The use and popularity of both zirconia and lithium disilicate have increased dramatically over the last several years (Fig. 1). In fact, if current trends hold¹ it is entirely possible that that the escalating use of both zirconia and lithium disilicate will soon lead to the demise of traditional PFM’s. This article will focus specifically on zirconia. Zirconia has many positive attributes not the least of which is high strength. In fact, the flexural strength of monolithic zirconia crowns can be anywhere from five to well over 10 times that of conventional PFM’s.2-4 Likewise, the fracture toughness (ability to resist crack propagation) of zirconia is significantly higher than both lithium disilicate and PFM restorations.5 In addition, zirconia can be bonded or conventionally cemented, is very wear-friendly to opposing tooth structure when properly polished6-8, is compatible with CAD-CAM technology, and can be used in the monolithic form to maximize strength or as a supportive substructure and layered with various ceramics to optimize esthetics. This article will focus on just what zirconia is, its advantages and disadvantages, recent improvements in optical properties, misconceptions some may have, how to optimize the zirconia surface prior to placement, and various cementation options.
What is zirconia?
Zirconia is often referred to as “white metal” or “white steel”. This terminology may have originated with the introduction of razor-sharp zirconia knives used as cutlery with blades that are typically harder and more corrosive resistant than those of their steel counterparts (Fig. 2). Of course, zirconia is not steel. In fact, it is not even a metal. Zirconia is the oxide of the element zirconium (element 40 in the periodic table). While the element zirconium is a tough hard silvery white metal (Fig. 3) its oxide, the zirconia we use in dentistry, is not. Zirconium is not found in nature in its pure elemental form. It exists combined with other elements forming minerals such as zircon (ZrSiO4) and baddeleyite (mostly ZrO2). These and other minerals are mined, refined, and purified through a number of complex physical and chemical processes to produce zirconium oxide powder (ZrO2) (Fig. 4). This powder (a polycrystalline ceramic) can be shaped, pressed, and fully or partially sintered to create zirconia pucks and blocks (Fig. 5) that can be milled using CADCAM technology to create zirconia dental restorations and supporting frameworks.
Types of zirconia
Zirconia exists in three distinct temperature and pressure dependent crystalline configurations or phases (monoclinic, tetragonal, and cubic). At normal temperatures and pressures zirconia exists in the monoclinic crystalline form. While this is the most stable configuration of zirconia it lacks the physical properties required for dental use. When heated to approximately 1170°C the monoclinic powdered form of zirconia will coalesce into a solid (sintering). During this process the zirconia crystals undergo a phase transformation from the monoclinic to the tetragonal crystalline configuration (Fig. 6). This form of zirconia is very strong, biocompatible, corrosion resistant, can be milled, and has the physical properties necessary for use as a dental restorative or supporting structure. When further heated to approximately 2,100°C another phase transformation occurs and the tetragonal crystals transform to the cubic crystalline configuration forming a very hard, translucent, and somewhat brittle material widely known as cubic zirconia (Fig. 7).
One of the problems with the tetragonal form of zirconia used in the fabrication of dental restorations is it is not inherently stable and readily converts back to its weaker, more stable, and lower energy state monoclinic crystalline form. Various dopants such as yttrium oxide (Y2O3) and aluminum oxide (Al2O3) are added in very small amounts to stabilize the tetragonal crystalline lattice as well as modify optical properties. This so called “yttria-stabilized zirconia” is in fact, not entirely stable. It is actually “metastable” meaning that given the right conditions the tetragonal crystals can convert back into their monoclinic configuration. On a small scale this is actually a desirable property as it makes zirconia resistant to crack propagation via a property called transformation toughening. Simply stated, as a crack is initiated on the zirconia surface and begins to propagate there is a localized conversion of the tetragonal crystals around the advancing crack front back into the monoclinic form. This results in a localized volumetric expansion of the crystals (around 4%) surrounding the crack essentially compressing and sealing the defect and mitigating further crack advancement. So, on a small scale the metastable nature of zirconia can be a positive attribute due to transformation toughening. However, on a large scale it can be a negative as excessive tetragonal to monoclinic phase transformation can weaken overall assembly strength potentially leading to catastrophic failure. Indeed, in the early part of this century thousands of Prozyr zirconia femoral heads (used in hip replacement surgery) were recalled due to numerous instances of spontaneous fracturing of the zirconia femoral heads within 27 months of placement (expected lifespan was at least 10 years). The failures were attributed in part to minute changes in processing temperatures during manufacture that resulted in excessive tetragonal to monoclinic phase transformation over-time9-11. These failures highlight the importance of using zirconia produced by reputable manufactures (all zirconia is not created equally).
