Peter Schouten

Unlock the power of zirconia: perfect for adhesive cementation, the ideal material for a wide range of indications, and essential in minimal invasive dentistry. Time to trust zirconia bonding!

This article demystifies zirconia bonding, providing clear, practical steps to ensure long-term functionality and patient satisfaction, all based on scientific research. Master the three adhesion pillars: mechanical retention, chemical activation, and wetting capacity. Discover how to successfully prepare zirconia surfaces, avoid pitfalls like misapplying silica coating and silane, and choose proven bonding systems for optimal results. Optimise retention even with minimal tooth preparation and achieve reliable zirconia restorations. Say goodbye to doubts and hello to successful zirconia bonding!

Factors influencing retention

Loss of retention due to de-cementation or debonding is a common cause of dental prostheses' failure.  First, let’s have a look at how to cope with the three main factors significantly influencing retention: tooth preparation, restoration pre-treatment, and cement type/bonding.

Tooth preparation

The abutment tooth's height, angle, and surface texture must be considered to achieve sufficient retention and resistance from the preparation. The retention form counteracts tensile stresses, whereas the resistance counteracts shear stresses ⁴. With the proper preparation, a restoration resists dislodgement and subsequent loss.

Full coverage restorations

To achieve sufficient retention and resistance for full-coverage crowns, the tooth abutment should be at least 4 mm high, and the convergence angle should range from 6 to 12 degrees with a maximum of 15 degrees ^{1, 5-8}.

Source; Conventional cementation or adhesive luting - A guideline, Dr. A. Elsayed, Prof. Dr Florian Beuer 

Adhering to the tooth preparation guidelines is crucial for full-coverage restorations (e.g., crowns, and FDPs). These practical guidelines are designed to achieve the required retention and resistance to make conventional luting possible. However, optimal retention and resistance are, in reality, hard to achieve. An unwanted amount of sound tooth substance often should be removed to achieve a highly retentive preparation. Moreover, several studies^2,3 show that, in daily practice, the preparation angle often exceeds 15 degrees.

Minimal-invasive restorations

Minimal-invasive restorations, such as single retainer FDPs, veneers, table-tops and inlay-retained FDPs, are based on a non- or low-retentive preparation form. In this case, retention shifts from (macro-)mechanical to micro-mechanical and chemical, necessitating the use of adhesive techniques ^9-11. Even though the preparations for minimal-invasive restorations largely lack mechanical retention, the long-term success of these types of restorations is well-documented when using a suitable resin cement (e.g. PANAVIA™, Kuraray Noritake Dental, Japan), including a proper pre-treatment and bonding procedure ^{10, 11}.

In high-retentive situations, conventional luting is acceptable for full-coverage restorations*. In all other cases, choosing a resin cement is a better solution. With proper tooth preparation (e.g., shaping, (self-)etching, abrasion) and the right adhesive resin cement system, a non-retentive preparation form provides a reliable basis using mainly chemical retention and micro-mechanical retention instead of macro-mechanical retention.

*Please review the articles available regarding the debate over whether to use a conventional cementation procedure, adhesive cementing, or selective adhesive luting

Restoration pre-treatment

Zirconia is densely sintered and does not contain a glass phase. Therefore, it cannot be etched with hydrofluoric acid to create a micro-retentive etching pattern. In addition, silanes cannot effectively promote zirconia bonding. Several studies have shown that air abrasion with 50-µm alumina at a reduced pressure of 0.5 bar (0.05 MPa; 7 psi) will create a sufficient micro-retentive pattern¹² and greatly enhances the wetting capacity.

In addition to air abrasion, chemical coupling agents such as bifunctional phosphate resin monomers are used on air-abraded zirconia. Bonding with phosphate monomer-containing adhesive resin systems gives very reliable results^27,28. The use of phosphate monomer-based resin cement systems (e.g., Panavia [Kuraray Noritake Dental, Tokyo, Japan]) and/or phosphate monomer primers, such as CLEARFIL CERAMIC Primer Plus (Kuraray Noritake Dental, Tokyo, Japan)  on freshly air-abraded zirconia, offer the most reliable bonding methods today ^13,27,28. We therefor consider MDP-based composite resin cements the material choice for our bonding procedure. However, it must be stressed that contamination of the air-abraded zirconia with saliva, phosphoric acid or other contaminants will limit the formation of chemical bonds and, therefore, must be avoided.

