Cementation of Indirect Restorations: An Overview of Resin Cements

Cementation is a crucial step in the process of ensuring the retention, marginal seal, and durability of indirect restorations.¹ Since the introduction of the first all-porcelain crown in the early 1900s, various cements have been used to adhere porcelain crowns to tooth structure. Initially, only luting cements were available for use, which led to many failures. Presently, another category has been added—the adhesive cements. Resin cements fall into this category of adhesive cements. Adhesive cements must bond to a variety of different substrates, including dentin and enamel, porcelain and other ceramics, gold and other metal alloys, and indirect resin composites.²

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.³ Flexural properties—including modulus and strength—are important to prevent de-bonding 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.^4,5

The disadvantages of resin cements are associated with their technique sensitivity and difficulty with clean-up. Resin cements may change shade during curing and can darken during their lifetime. This can be a crucial factor, especially since esthetics is a particularly important characteristic for all-ceramic restorations.⁶ 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.⁶

Resin cements may be classified according to their polymerization mechanisms into light-cured, chemical-cured, and dual-cured (Table 1). They can also be classified by their adhesive scheme: total-etch, self-etching, and self-adhesive (Table 2).⁷ The self-adhesive resin cements may be referred to as “all-in-one” resin cements or universal cements.⁶

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.⁶

Light-Cure Resin Cements—Light-cure resin cements utilize photo-initiators, which are activated by light. The ability of light to penetrate all areas and activate the photo-initiators 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.⁶

Another advantage of light-cure cements is their color stability compared to dual-cure or chemical-cure resin cements.⁸ These cements are, therefore, suitable for esthetic restorations and metal-free restorations.^9,10 Light-polymerized resins are recommended when cementing ceramic that is thin and fairly translucent, allowing the transmission of light through it to reach the resin cement.¹

Examples of light-polymerized cements include: RelyX™ Veneer Cement (3M ESPE, www.3MESPE.com); Variolink^® Veneer (Ivoclar Vivadent Inc., www.ivoclarvivadent.com); and Choice™ 2 Light-Cured Veneer Cement (BISCO, Inc., www.bisco.com).

Dual-Cure Resin Cements—Dual-cure resin cements are capable of being cured by means of both chemicals and light. Self-cure initiators that can cure the cement are present. In addition, a curing light can be used to activate the photo-initiators that are present in the cement.⁶ Dual-polymerized resin cements are indicated when the ceramic is too thick or too opaque to allow transmission of light through it.¹¹ Studies have shown that these dual-cure resin cements still require light-curing to reach a high degree of polymerization.^12,13 These cements are used for metal-free restorations where light-curing may be performed to quickly seal margins.⁶

Examples include: NX3 Nexus^® Third Generation (Kerr Corporation, www.kerrdental.com); RelyX™ ARC Adhesive Resin Cement (3M ESPE); and Variolink^® II (Ivoclar Vivadent Inc.).

Chemical-Cure Resin Cements—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.⁶ 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.⁶

Chemically polymerized resin cements do not offer much selection in terms of shade and translucency; therefore, dual-polymerized resin cements can be beneficial. Additionally, accessible areas benefit from light polymerization with dual-polymerized resin cements.¹⁴

Examples include: Panavia™ (Kuraray Dental, www.kuraraydental.com); and C&B™ Cement (BISCO, Inc.).

Total-Etch Resin Cements—Total-etch resin cements use a 30% to 40% phosphoric acid-etch to etch dentin and enamel. This etching procedure removes the smear layer, and dentinal tubules are opened.² After etching, the adhesive is then applied to the preparation to bond the cement to the tooth. These cements and the adhesives used with them can be light- or dual-cured.² 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 This category provides the highest cement-to-tooth bond but also requires the most steps to bond ceramic, composite resin, or metal to the tooth. This multi-step application technique is complex and consequently might compromise bonding effectiveness,⁷ because each step represents a possible contamination point.⁷

Examples include: RelyX ARC (3M ESPE); Variolink II (Ivoclar Vivadent Inc.); Choice 2 (BISCO, Inc.); and Calibra^® (DENTSPLY Caulk, www.caulk.com).