To take full advantage of the physical properties of zirconia it makes sense to use them as full contour monolithic restorations whenever possible. That is, to avoid layering or pressing ceramics over zirconia (as is often done to optimize esthetics) because the layering ceramic, along with the interface between the zirconia and layered ceramic, are weak links in the restorative assembly. In addition, monolithic zirconia restorations that can be milled full contour keep costs low as no additional time is required for layering by a ceramist. While high strength monolithic zirconia restorations make sense, esthetics can be an issue in the anterior regions of the mouth where the inherently high value and stark white colour of some monolithic zirconia restorations, coupled with a lack of translucency and fluorescence, can make them unsuitable when optimal esthetics is required. The esthetics of monolithic zirconia has improved significantly in recent years with the introduction of so-called “translucent” zirconia. Manufacturers employ a number of techniques to improve the translucency of zirconia including reducing grain size, reducing alumina content, modification of processing and pressing techniques, altering sintering times and temperatures, and manipulating dopant levels to increase the percentage of the more translucent cubic crystals relative to the more opaque tetragonal crystals. The trade-off is as the translucency increases, flexural strength and fracture toughness decrease. This is because as the percent of cubic phase crystals is increased (typically by increasing the yttria concentration) to improve translucency, there are relatively fewer tetragonal crystals available for transformation toughening should a crack or defect develop in the zirconia (cubic crystals do not undergo transformation toughening). This makes translucent zirconia more vulnerable to crack propagation. In addition, as the cubic phase is increased surface preparation such as sandblasting can further decrease physical properties. To mitigate this, some researchers recommend that the surfaces of translucent and conventional high strength zirconia be handled somewhat differently prior to placement.12 Additionally, some researchers recommend that translucent zirconia restorations be adhesively bonded with resin-based cement (as opposed to conventionally cemented) during placement to maximize assembly strength.13 Both these issues will be discussed in greater detail below. It should be pointed out that even though the physical properties of translucent zirconia are less than that of their higher strength and more opaque zirconia counterparts, they still exceed those of lithium disilicate and traditional PFM restorations. Translucent zirconia (5 mol.% yttria concentration) is typically about midway between traditional high strength zirconia (3 mol.% yttria) and lithium disilicate in terms of both strength and translucency.14
Sandblasting zirconia prior to placement
It is the author’s strong opinion that the zirconia surface should be particle abraded (sandblasted) prior to placement no matter what type of conventional or resin-based cement is used. There is significant support in the literature for this recommendation.15-18 While there is some concern that sandblasting has the potential to induce surface and subsurface cracks and/or defects that could reduce physical properties19,20, the author is unaware of any studies that demonstrate this to be a clinical problem assuming appropriate blasting pressures, particle types, and particle sizes are utilized. In fact, some studies found that sandblasting actually increases the flexural strength of zirconia (due to transformation toughening).12,21 Sandblasting zirconia is useful in terms of cleaning the target surface of impurities, increasing surface roughness and surface area, raising surface energy, improving the bond to zirconia primers and adhesives, and generally optimizing the surface prior to bonding or conventional cementation.22 Having said this, the term “sandblasting” is very ambiguous. Sandblast with what exactly? At what pressure and distance? The general consensus among researchers and opinion leaders is that traditional high strength zirconia (3-4 mol% yttria concentration) can be safely and effectively sandblasted with 30-50 micron aluminous oxide (Al2O3) using a blast pressure of 1.5-2.8 bar (approximately 20-40 psi) from a distance of 1-2 cm and for a duration of 10-20 seconds.18,23,24 In addition, some researchers recommend the nozzle head be held at an angle of approximately 60 degrees.24 When dealing with translucent zirconia (5 mol% yttria concentration) that has a reduced capacity to undergo transformation toughening, blasting pressures should be in the lower range (approximately 20 psi) to minimize any surface damage that could lead to a reduction in physical properties. Burgess and McLaren have suggested using 50-micron silica glass beads as an alternative to harder aluminum oxide particles when sandblasting translucent zirconia.25 Their testing showed no reduction in the physical properties when glass beads were used. Additional testing is needed to confirm and refine optimal protocols for sandblasting both translucent and high strength zirconia.
