Surface Treatments of Zirconia to Enhance Bonding Durability

The progressive improvement in the properties of dental ceramics has led to an increase in metal-free restorations. These properties include improved esthetics, chemical resistance, hardness, compression resistance, and biocompatibility.^1-3 

Several studies^4-7  have investigated the bond strength and the durability of various bonding methods to dental ceramics. Bonding to traditional silica-based ceramics, generally employing both mechanical and adhesive retention, has been well researched, and bond strengths are predictable.

While hydrofluoric acid (HF) etching along with methacryloxypropyl trimethoxysilane (MPS)application is a commonly recommended method used to roughen the surface of silica-based ceramics and to increase their wettability,⁶  zirconia is a polycrystalline nonetchable material.^8-12  Adhesion is still an issue, since a very low and unstable resin bond is promoted when the Y-TZP ceramic is untreated or has received primer application only.^13-16  Clinically, crown debonding (loss of retention) is a type of failure of zirconia-based restorations, and the search for surface treatments that improve resin adhesion to zirconia has increased in intensity.¹²  In spite of that, there is little information about the most effective and durable bonding methods,^15,16  namely the new treatments such as nano-film deposition of silicon oxides,^17,18  the glaze-on technique (application of a thin, low-fusing glassy porcelain layer rich in silicon oxides),^19,20  heating silanes,²¹  and chemical etching used for bond improvement to zirconia.^22,23  A previous report²⁴  of the literature showed that zirconia bonding was durable when air abrasion and a 10-methacryloxydecyl dihydrogen phosphate (MDP) component were combined, but this report was less informative with regard to the new surface treatments.

Thus, the aim of this review article was to search for the long-term response of the surface treatments used on Y-TZP zirconia surfaces for adhesion, from the most traditional to the newest ones, so that clinicians can discern which can provide the most durable zirconia restorations. Medline database (PubMed) was used as the main source of information for this literature review. The terms used were “zirconia and bond strength durability” and “zirconia and surface treatments.” We excluded articles that were not in the English language and those that had been published before 1999, as well as articles regarding the durability of bonding between zirconia and veneering ceramics.

So far, the surface treatments most indicated for dental zirconia are chemical modification of the surface, micromechanical interlocking through air abrasion, or a combination of both. Ideally, utilizing chemical adhesion in addition to mechanical retention is required for zirconia, mainly because of zirconium dioxide's (ZrO₂) nonpolar surface, which leads to a nonstable (hydrolyzable) interface.¹⁸ 

Inokoshi and others²⁵  evaluated the effect of mechanical (tribochemical silica coating/Cojet and Rocatec, 3M ESPE, Seefeld, Germany) and chemical (silane/MDP–combined ceramic primers) surface pretreatment on the bond durability of two composite cements (bisphenol A diglycidyl ether dimethacrylate [Bis-GMA]–based and MDP-based cements) to dental zirconia. The combined mechanical and chemical surface pretreatment of zirconia improved the bond durability of both composite cements bonding to zirconia after storage for six months.

Attia and Kern²⁶  investigated the durability of the bond strength of adhesive luting cement to zirconia ceramic after the application of different ceramic primers. They concluded that after storage for 150 days in water without thermal cycling, a new universal primer provided significantly better long-term resin bonding to zirconia ceramic than did a conventional silane. They also concluded that cleaning methods had little effect on long-term resin bonding to zirconia ceramic. They also showed that a combined mechanical and chemical pretreatment is the best recommendation for a durable bond to zirconia.

There is some evidence that a minimum bond to Y-TZP ceramics is obtained using resin cements with phosphate ester monomers (MDP) alone. Wolfart and others²  evaluated the durability of the bond to zirconia with two resin cements (Bis-GMA–based and MDP-based) after using different surface conditioning methods. The use of the MDP-containing composite resin on air-abraded zirconia ceramic presented the highest bond strength to zirconia surfaces after 150 days of water storage and was recommended as a promising bonding method. On the other hand, when evaluating the shear bond strength between zirconia and the dual-cured resin cement, Yoshida and others²⁷  reported that the application of the mixture primer and zirconate coupler was effective.

As mentioned previously, air abrasion in combination with phosphate ester monomer (MDP)–containing luting agents results in high, durable bond strengths, because the phosphate ester group chemically bonds to metal oxides such as zirconium dioxide. Sarmento and others²⁸  also reported that when zirconia was air-abraded with aluminum oxide (Al₂O₃) (110 μm) it resulted in higher roughness values, but air-abrasion protocols with silicon dioxide (SiO₂) (110 μm; Rocatec, 3M ESPE) promoted better adhesion to MDP-based resin cement (Panavia F/Kuraray, Kurashiki, Okayama, Japan).

In addition to that, heat treatment of the zirconia pretreated with the silane primer also resulted in higher bond strengths and bonding durability than did those created with the use of acidic primers.²¹ 

Ferracane and others²⁹  compiled a series of studies about self-adhesive resin cements, showing that they have been extensively studied on zirconia surfaces. Among all cements, Unicem (3M ESPE, St Paul, MN, USA [currently marketed as RelyX U200]), which was the first cement to contain a methacrylated phosphoric ester as a functional monomer, was the most promising material in terms of adhesion to zirconia, presenting results comparable to those of Panavia F (Kuraray America, New York, NY, USA). However, the authors²⁹  stated that the bonding stability is better when silica coating is used and that the decrease in bond strength depends on the aging protocol used.

