Multilayered Translucent Zirconia Crowns

In order to better understand this material, a brief history of dental zirconia should be described. The first generation of zirconia, which was very opaque and strong, was used only for frameworks and copings. Later, a second generation of zirconia was produced that was translucent enough to be used for monolithic posterior restorations. Because this second-generation zirconia was more translucent than framework zirconia, many companies referred to it as translucent zirconia. However, that zirconia should not be confused with the newest iteration: third-generation zirconia. Third-generation zirconia contains approximately 50% cubic phase zirconia. This is the same type of cubic zirconia that is used in the manufacturing of fake diamonds. The addition of the cubic phase material makes this anterior zirconia much more translucent and enamel-like, but slightly weaker than its predecessors.

Another advantage of third-generation zirconia is that it is available in multilayered discs. This means that the basic color gradient of the zirconia is built into the raw material. In a monolithic crown made from a previous generation zirconia, all color characterization was added onto the surface of the crown with external stains. For these crowns, if the patient had aggressive brushing habits or possessed an acidic oral environment, the external stains could be removed to reveal the monochromatic restoration. With a multilayered zirconia, even if all of the external stains are removed, the internal color gradient remains.

A middle-aged female patient presented to the University of Alabama at Birmingham Prosthodontics Clinic for treatment of erosive and carious lesions on teeth Nos. 8 through 10 (Figure 1 and Figure 2). Initially, veneers were considered, however, once all caries and previous restorations were removed, there was insufficient remaining enamel to justify placement of bonded ceramic veneers. Therefore, build-ups were placed, and crown preparations were finalized. To compensate for the dark hue of the cervical aspect of the preparations, all crown margins were planned to be slightly subgingival. Due to their polychromatic nature, an image of the tooth preparations with an accompanying shade tab was sent to the laboratory (Figure 3).

The patient presented with a complete dentition, canine guidance, and a shallow overbite. The unprepared neighboring teeth displayed a glassy surface with some degree of incisal translucency. Signs of erosive wear were observed on some incisal edges, cervical surfaces, and posterior cusp tips. For several reasons, a layered anterior zirconia (KATANA^™ Zirconia STML disc, Kuraray Noritake Dental Inc.) was prescribed for this case. The use of a multilayered disc was chosen because the erosive potential of this patient could lead to the breakdown of surface stains. KATANA Zirconia STML was chosen to mimic the translucency of the opposing dentition. Given the favorable occlusal situation in this case, the choice was made to apply a small microlayer of porcelain on the incisal edge to obtain ideal esthetics.

Three monolithic KATANA Zirconia STML restorations were milled to full contour and sintered. Next, a skeleton-style, cut-back technique was performed, predominantly on the facial-incisal third area of the crowns (Figure 4). The internal color gradient of the KATANA Zirconia STML can be observed at this stage. A skeleton build-up technique was performed with porcelain (CERABIEN^™ ZR, Kuraray Noritake Dental Inc.) to layer the incisal third of the crowns (Figure 5). Due to the multilayered design of the KATANA Zirconia STML disc, the technician was able to achieve a fully layered result by only adding additional layers to the incisal third area. A small amount of external stain and a very thin glaze were applied to the facial surfaces of the crowns (Figure 6). Although the lingual surfaces of the crowns were not glazed, they were hand polished. Laboratory testing has shown that polished zirconia is more wear friendly to opposing enamel than glazed zirconia or porcelain.² In addition, a recent study demonstrated that KATANA Zirconia is as equally wear friendly as previous generations of zirconia and natural enamel.³

Once returned from the lab and tried-in, the crowns were sandblasted with 50 micron alumina at 30 psi (ie, 2 bar) to clean and roughen the interior surface. Although recent research has shown that sandblasting anterior zirconia will cause some weakening of the material, if the sandblasted crown is bonded with a resin cement, the strength is returned.⁴ If the crown will be cemented with conventional glass ionomer or a resin-modified glass-ionomer cement, sandblasting with alumina should be avoided.

Following sandblasting, the crowns were bonded with a resin cement (PANAVIA^™ V5, Kuraray Noritake Dental Inc.). This cement is supplied with a self-etch primer and a universal ceramic primer. The ceramic primer was applied to the crowns because it contains the molecule MDP, which is responsible for forming a chemical bond with zirconia. After the crown preparations were coated with a layer of the self-etching tooth primer (Figure 7), the crowns were then filled with PANAVIA V5 and seated (Figure 8). Because the cement is dual-cure, light curing is optional. The patient returned to the office 2 weeks after delivery of the crowns for a follow-up appointment and expressed satisfaction with the final result (Figure 9).

Celin Arce, DDS, MS, FACP
Assistant Professor
Restorative Sciences Department
University of Alabama at Birmingham
School of Dentistry
Birmingham, Alabama

Sean Park, MDC
SP Dental Arts
Los Angeles, California

Nathaniel C. Lawson, DMD, PhD
Director
Division of Biomaterials
University of Alabama at Birmingham
School of Dentistry
Birmingham, Alabama

For more information, contact:
Kuraray Noritake Dental Inc.
kuraray.us.com
800-423-9762