Bonded Cementation of Veneers and Onlays
Cementing Bonded Restorations
Bonded cementation is a crucial step in ensuring correct retention and marginal seal of bonded restorations. It is the bonded cementation of indirect onlays or veneers that tends to greatly concern and sometimes dissuade clinicians from embracing this newer technique. The fact that traditional crowns and bridges can be cemented very successfully with traditional crown and bridge cements such as resin-reinforced glass ionomer cements, which are easy to use and rarely cause sensitivity [2,3], can be a reason why many people prefer a full crown procedure. They see those familiar procedures as easier. Nevertheless, the price paid by the tooth and the gingiva for this apparent simplicity at cementation may be too high (Figure 9.1).
It is easy to understand why some clinicians stay away from bonded restorations. First, postoperative sensitivity continues to irritate dentists and patients after the bonded cementation of indirect bonded restorations [4,5]. Second, the importance and complexity of isolation can be challenging. Some educators suggest, without good literature support, that unless a rubber dam is used, the adhesive procedure will not succeed or will be of inferior quality. Such statements can easily make many clinicians shy away from adhesive procedures. According to Christensen, less than 10% of restorative dentists use a rubber dam routinely . Third, resin cementation is perceived to be a very sensitive and difficult technique, especially the removal of resin cement, because once it acquires its full hardness, removal of any excess can be very difficult. Additionally, previous failures may also be clear in some clinician’s minds, which may prevent them from adopting these newer techniques. These concerns can easily be overcome with some additional training. It is important to remember that proper preparation will lead to a much easier cementation. Aggressive preparations with a great deal of dentin exposure and subgingival margin placement will make the cementation procedure very difficult. The implementation of a supragingival protocol can greatly simplify this process, as can choosing the appropriate materials correctly (Figure 9.2a–d).
Postoperative sensitivity after the cementation of bonded onlays and veneers is well documented [7,8]. One important reason for this sensitivity is the incorrect use of geometric preparation with mechanical retention, which can prevent full seating of the restoration, owing to the film thickness of the adhesive cement. Polymerization and contraction of the cement will continue after maximum seating and will form internal gaps, leading to bite pressure sensitivity if the dentinal tubules are not perfectly sealed (Figure 9.3). Nevertheless, supragingival minimally invasive preparation techniques, combined with correct bonded cementation techniques can virtually eliminate the problem of postoperative pain. Occlusal adjustment is also a contributory factor in postoperative sensitivity and is discussed in this chapter.
Excellent isolation during bonded cementation is a must, as any contamination can lead to failure. This may lead some to suggest that unless a rubber dam is used, failures will occur. This is not supported by the literature or through clinical experience. Experience and the literature attest equal results with or without a rubber dam [9,10,11]. Correct preparation, choice of adhesive [12,13] and technique can play a key role in simplifying isolation. The implementation of a supragingival protocol during preparation makes cementation easier. The choice of self-etch bonding systems is of great importance because the acidic monomer of this system is designed to work in multiple levels of moisture on the teeth, making the use of an oral humidity controller (rubber dam) unnecessary . A number of simple devices for isolation are available and can be used during bonded cementation, including traditional cotton roll isolation (Figure 9.4a–e). Adhesive cementation requires excellent isolation.
Bonded Cementation has Multiple Layers
It must be remembered that when bonding an indirect restoration there will always be two substrates being bonded together by a resin cement and an adhesive, so there are multiple layers that require great attention (Figure 9.4a–e). The correct choice of adhesive and cement is of great importance. The two substrates or surfaces that are being joined together, the tooth surface (enamel and/or dentin) and the restoration surface or intaglio surface must be prepared prior to bonded cementation.
Choosing the Correct Resin Cement
Resin cements have characteristics which make them the ideal type of cement for bonded indirect restorations, like high tensile strength, high compressive strength, low modulus of elasticity and low solubility [15,16]. The use of these strong adhesive resin cements in fact enhances the strength of composite, feldspathic porcelain and lithium disilicate restorations , although they have minimal effect on zirconia and alumina. There are a number of different types of resin cements, each with their own advantages and disadvantages. Depending on the type of activator used for polymerization, the resin cements can be categorized as chemically (self) cured, light-cured and dual-cured. Most resin cements do not adhere to the tooth without the help of a dental adhesive or bonding system, with the exception of the self-adhesive type cements. There are many other important differences between resin cements, including filler particle size (which affects the ability to polish the tooth), available colors, opacity, consistency, speed of curing . A crucial characteristic of a resin cement required for the implementation of supragingival dentistry is optimal translucency, as rule 5requires the proper use of translucency to achieve a good supragingival restorative margin blending, and the cement is one of the three components of this rule . Attention must be paid to avoiding the use of opacious cements, which will decrease the translucency of the restorative material of choice, and will force the need to hide the margins subgingivally (Figure 9.5a–c). Finally, choosing a cement with excellent chameleon characteristics, the ability to blend with other colors and absorbing them is very desirable, as it simplifies the choice of cement shade.