Some studies^19,20,30  reported that selective infiltration etching (SIE), based on ceramic infiltration by molten silica and posterior removal with hydrofluoric acid creating micromechanical irregularities, can enhance the zirconia ceramic-to-resin cement bonds. Aboushelib and others³¹  evaluated the zirconia/resin bond strength and durability using this technique and observed that the bond strength of the SIE specimens was stable and strong after 26 weeks of water storage and thermal cycling (51.9 MPa). They also demonstrated a good seal against silver nitrate penetration across the zirconia/resin interface.

The idea of infusing silica on zirconia, making it chemically reactive to the Bis-GMA–based cements, is also present in several other methods that either lack long-term studies or were not sufficiently stable after aging. Examples of such methods are the pyrosilpen method, which promises a firmly fixed adhesive silicate coating after flame treatment³² ; the “glaze-on technique” that uses a thin intermediary coating of acid-etchable glasses^33,34 ; the silica nano-coating; and³⁵  the sol-gel silica deposition.³⁶  A cold sol-gel process performed in the current authors' laboratory was used to infiltrate and grow a silica layer on pre-sintered zirconia surfaces that become etchable and treatable with silanes (Figure 1).

From the simplicity standpoint, the SIE looks very intricate and is still not accessible for more experimentation. The creator of the SIE method²⁰  stated “… SIE requires an investment of time and effort in order to achieve the required surface properties, and remains sensitive to the handling procedure during every step of the technique.”

On the other hand, the “glaze-on,” or vitrification, technique showed good bond strength results and can be easily performed, even though the vitrification of the intaglio surface of a Y-TZP–based crown might affect the seat of the crown, resulting in margin misfit. Unpublished results from this research group showed debonding of zirconia crowns with internal vitrification after 2 x10⁶ cycles, apparently between the cement and the zirconia intaglio (Figure 2). Thus, both of these surface treatments still need more investigation.

The application of an experimental hot etching solution (methanol: 800 mL; 37% hydrochloric acid: 200 mL; and ferric chloride: 2 g at 100°C) changed zirconia surface morphology (Figure 3) and induced a significant improvement in surface roughness, comparable with those of the well-known surface treatment modalities. This was first reported by Casucci and others,²²  who studied the influence of different surface treatments on the microtensile bond strength of resin cement to zirconia ceramic. They reported that conditioning the high-strength ceramic surface with SIE and experimental hot etching solution treatments yielded higher bond strengths than were obtained with airborne particle abrasion or untreated specimens. To date there are no long-term studies involving this method.

Other options for increasing bonding to zirconia are just not suitable for clinical applications, mainly because of the hazardous materials involved. An example is the Piranha solution (mixture of sulfuric acid and hydrogen peroxide) that was able to improve the hydroxylation and bond strength to adhesive monomers. But more intriguing than that was the use of 40% HF to improve bond strength.³⁷ 

Table 1 lists some of the most recent studies about surface treatments of zirconia for improved bonding and their long-term performance.

The treatments inflicted on the ceramic surface are as diverse as the methods of aging and the testing protocols. The only study in which all specimens failed before testing, after aging, was the one in which the topography was not altered and bonding was only achieved by chemical interaction.¹⁶  A few recent protocols had not been submitted to any type of aging at the time this review was performed.

With regard to the methods of testing bond strength, the tensile test was shown to be more sensitive in detecting differences among the bonding effectiveness of several surface treatments after aging.³⁸ 

One study was found that involved testing the load for debonding of zirconia crowns supported by teeth or composite core after thermal cycling and under tensile testing. The zirconia surfaces had been submitted to silicatization, or glazing, of the internal surface and had been cemented with various types of cement. The results showed that the surface treatment effectiveness depended on the resin cement used, as glazing improved the retention force for a 2-hydroxyethyl methacrylate–based cement.³⁹ 

A nondestructive method to treat the ceramic surface was the deposition of the oxide-fluoride phase on the zirconium oxide surface in a plasma reactor, which extracted HF in the presence of water and facilitated the Zr-hydroxylation. This improved the reaction with the silanes and, consequently, the bond strength, although no long-term results were presented. ⁴⁰ 

Although this review is about bond strengths to zirconia and their durability, it is important to recall that several studies observed that some surface treatments can affect the mechanical resistance of zirconia-based ceramics. Guazzato and others ⁴¹  investigated the influence of sandblasting, grinding, grinding orientation, and polishing on the flexural strength of zirconia ceramics. They observed that sandblasting and grinding increased the strength of dental zirconia. The increase in fracture strength after air abrasion was attributed to the residual compressive stress layer that promotes the transformation from the tetragonal to the monoclinic phase.^42,43  On the other hand, other studies ^44,45  reported a decrease in strength as a result of surface damage after air abrasion.

Moreover, air abrasion leads to the tetragonal and then to the monoclinic phase change on the surface of zirconia that in the long term can be detrimental to the restoration, not only because of the defects it creates ⁴⁵  but also because of the low temperature degradation suffered by zirconia. ⁴⁶  Thus, zirconia air abrasion with Al₂O₃ particles, large particles (>110 μm), and under high pressures (>3 bar) should be avoided, and an effective chemical component should be used. In the authors' search, no clinical studies about the specific effects of different surface treatments on zirconia restorations were found, signaling the urgent need for such trials.

Regarding the bond to zirconia, current literature states the following:

• 1. Silica coating of the surface via air abrasion of 30-μm particles associated to MDP-based primers or cements may result in more durable bonding;

• 2. SIE seems to be promising but not simple to perform;

• 3. The thin, low-fusing glassy porcelain application is also promising and simple to perform, even though new improvements should occur for preventing crown misfit; and

• 4. The methods of aging are very diverse, and the treatment efficiency may vary according to the study.

The authors of this manuscript certify that they have no proprietary, financial, or other personal interest of any nature or kind in any product, service, and/or company that is presented in this article.