Self-Etch Resin Cements—Self-etch systems apply a self-etching primer to prepare the tooth surface, and mixed cement is applied over the primer. Bonds to tooth structure using this category of cements are almost as high as those of the total-etch cements.²

Self-etching systems are popular among dentists because they are easy to use, but as a general category they have demonstrated bond strength to enamel that is weaker than that of total-etch systems.¹⁶ Therefore, the total-etch, three-step adhesive system of some 30 years ago still sets the standard in terms of versatility and long-term predictability.¹⁷

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.¹⁸ However, it is imperative to follow the manufacturer’s instructions during adhesive cementation, including use of the manufacturer’s adhesive and resin cement combination, because investigators have found incompatibilities between some dual-cure resin cements and simplified adhesive systems.¹⁹

Self-Adhesive Resin Cements—A number of resin cements have been introduced as one-component “universal adhesive cements”; they are said to have good bond strengths to dentin, enamel, and porcelains without the need for separate bonding agents.^20-22 These self-adhesive cements can bond to an untreated tooth surface that has not been micro-abraded or pretreated with an etchant, primer, or bonding agent; thus, cementation is accomplished in a single step. These cements 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.⁹

Data from Burgess et al (2010)⁷ shows that most of these cements bond better to dentin than to enamel. With most of the cements in this category, the bond to enamel is improved when an etchant and bonding agent are applied.²³ This “selective-etch” approach uses an etchant or a self-etching primer before applying the self-adhesive resin cement. In other words, “selectively etching” enamel and/or dentin surfaces and indirect restorations may be incorporated to improve the bond of these self-adhesive resin cements. However, a lower bond strength to dentin was shown when the phosphoric acid pre-etch was applied.²³ In contrast to enamel, when dentin is etched with phosphoric acid and a bonding agent is applied with one of these cements, the bond decreases.⁷ This negative effect of pre-etching of dentin for self-etch adhesives has been shown many times in the literature.²³ When tested without the pre-etch, self-adhesive resin cements have been shown to produce fairly strong bonds to dentin.^23-26 Examples include: RelyX™ Unicem (3M ESPE), BisCem^® (BISCO, Inc.), Maxcem Elite™ (Kerr Corporation), SpeedCEM™ (Ivoclar Vivadent Inc.).

In Simon et al (2011), it appears that the self-adhesive cements are on par with the values seen for resin cements. A possible explanation for this observation is that the self-adhesive cements are changing. For instance, older tests used RelyX Unicem cement in capsule format, whereas in this newer test the Unicem Clicker™ Dispense (3M ESPE) system was used. The cement was reformulated to accommodate the new delivery system. The changes in observed strengths may be due to incremental improvements made by the manufacturers.²⁷

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.⁸

The excellent physical properties and ability of resin luting materials to facilitate “adhesive” luting of restorations are distinct clinical advantages, because the clinician may not always be able to achieve an ideal taper or crown height.²⁸ In a recent study by Simon et al,²⁷ the tensile bond strength of ceramic crowns milled with computer-assisted design/computer-assisted machining (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.²⁷

The efficiency by which the self-adhesive resin cements adapt to and seal margins is critical for their success.² Studies have found conflicting results when analyzing the marginal seal and micro­leakage of resin cements.^29,30 However, self-etch and total-etch resin cements tend to have adequate marginal adaptation compared to glass ionomers, resin-modified glass ionomers (RMGIs), and zinc phosphate cements.³¹

Because self-adhesive cements bond to tooth structure, excess cement should be removed before setting to avoid damaging the weaker early bond.⁷ Self-adhesive cements are dual-cured, and like all dual-cured cements, have reduced bond strengths, color stability, and wear resistance in the self-cure-only mode.⁷ Therefore, the clinician should light-activate all dual-curing cements at accessible restorative margins to improve marginal integrity and wear resistance and to reduce staining.⁷

The key points for clinicians to bear in mind regarding resin-based cements are:

• Bond strengths vary among specific cements, but total-etch cements generally provide the greatest retention; self-etching systems are intermediate; and self-adhesive cements can provide bond strength nearly equal to self-etching systems.⁷
• Self-adhesive resin cements can create bond strengths to dentin that exceed the strength of the ceramic material.²⁷
• High bond strengths can be achieved with self-adhesive resin cements on non-retentive teeth.²⁷
• Large coefficients of variation indicate that the bond strength cannot consistently be achieved.²⁷

While in vitro data may show some cement bond differences among investigators, self-adhesive cements perform well clinically.³² Although clinical evaluations are few and short-term, the result of this review of self-adhesive resin cements would suggest that these materials may be expected to show clinical performance similar to that of other resin-based and nonresin-based dental cements.² The handling properties of these materials appear to be excellent, and their acceptance by the profession is increasing.²

Catherine Stamatacos, DDS
Assistant Professor
Director
Research and Education
Department of Restorative Dentistry
College of Dentistry
University of Tennessee Health Science Center
Memphis, Tennessee

James F. Simon, DDS, Med
Professor
Director, Division of Esthetic Dentistry
College of Dentistry
University of Tennessee Health Science Center
Memphis, Tennessee