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Among the many factors involved in accomplishing a successful restoration, selecting the proper cement to achieve adequate bond strength is highly critical. The choice of the correct cement for a given application has become increasingly complex as new cements are being created that are designed to be easier to handle, better performing, and more reliable than products that have been available in the past. As new cement materials continue to be developed, application procedures are being modified and the shift to more conservative preparations aimed at maintaining tooth structure progresses.
Properties of Cements
Several properties of cements affect the clinical outcome of a restoration. With older cements, attachment of the restoration to the tooth from the cement was typically mechanical in nature. Most newer resin cements use bonding agents and provide a chemical bond to tooth structure. Resin cements may change shade during curing and can darken during their lifetime. This can be a crucial factor, especially since esthetics is particularly important for all-ceramic restorations.
A film thickness varying between 5 µm and 25 µm is optimal in order to fully seat a restoration. Viscosity and ease of handling are important properties in order to ensure that the restoration may be seated. While glass ionomers and resin-modified glass ionomers (RMGIs) are more tolerable of moisture, resin cements should be used with isolation. Additionally, it is desirable to use cements with low water sorption to prevent expansion.
Flexural properties, including modulus and strength, are important to prevent debonding during function, and resin cements have both a high modulus and strength. In fact, they have the highest strength of the cements currently in use.1,2
For many years, early materials used to fix a restoration into place were limited to zinc oxide-eugenol and zinc phosphate cements. The primary retention for these restorations was dependent upon the preparation design, and the function of the cement was to fill the gap between the restoration and the tooth. Newer materials have the ability to do more. The term "luting" is now being used for these materials, as they are able to bond to both the tooth and the indirect material to help increase the retention of the restoration and provide a less water-soluble barrier at the margin of the restoration.3 This has enabled a change in the preparation design principles, allowing for more conservative tooth reduction while putting a greater dependence on the long-term bond strength of the luting agent.
Some of the more desirable properties of luting agents4 are listed in Table 1. Many newer luting agents are approaching fulfillment of all of the criteria listed in this table; however, none have completely overcome all of the problems of working in the oral cavity.
For a long time, zinc phosphate was the most popular cement. It was clinically successful for many years in spite of such shortcomings as potentially causing pulpal irritation due to low pH, being relatively soluble in the oral environment, and having no adhesive qualities nor antibacterial action.5 Zinc phosphate can be used to cement all-metal or porcelain-to-metal crowns and gold posts.
In an attempt to address some of these issues associated with zinc phosphate cement, zinc polycarboxylate cement was introduced. This cement featured such advantages as being somewhat able to bond to tooth structure, provide minimal postoperative sensitivity, and offer a slight amount of antibacterial action. The primary disadvantages of zinc polycarboxylate cement were its handling characteristics–it had a short working time and long setting time, and clean-up was difficult.6
Because of the handling issues with zinc polycarboxylate cement, it was readily replaced with the glass-ionomer and RMGI cements upon their introduction into the marketplace. These cements have the advantages of easier mixing and handling properties, fluoride release, and potential adhesive properties. The hybrid ionomers improved handling characteristics, allowed for dual-curing, and provided a higher flexural strength and bond to the tooth. Like conventional glass ionomers, hybrids release clinically significant amounts of fluoride7 and have been shown to clinically perform well due to their physical properties.8,9 Hybrid ionomer cements have maintained a significant role in dentistry because of their fluoride release. These cements can be used for all-metal posts and crowns, porcelain and metal crowns, and reinforced ceramic crowns. They are not, however, recommended for all-ceramic crowns. Restorations with intracrevicular margins in molar regions where crevicular fluids, salivary flow, and/or tongue control can present clinical challenges to the dentist in maintaining a dry operating field are good applications for considering using an RMGI cement.10
The final group of luting materials is resin-based cements. Resin cements are composed of the same basic components as composite restorative material but with lower concentrations of filler particle (50% to 70% by weight with glass or silica).11 This group has the advantages of high strength, low solubility, and high micromechanical bonding to tooth and ceramic,6 however the problems with resin cements arise with their technique sensitivity and difficulty with clean-up. These cements can be light-cured, chemical-cured, or a combination of both. Because these materials depend upon bonding, the operator must be careful to follow all steps in proper order and with the recommended time for each step.
Table 2 provides a summation of cement types and their applications.