Avoiding contamination

For optimal moisture control, absolute isolation of the working field is crucial. Minimising  the risk of contamination, avoiding exposure to oral fluids. Before restoration placement, a thorough cleaning of the abutment tooth is essential. Following trial placement, a meticulous recleaning step is recommended to remove any potential introduced contamination. KATANA Cleaner (Kuraray Noritake Dental, Tokyo, Japan) is an ideal choice due to its unique properties. Its slightly acidic pH of 4.5 allows for effective cleaning intraoral and extraoral adhesion surfaces. Additionally, the incorporation of MDP monomer technology makes it highly efficient. The MDP salt in this product effectively bonds with contaminants, breaks them down and results in easy removal by water rinsing.

Cement type/bonding

After pre-treatment of surfaces to optimise the , it is important to understand that the properties of highly translucent zirconia differ highly from those of earlier generation zirconia. Early-generation zirconium oxides, including 3 mol% yttrium oxide (3Y-TZP), are high in strength and low in translucency. With the increase in yttria, creating 4-5 mol% yttria, or higher, zirconium oxides, the number of cubic crystals increases, resulting in higher translucency but leading to a reduction in strength. Therefore, attention must be paid to zirconia type, material thickness, restoration type, and application area. These factors may influence the choice of cement based on the adhesive properties demanded for lasting restorations and high aesthetic outcomes.

PANAVIA™ V5

For a resin cement system to deliver a strong bond, it is not always enough to have it contain an appropriate adhesive monomer. It is necessary for that adhesive monomer to be polymerised effectively under different circumstances. The PANAVIA™ V5 system contains an innovative “ternary catalytic system” consisting of a highly stable peroxide, a non-amine reducing agent* and a highly active polymerisation accelerator. Since this catalytic system is amine-free, the hardened cement has unsurpassed colour stability. In addition, the highly active polymerisation accelerator, one of the components in PANAVIA™ V5 Tooth Primer, is not only an excellent reducer that promotes polymerisation effectively, but it is also capable of coexisting with the (in this product) acidic MDP. This makes it possible to create a single-bottle self-etching primer. This accelerator is also responsible for the so-called touch-cure reaction when it comes into contact with the paste. Resulting in the sealing of the dentin interface and, at the same time, allowing the paste to set even in situations where light curing is limited.

*PANAVIA™ V5 Tooth Primer applied and left for 20 seconds, followed by air drying.

The second primer in the PANAVIA V5 system is CLEARFIL™ CERAMIC PRIMER PLUS, which incorporates Kuraray Noritake Dental’s original MDP and a silane. This product is used to prime zirconia but is also an excellent choice for priming silica-based ceramics, composites, and metals.

CLEARFIL™ CERAMIC PRIMER PLUS, which contains the original MDP, applied and dried.

The PANAVIA™ V5 full adhesive resin cement system consists of all three above-mentioned components, always used in the same way, independent of the material, for a straightforward procedure to ensure reliable bonding. The PANAVIA^TM V5 systems offer try-in pastes to visualise the final results before final cementing and confirm the appropriate shade of the resin cement to be used.

PANAVIA™ VENEER LC

Offering a flexible workflow and high bondability of thin, translucent restorations like veneers but also inlays and onlays, PANAVIA™ Veneer LC was designed. It is a light-curing resin cement system allowing a long working time of 200 seconds under ambient light*. This allows multiple veneers to be placed simultaneously without racing against the setting. The final light-curing can be started anytime after positioning the provisions. The PANAVIA™ Veneer LC cementing system includes PANAVIA™ Tooth Primer and CLEARFIL CERAMIC PRIMER Plus as primers to chemically interact with the adhesive surfaces.

PANAVIA™ Veneer LC Paste applied and the laminate veneer seated. In this case six veneers were simultaneously placed during one session.

Unpolymerized excess paste removed with a brush. PANAVIA™ Veneer LC Paste is a light-cured type rein cement, designed to provide sufficient working time.

This photo shows the results after the final light curing. Since the excess cement was easily removed, there were almost no cement residues.