As far as when to sandblast, the author prefers to sandblast the intaglio surface of zirconia restorations after try-in and any adjustments, and just prior to conventionally cementing or adhesively bonding the restoration into place. In this regard, the author strongly recommends that dentists invest in a quality chair-side sandblasting unit (i.e. Microetcher II, Danville Materials) and dust cabinet with a built-in fan filtration unit (i.e. Microcab, Danville Materials). If the dentist does not have a sandblaster then the author recommends having the dental laboratory sandblast the restoration just before shipment. Of course, this requires a high degree of faith that the dental lab is sandblasting the zirconia correctly. Once the zirconia restoration is ready for placement dentists have several important decisions to make: 1) Should the restoration be conventionally cemented or adhesively bonded into place? 2) Should a separate zirconia primer such as 10-Methacryloyloxydecyl Dihydrogen Phosphate (10-MDP) be applied at some point after sandblasting? 3) Is it necessary to clean the zirconia surface before cementation with alkaline solutions such as Ivoclean (Ivoclar) or ZirClean (BISCO)?
Are alkaline cleaning solutions necessary prior to zirconia primer application?
It is the author’s strong opinion that if you want to predictably and durably bond to zirconia with resin-based cements then it must be sandblasted and a zirconia primer placed. The primer can take the form of a separately applied solution that contains a phosphate ester zirconia primer such as 10-MDP (i.e. Z-Prime/BISCO, Monobond Plus/Ivoclar, Clearfil Ceramic Primer Plus/Kuraray, various universal adhesives22, etc.) or by using a resin cement that incorporates a zirconia primer directly in its chemical makeup (i.e. PANAVIA SA Cement Plus Kuraray, Unicem 2/3M ESPE, TheraCem BISCO). A recent study found that 10-MDP zirconia primers chemically interact with the zirconia surface by both hydrogen and ionic bonding mechanisms.26 This chemical interaction requires that terminal phosphate groups in zirconia primer molecules such as 10-MDP (Fig. 8) can freely interact with reactive sites on the zirconia surface. Zirconia has a remarkable affinity for phosphate ions.27 This affinity extends not only to the phosphate groups in zirconia primers but also to phosphate groups and ions that are inherent in saliva. When zirconia restorations are tried in and the intaglio surface is contaminated by saliva, the phosphate ions from the saliva bind to, and occupy, the same reactive sites that zirconia primers require for chemical interactions. This competition for reaction sites significantly decreases the efficacy of zirconia primers and it is necessary to “free-up” these sites so zirconia primers can function optimally. This can be accomplished by sandblasting the restoration after saliva contamination and/or the use of strongly alkaline cleaning solutions such as Ivoclean (Ivoclar Vivadent) or ZirClean (BISCO). It should be pointed out that vigorous rinsing with water, or the use of acetone and alcohol, is not effective in cleaning zirconia surfaces that have been contaminated with saliva.28 Products such as Ivoclean and ZirClean essentially work by having a higher affinity for phosphate ions than does the zirconia itself. In effect, the cleaning agent chemically scavenges phosphate ions from the zirconia surface and in so doing frees up reaction sites that now become available for chemical interaction with subsequently placed zirconia primers. If the dentist is sandblasting the zirconia restoration themselves (after it is tried in and just prior to placement), then the use of a separate cleaning agent is not necessary (but still an option) as the sandblasting alone is sufficient in terms of freeing up reaction sites. If the dentist does not have a sandblaster, and had the dental lab sandblast the restoration before shipment, then the restoration SHOULD be treated with a cleaning solution such as Ivolean or ZirClean (after it has been tried in and prior to primer application). To reiterate, studies show that the best way to treat saliva-contaminated zirconia surfaces is by sandblasting and/or the use of strongly alkaline cleaning solutions such as Ivoclean or ZirClean.29,30 Two last important notes: 1) While phosphoric acid (H3PO4) is an effective cleaning agent for saliva-contaminated silica-based ceramics (such as stacked porcelain and lithium disilicate), it is contraindicated for cleaning zirconia surfaces. This is because, just as in the case of saliva, the phosphate ions from the phosphoric acid remain bound to the zirconia surface (even after rinsing) and tie-up reaction sites required for chemical interaction with zirconia primers. 2) While silane is an effective primer for silica-based ceramics31 it is not effective for priming zirconia surfaces (use phosphate ester primers such as 10-MDP).
