The Zirconia-Based Porcelain Veneer

Dentistry is both an art and science requiring today’s practitioner to be versatile and ready to adapt. Not every patient fits into the same treatment box, calling for an identical approach. A comprehensive examination is needed to discover and establish a list of each patient’s requirements.
Generally, most patients’ dental treatments call for conventional and time-tested procedures. However, at times, we are faced with challenges that require us to be able to think and operate “outside the box,” relying on our ingenuity and creativity. As practitioners gain experience, they gather their own personal “bag of tricks” to turn to. New procedures and innovations in dental materials provide us with increasing options.

CASE REPORT
Diagnosis and Treatment Planning
Our patient presented with a discolored upper left central incisor (Figure 1). His dental history noted that trauma occurred to the tooth in his youth. His tooth tested vital and had never been restored. Although the dark tooth bothered the patient, he had always been reluctant to compromise its integrity. Full-coverage crowns, pressed-ceramic veneers, and bonded-composite restorations had been previously proposed as possible aesthetic solutions. However, these restorations called for significant tooth reduction in order to mask the dark underlying dentin. Since his tooth was healthy and asymptomatic, he had been seeking a more conservative option.

Figure 1. A discolored, vital, unrestored left central incisor.

Presented with the desires of the patient, we studied his restorative choices and considered the currently available dental materials. An ultra-thin zirconia core veneered with porcelain was acceptable to him.
Zirconia has recently become a popular dental material as it has high strength, is resistant to fracture, and being opaque has the ability to mask discolored underlying tooth structure.¹ Its flexural strength of 900 to 1,100 MPa and fracture toughness of 8 to 10 MPa make it a suitable material to create both conservative aesthetic restorations as well as pontic spans for bridge work. It can be milled to a 0.2 mm thickness and then layered with a compatible porcelain to yield a restoration of less than 0.6 mm in total thickness. A restoration of this nature would be minimally invasive, with the ability to alter the color of the dark tooth to approximate that of the other central incisor. However, in order to achieve a restoration of this description, we require the necessary chemistry to create adhesion between the zirconia and the tooth.
To understand the challenges of the required chemistry, we must realize that enamel and dentin are living tissue requiring primers and adhesives that infiltrate into created microretention on the surface. Zirconia is a nonliving, oxide-based substrate requiring a primer with phosphate comonomers to covalently bond to the oxide. In order to fuse the zirconia to the tooth, we require a cement interface which cohesively recreates the dentoenamel junction among the living enamel, dentin, and the inert zirconia oxide-based substrate. The resin cement chosen should be hydrophobic and dual-curing (light- and self-curing), since zirconia is opaque and may not allow light to totally pass through it to fully cure the resin.^2-4
Until recently, the zirconia-to-tooth bonded interface was not possible, as hydrophobic cements do not adhere to oxides. New products, such as Z-PRIME (BISCO Dental Products), have been developed. These are primers that utilize a combined phosphate and carboxylic monomer to create a cohesive interface, allowing the hydrophobic resin cement to create a linked, cohesive hydrophilic seal between tooth and the indirect zirconia restoration. New universal primers (such as Monobond Plus [Ivoclar Vivadent]) have also been developed to accomplish this task with zirconium oxide, metals, porcelain glass, and lithium disilicate.

Clinical Treatment
An initial reference shade was taken and photographed before any preparation to the tooth occurred. The other incisors matched a Vita A1 shade (Figure 2).
An impression of the unprepared teeth was then taken using a clear plastic tray (CLINICIAN’S CHOICE) in order to have a matrix ready for subsequent bis-acryl temporary fabrication. This clear silicone impression material Affinity Crystal (CLINICIAN’S CHOICE) allows the dentist to visualize the underlying teeth as a reference, making it easy to reposition in the patient’s mouth when filled with the provisional material (Figure 3).
After labial reduction of 0.6 mm, we had removed the outer layers of translucent enamel, thus exposing the darker underlying hue of the dentin that creates the tooth’s color. Zirconia oxide is typically a very opaque material that does not provide for natural translucency and light reflection. It should be noted that several translucent zirconia-based materials have been recently introduced into the marketplace to address certain aesthetic concerns (such as inCoris TZI [Sirona], Zenotec CAD/CAM [Wieland], and Bruxer Esthetic [Glidewell Laboratories]).

Figure 2. Shade selection (VITA Classical Shade Guide [Vident]).	Figure 3. Matrix impression was taken using a clear silicone impression material (Affinity Crystal [CLINICIAN’S CHOICE]) in a clear plastic tray; this was made for later fabrication of the bis-acryl provisional.
Figure 4. Creative tooth preparation, allowing room for the ceramist to build in (layer) opacity and transparency.	Figure 5. Stump shade selection was done after preparation.

