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The world of cements (ie, luting agents) in dentistry has rapidly evolved over the years. Thirty years ago things were fairly simple. We had three basic types of cements: cements based on zinc oxide (zinc oxide-eugenol, zinc oxyphosphate, and polycarboxylate), cements based on alumino-silicates (glass ionomers and silicate cements), and resin cements. Our choice of substrates was simple as well, in that we were trying to cement to tooth structure with basically one of two types of restorations: metal castings/porcelain-fused-to-metal (PFM) castings or “porcelain jacket” (PJC) type crowns. Back then we learned quickly that PJCs were brittle and problematic, but they offered esthetic benefits that no other restoration could provide at that time. We learned that resin cements worked well for both retentive and “non-retentive” preparations and gained favor over the course of time. But they also had inherent challenges, such as technique sensitivity, difficulty in cleaning up the resin, potential for shade changing during curing, and a darkening of the resin over time.1
Today, with the introduction and use of newer ceramics (lithium disilicate, zirconia, etc.) being added to traditional full-cast and PFM crowns, some complexity has been added to the mix. It is crucial that clinicians understand the steps required for lithium-disilicate or zirconia-based crowns to properly adhere to the tooth structure and the luting cements being used. They need to be handled and treated properly to prevent delamination and failure. Do you know the best ways to prepare the intaglio surface and the prepared tooth for cementation and which type of luting agent to maximize your success?
Some key considerations that every clinician must embrace:
1. Understand if your preparation is a retentive or non-retentive type.
2. Understand how to prepare the intaglio surface properly before cementation.
3. When in doubt, the doctor must read the instructions for use.
In addition to the three types of luting agents previously mentioned, there have been additions to the possible choice of luting agents by many new resin-modified cements, continued improvements to resin cements/bonding systems and the introduction of two bioactive luting agents: one a “nanostructurally integrating cement” and another that is a universal “bioactive” cement. These later two bring many new interesting considerations to the cementation equation.
Resin Cementing Processes
When working with porcelain, lithium disilicate, and silicate restorations and using resin cement for the luting process, it is imperative to treat the intaglio surface of the crown properly to ensure effective adhesion to the restoration and the tooth. According to Dr. Byung Suh, there are crucial steps that must be taken with these types of indirect restorations to maximize the performance of these resin cement systems (Table 1).2
First, every porcelain, lithium-disilicate, and silicate restoration should be etched with hydrofluoric acid. The laboratory typically does this prior to shipping the restoration. It is recommended that the etched restoration be silanated as soon as it is received from the laboratory. If the laboratory does not etch these types of restorations before shipping, then they must be etched in the office. Typically for lithium disilicate, 4% HF is placed into the dry intaglio surface for 25 seconds, rinsed and dried, then silanated.3 Why is silanation so important to do so far in advance from delivery of the restoration? It is because silane works via a condensation reaction, removing a water molecule from the porcelain silane interface, which then enables a prepared interface of the porcelain to accept the resin cement. This condensation reaction takes time to occur, so placing silane on these restorations upon arrival from the laboratory will enable this condensation process to occur. The use of pure silane is highly recommended because the condensation reaction will not occur in the presence of other molecules.4
Second, after the restoration has been tried in, it must then be decontaminated using phosphoric acid or placing the restoration in ethanol and ultrasonically cleaned for 2 minutes. Once this is done, the restoration is rinsed and dried, followed by the placement of a bonding agent that will work with the resin cement being used and the correlating resin cement to be used for cementation. Again, it is imperative that the information for use be properly followed to maximize the short- and long-term effects of resin bonding of the restoration.
The popularity and use of zirconia restorations has grown exponentially in the last few years. Yet, zirconia is an entity all to itself and must be treated very differently from the previously discussed porcelain, lithium disilicates, and silicate restorations. The intaglio surface of a zirconia restoration must never be etched with phosphoric acid. Using phosphoric acid on the intaglio surface of a zirconia restoration will deplete zirconia sites for the 10-methacryloyloxydecyl dihydrogen phosphate monomer to react, which is necessary for resin bonding to occur. Saliva also contains phosphates and will create zirconia site reduction after try-in. This contamination must be removed before cementation.