The Ideal Cement for Bonded Onlays
The first important consideration when choosing a resin cement is whether it will be light-curing, self-curing or dual-curing. Full self-curing cements are more hydrophilic, less color stable and harder to use because, after the restoration is fully seated, the clinician has to wait until the cement has set. This may compromise isolation, as the longer that isolation needs to be maintained the more difficult it is. The fact that onlays can vary in thickness anywhere from 2 mm to 5 mm (Figure 9.6) clearly makes light-cured cements unpredictable. Research has shown that an insufficient amount of light may reach the cement in thicker and darker restorations and may adversely affect the degree of conversion [20,21]. With light-cured cements it becomes worrisome, as there may be some parts of the cement under the onlay that will not cure correctly or fully. Some clinicians suggest warming the restorative composite regularly. This is impractical in a busy practice, as composite has to be warmed in advance to make it less thick. Additionally, the rigidity of a traditional resin cement may be undesirable for cementation, due to polymerization shrinkage.
For all of the above reasons, dual-cure resin cement is preferred (Figure 9.7). It sets on command, making isolation simpler and, in cases of thick porcelain areas where light may not fully penetrate, a chemical activator will polymerize the cement. The literature points to the fact that dual-cure cements that are not light activated will have less desirable mechanical properties [22,23].Attention must be paid to ensuring that the cement is compatible with self-etch bonding systems, as the acidity of self-etch can have a negative effect on some dual-cure cements [24,25]. I have used Clearfil™ Esthetic Cement (Kuraray) extensively with success. Simplicity in practice is a must and for posterior restorations only one shade is necessary: Universal. The great chameleon ability of this cement allows for an enhanced blending ability of the restoration, providing simple and predictable results. Other cements that work well for onlays are Bifix QM (VOCO), Multi-Link (Ivoclar), Starfil 2B (Danville), NX3 Nexus™ Third Generation (Kerr) and Panavia V5 (Kuraray). Panavia F 2.0 also has the capability of direct adhesion. No primer is needed with metal oxides such as zirconia, alumina, and base metal (“non-precious”) alloys. On these substrates (metal oxides), only sandblast conditioning is needed .
Self-adhesive resin cements also have a history of successful use, when used as indicated and in appropriate cases, such as mechanically retentive preparations with supragingival margins, where contamination and lower adhesion may not be a problem [27,28].A decrease in bond strength to enamel has been shown, so it may be risky to use self-adhesive resin cements with preparations with no mechanical retention . With these materials, selective enamel etching is contraindicated because if any dentin is etched inadvertently, an important decrease in dentin bond strength will occur.
The Ideal Cements for Porcelain Veneers
Light-cured cements are considered to be more color stable because they do not include tertiary amine activators . Combined with the fact that anterior porcelain veneers should be thin, light activation is predictable, which makes light-cured cement ideal for porcelain veneers. The most important characteristic of the cement becomes the proper translucency and the ability of the cement to enhance the blending of a thin, translucent restoration. Opacious cement would be contraindicated. The market is replete with veneer cement systems which offer different colors and levels of translucency. The most important determinants of the final shade and opacity of the finished restorations are the tooth (preparation) color and the type, and thickness of the ceramic veneer . The veneer cement will influence the final color  to a lesser extent, unless opacious cements are used, but we already know that if supragingival margins are desired, opacious cement is contraindicated. Some techniques suggest using the cement to dramatically alter the color and value of the final restoration, or to mask the color under the restoration. This technique can be very complicated and can lead to some esthetic failures. Especially problematic are cases where there are variations of shade between the prepared teeth (Figure 9.8), and the different veneer and cement thickness, all which will make the perfect selection of different cements difficult and unpredictable. It is preferable to alter the color of the teeth prior to preparation by bleaching, internal bleaching or other means, and to allow a knowledgeable technician to undertake the necessary masking on the porcelain, as this will greatly simplify the results. Occasionally, a very difficult area such as a deep stain can be blocked out with a fully opaque flowable composite, (such as Accolade OP Mask™), which can be placed on the tooth before the impression is taken. The technician should always receive a photograph of the teeth next to a shade tab or tabs, if there is difference in shade between the teeth or parts of a tooth.