Understanding the properties of our restorative materials and their limitations allows us to work creatively within the parameters that we are given. Incisal reduction of 1.0 mm was created so that the incisal edge of the veneer could be fabricated in pure feldspathic porcelain. This design would allow our ceramist to create transparencies and internal coloration similar to that of the adjacent natural tooth. In addition, deeper preparation on the mesial and distal line angles would enable the ceramist to develop a translucent effect in these areas (Figure 4). The areas of discolored dentin would be masked out with the opaque zirconia.
Next, the shade of the underlying prepared tooth was selected and photographed in order for the ceramist to create a corresponding die as a color reference (Figure 5).
Before taking the final impression of the prepared tooth, our procedure protocol is to fabricate a preliminary provisional restoration using a bis-acryl provisional material (such as Luxatemp Ultra [DMG America], Protemp Plus [3M ESPE], Integrity [DENTSPLY Caulk]). This initial provisional serves both as a preparation check, allowing the clinician to assess if enough tooth structure was removed to accommodate the desired restoration, and to also verify that the correct shade of bis-acryl provisional material was chosen. Our office protocol calls for the dental assistants to use an operatory timer that is set to reflect the setting time as noted in the manufacturer’s instructions for the material being used. The timer was set for 2 minutes (as required for the Luxatemp Ultra, used in this case) (Figure 6a), and then the bis-acryl material was injected into the previously obtained clear silicone matrix impression (Figure 6b), and positioned over the prepared tooth. The provisional material was allowed to set on the tooth so that we could check to see if we had achieved an adequate and uniform preparation to accommodate the required thickness of the restorative material selected for this case (Figure 7). Any modification needed can be done to the preparation by directly cutting through the bis-acryl which serves as our reference. In this case, we needed to reduce the tooth a bit more on the labial surface (Figure 8).⁵ After the preparation was completed, a final provisional restoration was fabricated and set aside to harden.

Figure 7. Using the provisional to assess the preparation.	Figure 8. Modifying the preparation through the provisional (further labial reduction needed, as noted in the preceding prep assessment).

It is important to emphasize that, by using the bis-acryl provisional as a preview, we eliminated the need for our dental laboratory team to inform us of underpreparation/insufficient room; this also obviated the need for the patient to return to our office to modify the tooth and to take a new impression. Having to reanesthetize, retemporize, reprepare, and reimpress (the author refers to this as “redo-a-dontics”) is not fun for either the patient or the dentist!
The patient’s bite was taken using a rigid bis-acryl bite registration material (LuxaBite [DMG America]) placed in the anterior region from canine to canine (Figures 9a and 9b). This is a thixotropic material that offers minimal resistance to closure; and once set, it has a D-69 Shore scale hardness that prevents any flexure/distortion, allowing the accurate transfer of the patient’s bite to the articulator. Unlike many silicone bite registration materials, it is a bis-acryl material designed to yield precise mounting and orientation of the plaster models.⁶ Again, the assistant set the operatory timer to a setting time of 2 minutes, according to the manufacturer’s instructions. There was no reason to extend the registration beyond the canines as the posterior bite was extremely stable; and in this way, it was easily visually verified that the patient was biting in maximum intercuspation.

Figure 10. The milled zirconia core (Zenotec TI 7-axis CAD/CAM milling system [Wieland]).	Figure 11. Application of the zirconia primer (Z-PRIME [BISCO Dental Products]) to the internal surface of the restoration.

Figure 12. Verifying the restoration for form and color, with the provisional still in place.

The final upper impression was taken using an extremely firm vinyl polysiloxane (VPS) impression material (Honigum Mixstar Putty [DMG America]). The Honigum Mixstar Putty was dispensed from an automatic mixer (Mixstar Emotion [DMG America]) and has a setting time of 5 minutes. It easily displaced the very viscous Kopy Single Crown Light Body (Dental Savings Club) into the subgingival sulcus, yielding a sharp and accurate impression. The author has found that this impression material is pleasant tasting and is easily removed from the mouth. The opposing lower model was taken with a VPS alginate substitute (Status Blue [DMG America]). Totally rigid and inflexible metal impression trays (Rim-Lock Metal Trays [DENTSPLY Caulk]) were used for both impressions, thus minimizing distortions and inaccuracies.
Next, the patient’s tooth was cleaned and treated with a desensitizing agent (Gluma [Heraeus Kulzer]); then the provisional restoration was cemented with a translucent, dual-cured, eugenol-free resin temporary cement (TempBond Clear [Kerr]).

Brief Synopsis of the Dental Laboratory Protocol
At the dental lab, a sintered zirconia core (keyed to the desired A1 shade) was milled using the Zenotec TI 7-axis CAD/CAM milling system to a thickness of 0.2 mm (Figure 10). A feldspathic porcelain (CZR Cerabien [Noritake]) was used to create a 0.4-mm porcelain buildup (laminate) over the zirconia substructure, yielding a total restoration thickness of 0.6 mm. The Noritake feldspathic porcelain has a coefficient of thermal expansion carefully matched to the zirconia, permitting the ceramist to create lifelike aesthetics that display internal detailing and transparencies. The finished zirconia veneer offers precise marginal fit, excellent aesthetics, and high strength.