After try-in, the intaglio surface is cleaned with either a pumice scrub, ultrasonic cleaning (with alcohol or acetone), steam cleaning, or sand blasting with aluminum oxide 30-µm to 100-µm grit at 30 to 45 psi. Again, do not etch zirconia restorations with phosphoric acid.
Use a zirconia primer in the zirconia restoration. This can be done immediately after trying in the restoration because the reaction to zirconia works via an addition reaction, which is typically quite quick. Dry the restoration for 3 to 5 seconds and then place the bonding agent and resin cement as directed by the manufacturer.
There are “cleaning pastes” available to help remove the phosphate contamination prior to cementing without the cleaning process needed as described above. These pastes contain zirconium oxide and other ingredients to help expedite the decontamination process required prior to using a resin cement, and have been shown to be successful and very simple to use. After try-in, the cleaning paste is applied to the intaglio surface and left to react for 20 seconds. This paste is then rinsed and the restoration is dried; then the resin bonding agent and resin cement is applied as directed.
Glass-Ionomer Cements
The first glass-ionomer cement was developed in 1972 by A.D. Wilson and B.E. Kent.5 These powder/liquid materials were mixed manually. The appropriate ratio of powder to liquid was critical for optimal strength and working time. This created some difficulties with the consistency of the material, and eventually led to the use of capsule delivery systems because the materials mix could be more consistent and predictable; this is still the predominant delivery method used today. Resin-modified glass-ionomer cements were developed to add some of the beneficial properties, such as increased strength, of resins to the glass-ionomer cement materials. The main benefit to using glass-ionomer cement is the chemical fusion of the material to the tooth surface via ion transfer between the tooth and the cement/restoration material.6
There is no shrinkage associated with glass-ionomer cement use and the material can be bulk-filled. However, because glass-ionomer cements are acidic in nature, their proper setting and cure require water. If the tooth is desiccated when cementing, severe sensitivity can often result. This is because any remaining moisture in the dental tubules is removed to enable the completion of the chemical reaction and puts the odontoblasts in the pulp chamber in stress, so care must be taken to not over dry the tooth during the cementation process.
There is an added benefit with the use of glass-ionomer cement, and that is the release and recharging with fluoride. This made glass-ionomer cement a bioactive material. However, glass-ionomer cements do not contain calcium or phosphates and they cannot create apatite, which is being shown to be a beneficial feature when using some of the more recently developed cements.7
Bioactive Cements
The latest trends in cementation now incorporate bioactive cements that do not form a hybrid layer or require a bonding agent to enable adhesion to the crown or tooth structure. In fact, these materials can make cementation of our newer indirect restorations much easier, and are very effective in their use with conventional metallic and porcelain restorations.
One type of material is called a nano-structural integrating bioceramic (NIB) cement, which is a calcium aluminate (CaOAl2O3)-based cement, and the other is called an “universal bioactive cement.” These cements are both bioactive in that they enable the creation of hydroxyapatite at the cement/tooth interface8,9 and also minimize leakage at the margins of the restorations.10 They have also been shown to have antimicrobial properties11 and compressive and retentive strengths to all substrates have been demonstrated to be equal to or higher then resin cements.12
Additionally, these cements contain no HEMA, BPA, or BPA derivatives, thus providing an added bonus of biocompatibility in the oral cavity.
The calcium aluminate-NIB cement contains nanocrystals, preformed katoite (Ca3Al2(OH)12) and gibbsite (AL(OH)3), which are activated with water. These crystals then will interact with the tooth surface to create a mixed zone of chemically formed apatite, which then create a stable sealing interface between the restoration and the tooth structure. This interface is acid-resistant because of its alkaline (pH 8.5) properties. The material has a film thickness of 15 µm, compressive strength of 160 MPa, and radiopacity of 1.5 mm Al.
