDS, and I have written a book, "Supra-Gingival Minimally Invasive Dentistry," published by Wiley (sold reasonably on Amazon). I am the contributing editor.

The book summarizes our thoughts and presents the techniques in detail.

Pictured Below: An example IPS E.max (lithium disilicate) restoration was taken from the book, and a Milicich compression dome supra-gingival onlay (1), diagram as an alternative to the crown. The onlay is made of E.max HT (high translucency) and bonded to the prepared tooth without significant resistance or retention form.

This adhesion onlay was kept supra-gingival, resting on a solid "biorim," the 2-3 mm of tooth structure located above the maximum diameter of the tooth. Enamel acts as a stress distributor. The load is transferred to the root vertically and horizontally to the crown through the dentinoenamel junction. In the compression dome concept, since the biorim supports the base of the dome, it is crucial to preserve the biorim. When there is a defect located gingival to the planned restoration, "margin elevation," (2) of that area would be utilized to disturb the function of the biorim minimally.

So why choose E.max and not zirconia? Zirconia can be a reasonable choice when replacing an entire crown, but there are usually better choices for compression domes and other occlusal onlays. When preparations are well-designed, and bonding is utilized, E.max can achieve about 75% of the strength of Y3 zirconia (3). We have excellent clinical success with bonded E.max as thin as 0.6 mm on the occlusal. Of course, E.max is more esthetic and much easier to polish than zirconia. Contrary to popular opinion, zirconia can be bonded very well (4), around 30% better than phosphoric acid etched enamel bonds.

The Properties and Best Uses of Zirconia

Zirconia is strong when it is "yttria-stabilized zirconia" in a monolithic form, not veneered with another ceramic. This form is known as Y3 since it contains 3 mol percent yttria. BruxZir, a well-known product introduced by Glidewell Lab, is in this group and has clinically proven to be very durable. More esthetic zirconia, with enhanced translucency, having higher yttria content, such as Y5 and Y8, is not the same ceramic and, at this point, is clinically unproven or perhaps proven to fail when not supported by bonding to the substrate frequently.

Cemented Y5 Zirconia Crown Fractures

For all ceramic adhesion (almost no pep) anterior bridges, Y3 zirconia is the material of choice (see below). For conventional bridges, it may be a good choice for the frame, but well-proven "porcelain-fused to the metal" is an excellent alternative to consider. These can be made with ceramic margins. If you use Y3 zirconia as a framework, Noritake CZR Press is the veneering ceramic of choice. It has a much lower failure rate (by chipping off) than all other available ceramics known to the author.

A Revolution in Adhesion Bridges

Professor Matthias Kern of Germany has just published his research results in a new English edition book by Quintessence. There is also a German edition. I did some editing and translating in the English version.

Professor Kern shows 100% success with anterior, cantilevered, bonded zirconia adhesion bridges at ten years in the book. That 100% success counts the 5% debonded and successfully re-bonded, with the debonds primarily due to trauma. There was zero zirconia breakage. These ceramic bridges are much like the metal-backed, anterior Yamashita adhesion bridges(5), which I have taught for over 25 years.

I have zero debonds out of nearly 200, placed as long as 30 years ago. The grooves I place for Yamashita bridges differentiate them from the better-known Maryland bridges. Grooves are not used with zirconia. Kern has reasoned that the grooves stiffen the metal to prevent peeling. Since zirconia is a lot stiffer than metal, no stiffening is required. (Alumina is even stiffer and more bondable but breaks.)

Bonding is done by the same procedure that I use for metal: alumina blasting, followed by the application of Panavia.  Kern has used Panavia 21, which is understandably not about to change due to his excellent track record. I prefer Panavia F2.0 since it is very similar to 21 but dual-curing, making it light-curable at the margins.

The light-curing also facilitates placement by not having to stabilize the bridge during placement with self-curing Panavia 21. The margins must be covered with Oxyguard if using Panavia 21 since they cannot be light-cured, do not confuse them with Panavia SA cement. It is different from 21 or F2.0, not even close. It is not strong enough for adhesion bridges.

Panavia V looks promising in the short term. However, I have not used or taught it since 21, and F2.0 has established track records of success, and V requires separate primers, even on non-precious metals.

Pictured Below: An excellent example of the cantilevered adhesion bridge provided as an alternative to an implant (Courtesy of Charles Ruefenacht DDS):

Conclusion

Zirconia can is very strong, especially zirconia Y3. Its high strength can be utilized successfully for conventional and adhesion bridge frames and full crowns. Y3 zirconia's strength is compromised when yttria is added to make it more esthetic (translucent). For tooth-conservative restorations, full zirconia may not be the best choice. Proven and more desirable alternatives are available.

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Author: Professor Ray Bertolotti is a world-renowned expert in adhesive dentistry and annually presents at many international conferences.

References

1. The compression dome concept: the restorative implications. Milicich G. Gen Dent. 2017 Sep-Oct;65(5):55-60.
2. Deep Margin Elevation: A Paradigm Shift. Magne P, Spreafico RC.  Am J Esthet Dent 2012; 2:86-96.
3. Load-bearing properties of minimal-invasive monolithic lithium disilicate and zirconia occlusal onlays: finite element and theoretical analyses. Ma L. et al., Dental Materials 2013 July:29 (7): 742-751.
4. Durability of the resin bond strength to zirconia ceramic after using different surface conditioning methods. Wolfart M et al., Dental Materials 2006;23:45-50
5. Adhesion bridge, background, and clinical procedure. Yamashita A, Yamami T, Proceedings of the International Symposium on Adhesive Prosthodontics, Academy of Dental Materials, 1986.