Peter Schouten

Zirconia ceramics are increasingly being used for dental prostheses. In the past, they were used as a substructure that was baked with porcelain; nowadays, monolithic zirconia prostheses have been made possible by the introduction of aesthetic zirconia ceramics varieties. They are used more and more, even as RBFDPs (Resin Bonded Fixed Dental Prosthesis).

There is currently quite a lot of discussion about zirconia. So much the better, because it keeps the dynamics of the subject alive. Is it possible to adhesively cement zirconia? Is it permissible to sandblast zirconia? Is it feasible to polish zirconia? Is wear of the antagonist an issue? And so on and so forth.

New dental technologies and materials have been developed in recent years to meet the demand for aesthetic, biocompatible and metal-free prostheses. Although porcelain has been used as an aesthetic dental material for more than hundred years, its restrictions are now well-known. It is the fragility of porcelain (low flexural strength and fracture toughness) in particular which restricts its application in areas where the exertion of massive force on the material is a factor. Zirconia has a high flexural strength and is therefore suitable for multiple dental applications.

Development

The first generation of zirconias used in dentistry belonged to the Partially Stabilised Zirconia (PSZ) class. This type of zirconia, stabilised with yttrium oxide, was composed of a mix of monoclinic, tetragonal and cubic crystals but is no longer used nowadays. The so-called Y-TZP variant was then used for many years, and still is. Its main characteristics are high flexural strength (> 1.000 MPa) and an opaque white appearance. For the most part, this variant is composed of tetragonal crystals of a few hundred nanometers. To keep the material stable at room temperature, approximately 3 mol% of yttrium oxide is added to the composition, which is why it is sometimes referred to as 3Y zirconia.

In order to improve its aesthetic properties, a start was made on the development of a new variety, so-called cubic zirconia. Cubic zirconia is characterised by a high translucence, meaning that a lower flexural strength must be accepted. Admittedly, the flexural strength of this variant is lower than that of the tetragonal variety, but still much higher than the flexural strength of lithium disilicate. It also contains more yttrium oxide; approximately 5 mol%.

Crystal phases

We currently know zirconia crystals in three varieties: monoclinic, tetragonal and cubic. In general, zirconia only exists in the monoclinic phase at room temperature. Monoclinic zirconia has low strength as well as low translucence. Tetragonal zirconia crystals are metastable and can only exist at room temperature after having been stabilised with yttrium oxide, among other metal oxides. The characteristics of tetragonal zirconia are that it is strong, but at the same time devoid of aesthetic
properties. Cubic crystals are stable and ensure improved translucence. Although prostheses made from cubic zirconia are not as strong as the tetragonal variety, they are highly aesthetic, and are even suitable for monolithic restorations in the aesthetic zones.

Multilayer

KATANA™ Zirconia ML, STML and UTML, as well as the latest KATANA™ Zirconia Block, are so-called multilayer zirconias. These products are built up of four layers with an ascending degree of translucency, ranging from the more opaque and coloured body layer (cervical), through two transitional layers, to the translucent incisal edge (occlusal/incisal). The invisible transition from one layer to another is achieved using a unique process. The secret is in the specific distribution of particles during the pre-sintered stage (continuous gap grading). In combination with the cold isostatic pressing method, this ensures material of an outstandingly high quality.

KATANA™ Zirconia Block

The use of KATANA™ Zirconia Block makes it possible to produce a complete monolithic zirconia prosthesis in a short time. This block was developed to be used within the CEREC workflow. After scanning, the prosthesis is ground for approximately 15 minutes and can then be sintered in the SpeedFire oven in just half an hour¹). KATANA™ Zirconia Block shares the properties of KATANA™ STML and can be used for the production of single-unit prostheses such as full crowns for both anterior and posterior applications.

Wear of the antagonist

The question of whether zirconia prostheses are harmful to the antagonist would seem to be justified; after all, this is a very hard material. Hardness, however, does not relate directly to the abrasive properties of a material. Smoothly polished zirconia is minimally abrasive, something which has been corroborated by multiple studies²). Glaze, porcelain, lithium disilicate and even enamel are all more abrasive for the antagonist.