When using resin-based cements, the internal surface of the restoration must be treated differently than the surface of the tooth, because the surface treatment depends upon the type of material (metal, ceramic, or zirconia) used for the restoration. The surface of the tooth may need to be treated with phosphoric acid, while the surface of the restoration may need to be treated with hydrofluoric acid, sandblasting, and silanization.12 Each of these steps needs to come together correctly to make a bonded restoration. Product manufacturers have been simplifying the technique and have now attempted to combine the steps to make it easier for the clinician to achieve a well-bonded restoration. These luting materials can be used for any type of restoration, but must be used when placing an all-ceramic restoration. Luting resins can be divided into two groups based on the application steps: total-etch and self-etch.
Resin Luting Agents
Resin cements bond to the organic phase of dentin. Resins are blends of polymerizable monomers of methacrylates, dimethacrylates, and polymethacrylates, and resin cements are similar to composite restorative materials but with lower percentages of filler particles.1 Bonding of these cements is based on the formation of a hybrid layer as seen in literature of dentin bonding with composites.4 These resin cements are the latest development in the area of luting agents. Manufacturers continue to modify and improve these products. As a result, the classifications by the manufacturers may be unclear. The main classifications of resin luting agents include two groups: one that requires the application of a dentin bonding agent prior to use, and another that does not require its use (a dentin bonding agent may be optional in this group). The latter group may be referred to as "all-in-one" resin cements, universal cements, and self-adhesive resin cements.
Recently, cements have been developed that do not use a dentin bonding system prior to placement. These cements are considered self-adhesive and self-etching. They contain phosphoric acid, which is grafted into the resin. Once mixing is initiated, the phosphoric acid reacts with filler particles and dentin in the presence of water, forming a bond. The resin is polymerized into a cross-linked polymer, as is the case with composite resin bonding.3 Resin cements that incorporate self-etching primers eliminate steps during application with the goal of reducing operator errors and technique sensitivity and increasing ease of use.13 With these cements, the clinician no longer has to pretreat both the tooth and the restoration before bonding.4 The bonding potential of these cements is somewhat less than that of resin cements that are accompanied by a separate self-etching primer. One study revealed shear bond strengths of 4.8 MPa to 7 MPa for self-etching resin cements, and shear bond strengths of 15 MPa to 28 MPa for resin cements with a separate dentin bonding application.14
Total-etch resin cements have increased the bond strengths of resin-based cements to nearly that of enamel bonding and have significantly reduced microleakage.15 Total-etch resin cements use 30% to 40% phosphoric acid to etch dentin and enamel. This etching procedure removes the smear layer, and dentinal tubules are opened. After etching, the adhesive is then applied to the preparation. Sensitivity has been a concern for some clinicians when using the total-etch technique. A study by Perdigão et al16 revealed no statistically significant difference in postoperative sensitivity for patients who had restorations cemented using the total-etch technique compared to those that had restorations cemented using the self-etch technique. However, some investigators13 recommend that use of the total-etch technique be limited to restorations that are mainly in enamel or when enamel margins are cracked.
These cements set by additional polymerization, and the polymerization reaction can be initiated by chemicals, light, or both (dual cure). The curing methods are a factor in dictating the potential uses of the cements. For example, in cases where very little or no light-cure is possible, chemical-cure cement is a better choice than either a dual-cure or, of course, a light-cure cement.
Chemical-cure resin cements polymerize with a chemical reaction and are referred to as self-curing. This means that two materials must be mixed together to initiate this reaction. As stated previously, these cements are especially useful in areas where light-curing is difficult. Some examples include metal restorations, endodontic posts, and ceramic restorations that prohibit the curing unit from adequately polymerizing the resin cement.
Light-cure resin cements have photoinitiators, which are activated by light. The ability of light to penetrate all areas and activate the photoinitiators is important with this type of cement. An advantage of light-curing cements is that there can be an increased working time compared to the other cure types. The clinician has the ability to remove excess cement before curing, and thus the finishing time required is decreased. Another advantage of light-cure cements is their color stability compared to dual-cure or chemical-cure resin cements.12 These cements are, therefore, suitable for esthetic restorations and metal-free restorations.3,17 An important factor is the thickness of the restoration.18 If the depth of cure is not large enough then the cement will not achieve its optimal strength, which could result in failure of the restoration. According to ISO standard 4049, the depth of cure of resin cements must be greater than 1.5 mm.19 However, some studies have shown that for thicker restorations, light-cured resins do not reach their maximum cure and either using a dual-cure resin or increasing the curing time may be recommended.20-23
Dual-cure resin cements are capable of being cured by means of both chemical and light. Self-cure initiators that can cure the cement are present. In addition, a curing light can be used to activate the photoinitiators that are present in the cement. Studies have shown that these dual-cure resin cements still require light-curing to reach a high degree of polymerization.24,25 These cements are used for metal-free restorations where light-curing may be performed to quickly seal margins.
