Keys to Bonding Esthetic, Durable Zirconia Restorations: Two Case Reports

As dentistry continues to evolve, practitioners are able to more easily deliver amazingly esthetic products while being less invasive in patients. Amalgam and gold have largely been replaced mainly due to patient-driven preferences for tooth-colored restorative alternatives such as glass-ionomers, composite materials, and all-ceramic restorations. These contemporary materials have made it easier for clinicians to create beautiful lifelike restorations, often requiring less time, effort, and skill than with past materials, while also potentially offering improved longevity of the tooth-restoration complex.

Polycrystalline metal oxide-based ceramics, typically referred to as zirconium dioxide (or zirconia) restorations, have advanced greatly in the past 20 or so years. Zirconia was once thought of as an opacious, white, artificial-looking, metal-free ceramic crown alternative to porcelain fused or gold alloy restorations. Originally defined as a solid ceramic core material that replaced traditional gold alloys in porcelain-fused-to-gold full-coverage crown and bridge restorations, zirconia has made considerable strides in its properties and appearance. Initially, similar beautiful feldspathic porcelains were used to create the final color characteristics and beauty when layered on to zirconia core materials.¹

Eventually, full-coverage high-strength monochromatic zirconia restorations became available. Their inherent opacious appearance, however, limited their use to mostly the posterior dentition. Continued development over the years brought about a marked improvement in the appearance of zirconia restorations. The traditional 3Y (3 mol% yttria-stabilized tetragonal polycrystalline) zirconia restorations, which offered high strength and acceptable cosmetics for posterior teeth and occasionally anterior teeth, were transformed into esthetic and realistic-looking restorations for the anterior thanks to 4Y and 5Y varieties of multilayer and improved translucencies.

Although zirconia restorations were originally utilized in full-coverage applications and with implementation of traditional cementation procedures, this limitation has since been removed. Thin translucent and multilayered zirconia veneers whose appearance rivals that of other high-end cosmetic restorative ceramics can now be created and used with modern resin adhesion to cement them.

Research has shown that chemical bonding to zirconia is achievable when using the MDP monomer (10-methacryloyloxydecyl dihydrogen phosphate).² However, unlike typical ceramics, zirconia is non-etchable, which means clinicians must handle it differently and use different materials with it. For example, using hydrofluoric acid to create microscopic mechanical retention does not work on zirconia; hence, mechanical retention must be created through the use of sandblasting instead. Also, zirconia does not contain silica, so silane primers are ineffective and must be replaced with zirconia ceramic primers. Thus, sandblasting and MDP zirconia primers may be used to achieve both mechanical retention and chemical adhesion. Products that contain MDP monomer will bond to zirconia when proper protocols are followed.

An MDP-formulated cement may be used when there is a good preparation design with axial walls that have 5 to 6 degrees of taper and heights of 4 mm or more. In instances of low retentive preparation designs, MDP-formulated primers can be used along with adhesives and resin luting cements to chemically adhere the restoration to the tooth to create a strong tooth-restoration complex, as is achieved in pressable and feldspathic ceramics.³

Another key aspect of zirconia is that saliva interacts with it in a way that can be detrimental to creating resin adhesion. This is because the saliva phosphates interact with the zirconia to form ionic bonds, depleting the number of available bonding sites on the zirconia surface, thus creating a weaker adhesion.⁴ Moreover, phosphate contaminants from saliva accumulate on the surface of the zirconia restorations during the try-in procedure. Simply rinsing the zirconia restoration with water, ethanol, or acetone will not effectively remove the phosphates due to their ionic bond to the zirconia. To neutralize and remove the phosphate contamination, the zirconia surface must be cleaned with an alkaline solution.^5,6

Before starting the cementation or adhesion procedure with any zirconia restoration, the retention of the preparation needs to be considered. A short tooth preparation design or one that has minimal retention will require a bonding procedure on the dental preparation with a bonding agent, an adhesive resin cement, and an MDP-containing primer. If there is good retention and resistance form in the preparation design and fit of the restoration, a self-adhesive resin cement may be used, and there will be no need for an MDP-containing primer.

Zirconia's inherent high flexural strength rivals any ceramic currently available and exceeds natural masticatory forces.⁷ Hence, it can be cemented with traditional glass-ionomer-based cements as well.

The continued improvements in ceramics, including metal oxides-based ceramics (zirconia), has allowed for the creation of additional options for minimally invasive, realistic-looking restorations not only in the posterior dentition but also in the anterior dentition. High-strength polycrystalline metal oxide-based ceramics like zirconium dioxide are characterized as having excellent mechanical properties that are substantially greater than those of silica-based ceramics.⁸

Flexural strength values of conventional yttria-stabilized tetragonal zirconia polycrystals range between 1,000 MPa and 1,500 MPa. The inherent strength of these materials allows for conventional cementation of full-coverage restorations. The early generations of zirconia for copings and frameworks that were veneered with a feldspathic veneering porcelain had success rates that were similar to porcelain-fused-to-metal restorations; however, recent trends favor monolithic ceramic restorations.⁹

Various manufacturers have created multilayered, variable-translucent zirconia restorative options that can mimic the appearance of natural teeth equally as well as-or, thanks to their physical properties, possibly better than-inherently weaker feldspathic, leucite-reinforced, lithium-silicate, and lithium-disilicate ceramic materials.^10,11 Newer high-translucency zirconia materials have inherent multilayer capabilities, creating realistic color transitions that mimic natural teeth. The latest zirconia generations benefit from changes of the yttrium oxide content. However, with more translucency comes a tradeoff in strength properties. While the traditional 3Y zirconia had flexural strength characteristics in the 1,000 MPa to 1,500 MPa range, the current 4Y and 5Y zirconias have flexural strength in the 800 MPa and 550 MPa ranges, respectively.