This cement is packaged as a capsule that is applied after trituration with an applicator device; the universal bioactive cement is used via an automix syringe. Both can be used on all indirect restorative intaglio surfaces. The cement recommends leaving the intaglio surface slightly moist when cementing, and no bonding agents are required. After try-in, the restoration is washed and dried with one quick air blast. There is no need to use silane or any other primers on the restoration. For lithium disilicate it is recommend that the intaglio surface be treated with 5% hydrofluoric acid for 20 seconds and rinsed before the application of the NIB cement. The working time after mixing is 2 minutes, at which time the gel state of the cement can be removed. The cement will fully set at 5 minutes. It is very easy to clean up the cement its gel phase, and there is typically no sensitivity experienced by the patient.
The “universal bioactive cement” works and is used differently then the calcium aluminate-NIB cement. This cement provides both calcium and phosphate components that are available immediately at the time of reaction, which creates an initial bond and apatite formation at the time of cementation to the tooth structure. This cement is also hydrophilic and has the ability to release and re-absorb ions in the oral environment. The “universal bioactive cement” also contains and releases fluoride, which the calcium aluminate-NIB cement does not.
After the zirconia restoration is tried in, the intaglio surface is decontaminated via sand blasting with aluminum oxide 30 µm to 100 µm grit at 30 to 45 psi or with the zirconia cleaning paste previously described. There is no use of primers or bonding agents before cementation. With lithium disilicates, the use of 5% hydrofluoric acid for 20 seconds, rinsed thoroughly and air-dried, is recommended.13,14 The intaglio surface is then primed with a silane-based primer. The cement is dispensed from the automix syringe and applied to the intaglio surface of the restoration and placed on the preparation within 30 seconds of dispensing the material. Gentle finger pressure is used to seat the restoration, and the buccal and lingual margins are flash-cured for 10 seconds with a light-curing unit for a total of 20 seconds. The excess cement is then removed from the buccal, lingual, and interproximal surfaces. The cement will fully cure in 2 minutes.
Conclusion
The indirect restorative materials we have available have significantly changed over the last 30 years, and our luting agents have evolved and changed with them. The possible available agents are still varied and many, but the advent of bioactive luting agents with strengths and durability similar or better then resin cements is providing clinicians a real choice that has not been available before, and can enable cementation procedures to be easier for the doctor and very likely better for the patients being treated.
About the Author
Dr. Comisi has been in private practice in Ithaca, New York, since 1983, and is the president and CEO of Dental Care with a Difference®, PC, where “Knowledge Brings Health”®. He is also Dental and Oral Medicine Liaison for Vigilant Biosciences, Inc., and is a clinical instructor in dentistry at the University of Rochester School of Medicine and Dentistry. He is a member of the National Dental Practice Based Research Network and the International and American Associations of Dental Research.
References
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3. Wilson AD, Kent BE. A new translucent cement for dentistry. The glass ionomer cement. Br Dent J. 1972;132(4):133-135.
4. Fruits TJ. An Atlas of Glass-Ionomer Cements: A Clinician's Guide. 3rd ed. London: Martin Dunitz Publishers; 2002.
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7. Engstrand J, Unosson E, Engqvist H. Hydroxyapatite formation on a novel dental cement in human saliva. ISRN Dentistry. (2012): Article ID 624056.
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10. Pameijer CH, Jefferies SR, Lööf J, Hermansson L. A comparative crown retention test using XeraCem™. J Dent Res. 2008;87(B):3099.
11. Ceramir Instructions for Use. http://www.ceramir.se/web/Bruksanvisning_1.aspx.
12. BioCem Instructions for Use. http://www.nusmilecrowns.com/pdf/IFU_28_BioCemCZ_TechnicalGuide_Rev1_Web.pdf.