PANAVIA™ SA CEMENT Universal

Still, clinicians seek efficiency and effectiveness in everyday practice by using a straightforward but durable resin cement solution. PANAVIA™ SA Cement Universal is developed to offer this ease-of-use property without losing focus on bonding properties.  PANAVIA™ SA Cement Universal is developed with the original MDP monomer in the hydrophilic paste compartment, allowing for chemical reactiveness with zirconia and tooth structure. The other compartment contains the hydrophobic paste, to which a unique silane coupling agent, LCSi monomer, is added, which allows the cement to deliver a strong and durable chemical bond to silica-based materials like porcelain, lithium disilicate and composite resin*. Furthermore, PANAVIA™ SA Cement Universal is less moisture sensitive than full adhesive resin cement systems. This also makes it the ideal cement in situations where rubberdam isolation is difficult.

*The product is available in both auto mix and hand mix options.

*Old PFM bridge (shown here) removed, and existing preparations modified to accommodate a 3-unit KATANA™ Zirconia bridge. The upper right canine was prepared to receive a single-unit KATANA™ crown.

Before

After. Seating & Final Smile. PANAVIA™ SA Cement Universal and CLEARFIL™ Universal Bond Quick were used for cementation and bonding. “I love the ease of use and clean-up with PANAVIA™ SA Cement Universal, and its MDP monomer creates a strong chemical bond to the tooth structure and zirconia. CLEARFIL™ Universal Bond Quick has a quick technique without reducing bond strengths, releases fluoride and has a low film thickness. I simply rub CLEARFIL™ Universal Bond Quick into the tooth for a few seconds and air dry. There is no need to light-cure, since it cures very well with PANAVIA™ SA Cement Universal. The patient was very happy with the results. She loved that she no longer saw metal margins, and her smile was much more uniform and lifelike.” Dr. Kristine Aadland

*Images are a part of a case by Dr. Kristine Aadland; 3-Unit anterior maxillary

Bonding to zirconia in three steps

Over the last century, the popularity of highly translucent zirconia has skyrocketed due to its excellent properties and wide range of anterior and posterior clinical applications. Because zirconium oxide prostheses are, if processed correctly, antagonist-friendly and easy (and relatively inexpensive) to fabricate, the material keeps gaining popularity in dentistry.

Several steps need to be taken into account for reliable and durable bonding. Years of research on achieving high and long-term bond strength to zirconia have concluded into three practical steps, summarised as the APC concept¹³ as a reliable procedure guideline.

APC-Step A

Zirconia should be air-particle abraded (APC-Step A) with alumina or silica-coated alumina particles; the sandblasting or micro-etching procedure. Air abrasion with a chairside micro-etcher using aluminium oxide particles (size: up to 50 μm) at a low pressure of 0,5 bar (0.05 – 0.25 MPa) is sufficient.^14,18,25-27

APC-Step P

The subsequent step includes applying a special ceramic primer (APC-Step P), which typically contains specially designed adhesive phosphate monomers, onto the zirconia adhesive surfaces.^29,30 The MDP monomer has been shown to be particularly effective at bonding to metal oxides like zirconium oxide.

APC-Step C

Dual- or self-cure resin cement systems should be used to reach an adequate C=C conversion rate underneath the zirconia restoration since the lack of translucency in zirconia reduces light transmission.¹³ However, in cases where high-translucent zirconia (HTZr0₂) is used, the zirconia transmits light so that the shade of composite or resin cement might influence the final appearance of such restorations. It is, thereforebased on the individual situation and shade of the abutment tooth.

The APC zirconia-bonding concept is not limited to intra-oral situations and can also be applied in the laboratory for implant reconstructions that include cemented zirconia components.

Conclusion

Rapid developments in high-quality translucent zirconia have made the utility and reliability of adhesive cementing systems even more crucial. This applies to fully opaque restorations but also minimally invasive and ultra-translucent restorations of low thickness. In all cases, the longevity of the bonding and, thus, the provision directly affects patient  satisfaction. By taking into account the three primary parameters we have discussed in this article and following the predictable APC protocol, you will successfully realise durable bonded zirconia restorations from now on.

