Zirconia Reinvented

Every dental material ever created by a manufacturer has gone on a journey—some longer than others. Zirconium dioxide (zirconia) has had a longer journey than most and has been steadily advancing since its inception. For many years, ceramists who worked with zirconia would tell you the same things: it’s changed how restorations are fabricated and it has very impressive strength, but the aesthetics are so-so.

During the last few years, we have advanced greatly in material science, capitalizing on monolithic zirconia restorations to improve upon the layered interface approach while maximizing the strength for which this material is known. Simultaneously, the introduction of some very advanced dyeing liquids has elevated zirconia’s aesthetics considerably. However, ceramists who have used these materials will stain and glaze the restorations to improve the aesthetics, but the end results can still be a little opaque. In other words, even though they do the best they can, the aesthetic outcome is still compromised. So, how could zirconia be taken to the next level?

ADD-ON VERSUS BUILT-IN
The very reason why dentists like a full-contour zirconia crown is because there are no weak areas. When it’s milled and fully sintered as a monolithic restoration by the dental laboratory team, it is as strong as it will ever be—that’s step one. Prior to this, you must think about the layers of a tooth structure. How can we get those layers out of a monolithic crown? Enter the shade gradient, where we have built-in translucency throughout; a zone with enamel shading followed by a transition zone, and then a body-shaded layer worked right into the zirconia disc (Figure 1).

Figure 1. The cross-section of the zirconia disc (Lava Esthetic Fluorescent Full-Contour Zirconia [3M]).

A recently introduced zirconia material (Lava Esthetic Fluorescent Full-Contour Zirconia [3M]) has incorporated the advanced technology in which color ions found in dyeing liquids are combined as tiny colorant clusters dispersed within the disc in a manner that results in a natural gradient shading after sintering. By using color ions, the zirconia is shaded while minimizing light scattering, which helps maintain translucency for aesthetics, flexural strength, and fracture toughness. If the potential of a restoration to mimic the natural tooth is based on translucency and color, this process gets us much of the way there via built-in features. This equates to not needing to incorporate add-ons, such as stains, which take more time and are subject to inconsistencies.

The Missing Link: Fluorescence
So, if the aesthetics are most of the way there, what’s missing? Well, beyond the basics of understanding lighting conditions, we also know that sunlight has an ultraviolet (UV) component and that natural dentition interacts with the UV light and fluoresces. This fluorescence is what gives natural teeth their look of lifelike vitality. While porcelains and dental restorative materials have been fluorescent for years, the ability to produce zirconia-based restorations with fluorescence has been limited to the use of special dyeing liquids, which have been ineffective on darker shades, or the use of surface-applied fluorescent glazes, which can wear with the passage of time or even chip off. Therefore, from an aesthetic standpoint, what is needed for zirconia to be ideal is to have inherent fluorescence throughout the entire restoration so that it interacts with light like natural teeth (Figure 2).

Figure 2. Inherent fluorescence of Lava Esthetic Zirconia.	Figure 3. As presented, the patient’s fractured onlay on tooth No. 4.

In the following case, we used the new Lava Esthetic Fluorescent Full-Contour Zirconia. We found that it fits better than other materials that we have previously used due to it being digitally designed and milled. Furthermore, this material is as translucent and aesthetic as materials that are significantly weaker than it. In addition, it incorporates the elements we discussed above (full-contour, monolithic, built-in shade gradient, and inherent fluorescence) and has all the advantages of traditional zirconia—plus it looks better!

CASE REPORT
A 42-year-old female presented with a fractured leucite-reinforced all-ceramic onlay on tooth No. 4 (Figure 3) that had been placed 7 years prior. She had fractured the buccal aspect of the onlay while eating. Due to lack of retention of the preparation, we treatment planned for a full-coverage restoration.