Delivery of the Final Restoration
Upon receiving the restoration from the laboratory, its fit was verified on the laboratory work model to ascertain that it met our expectations. We then telephoned the patient to schedule an appointment. (In our office protocol, delivery appointments are not prescheduled according to the lab return dates, as we always want to have the flexibility of being able to return the case to the dental laboratory team if obvious corrections are needed.) In this case, the restoration was acceptable as received from the laboratory, so the internal surfaces were treated with a zirconia primer (Z-PRIME) (Figure 11). The phosphate monomer in this special primer interacts with the oxide surface of the zirconia to form a covalent bond, thus optimizing the bond strength between the resin cement and the zirconia surface.

With the patient in the chair, and before removing the provisional, we first assessed the aesthetics of the restoration next to the natural teeth to see if they looked the same (Figure 12). If they had not, we would have returned the case to the dental ceramist with photos and a description of the changes required.
Once approved, the provisional was removed by first placing a slit in the labial surface using a flame-shaped diamond (KUT 3205 C [Dental Savings Club]); it was then separated and removed using an EB134 (Brasseler USA) instrument (Figures 13a to 13c). The underlying prepared tooth was cleaned with pumice, and then treated with a 32% phosphoric acid gel (UNI-ETCH [BISCO Dental Products]) containing benzalkonium chloride an antimicrobial agent (Figure 14). A universal ad (ALL-BOND UNIVERSAL [BISCO Dental Products]) with low film thickness was then brushed on the tooth structure (Figure 15). Next, a resin cement (DUO-LINK [BISCO Dental Products]) was applied to the internal surface of the veneer (Figure 16), and then it was placed on the tooth and light-cured (VALO LED Curing Light (Ultradent Products) (Figure 17). The VALO LED light produces a high degree of polymerization.^7,8 Furthermore, its curing tip can be easily placed at a 90°-angle to the labial surface of the tooth, allowing optimum light penetration.

Figure 14. Etching the tooth with a 32% phosphoric acid gel (UNI-ETCH [BISCO Dental Products]) containing benzalkonium chloride an antimicrobial agent.	Figure 15. Placing a universal adhesive (ALL-BOND UNIVERSAL [BISCO Dental Products]).
Figure 16. Placing the resin cement (DUO-LINK [BISCO Dental Products]).	Figure 17. The completed restoration.

Figure 18. The zirconia-based porcelain layered veneer demonstrating natural lifelike details.

Designing the tooth preparation specifically to accommodate the application of the ceramic materials chosen in this case was very important. This thoughtful protocol resulted in areas of translucency, transparency, and internal detailing, providing a definitive restoration with lifelike aesthetics that satisfied our patient’s aesthetic goals (Figure 18).

CLOSING COMMENTS
No single restorative material can satisfy every aesthetic need. By having current knowledge of procedures and materials available, the practitioner is better able to adapt and to satisfy the patient’s desires. Porcelain-layered zirconia veneers can be a useful addition to our dental “bag of tricks.”

Acknowledgement
The author would like to thank Mr. Adrian Jurim, CDT, MDT, for inspiration and the exquisite technical fabrication of this zirconia-based layered veneer (Zeneer [Jurim Dental Studio]).

References

1. Denry I, Kelly JR. State of the art of zirconia for dental applications. Dent Mater. 2008;24:299-307.
2. Tanaka R, Fujishima A, Shibata Y, et al. Cooperation of phosphate monomer and silica modification on zirconia. J Dent Res. 2008;87:666-670.
3. Yoshida K, Tsuo Y, Atsuta M. Bonding of dual-cured resin cement to zirconia ceramic using phosphate acid ester monomer and zirconate coupler. J Biomed Mater Res B Appl Biomater. 2006;77:28-33.
4. Dérand P, Dérand T. Bond strength of luting cements to zirconium oxide ceramics. Int J Prosthodont. 2000;13:131-135.
5. Gürel G. Predictable, precise, and repeatable tooth preparation for porcelain laminate veneers. Pract Proced Aesthet Dent. 2003;15:17-24.
6. Chan CA. Bite-management considerations for the restorative dentist. Dent Today. 2008;27:108, 110-113.
7. Price RB, Felix CA, Andreou P. Third-generation versus a second-generation LED curing light: effect on Knoop microhardness. Compend Contin Educ Dent. 2006;27:490-496.
8. Boksman L, Santos GC Jr. Principles of light-curing. Inside Dentistry. 2012;8:94-97.

Dr. Mechanic received his bachelor of science (1975) and doctor of dental surgery (1979) degrees from McGill University. He practices aesthetic dentistry in Montreal, Canada, and he is the aesthetic editor of Canada’s Oral Health dental journal and is on the editorial board of Dentistry Today. He also is the cofounder of the Canadian Academy for Esthetic Dentistry, program coordinator of the University of Toronto Advanced Restorative Continuum, and is recognized as a leader in continuing dental education. Dr. Mechanic maintains membership in numerous professional organizations, including the American Academy of Cosmetic Dentistry, the Academy for Dental Facial Esthetics, the American Society for Dental Aesthetics, and the European Society of Cosmetic Dentistry. His work has been profiled in magazines, television, and radio. He can be reached via e-mail at info@drmechanic.com.

Disclosure: Dr. Mechanic reports no disclosures.