References

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3. Florian BEUER, Daniel EDELHOFF, Wolfgang GERNET, Michael NAUMANN, Effect of preparation angles on the precision of zirconia crown copings fabricated by CAD/CAM system, Dental Materials Journal, 2008, Volume 27, Issue 6, Pages 814-820

4. Muruppel AM, Thomas J, Saratchandran S, Nair D, Gladstone S, Rajeev MM. Assessment of Retention and Resistance Form of Tooth Preparations for All Ceramic Restorations using Digital Imaging Technique. J Contemp Dent Pract. 2018;19(2):143-9.
   
   
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8. Uy JN, Neo JC, Chan SH. The effect of tooth and foundation restoration heights on the load fatigue performance of cast crowns. J Prosthet Dent. 2010;104(5):318-24.
   
   
9. Blatz MB, Vonderheide M, Conejo J. The Effect of Resin Bonding on Long-Term Success of High-Strength Ceramics. J Dent Res. 2018;97(2):132-9.
   
   
10. Chaar MS, Kern M. Five-year clinical outcome of posterior zirconia ceramic inlay-retained FDPs with a modified design. J Dent. 2015;43(12):1411-5.
    
    
11. Kern M, Passia N, Sasse M, Yazigi C. Ten-year outcome of zirconia ceramic cantilever resin-bonded fixed dental prostheses and the influence of the reasons for missing incisors. J Dent. 2017;65:51-5.

12. Kern M, Dr Med Habil, M. BONDING TO ZIRCONIA. Jerd_40. 3DOI 10.1111/j.1708-8240.2011.00403.x VOLUME 2 3 , NUMBER 2 , 2011

13. Blatz MB, Alvarez M, Sawyer K, Brindis M. How to Bond Zirconia: The APC Concept. Compend Contin Educ Dent. 2016 Oct;37(9):611-617; quiz 618. PMID: 27700128.

14. Blatz M.B., Oppes S., Chiche G., et al. Influence of cementation technique on fracture strength and leakage of alumina all-ceramic crowns after cycling loading. Quintessence Int. 2008; 39(1): 23-32

15. Burke F.J., Fleming G.J., Nathanson D., Marquis P.M. Are adhesive technologies needed to support ceramics? An assessment of the current evidence. J Adhes Dent. 2002;4(1)): 7-22

16. Blatz M.B. Sadan A., Maltezos C., et al. In vitro durability of the resin bond to feldspathic ceramics. AM J Dent 2004;17 (3):169-172

17. Blatz M.B., Bergler M. Clinical applications of a new self-adhesive resin cement for zirconium-oxide ceramic crowns. Compend Contin Educ Dent. 2012;33(10):776-781

18. Maggio M., Bergler M., Kerrigan D., Blatz M.D. Treatment of maxillary lateral incisor agenesis with zirconia-based all-ceramic resin bonded fixed partial dentures: a case report. Amer J esthet Dent. 2012;2(4):226-237
19. Ozer F., Blatz M.B., Self-etch and etch-and0rinse adhesive systems in clinical dentistry. Compend Contin Edus Dent. 2013;24 (1):12-20

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21. Kern M., Wegner S.M., Bonding to zirconia ceramics: adhesion methods and their durability. Dent Mater. 1998;14(1):64-71

22. Wegner S.M., Kern M. Long-term resin bond strength to zirconia ceramic. J Adhes Dent. 2000;2 (2):139-147

23. Blatz M.B., Sadan A., Martin J., Lang B. In vitro evaluation of shear bond strength of resin to densely-sintered high-purity zirconium-oxide ceramics after long-term sorage and thermos cycling. J Posthet Dent. 2004;9(4):356-362

24. Blatz M.B., Chiche G., Holst S., Sadan A. Influence of surface treatment and simulated aging on bond strength of luting agents to zirconia. Quintessence Int. 2007;38 (9):745-753

25. Quaas A.C., Yang B., Kern M., Panavia F 2.0 bonding to contaminated zirconia ceramic after different cleaning procedures. Dent Mater. 2007;23(4):506-512

26. Song J.Y., Park S.w., Lee K., et al. Fracture strength and microstructire of Y-TZP zirconia after different surface treatments. J Prosthet Dent. 2013;110(4):274-280

27. Koizumi H., Nakayama D., Komine F., et al. Bonding of resin-based luting cements to zirconia with and without the use of ceramic priming agent. J adhes Dent. 2012;14(4):385-392

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29. Alnassar T., Ozer F., Chiche G., Blatz M.B. Effect of different ceramic primers on shear bond strength of resin-modified glass ionomer cement to zirconia. J Adhes Sci Technol. 2016;DOI:10.1080/01694243.1184404

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Article by Peter Schouten.