Cementing and bonding zirconia restorations
The fact is there is not one specific universal protocol to use when it comes to the placement of zirconia restorations. The optimal way to treat both the zirconia and tooth surfaces prior to placement is contingent on many factors including, the specific clinical conditions, how retentive the preparation is, the nature of the conventional or resin-based cement being used, the minimum occlusal thickness, whether the dentist or the lab sandblasted the zirconia, the type of zirconia being placed (conventional vs. translucent), and esthetics (will the colour of the cement affect the esthetic result). As previously discussed and as a general rule, the author recommends that the intaglio surface of all zirconia restorations be particle abraded (sandblasted) and a zirconia primer placed (typically a phosphate ester such as 10-MDP). However, this is not true in every situation and the use of a separate zirconia primer is actually contraindicated or not necessary with some materials. As an example, Ceramir C&B (DOXA Dental) is a “bioactive” glass ionomer/calcium aluminate hybrid cement that is very hydrophilic in nature. Hydrophilic surfaces generally interface well with other hydrophilic surfaces (“like likes like”) but are generally less or non-interactive with hydrophobic surfaces. For example, water (hydrophilic) does not mix well with oil (hydrophobic). Properly sandblasted zirconia has a high energy hydrophilic surface. Once a zirconia primer is placed the hydrophilic surface becomes hydrophobic (Fig. 9). This is advantageous when using methacrylate-based resin cements that are also hydrophobic but can be a detriment with hydrophilic non-resin containing materials such as Ceramir C&B. Indeed, there are a number of anecdotal reports of zirconia crowns loosening or falling out when a zirconia primer was used prior to cementation with Ceramir C&B. For those dentists using Ceramir C&B, the zirconia surface should still be sandblasted to optimize surface conditions, but a zirconia primer should NOT be used.32 Likewise, conventional glass ionomer cements (i.e. Fuji II/GC, Ketac CEM/ 3M ESPE) do not require the use of a separate zirconia primer.
However, zirconia primers (i.e. Z-Prime, BISCO, Monobond Plus, Ivoclar) have been shown to increase bond strength of zirconia to both RMGI33,34 and methacrylate based resin cements.35 RMGI (resin-modified glass ionomer) cements have many positive attributes including good physical properties, low solubility, some chemical bond to tooth structure, low film thickness, fluoride release, anti-microbial activity, good long-term clinical track record, and low incidence of postoperative sensitivity. Perhaps the biggest clinical advantages of RMGI cements is that they are very easy to mix, place, and clean. In fact, cement cleanup is generally much easier compared to resin cements, and this fact alone makes RMGI an attractive cementation option. Indeed, according to a 2018 survey of 1,026 dentists, RMGI cements (i.e. Rely X Luting Plus/3M, FujiCEM 2/GC) are currently the most popular cement type used in North America36 (Fig. 10). The author personally considers RMGI to still be one of the best cementation options for high strength zirconia assuming the preparations have proper resistance and retention form and a minimum occlusal thickness of 1 mm or more. Even though studies appear to support the application of a separate zirconia primer after sandblasting to enhance the bond of RMGI to zirconia, the actual clinical relevance and benefit of this extra step is unclear and open for debate. The author’s personal preference, at least at this time, is to apply a separate zirconia primer (Z-Prime, BISCO) after sandblasting when cementing zirconia restorations with a RMGI. The author also recommends a warm air dryer be used to evaporate primer solvents from the zirconia surface after primer application. Warm/dry air is simply very effective at removing solvent carriers and by “heating up” the substrate one can speculate that reaction rates will be accelerated, molecular interactions become more frequent, and greater numbers of chemical bonds are formed.
