Cements for Today’s All-Ceramic Materials
A review of cementation for esthetics and strength
Matching cements to restorations can be challenging because there are many choices for the wide variety of restorations that are cemented into and onto teeth. Our previous article (Inside Dentistry, November 2013) described cementation choices and techniques for porcelain-fused-to-metal (PFM) and all-metal fixed prosthodontic restorations, as well as for the cast metal posts and cores and prefabricated metal posts used when restoring endodontically-treated teeth. This article will address cementation for today’s all-ceramic restorative materials, emphasizing important clinical concepts for clinicians and chairside assistants that will contribute to successful outcomes.
Cementation Today
All-ceramic is a catch-all category, but it really describes restorations fabricated without cast metal. All-ceramic restorations can be crowns, bridges, inlays/onlays, and veneers. These restorations are typically cemented adhesively using glass ionomer cements and bonded (adhesive) composite resin cements. Bonded composite resin cements are used for the cementation of fiber posts as well.
A critical element that contributes to restoration longevity is the correct choice of cement when placing the restoration. Adhesive cements used for different types of all-ceramic restorations require different surface treatments of the tooth or the tooth and restoration to optimize bonding. Even our best adhesive cements are not “permanent.” When describing the cementation technique to your patients, avoid using that phrase. A better description for the cementation step is “definitive cementation.”1
The planning and implementation of restorative procedures for crowns and bridges and restoring implants are among the most complex that a dentist provides a patient. Planning based upon periodontal health, tooth position, pulpal status, and occlusion necessitates a thorough evaluation. Tooth preparations require specific reductions and margin design and clearances to establish fit, shape, and contours that are compatible with gingival health and esthetics. Once fabricated by the laboratory or in the office with a CAD/CAM system, the restoration can be adhesively cemented. In the course of treatment, cementation of the restoration would be less than 5% of the overall time spent on all the clinical steps.
Although we would like all restorations to return from the laboratory with a “perfect” fit, this is generally not the case. Today’s contemporary adhesive cements provide for excellent marginal integrity to compensate for any discrepancies between the restorative material and tooth preparation.2,3 The contemporary cements used today are more retentive than cements used in the past.4 One significant clinical issue after cementation is the presence of postoperative sensitivity. To help avoid sensitivity after cementation, it has been recommended to not over-dry the tooth5 or to use a compatible desensitizing agent.6,7 This article will focus on the contemporary cements—glass ionomers and adhesive composite resins.
Ceramics for Restoration Fabrication
“All ceramic” is a broad description for a class of materials that are fabricated and finalized with high heat to fuse the particles into a restoration. Not all ceramics are the same. All-ceramic restoration materials can be divided into two distinct classifications: silicate ceramics and oxide ceramics.8
Silicate ceramics refer to porcelains with a glass-rich silicate matrix that have a multiphase structure. The key to these ceramics is that they are made from silica-containing glasses, similar to the sand at a beach. This silica structure has additives to improve the physical properties of the ceramic material so it can function in the mouth. These materials are then melted at relatively low sintering temperatures, generally less than 1000°C. Silicate ceramics are typically of two types: feldspathic glass used as the porcelain on PFM crowns and bridges or applied to oxide ceramic (zirconia or alumina) cores. Feldspathic porcelains were used, and are still used, for porcelain veneers, but are not as popular as glass ceramics. Glass ceramics are the most popular today for all-ceramic crowns and veneers because of their esthetics and high strength.
Oxide ceramics are fabricated from metal oxides, zirconia, or alumina. They are characterized as monophase and single-component, with metal oxides comprising more than 90% of the structure. These are polycrystalline ceramics that require sintering temperatures greater than 1400°C to fuse the components. These ceramics have very high strengths but are generally more opaque in appearance and not as esthetic as the silicate ceramics.
Interestingly, silicate ceramics when cemented with composite resin cements can be etched with hydrofluoric acid so that can be bonded to the tooth from the inside out. Although there are some primers for oxide ceramics, the adhesion is not as good at the interface between the restoration and the adhesive composite resin cement.
Dental Cements for All-Ceramic Restorations
For definitive cementation of all-ceramic crowns, bridges, inlays, and onlays, the dentist can choose resin-modified glass ionomers (RMGIs) or composite resins. These cements can be self-setting or dual curing, where a light can be used to set the cement at the margin of the restoration. For porcelain veneer cementation, the recommendation is to use an adhesive technique with a light-curing composite resin cement. Composite resin cements used today have a chemistry based upon our current generation of composite resins and bonding adhesives.9 Resin-based cements include composite resin cements that are self-adhesive and self-setting or those that require an additional bonding adhesives used before the cement is placed. These cements can be light curing only, self-curing, or dual cured.
The challenge to the chairside assistant for all-ceramic restorations is what steps are necessary both for pretreatment of the restoration and in preparation for cementation. Some cements are in pre-dosed capsules that can be mixed in a triturator, whereas others are dispensed onto a paper pad and are mixed with a cement spatula. Most composite resin cements and some RMGI cements can be mixed by extruding from a double-barrel system into automixing tips. No matter which cement is being used, these cements have physical properties that allow for a consistency and viscosity consistent with cementation and a film thickness that will allow for complete seating of a restoration during cementation.