I am frequently asked questions about the intraoral repair of porcelain chippings. To achieve success in repairs, it is essential to consider several important issues.

Perhaps the single most crucial issue to recognise is why the chipping occurred in the first place. For example, if loading stress is the leading cause, this should be considered during the repair.

Other issues to consider are removal of contamination, optimal roughening and chemical activation of the surface, and the prevention of contamination during the repair. Also, a rubber dam should be used to isolate the working field.

FUNDAMENTALS OF ADHESION

Adhesive procedures can be only successful by using the proper substances and methods. Different kinds of surfaces often need different treatments for success. However, the three basic fundamentals of adhesion must be respected to achieve the best results.

1) Mechanical retention through a roughened surface.

2) Chemical activation through chemically active substances.

3) High energetic bonding surface allowing for optimal interaction (wetting capacity) between the surface and the applied medium. Contamination will lower the bonding capacities and must be avoided or removed in any case.

TYPES OF FRACTURE

The most frequent fractures are porcelain only and those that include exposure of the substructure in PFZ or PFM prosthesis.

Many cases present with only limited chipping to the porcelain, for example, at the incisal edge. To achieve a durable repair in this instance, start by increasing the bonding/repair area using a fine diamond burr to create a large bevel. A fine grit burr is preferable over a medium or coarse version because a higher number of shallow grooves deliver a more optimal bonding surface than lesser deeper ones do. Additional roughening of the adherent surface by sandblasting with alumina (50 µm grain size, 2 bar pressure) is highly recommended to increase the surface area further.

When repairing porcelain chippings where the substructure is exposed, it is essential to be aware that multiple substrates are dealt with, indicating a need to adjust the repair protocol accordingly.

Clean the roughened fractured surface thoroughly. KATANA™ Cleaner is the product of choice. It is a safe and easy to use product with high cleaning power. It can be used both intra and extra orally on all kinds of dental substrates. After rinsing and thorough drying, the surface is ready for the next step, chemical activation.

CLEARFIL™ CERAMIC PRIMER PLUS contains both silane and MDP and effectively treats both silica-containing ceramics and metal oxides (zirconia) and metals. After application and thorough drying, the composite restoration can be carried out immediately without an extra bonding step. The composite of choice is a durable flowable, CLEARFIL MAJESTY™ ES Flow. It has high flexural strength, even higher than most paste-type composites. Besides that, it adapts to the surface better and easier. Quick and easy polishing and gloss retention are other highly valuated qualities of CLEARFIL MAJESTY™ ES Flow.

HINTS AND TIPS

• Isolate the working field by using rubber dam
• Bevel the chipping extensively using a fine diamond burr
• Roughen the adherent surface, preferably by sandblasting
• Clean the bonding area with KATANA™ Cleaner
• Apply CLEARFIL™ CERAMIC PRIMER PLUS to the entire bonding area (including exposed zirconia or metal) and dry thoroughly
• Cover exposed metal with a thin layer of CLEARFIL™ ST OPAQUER and light cure
• Repair with a strong flowable composite, such as CLEARFIL MAJESTY™ ES Flow

REPAIR OF PORCELAIN CHIPPINGS VIDEO

BY PETER SCHOUTEN

ZIRCONIA
CRYSTALLISATION

Zirconia is here to stay in dentistry. Since its introduction some two decades ago, the possibilities for use of zirconia have increased enormously. But how is this compound produced? What happens during sintering, what types are available, and how are the desired shape and shade of the final product achieved?