CAD/CAM

Thanks to emerging digital solutions (CAD/CAM) combined with the possibility of executing fixed partial dentures (FPDs) in monolithic zirconia, a fully digital workflow - from scan to prosthesis - is now feasible.

Sandblasting

Sandblasting using alumina is the standard method for the roughening of zirconia. In order to reduce the decline from the tetragonal crystal stage to the monoclinic stage, it is recommended that sandblasting be carried out at a maximum pressure of 2.5 bar using 50 micron aluminium oxide particles as a maximum³). In practice, zirconia is also frequently sandblasted tribochemically, for example, by means of CoJet (3M). The results achieved with this method are variable. In the case of limited pressure (2.5 bar as a maximum) there is a risk that the energy will be insufficient to achieve an effective and complete fusion between the silica and the zirconia surface. It is therefore recommended that CLEARFIL™ Ceramic Primer Plus be used for adhesive connection with the sandblasted surface. The silane in this ceramic primer bonds chemically with the silica, while the MDP bonds with the areas of zirconia not covered by silica. The use of silane alone does not provide an adequate basis for a reliable result⁴). Sandblasting with alumina, however, followed by priming with CLEARFIL™ Ceramic Primer Plus, is a well-proven and reliable method to ensure effective bonding to zirconia⁵).

Not etchable, but certainly bondable

Zirconia is not – or is barely – etchable with the etching technology available (HF). In addition, it cannot be recommended that zirconia be treated with phosphoric acid, because of the bonding of phosphates to the surface of the zirconia, which will inhibit a chemical activation of the surface for some adhesive cementation systems. The question of whether an etching product should be used is a valid one; after all, chemical etching is not necessary to achieve surface roughness. Achieving surface roughness is, however, a logical option, and roughening by means of sandblasting would seem to be the most appropriate method for zirconia. Thereafter, chemical bonding is ensured using a phosphoric functional monomer, preferably MDP.

Adhesion principles

Three basic factors are required to achieve the effective bonding of different materials. In keeping with the principles of adhesion, these are: surface enlargement (micromechanical retention), chemical activity (adsorption and electrostatic bonding) and a high-energetic surface to guarantee proper adaptation.

Adhesive bonding

The best method of adhesive bonding for zirconia is the use of a composite cement containing MDP. Professor Mathias Kern published an article about bonding to zirconia using MDP as early as 1998. This study showed that sustainable resin bonding to zirconia can only be achieved using a composite cement with a special adhesive monomer. The monomer used was in fact MDP. In the meantime, many studies have been published about bonding to zirconia.

On the one hand, these studies concern bonding with phosphate monomers (MDP); on the other hand, attention has been paid to the modification of the zirconia surface to render it etchable. The simplest and most reliable method is to sandblast the surface at a low pressure using aluminium oxide, followed by priming with an MDP-containing primer⁶).

PETER SCHOUTEN,
Technical Manager Kuraray Europe Benelux

1) Depending on the method of grinding/milling.
2) Mörmann W.H. et al. Wear characteristics of current aesthetic dental restorative CAD/CAM materials: Two body wear, gloss retention, roughness and Martens hardness. Journal of the mech. Behavior of Biomedical Materials 20 (2013) 113-125, Janyavula S. et al. The wear of polished and glazed zirconia against enamel. J Prosthet Dent 2013; 109;22-29 Stawarczyk B. et al. Comparison of four monolithic zirconia materials with conventional ones: Contrast ratio, grain size, four-point flexural strength and two-body wear. Journal of the mech. Behavior of Biomedical Materials 59 (2016) 128-138 D’Arcangelo C. et al. Wear Evaluation of Prosthetic Materials Opposing Themselves. Oper Dent 2017 antagonistic surface roughness on the wear of human enamel and nanofilled composite resin artificial teeth. J Prosthet dent 2009; 101: 342-349.
3) Attia A. / Kern M. Effect of cleaning methods after reduced-pressure Air Abrasion on Bonding to Zirconia Ceramic J Adhes Dent 2011; 13: 561-567.
4) Pilo R. Effect of tribochemical treatments and silane reactivity on resin bonding to zirconia. Dent Mater (2017).
5) Kern M. Bonding to oxide ceramics - Laboratory testing versus clinical outcomes. Dent Mater (2014).
6) M. Kern et al., Bonding to zirconia ceramic: adhesion methods and their durability. Dent Mater 14: 64-71, Jan.1998.

