Abstract: Zirconia has been used in clinical dentistry for several years. However, a recent surge of interest in metal-free alternatives in esthetic/restorative dentistry has led to the increased use of zirconia-based restorative materials due to the material’s strength, versatility of clinical indications, and ability to be CAD/CAM-milled, as well as the increasingly high costs of precious metals. Zirconia, or ZrO2, is a silica-free, acid-resistant, polycrystalline ceramic that has no amorphous silica glass component. Traditional ceramic surface treatments, such as hydrofluoric acid etching and/or silane primer application, are ineffective on the silica-free surfaces of zirconia, alumina, and metal. For slightly retentive or nonretentive designs, traditional adhesive protocols are required. Optimizing adhesive performance in less-than-retentive preparation designs requires both the tooth and indirect substrate be addressed with specific adhesives and primers. Dentin and enamel demand the use of self-etch or total-etch adhesives, such as All Bond® SE (Bisco, https://www.bisco.com) or All Bond® 3 (Bisco). On the indirect substrate, research supports the use of a 10-methacryloxydecyl dihydrogen phosphate (MDP)-containing primer on the zirconia surface, such as Z-Prime™ Plus (Bisco). Hydrophobic dual-cure resin cements, such as Duo-Link™ (Bisco), are used to facilitate a cohesive interface between both substrates. When preparation designs are fully retentive and strong adhesion is not critical, self-adhesive resin cements, such as BisCem® (Bisco), Maxcem Elite™ (Kerr, https://www.kerrdental.com), RelyX™ Unicem (3M ESPE, https://www.3mespe.com), and SmartCem™2 (Dentsply Caulk, https://www.caulk.com), can be used. Self-adhesive resin cements are dual-cured and contain organophosphate monomers, which have been shown to have a mild affinity for bonding to zirconia, but lack the strength of traditional resin cements. Alternatively, glass ionomer cements have minimal bond strengths (4 MPa) to zirconia, but are more susceptible to water degradation due to the nature of the chemistry.
Adhesion is created. The goal of replicating the cohesive hydrophobic interface (dentinoenamel junction) with the use of resin luting cements depends on the clinician’s addressing the individual needs of the tooth substrate (dentin, enamel) and indirect substrates (zirconia, alumina, ceramic, and metal). Adhesive bonding agents on the tooth substrate and primers on the indirect substrate are critical to optimizing this cohesion.
While zirconia has been used successfully in clinical dentistry for several years,1-8 the difficulty of creating adhesion to non-silica-based oxide ceramics such as zirconia and alumina has limited the use of these materials.9-14 However, the understanding of zirconia is evolving. Zirconia is a silica-free, acid-resistant, polycrystalline ceramic. It does not contain amorphous silica glass (unlike feldspathic porcelain, leucite-reinforced ceramics, and lithium disilicate ceramics), thus traditional ceramic surface treatments, such as hydrofluoric etching followed by silane application, are ineffective.9-12,14 The use of pyrochemical (Pyrosil®, SURA Instruments, https://www.sura-instruments.de)15,16 or tribochemical treatments (CoJet/Rocatec, 3M ESPE)12,14,17-22 to create a pseudo-silane attached surface is an alternative. Literature studies and unpublished internal research at Bisco Dental Products on pyrochemical and tribochemical bonding showed neither method offered improved bonding and they could be prone to degradation. However, other research has demonstrated tribochemical bonding with the use of primers improved bonding.19
Glass ionomer cements have minimal bonding strengths to zirconia (4 MPa) and are susceptible to water degradation due to their chemistry.23-26 Phosphate monomers in self-adhesive cements have been proven to be effective for adhering to the non-silica-based polycrystalline materials of zirconia, alumina, and metal. Based on this knowledge, primers specific to zirconia, alumina, and metal were created. Numerous studies have shown phosphate monomers are promising chemical agents for improving zirconia bonding.17-20,27,28 The possible mechanism is the ability of phosphate monomers to form chemical bonds with zirconia, alumina, and metal surfaces, and have polymerizable resin terminal end groups (eg, methacrylate), which enable cohesive bonding to appropriate resin cements (Figure 1).28,29
Self-adhesive resin cements, such as BisCem, RelyX Unicem, and SmartCem 2, are dual-cured, contain organophosphate monomers, and can be used when preparation designs are fully retentive; however, these cements are hydrophilic due to the acidic resin components and have lower physical and mechanical properties than resin cements. Self-adhesive resin cements differ in viscosity, smell, and self-curing efficiencies. Bond strengths are lower than those of bonded resin cements (with zirconia primer). However, in retentive preparations, the ease of placement is a compelling benefit (Table 1). Self-adhesive resin cements may not be strong enough to be used alone on both surfaces (tooth and zirconia) when cementing a nonretentive zirconia restoration. Primers may be beneficial for improved adhesion of self-adhesive cements to zirconia.
For slightly retentive or nonretentive designs, traditional adhesive protocols are time-tested and required. Optimizing adhesive performance is the goal in less-than-retentive preparation designs and demands the use of dentin adhesives, including self-etch (ACE/All Bond SE, Bisco) or total-etch (All Bond 3), primers specific to zirconia/alumina/metal (Z-Prime Plus), and the use of dual-cure hydrophobic resin cements (Duo-Link).
Primers that address the specific needs of nonsilica oxides (zirconia and alumina) and metals are highly beneficial and warranted for restorations when the retention/resistance form is compromised. In laboratory testing, ceramic and metal primers have been shown to be important to the success of bonding to these indirect materials. Clinical experience with primers has indicated improved bonding to both direct and indirect substrates (Table 2).
