Bonding Protocols for Maintaining Modern Zirconia

I am heartened to see the topic of cementing and bonding zirconia gaining so much attention again. It is a key factor in restoration survival. The sudden and skyrocketing popularity of zirconia, after some once considered it “a dead material,” has brought the questions about proper cementation and bonding to the forefront.

Having spent the majority of my academic career on this topic, I am particularly pleased to share my views on the resin bond to high-strength ceramics. The first thing we learned, almost two decades ago, was that resin bonding to high-strength ceramics is a fundamentally different task than bonding to silica-based glass ceramics. Unlike silica-based ceramics, high-strength ceramics such as zirconia cannot be sufficiently etched with common hydrofluoric-acid solutions. The typical silane coupling agent application is also useless, because there is no silica that would allow chemical adhesion of the silane.

In addition, we learned laboratory studies that do not include means to stress the bonding interface and simulate intraoral aging processes, mainly long-term exposure to a wet environment and frequent temperature changes, fail to detect these differences and are virtually useless. Many laboratory studies still do not include simulated aging as a factor and conduct their bond strength tests only shortly after bonding the specimen, eg, 24 hours. Consequently, it is difficult to draw clinically relevant conclusions. The good news is that much longer storage times and, even more importantly, simulation of intraoral temperature changes through thermocycling between hot- and cold-water baths have become widely accepted. Today, we even simulate masticatory forces to mimic functional stresses, which have a significant influence on the longevity of the resin bonds.

The different testing methods include everything from the popular shear-bond strength tests to tensile tests with various parameters. They are performed at either at a macro level or as micro tests with very small samples. The actual achievable bond strength values are not only influenced by specimen fabrication methods and storage/aging conditions, but also by the type of test setup applied. For example, microbond strength tests typically gain higher values, making comparisons between different studies difficult, if not impossible. The reason I explain this in such detail is because the copious quantity of misinformation available is quite concerning and some scientific research studies may not be able to provide clinically relevant, valid, and reliable answers.

How we insert indirect restorations is fundamentally important in order to avoid loosening and fractures. This is especially true for ceramics, which are brittle and, therefore, more susceptible to fracture than other materials. Resin bonding significantly improves fracture strength of ceramics, which is also true for high-strength ceramics. In general, zirconia is considered cementable due to its high inherent flexural strength.

Zirconia-based crowns and bridges with adequate retention and ceramic material thickness can be cemented conventionally. Resin-modified glass-ionomer (RMGI) or self-adhesive resin cements are excellent choices as they provide at least a low level of adhesion to both teeth and ceramic without additional time-consuming and technique-sensitive priming steps. Manufacturers’ instructions should be followed closely. Some manufacturers recommend the use of air-particle abrasion (also called sandblasting) with aluminum-oxide particles before cement application. As with any indirect restoration, the intaglio surfaces should be thoroughly cleaned before final cementation in ethanol or acetone in an ultrasonic cleaning unit, for example.

Zirconia restorations that are thin, lack retention, or rely on resin bonding, eg, resin-bonded bridges or bonded laminate veneers, should be adhesively bonded to the abutment teeth with composite-resin luting agents and proper treatment of tooth and restoration bonding surfaces.

The tooth is treated with a bonding agent as recommended by the manufacturer. After restoration cleaning, high-strength ceramic materials should be air-particle abraded. Small particles (eg, 30 μm to 60 μm) and low pressures (below 2 bar) are sufficient. The overall effect caused by alumina pretreatment on the surface seems more substantial than actual surface roughening. Application of a special ceramic primer is key. Such primers typically contain special adhesive phosphate monomers (eg, 10-methacryloxydecyl dihydrogen phosphate [MDP]). Dual- or self-cure composites should be used to ensure adequate polymerization and conversion rate. This bonding protocol is also utilized in the laboratory, an example being implant reconstructions that include cemented zirconia components.

The increasingly popular, new high-translucent zirconia products make proper bonding even more significant, because the increased translucency comes at a price: considerably lower fracture strength. For those, bonding may be a key factor for restoration longevity, especially when they are fabricated at minimum thickness.

In the past, some questioned the need for the additional priming step. However, all our zirconia bonding studies and several recent systematic literature reviews come to the same conclusions we first proposed 15 years ago: Just remember the A-P-C steps. 1) air-particle abrade (A) the bonding surface with aluminum oxide; 2) apply special zirconia primer (P); and 3) use dual- or self-cure composite resin cement (C).