Peter Schouten, Technical Manager at Kuraray Europe Benelux

Before universal adhesives were available, two major techniques were used: total-etching adhesives, which basically work with phosphoric acid etching on enamel and dentine; and self-etching adhesives, which can be used with or without the option of etching enamel selectively. Prior to the introduction of self-etching adhesives in the late 1990s, dentists used mostly total-etching techniques. While this procedure achieves strong enamel bonding, it can also be very technique-sensitive and involves several steps. As a consequence, dentists welcomed the development of simplified adhesives. In 2011, the new generation of universal adhesives was introduced, with the aim of replacing all previous generations.

The development of universal adhesives was firstly due to the success of self-etching adhesives, but total etching was still advocated. The result, a universal adhesive, must be considered a self-etching adhesive with a phosphoric acid conditioning option on enamel and/or dentine. Maximum flexibility resulting from the freedom of choice in etching technique and the preference of the practitioner was thus obtained.

CLEARFIL Universal Bond Quick, manufactured by Kuraray Noritake Dental, is a single-component light-curing bonding agent indicated for all direct and indirect restorations in combination with all etching techniques (total-etching, self-etching or selective-etching). The adhesive is also indicated for the surface treatment of zirconia- and silica-based ceramics. When compared with other one-bottle universal adhesives, CLEARFIL Universal Bond Quick exhibits RAPID BOND TECHNOLOGY. We asked Peter Schouten, Technical Manager at Kuraray Europe Benelux and a chemist with decades of experience in the dental industry, about his views on universal adhesives and CLEARFIL Universal Bond Quick.

Dental Tribune: Since the introduction of the first universal adhesive, a new generation of adhesives has been created that has enjoyed increasing popularity since then. What is your opinion about the system?

Peter Schouten: For me, the term “universal” remains debatable. There is no clear definition of a universal bonding system yet. When we look at what different manufacturers are saying about universal bonding systems, the term to me applies primarily to the etching technologies and the ability to adhere to all substrates currently used in dentistry, such as silica- or metal-based materials. In this case, we can really speak of “universal”.

What are the advantages and disadvantages of self-etching and total-etching technologies? How are universal adhesives positioned in relation to them?

In my home country of the Netherlands, there is a large group of self-etching users. In many other countries, most dentists still use the total-etching approach. And, of course, there are reliable three-step total-etching systems on the market. However, etching of dentine removes hydroxyapatite and creates a layer of collagen. Afterwards, the dentist tries his or her best to penetrate this layer again with a bonding system. Why not preserve the hydroxyapatite and create a reliable bonding to the hydroxyapatite itself? This is the basis of the gold standard two-step self-etching bonding, our CLEARFIL SE BOND. Now universal adhesives—at least CLEARFIL Universal Bond Quick—provide the advantage that dentists can use any etching technique without worrying about results that are less than optimal. It really is an open system.

How do universal adhesives perform on wet and dry dentine, as well as enamel, in combination with all etching technologies?

I am a strong believer in the self-etching technique as we have proven already. The wetness and dryness of dentine is always an issue in total-etching techniques. With CLEARFIL Universal Bond Quick, the instruction is to rinse and dry. Our universal adhesive has the capability of penetrating the dentine surface quickly and completely.