I prefer to use digital intraoral scanning as an alternative to traditional impressions. To produce a zirconia crown, the dental lab team must convert a traditional impression into a digital file. Therefore, by doing an in-office scan, time is saved and the chance for errors associated with pouring stone models is reduced. This can also frequently result in lower lab costs. I like to use retraction paste to passively retract tissue prior to scanning. Preferably, the paste (such as Retraction Paste [3M]) incorporates a hemostatic agent. In this case, we placed the paste around the margin (Figures 4 and 5) with the convenient dispensing capsule which has a unique rounded corner tip that easily inserts into the sulcus. The paste is allowed to sit in place for 2 minutes, and then it is rinsed away with water.

Figure 6. Selection of the stump shade.	Figure 7. Scanning spray applied to preparation.
Figure 8. The milled restoration.	Figure 9. The sintered restoration.

Figure 10. Filling crown intaglio with the cement.

After taking a stump shade, which is highly recommended when prescribing translucent materials (Figure 6), a light dusting of contrast powder (High-Resolution Scanning Spray [3M]) was applied (Figure 7) prior to taking the digital impressions (True Definition Scanner [3M]). Note that this is not the old “powdering technique,” but rather a much lighter, simpler approach that helps speed up the process while achieving the accuracy that we are looking for in our impressions. The digital impressions were then sent electronically to the dental lab where everything was reviewed and confirmed quickly and simply—one of the many benefits of intraoral scanning.

The dental lab team (Utah Valley Dental Lab [Provo, Utah]) positioned the restoration in the disc to achieve the desired thickness of enamel shading (Figure 8). Then the milled restorations were subsequently sintered to achieve their final physical properties and, if needed, additional characterization was applied. In this case, no additional staining was needed (Figure 9). To save time and to facilitate the cementation process, I requested that the lab team sandblast the inside of the crown following the manufacturer’s recommendations (50-µm alumina using a pressure of 2 bar).

Figure 13. Removing excess cement after tack cured.	Figure 14. Cleaning interproximal contacts with dental floss.

Figure 15. The tooth was rinsed, and a finishing strip was used for final interproximal finishing.

At the seating appointment, the crown was tried-in. Because we had used an intraoral scanner, combined with the lab team using digital design and manufacturing, the contacts were good and adjustments were minimal. After try-in, the internal surface of the restoration was cleaned. Although not always necessary, my preference is to place a primer on this internal surface and then air dry thoroughly. For final cementation, we utilized a self-adhesive resin cement (RelyX Unicem 2 Self-Adhesive Resin Cement [3M]) (Figure 10). The crown was seated and then spot tacked using a 2.0-mm curing tip for 10 seconds (Figure 11). The margins were light cured for 2 seconds on the lingual and 2 seconds on the facial (Figure 12).

With this accomplished, I find the gelled RelyX Unicem 2 cement easier to remove using a No. 12 Bard-Parker blade (Aspen Surgical) or scaler (Figure 13). Additionally, floss was used interproximally to remove partially polymerized cement prior to the final cure (Figure 14). After a final polymerization of the cement for 20 seconds on the occlusal, lingual, and buccal surfaces, the tooth was rinsed and a finishing strip (Sof-Lex [3M]) was used for final interproximal finishing (Figure 15).

CLOSING COMMENTS
We owe our patients no compromises. They deserve a restoration that is strong, fits unbelievably well, and does not compromise aesthetics. This new zirconia material has inherent fluorescence like natural teeth and, thus, interacts with light like natural teeth.

With this new zirconia, we can confidently and predictably deliver long-lasting restorations that will be more undetectable than ever before (Figures 16 and 17).

Acknowledgment
The author would like to thank Utah Valley Dental Lab in Provo, Utah, for consistently providing the standards of excellence.

Dr. Hornbrook is a gifted clinician and product researcher turned mentor who guides meeting participants to the realization that they too can practice aesthetic dentistry at the very highest level. He is a pioneer in hands-on, live-patient, clinical education as the founder and past director of the Pacific Aesthetic Continuum (PAC~live) and the Hornbrook Group in La Mesa, Calif. He is the Clinical Director of Education and Technology for Utah Valley Dental Lab in Provo. He lectures internationally, consults with numerous manufacturers, and has published articles in many leading dental journals. He can be reached at (619) 813-3869 or via email at david@hornbrook.com.

Disclosure: Dr. Hornbrook reports no disclosures.

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