PSZ or Partially Stabilised Zirconia was the first of the zirconium oxides to be used in dentistry. This type of zirconia was stabilised using yttria, and is currently no longer in use. It consisted of a mix of monoclinic, tetragonal and cubic crystals. For many years, the so-called Y-TZP variety was used (yttria tetragonal stabilised zirconia polycrystal). This has outstanding flexural strength (> 1,000 MPa), however, a non aesthetic opaque white appearance. It consists for mostly of tetragonal crystals with a diameter of some hundreds of nanometres. Approximately 3 mol% yttria is added to keep the material stable at room temperature;
this variety is therefore sometimes described as 3Y zirconia. New varieties, such as cubic zirconia, were developed with the aim of improving the aesthetic
properties of the material. Cubic zirconia is characterised by high translucency but has a lower flexural strength. However, although the flexural strength of this type is considerably lower than that of the tetragonal version, it is nevertheless much higher than that of lithium disilicate. This form also contains a higher amount of yttria, ranging from 4 to more than 5 mol%.

CRYSTAL PHASES

We currently know of zircona crystals in three major shapes.

Monoclinic

Normally, zirconia exists only at room temperature in the monoclinic phase. Monoclinic zirconia has low strength and translucency.

Tetragonal
These zirconia crystals are metastable and only exist at room temperature after stabilisation, mainly by adding yttria.
Although tetragonal zirconia is strong, it has limited aesthetic properties.

Cubic
Cubic crystals are stable and provide improved translucency. Due to the addition of a higher percentage of yttria, cubic
zirconia work pieces are not as strong as tetragonal ones. On the other hand, they are highly aesthetic and therefore
suitable for monolithic restorations, even in the anterior zone.

CRYSTALLISATION PROCESS
Zircona is almost invariably offered as a partially sintered material. In this form it is always opaque white, and the true
colour and translucency is only apparent after sintering. During sintering, the material shrinks to its ultimate size; a
multitude of small crystals merg to form larger crystals, but although the crystals themselves increase in size, the total
volume decreases. This shrink factor is therefore taken into account in the software, so that the end product is not only
colourfast after sintering, but is also sizewise correct.

Small crystals merge into larger ones during the sintering process.

KATANA™ Multilayer HTML before (left) and after sintering.

By Peter Schouten, Technical Manager Kuraray Europe Benelux

Kuraray Noritake’s self-adhesive cement series, PANAVIA™ SA Cement Universal, has evolved through several stages of development since it was first introduced as “CLEARFIL™ SA Cement”. Over time, various improvements have been implemented, bringing us to our latest product: PANAVIA™ SA Cement Universal. Improvements include: strengthening of the bonding power to dental tissue, increasing the storage temperature to room temperature and extending the shelf life to three years. What remained the same - the easy removal of any excess cement, the moisture tolerance and the integration of the original MDP monomer.

The original MDP enables PANAVIA™ SA Cement Universal to be bonded to dental tissue as well as to metals and zirconia ceramics. Until recently, the chemical bonding of glass-based materials such as porcelain, glass ceramics, including lithium disilicate, and composite, was only possible with an additional silane-based primer.

With the introduction of PANAVIA™ SA Cement Universal the silanisation step has become redundant. Instead, the silane has been incorporated into the PANAVIA™ SA Cement Universal paste. As a result, there is only one single universal procedure, without the need for separate primers.

LCSi

Adding silane to the paste sounds easy. However, there are some limiting factors. For example, keeping silane in the form of γ-MPS (γ-methacryloxypropyltrimethoxysilane) active for a long time is a challenge. This is partly due to γ-MPS’ hydrophilic nature. Contact with water under acidic condition results in hydrolysis of alkoxy groups of silane. Therefore, it is best to use a more hydrophobic and thus stable silane and to avoid bringing it into contact with water and acid prematurely.

We have been using a long-chain silane (LCSi) for some time in a number of our superior composites, such as CLEARFIL™ MAJESTY™ Posterior and CLEARFIL™ MAJESTY™ ES Flow. However it’s the first time we have used it in a cement.

This unique silane has long hydrocarbon spacer (the hydrocarbon chain between the silanol group and the methacrylate group) which makes it more hydrophobic and stable than the small γ-MPS molecule. The reaction with the silica particles in the glass-based materials is expected to be also more orderly and faster. The result is a more optimally bonded surface with a greater resistance to hydrolysis.