It should be noted that there is variability in the handling characteristics of each class of cement, and even differences within the same class of cement. In almost all cases, the clinician should not assume that cements of the same class are mixed and manipulated the same. It is critical that the dentist and their chairside assistant read the instructions as it relates to material dispensing and mixing before it is used for cementing the restoration.1,10
Cements are recommended so that they match the physical properties of the restorative material being used. It is important to follow the manufacturer’s recommendations to achieve a clinically successful result.
Silicate Ceramics
Silicate ceramics have the unique chemistry that means once it is etched with hydrofluoric acid and bonded, the porcelain has improved physical properties.11 Due to the brittle nature of silicate porcelains in function, they can develop microcracks. The use of an adhesive composite resin cement strengthens the etched porcelain by interlocking into the microscopically roughened surface, not allowing the microcracks to propagate through the porcelain, which would create a fracture.
Porcelain veneers are typically fabricated from silicate ceramics—feldspathic porcelain, leucite-reinforced porcelain, and lithium disilicate porcelains. Silicate ceramics are more translucent than oxide ceramics and can be etched using hydrofluoric acid so that these thin veneers can be better bonded to the tooth. The cement of choice for porcelain veneers is a light-cured composite resin cement. These cements are usually provided in kits that contain an etch-and-rinse adhesive, a porcelain primer, and the light-cured cement in shades that are matched to the ceramic.
Crowns fabricated from leucite-reinforced ceramics can be cemented with a light-cured, self-cured, or dual-cured composite resin cement. Self-adhesive resin cements are typically not used. Since the introduction of lithium disilicate ceramics, a high-strength durable porcelain, the use of leucite-reinforced ceramics has been declining. Although some recommend the cementation of lithium disilicate ceramic restorations with an RMGI, these restorations can be cemented with any of the composite resin cements available on the market. Many lithium disilicate restorations are more slightly more opaque than leucite reinforced ceramics. Although veneers fabricated with lithium disilicate can be cemented with light-cured composite resin, it is not recommended that a light-cured resin cement be used for crowns because of their greater thickness. Dual-cured, self-cured and self-adhesive resin cements can be used.
Oxide Ceramics
Oxide ceramics are fabricated with metal oxides. These ceramics are very resistant to fracture. Unlike silicate ceramics that can be etched with hydrofluoric acid, oxide ceramics are resistant to etching.12 Using specialized air abrasive aluminum oxide/silicate coated particles, sand blasting can be effective in making the surface more adhesive.12,13 Cementation of oxide ceramics can be done with the same cements that are used for PFM restorations. Oxide ceramics, typically referred to as zirconia restorations, can be cemented with conventional glass ionomer, RMGIs, self-adhesive composite resins, dual-cured composite resins, and self-cured composite resin cements. In the experience of the authors, the easiest cements to use with zirconia restorations are RMGIs and self-adhesive composite resin cements. With both these types of cements, there is no need for the additional steps of etching and use of a separate bonding agent. Some manufacturers provide a separate surface primer for the zirconia before using a composite resin cement.
Conclusion
Success with cementation depends on the chairside assistant and dentist’s attention to detail. Table 1 provides practical tips for cementation. With the increase in the number of contemporary cements used for indirect restorations, clinicians are faced with an important choice when deciding which cement to use for each clinical situation. Although no one cement fulfills all the needs for all cementation, understanding the differences between each class of dental cement will contribute to clinical success of the restoration.
References
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8. Kunzelmann KH, Kern M, Pospiech P, et al. All-Ceramics at a Glance. 1st English ed. Wiesbaden, Germany: Society for Dental Ceramics; 2007:6-8.
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10. Strassler H, Suh BI, Wang Y. Compatibility of oxalate desensitizer with glass ionomer cements. J Dent Res. 2006;85(special issue A):abstract 1885.
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12. Baldissara P, Querzè M, Monaco C, et al. Efficacy of surface treatments on the bond strength of resin cements to two brands of zirconia ceramic. J Adhes Dent. 2013;15(3):259-267.
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14. Strassler HE. Fiber posts: a clinical update. Inside Dentistry. 2007;3(3):70-77.
15. Strassler HE, Campbell R, Wycall B, VonFraunhofer JA. Evaluation of two techniques for cement application into root canals. J Dent Res. 2000;79:435 (IADR abstracts):abstract 2332.
16. Asmussen E, Attat J, Degrange M. Adherence of resin based luting agents assessed by the energy of fracture. Acta Odontol Scand. 1993;51(4):235-240.
17. Wang C, Millstein PL, Nathanson D. Effects of cement, cement space, marginal design, seating aid materials, and seating force on crown cementation. J Prosthet Dent. 1992;67(6):786-790.
18. Strassler HE, Levine E. Contemporary dental cements. In: Freedman G, ed. Contemporary Esthetic Dentistry. St. Louis, MO: Elsevier; 2012:537-546.
About the Authors
Howard E. Strassler, DMD, FADM, FAGD
Professor and Director of Operative Dentistry
Department of Endodontics, Prosthodontics, and Operative Dentistry
University of Maryland Dental School
Baltimore, Maryland
Roseanna J. Morgan, CDA
Postgraduate Prosthodontics
University of Maryland Dental School
Baltimore, Maryland