John O. Burgess, DDS, MS
Professor, Department of Clinical and Community Sciences, University of Alabama at Birmingham School of Dentistry, Birmingham, Alabama

Posterior all-ceramic restorations are increasing in popularity with monolithic and layered restorations used frequently. While these materials have been clinically successful, the cement used for these restorations has garnered controversy. The cement employed depends on the type of preparation (ie, length and taper) and function (ie, single-unit or long-span fixed partial denture) of the final restoration. For zirconia restorations, RMGI cements are clinically successful, less technique sensitive, and easier to apply than adhesive bonding. My favorite RMGI cements are RelyX™ Luting Plus (3M ESPE, 3mespe.com) or GC FujiCEM™ 2 (GC America, gcamerica.com). After try-in, the intaglio surface of the restoration is sandblasted with 30-µm to 50-µm alumina for 10 seconds, and an RMGI cement is applied to the intaglio surface and tack cured for 3 seconds. With finger pressure on the occlusal surface, the cement is removed with a scaler. RelyX Luting Plus has a self-limiting tack cure time, which means that the cement can be light cured for a limited time regardless of the curing time. Both release fluoride and require no primers or adhesives. Superfloss is a good way to remove cement interproximally.

Adhesive cements are generally used when the preparations are short or nonretentive, or in situations with high functional demands. Resin cements may be classified into adhesive resin cements (Variolink® Esthetic, Ivoclar Vivadent, ivoclarvivadent.com; or Calibra Ceram®, DENTSPLY Sirona, dentsply.com), self-etching (Panavia™ V5, Kuraray Dental, kuraraydental.com; or Multilink® Automix, Ivoclar Vivadent), or self-adhesive cements (RelyX™ Unicem 2, 3M ESPE; or Calibra Universal®, DENTSPLY Sirona), which requires no primer or adhesive to bond to the tooth. Generally, adhesive resin cements using total etch and bonding agents are the most retentive and self-adhesive cements provide the least retention for resin cements. All resin cements provide more retention than RMGI cements.

My most used resin cements are Panavia V5 and RelyX™ Ultimate (3M ESPE). With either cement, the intaglio surface of the traditional zirconia is sandblasted, ie, air abraided, with 30-µm to 50-µm alumina for 10 seconds at 30 psi (2 bar) to clean the bonding surface, which has been contaminated with phosphate-containing saliva that blocks the bonding sites to zirconia. Sandblasting with alumina cleans and creates micromechanic retention in the zirconia and produces the greatest long-term bond. If the newer translucent zirconia is bonded, sandblasting the surface with alumina weakens the flexural strength of the restoration and glass beads should be used to clean the surface and produce some microscopic roughening of the intaglio surface. The Panavia V5 cement uses two primers, one for the ceramic and the other for the prepared tooth. Both are applied and dried, and the zirconia restoration is filled with cement and pressed into place. RelyX Ultimate is used similarly as ScotchbondTM Universal Adhesive (3M ESPE) applied to the tooth surface and the ceramic surface. In this situation, Scotchbond Universal was applied to the tooth and the restoration intaglio surface was air dried. The restoration was filled with the RelyX Ultimate cement cement and inserted over the prepared tooth.

Cleanup can be accomplished in two ways. The method I favor is to begin the removal of the cement with a brush and floss interproximally while applying firm pressure on the occlusal surface of the restoration. After the cement is removed, all light-accessible margins are light cured to produce the most color stable, wear-resistent margins. The second method to accelerate cement removal is to tack cure the cement with a curing light with a 1- to 3-second cure. Apply firm pressure on the occlusal surface of the restoration during cement removal and gently pull off the excess marginal cement. Do not remove the occlusal pressure on the crown during cement removal or the bond between the cement and the tooth will be reduced.

Gerard J. Chiche, DDS
Thomas P. Hinman Endowed Chair of Restorative Dentistry; Director, Augusta University College of Dental Medicine, Center for Esthetic and Implant Dentistry, Augusta, Georgia

It is generally accepted that zirconia restorations should be cemented using conventional cements such as RMGI cements. The primary reason is that zirconia has sufficient strength to withstand normal masticatory forces and does not need further reinforcement from bonding. This approach is valid under two conditions: 1) the tooth preparation has sufficient retention and resistance; and 2) the zirconia restoration has enough thickness (ideally 1.0 mm at the occlusal aspect). When the clinical situation deviates significantly from either parameter, then an adhesive technique becomes necessary to provide more adhesion to a preparation with compromised retention, or more strength to a crown with compromised thickness. A clinician seeking to increase the adhesion of a zirconia restoration to tooth structure should address three areas.