A great deal of thought has gone into combatting the degradation of silanes in PANAVIA™ SA Cement Universal. It was decided to separate the more hydrophilic components from the hydrophobic ones so they only come together when the paste is mixed. This is why we are able to achieve a three-year shelf life, even when stored at room temperature.

Research

It’s not only in-house data that show comparable or even better bonding of PANAVIA™ SA Cement Universal to glass-based materials, than with its previous version, PANAVIA™ SA Cement Plus, in combination with CLEARFIL™ CERAMIC PRIMER PLUS. The first results from independent research are also strongly supporting this.

With PANAVIA™ SA Cement Universal, there is now a self-adhesive resin cement on the market that allows virtually all indirect materials to be bonded directly without the intervention of primers or bonding.

By Peter Schouten, Technical Manager Kuraray Europe Benelux

CLEARFIL MAJESTY™ ES-2 is the composite that allows intuitive creation of natural-looking restorations. The combination of its self-adaptive capacity, thanks to the use of light diffusion technology, its natural fluorescence, high filler content, fine workability and exceptional polishability provide outstanding aesthetic results.

CLEARFIL MAJESTY™ ES-2 is a forgiving composite. It blends seamlessly with the adjacent tooth structure. This is because CLEARFIL MAJESTY™ ES-2 incorporates light diffusing technology. This makes it possible to cover the entire VITA range using only VITA A shades.

In our endeavors to reproduce nature, we need to step away from the notion that artificial materials do all the work for us. Especially enamel is a difficult kind of tissue to replace with artificial materials like composite. When you replace natural enamel with translucent composite you almost never reach the desired goal! Why is this? The most determinant factor is the difference in value. Natural enamel increases in value in proportion to its thickness, while for (translucent) composite the value decreases.

In certain situations, for instance for the reproduction of the incisal edge of a central incisor, where a translucent zone is naturally present, the use of a more or less translucent composite is needed. However, it is often the opacity of a composite that we wish to harness in order to cover up underlying (irregular) discoloration. To do this, the use of a translucent composite is clearly not desirable. In this situation, the most opaque variant is your first choice of composite.

You've probably been there: applying anatomical layering techniques with a composite system, only to arrive at a somewhat disappointing result that looks too gray. This can be explained by the fact that, as a manufacturer - and we're certainly not alone here - we have created some confusion by giving composites names such as Dentin and, worse still, Enamel. These names suggest that these materials can be used as a replacement for natural dental tissue. Due to the totally different optical properties of each dental tissue and composite, this can only be achieved within a frequently too narrow spectrum. Therefore, when it comes to the use of the more translucent composite variants, I recommend a cautious approach. Only use such variants where transparency is required.

...it is often the opacity of a composite that we wish to harness in order to cover up underlying (irregular) discoloration. To do this, the use of a translucent composite is clearly not desirable. In this situation, the most opaque variant is your first choice of composite.

Kuraray Noritake Dental offers three basic opacities within the CLEARFIL MAJESTY™ family. These three variants enable you to create transparency where necessary, or indeed to avoid it in zones where greater coverage is required. The most opaque variant within the CLEARFIL MAJESTY™ ES-2 family is Premium Dentin. The semi-opaque variant Classic and the semi-translucent variant Premium Enamel.

The challenging class IV restoration as an example. The tendency exists to use dentin composite up to the dentin-enamel junction. Don't do this, but instead apply it right up to the outermost edge of the restoration. This means also covering the beveled enamel. Depending on the desired end result, you should only use the more translucent variants, Classic or Premium Enamel, in the incisal third.

Working with different variants of CLEARFIL MAJESTY™ ES-2 in one restoration where more than one variant end at the surface the transition from one type to the other is totally seamless. Since all three variants have virtually the same high filler percentages and filler type. During finishing you won't even notice the transition between the different variants. The polishability of all the variants is also identical.

All these qualities make CLEARFIL MAJESTY™ ES-2 your aesthetic composite for the effortless creation of natural-looking restorations. Both anterior and posterior.

No shade taking needed. Just one shade to cover all posterior cases. Even the larger Class I's and II's!

Anterior cases from B1 to C4 done with only two shades, without the need for shade taking. Can you imagine? No additional opake materials (light blockers) needed too.

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.