Five ceramic primer systems intended for use with zirconia are on the market: AZ-Primer (Shofu Dental, https://www.shofu.com), Clearfil Ceramic Primer (Kuraray America, https://www.kuraraydental.com), Metal/Zirconia Primer (Ivoclar Vivadent, https://www.ivoclarvivadent.com), Monobond Plus (Ivoclar Vivadent), and Z-Prime Plus (Table 2). Z-Prime Plus is a proprietary formula containing both phosphate and carboxylate functional monomers. The synergistic coupling of these proven monomers results in stronger bonding coupled with improved durability as compared with other commercial primers. Enhanced bonds are shown in the laboratory setting to result in cohesive failures of luting cements rather than adhesive failures (Table 2 and Figure 2).
Most of the commercial primers contain phosphonate or phosphate monomers that bond to zirconia but differ depending on formulations and acidity levels (which is important for compatibility with self-cured and dual-cured modes of resin cements). Phosphate monomers form covalent bonds with the zirconia surface and have polymerizable resin terminal ends that copolymerize with the resin cements. Bond strengths are a function of the mode of curing, stability of the resin chemistry, and compatibility of the primer to cement. In addition, the potential for contamination of the substrate could be associated with the time required to institute the specific protocols of each ceramic primer: Metal/Zirconia Primer requires 3 mins, whereas Z-Prime Plus needs 10 secs.
Internal unpublished research at Bisco Dental Products showed Z-Prime Plus not only provides high bond strengths to zirconia with different surface treatments (ie, sandblasted or polished) but also is compatible with self-cured or light-cured cements, as shown in Table 2. Z-Prime Plus has been demonstrated to be a very versatile system, creating significantly enhanced bond strengths when used with competitors’ resin cements/composites (Table 2). In addition, Z-Prime Plus provided significant bond strengths (20 MPa ~ 30 MPa) to other indirect dental substrates, such as metals (eg, titanium, stainless steel, Rexillium III® [Pentron, https://www.pentron.com], gold), composites, and fiber posts.
Durable and reliable bond strengths are extremely important when bonding to dental restorations. Durable bonds are a function of the physical properties of luting cements. Luting cements that contain functional monomers or polyalkenoic groups (glass ionomer) are usually acidic, hydrophilic, and water absorptive when exposed in oral environments (ie, 37° C water), even after polymerization. Aging in boiling water can provide a fast and convenient method for predicting the durability of dental materials and dental bonding.30,31 Internal unpublished research at Bisco Dental Products showed the aging effect of 3 days in boiling water is equivalent to approximately 6.6 years at 37° C. The current study showed, following the harsh conditions of accelerated aging for 3 days in boiling water, the shear bond strength of Z-Prime Plus remained statistically the same, while all other commercial primers experienced a substantial decrease in bond strengths after aging (Table 3).
In addition to monomers for zirconia bonding, Clearfil Ceramic Primer and Monobond Plus contain a silane primer component, intended to expand their use to both zirconia and porcelain surfaces. Internal unpublished research at Bisco Dental Products has shown the formulas of Monobond Plus and Clearfil Ceramic Primer to be ineffective as porcelain primers alone, due to the instability of the silane component in the presence of acidic organophosphate zirconia monomers.32,33 Best results were obtained when the porcelain (glass-based oxide) was treated and bonded with a silane primer that was separate from zirconia primers (Figure 3). Bond strengths varied by brand and were dependent on the setting mode (light-cured/self-cured) of the resin cement, especially on aging. Z-Prime Plus and Duo-Link Resin Cement are the only commercial systems that do not require refrigeration.
The most important factor in bonding to zirconia is the polymerization (setting) properties of resin cements. In theory, the use of hydrophobic resin cements with polymerization properties that allow timely cleanup (5 to 12+ mins in self-cured mode) is better. The dual-cured mode is preferred over light-cured-only esthetic resin cements, removing the potential for limited light transmission through opaque zirconia copings. All dual-cured cements are not created equal. The ideal dual-cured cement performs equally well in light-cured and self-cured modes, is unaffected by aging, and has an appropriate setting time. A dual-cured resin cement that fully polymerizes in the self-cured mode within 6 mins allows for interproximal flossing, whereas one that sets in 10 to 12 mins requires appropriate measures not to interfere with the bond development. Internal testing at Bisco Dental Products supports the data showing self-cured modes of some resin cements significantly differ and many are affected by the material’s aging.
The ideal protocol for luting a zirconia crown onto a preparation lacking resistance/retention form would be:
1. Treat the zirconia surface with an MDP-based zirconia primer, such as Z-Prime Plus.
2. Treat the dentin and enamel surface with a bonding agent, such as total-etch adhesive All Bond 3 or self-etch adhesive All Bond SE.
3. Apply a dual-cured resin cement, such as Duo-Link Cement.
The cohesive linking of the direct and indirect substrates with the hydrophobic resin cement optimizes both bonding and sealing. In preparations with adequate retention/resistance form, self-adhesive resin cements such as BisCem or Clearfil SA Cement offer acceptable strength.
Understanding the interactions of resin cements, adhesives, and primers when used with zirconia, alumina, and metal is important for long-term success in esthetic restorative dentistry.
Dr. Suh is president of Bisco, Inc. Drs. Chen and Brown are employed by Bisco, Inc.
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