The first would be tooth-preparation management. We initially clean the tooth surface with a slurry of pumice and water. This is followed by sandblasting with small-particle size (25 µm to 30 µm) using either alumina or silica-treated alumina (CoJet™ System, 3M ESPE).

Next, zirconia-restoration management should be considered. We expect the dental laboratory to sandblast the crown using alumina (30 µm to 50 µm). After try-in, the crown is ultrasonically cleaned in acetone or alcohol for 5 minutes. Then, it is either sandblasted again or coated with Ivoclean (Ivoclar Vivadent). Finally, a ceramic primer containing 10-MDP monomer is used to coat the intaglio (eg, Clearfill™ Ceramic Primer, Kuraray Dental, kuraraydental.com; Monobond S Plus, Ivoclar Vivadent; Z-Prime™, Bisco, bisco.com).

The final area would be in regard to adhesive and cement management. When using a resin cement, prepack a small retraction cord in the crevice to contain cement residues and limit the intrasulcular depth of mechanical cleaning with a scaler or scalpel. If a self-etch adhesive is employed, it should be imperatively used in conjunction with the cement paired by the manufacturer so that they chemically cure together (eg, Scotchbond Universal/Rely X Ultimate; Multilink; Clearfil Universal Bond/Panavia SA Plus). It is also imperative to strictly follow the manufacturer’s recommendations because some advocate placing the adhesive both on the tooth preparation and the crown intaglio, while others advocate placing the adhesive on the tooth preparation only. If no adhesive bottle is used, then a self-adhesive cement should be used (eg, RelyX Unicem 2; Panavia SA Plus; Maxcem Elite™, Kerr Dental, kerrdental.com). While using these cements alone represents a simple option, it is critical to ensure that the dentin is moist prior to seating the crown to provide dentinal adhesion with the self-adhesive cement. It is also critical with both above options (with or without self-etch adhesive) to compress the crown during the entire time the cement is being cleaned, in order to avoid micromovements as these could significantly reduce the bond to tooth structure. Finally, once the adhesive and cement excess are cleaned off the margin periphery, the cervical area should be light cured for a minimum of 20 seconds to increase the hardness and mechanical strength of the cement at the margins. A final inspection for any cement excess is recommended after light-curing is completed.

J. William Robbins, DDS, MA
Private Practice, San Antonio, Texas

My recent experience has reinforced my beliefs. I inserted a full upper arch of porcelain crowns. Teeth Nos. 5 to 12 were being treated with lithium disilicate indirect restorations, and the rest were receiving layered zirconia. I bonded the lithium-disilicate crowns with a self-adhesive resin cement and cemented the layered zirconia crowns with a RMGI cement. This is my bias: I love cementing restorations and dislike bonding restorations. Granted, I have been bonding porcelain veneer restorations for 30 years, but that is different. I bond them, a few at a time, and the procedure is controlled. However, bonding crowns is distinctive. If bonded 2 at a time, the risks include getting resin cement on adjacent preparations and not placing crowns in exactly the correct alignment. When 6 or 8 crowns are bonded simultaneously, the alignment is correct, but the cleanup is tedious, difficult, and time-consuming. The problems are exponentially complicated if the restoration is in the posterior part of the mouth. It is much more difficult to isolate posterior teeth for bonding procedures and the cleanup is challenging. I understand lithium disilicate can be cemented. However, the restorations are clearly not as strong, especially if an endodontic-access preparation is required in the future. With current technology and my personal laboratory situation, I obtain more beautiful crowns with lithium disilicate; hence, that is my choice for the anterior. However, I routinely place layered zirconia crowns in posterior areas, primarily because they do not require bonding and they can be cemented.

As a profession, we are moving away from full-coverage posterior restorations to partial-coverage restorations. With current advances in technology, these restorations are generally bonded. Nonetheless, it is my belief that these restorations must have 100% enamel margins, and that the margins should almost always be supragingival. Therefore, the access for isolation, bonding, and cleanup is much easier for partial coverage compared with full-coverage restorations.

“Just remember the A-P-C steps. 1) air-particle abrade (A) the bonding surface with aluminum oxide; 2) apply special zirconia primer (P); and 3) use dual- or self-cure composite resin cement (C).”

Markus B. Blatz, DMD, PhD