It is generally considered that today's highly esthetic full-coverage zirconia crown and bridge restorations, with their high flexural strengths that exceed natural masticatory forces, would be cemented much like porcelain-fused-to-gold alloy restorations when proper retention and resistance form exists in the preparation design.¹² Although contrary to common belief, bonding to zirconia is possible and quite easy with the proper use of the right materials. The concepts are not much different than when bonding to other types of dental restorative materials. A clean surface is needed that offers some mechanical retention whereby a chemical interaction can be achieved by surface priming. From there the procedure is typically no different than using a universal bonding agent and dual-cured luting resin.

The following two case reports describe zirconia adhesion and zirconia cementation, respectively.

When poor retention exists for a zirconia restoration to be cemented, an adhesive protocol will need to be used. Many resin adhesive bonding agents, cements, and ceramic primers contain MDP chemistry, which allows adhesion to zirconia in cases where mechanical retention is not an option. In the present case, the patient presented with a desire to have natural-looking front teeth, Nos. 8 and 9. Crowns were recommended for both teeth due to the amount of missing tooth structure, cracks, and root canals.

Crown preparations were performed, impressions were taken, as were photographs and a facebow to mount the case on an articulator. The teeth were then provisionalized.Upon removal of the provisional restorations, the tooth preparations were sandblasted to clean and remove contaminants, cleanse the tooth surface, and create more surface area for adhesion (Figure 1).

After the try-in and approval of the appearance of the zirconia restorations, they were cleaned using a phosphate scavenger product (ZirClean^®, BISCO, bisco.com) (Figure 2) that contains potassium hydroxide, which removes the phospholipid contamination that occurs during intraoral try-in. This extraoral cleaning gel is designed for non-abrasive cleansing after intraoral try-in of the bonding surfaces of zirconia.

After thoroughly rinsing and drying the restorations, a zirconia primer (Z-Prime^™ Plus, BISCO) was then applied to the bonding surfaces of the zirconia crowns (Figure 3). This zirconia primer contains 10-MDP, the adhesion-promoting monomer that offers outstanding bond strength to zirconia, alumina, and metal restorations. This zirconia primer can be used with both light-cured and dual-cured resin luting cements, significantly enhancing their bond strength (data on file, BISCO, Inc.).

The teeth preparations were treated following the manufacturer's protocols for the application of a universal bonding agent (All-Bond Universal^®, BISCO), followed by light-curing. Next, a dual-cured adhesive resin luting agent (Duo-Link Universal^™, BISCO) was placed inside of the restorations (Figure 4). Some of the gross excess was wiped away leaving some excess on all the margins. The air-inhibition layer, which is not directly on the margin, was then removed after curing was completed. The crowns were delivered, optimally adhering to the tooth structure for longevity (Figure 5).

The use of modern bioactive cements can potentially benefit the tooth long term by helping to seal the margins, create hydroxyapatite, and strengthen the tooth all while aiding in avoiding acid attacks from the oral environment and cariogenic bacteria.^13-16 When adequate retention and resistance form is present in the way of axial wall height, taper, rounded box, or grooves, cementation may be used, as the clinician does not need to rely on adhesion to hold the restoration in place.

This case involved the maxillary first molar, tooth No. 3, which had decay. After removal of the existing crown, the molar was noted to have sufficient tooth structure. The tooth preparation modification had good axial wall height and angulation such that the final restoration would have good retention (Figure 6).

The crown was tried-in and then cleaned with ZirClean. A dual-cure self-adhesive resin cement (TheraCem^®, BISCO), which contains MDP and offers such adhesion properties as calcium and fluoride release and alkalinity, would be used on the posterior crown. While pumice and water or a sandblaster may be used to first clean the tooth, a sandblaster was used in this case. Then the resin cement was applied to the internal surfaces of the restoration (Figure 7). The restoration was seated and the margins were tack-cured (Figure 8). The excess cement was then removed after completion of the self-curing process. The author suggests that to aid in the removal of excess cement, the clinician initially light-cure the margins for 2 to 3 seconds, then perform light-curing for 20 to 30 seconds, or allow the cement to self-cure. (When using self-cure mode, light-curing the margins even briefly will expedite the final set.) Excess cement was removed, and occlusion and contacts were checked (Figure 9 and Figure 10).

Adhesive dentistry, whether with direct composite resins or any number of indirect ceramic restorations, can be challenging, especially when it involves modern zirconia restorations. Regardless of materials used, the effectiveness of the dentist's technique is crucial. Although many dentists may believe zirconia is unable to bond well using adhesive resins, with the correct materials and appropriate protocol and technique, adhering zirconia to tooth structure is fairly similar to the bonding of traditional feldspathic and lithium-disilicate ceramics done regularly.

This article was commercially supported by BISCO, Inc.

Todd Snyder, DDS
Private Practice, Laguna Niguel, California, and Las Vegas, Nevada; Fellow, American Academy of Cosmetic Dentistry