Kuraray is a pioneer in adhesive systems: the company introduced total-etching bonding in the 1970s and innovative self-etching technology in the 1990s. The secret to success of all universal adhesives seems to be the incorporation of the adhesive molecule MDP (10-methacryloyloxydecyl dihydrogen phosphate) developed in 1981 by Kuraray. What is the function of MDP?

Kuraray has over 40 years of experience in the development of phosphate monomers. In 1976, we had already developed Phenyl-P. By far the most important ingredient in our current bonding systems is the original MDP. We never would have reached the level at which we are today without this phosphate monomer. MDP is capable of creating a long-lasting bond to calcium in hydroxyapatite and to other metals. As MDP can chemically bond to Ca2+ ions, it forms stable, insoluble MDP–Ca salts present as nano-layers at the adhesive interface.

Kuraray Noritake Dental introduced CLEARFIL Universal Bond Quick at this year’s International Dental Show. According to the company, CLEARFIL Universal Bond is the best one-step adhesive ever developed. Briefly, what do you find noteworthy about the bonding agent?

The most remarkable thing is RAPID BOND TECHNOLOGY, enabling us to introduce the benefit of no waiting. This technology works in three steps: first, rapid penetration; second, fast polymerisation; and third, quick formation of a hydrophobic, hydrolytically stable bonding layer. It took our research and development lead Dr Yamato Nojiri many years to develop a cocktail of amide monomers that is superhydrophilic and turns after curing into a stable hydrophobic polymer. The addition of this amide monomer makes it possible to skip the waiting step. Fast polymerisation is achieved by a modified photoinitiator, releasing twice as many radicals in comparison with other initiators. The quickly formed stable bond derives from the combined action of MDP and the amide monomer. MDP bonds to calcium and amide monomer turns into a highly cross-linked hydrophobic polymer network.

The motto of CLEARFIL Universal Bond Quick is “Universal. Easy. Reliable.” Could you please explain that further?

I think that clinicians will be very satisfied with the bonding agent’s performance and wide indication range. It definitely will be used mainly for direct restorations with light-curing composite resins, but also for core build-ups, cavity sealing, treatment of exposed root surfaces and hypersensitive teeth. The pretreatment of the tooth can be done with any of the three etching procedures before applying this adhesive. For selective enamel etching and total-etching, phosphoric acid needs to be applied. I recommend K-ETCHANT Syringe etching gel. It is left in place for 10 seconds, followed by rinsing and drying. The product is very easy to use and not technique-sensitive. The result is a reliable bond because of the use of our proven MDP technology combined with the cross-linked hydrophobic polymer network.

Does CLEARFIL Universal Bond Quick adhere to any dental substrate (lithium disilicate, zirconia and metals)?

Yes, it does. CLEARFIL Universal Bond Quick is the ideal bonding agent in most situations. Bonding to tooth structure and to most direct and indirect filling materials can be performed with CLEARFIL Universal Bond Quick. For the pretreatment of silica-based ceramics (glass-ceramics); however, we advise the use of CLEARFIL CERAMIC PRIMER PLUS for the most optimal results.

What is known regarding combining universal adhesives with light-curing, dual-curing and self-curing composites without the use of primers? Can the dentist really combine them without any problems?

It can be used with all light-curing composites and compomers, with the exception of silorane-based composites. For use with self- and dual-cure composites, CLEARFIL DC Activator is needed. This catalyst activates the dual-curing mechanism of this adhesive. However, the addition of CLEARFIL DC Activator to the adhesive is not required when using it with CLEARFIL DC CORE PLUS or PANAVIA SA Cement Plus.

What do you see for the future of universal adhesives? 

As long as we still need adhesives to bond our composites to tooth structure, I'm almost certain that single-bottle universal adhesives will become the most used systems. In vitro tests have shown good results. A universal, easy-to-use adhesive with few treatment steps and a short working time reduces the risks of errors. Of course, relevant long-term clinical research results are needed to prove the quality. Kuraray Noritake Dental, with its leading adhesive technology, will surely remain the leader in this field.