There are several characteristics of resin cements that make them clinically superior luting agents. Resin cements may have high bond strengths both to tooth structure and porcelain, high tensile and compressive strengths, and the lowest solubility of the available cements.5 ISO 4049 states the physical properties of resin cements. The minimum flexural strength is 50 MPa, water sorption rates must be less than 40 µg/mm3 , and the radiopacity must be equivalent to aluminum.19 Just as with composite resins, these luting agents are technique-sensitive. Proper material selection and cementation procedures are crucial to the long-term success of the restorations.
Comparing Physical Properties
Several studies have been conducted to compare the physical properties of cements. One such study26 compared self-etch and total-etch resin cements with zinc phosphates, glass ionomers, and RMGIs. The resin cements had similar compressive and flexural strengths, but they were significantly stronger than zinc phosphate cements, glass ionomers, and RMGIs. Another study27 found that the compressive strength of a total-etch resin cement was higher than the compressive strength of a self-etch resin cement; however, the self-etch resin cement had higher flexural strength than the total-etch resin cement. Both resin cements had higher flexural and compressive strengths compared to polycarboxylate cement.
Studies have found conflicting results when analyzing the marginal seal and microleakage of resin cements.28,29 However, self-etch and total-etch resin cements tend to have adequate marginal adaptation compared to glass ionomers, RMGIs, and zinc phosphate cements.30 When crown margins are in dentin there is more leakage at the margin compared to when crown margins are in enamel.26 Studies comparing self-adhesive resin cements to other resin cements as well as other types of cements have shown that applying a phosphoric etch on the enamel margins may provide additional strength.31 When analyzing failure of cements under cyclic loading, Uy et al32 found that for each of the three resin cements tested, similar numbers of cycles were performed until failure. When analyzing where failures occurred, resin cements failed at the tooth–cement interface, while zinc phosphate cement failed at the cement–crown interface.
High-strength ceramic restorations with zirconia or alumina cores differ in both physical properties and composition compared to silica-based ceramics. They require a different surface treatment in order to roughen the surface.31 They are not etchable with phosphoric acid and cannot be bonded in the same way that other ceramics are. Some surface treatments used for high-strength all-ceramic restorations include hydrofluoric acid-etching, airborne particle abrasion, silane coupling, and various combinations of these methods. All-ceramic restorations are etchable with hydrofluoric acid (HF) and should always be bonded with silane primer, bonding agents, and resin cement.33 It has been shown that 2.5% to 10% HF applied for 2 to 3 minutes was successful in removing the glassy matrix of the ceramic.34
Silane coupling has been shown to be a major factor in the ability to create a durable bond between tooth structure and an all-ceramic restoration.35 Silanes are bifunctional molecules that bond with both the ceramic surface as well as the organic matrix in the resin.36 Studies have shown that restorations that underwent airborne-particle abrasion alone were not as retentive as those that had first been silanated.37 A study by Sorensen et al38 showed that etching and silanization had an additional benefit of decreasing microleakage.
The choice of cements is becoming increasingly complicated as manufacturers develop new materials and application procedures become altered. Not only is it important to select the proper cement, but the clinician must also understand the specifics of the material being used in order to employ the proper surface treatment for cementation.
In a recent study by Simon et al,39 the tensile bond strength of ceramic crowns milled with computer-aided design/computer-aided manufacturing (CAD/CAM) technology was tested on overly prepared teeth. This study found in a number of cases that the retention of the crowns supplied by three leading adhesive cements was stronger than the ceramic crown itself; however, these high bond strengths cannot be achieved on a consistent basis. This inconsistency can be a potential problem as dentistry moves to more conservative, less retentive preparations in an effort to maintain tooth structure. There are many steps to a successful restoration, and they all must be followed meticulously.
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About the Authors
James F. Simon, DDS, MEd
Division of Esthetic Dentistry
Department of Restorative Dentistry
University of Tennessee College of Dentistry
Laura A. Darnell, DDS, PhD
Department of Restorative Dentistry
University of Tennessee College of Dentistry