In situations where there is a lack of resistance and retention form, esthetics is an issue, or maximum adhesion is required, then self-etching self-priming resin-based cements (i.e. RelyX Unicem/ 3M ESPE, Maxcem/Kerr, Bis-Cem/BISCO, G-Cem/GC) or resin-based cements used in conjunction with a dentin bonding agent (i.e. Duo-Link/BISCO, RelyX Ultimate Adhesive Resin Cement/3M ESPE, Multilink/Ivoclar) are preferable over conventional cements. Resin-based cements have a distinct advantage over RMGI and other conventional cements when it comes to bonding restorations on, or in, minimally retentive preparations as they bond more durably and predictably to both tooth tissues and zirconia. In addition, they may be a better choice when dealing with translucent zirconia or zirconia restorations with minimal occlusal thickness as resin-based cements allow better stress distribution when loaded, may inhibit crack formation, and generally optimize overall assembly strength.25 On the downside, resin-based cements can be difficult to clean, are more technique sensitive, and require extra steps when used in conjunction with a separately placed bonding agent. While dual cure self-etching self-priming resin cements are popular with dentists because they do not require a separate bonding agent be placed on the tooth, dentists should be aware that the highest bond to tooth structure is achieved by the use of resin cements used in conjunction with a separately placed bonding agent. In fact, some studies have found that even the self-etching/priming cements (such as Unicem 2) that are designed to be used without a separate bonding agent perform better, in terms of bond strength to tooth structure, when a separate bonding agent is placed on the tooth first.37-39 There is some ambiguity as to the necessity of using a separate zirconia primer with some resin-based cements. This is because some self-etching resin cements such as Panavia SA Cement (Kuraray) and Unicem 2 (3M ESPE) already contain a phosphate ester zirconia primer inherent in their formulations. This may preclude the need for a separate dedicated zirconia primer. Indeed, some of these materials have shown promise bonding to both tooth tissues and zirconia without using a separately placed adhesive or primer.40,41
A misconception held by many dentists is that “you cannot bond to zirconia.” The fact is you can bond very predictably and durably to zirconia surfaces using a combination of sandblasting, a phosphate ester primer such as 10-MDP, and an appropriate resin-based cement42-46 (Figs. 11-13). The optimal way to treat zirconia and tooth surfaces prior to placement of zirconia restorations is contingent on many factors including, the specific clinical conditions, how retentive the preparation is, the nature of the conventional or resin-based cement being used, the minimum occlusal thickness, whether the dentist or the lab sandblasted the zirconia, the type of zirconia being placed (conventional vs. translucent), and esthetics (will the colour of the cement affect the esthetic result). Proper management of both the zirconia substrate and tooth tissues is crucial for predictable and durable clinical outcomes. As a general rule the intaglio surface of all zirconia restorations be particle abraded (sandblasted) and a zirconia primer placed (typically a phosphate ester such as 10-MDP). However, this is not true in every situation, and the use of a separate zirconia primer is contraindicated or not necessary with some materials. In this regard manufacturer instructions and recommendations should be followed precisely for optimal results. It is incumbent on all clinicians to familiarize themselves with optimal cementation options and protocols when placing zirconia restorations.
Oral Health welcomes this original article.
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About the Author
Dr. Alex is an accredited member of the American Academy of Cosmetic Dentistry and past president of the AACD New York Chapter. With a background in chemistry and adhesive technology, he is a consultant for numerous dental manufacturers and member of the IADR. He has studied occlusion extensively with Dr. Peter Dawson (Center for Advanced Dental Study) and the late Dr. Bob Lee (Lee Institute) and is a member of the AES (American Equilibration Society). He has been the director of “PAC Live Ultimate Occlusion” and “Aesthetic Advantage Occlusion and Comprehensive Dentistry” programs. He is co-founder of the “Long Island Center for Advanced Dentistry”.