Should We Trust in Dental Adhesives and Adhesive Dentistry?
Adhesion plays an indispensable role in modern dentistry. Much is owed to the pioneers of adhesive dentistry, who faced many obstacles in helping the profession accept these techniques [1,2,3]. When dentists fully trust adhesion, the need for mechanical retentive features, such as retention and resistance forms, is eliminated, and thus a more healthy tooth structure can be preserved. The health benefits do not end with tooth preservation. Through confidence in adhesion and bonding translucent restorative materials, clinicians can retain the restorative margins above the gum line by eliminating the need for traditional mechanical retentive features such as axial walls, boxes or ferrules, thus preserving periodontal health. Unfortunately, there is a current lack of trust that adhesion can truly replace retention and resistance mechanically retentive features. This lack of trust has prevented the widespread acceptance of partial coverage restorations retained solely by adhesion. Teeth continue to be drilled way past the point where caries has been fully removed, and often vital tooth structure is cut away for the purpose of traditional mechanical retention (Figures 4.1–4.3a,b).
Trusting in Adhesion is Key To Success
Supragingival minimally invasive dentistry depends upon adhesion and trust in adhesion. Lack of trust in dental adhesives should be reconsidered in light of the advances in adhesive technology and the trust that other industries place in adhesion. In modern airplanes, newer cars and even building construction, adhesives are increasingly replacing conventional joining techniques such as welding, riveting or bolting. Modern industries are placing great trust in adhesion and allowing crucial components to be held together by this means [4,5]. Modern society as a whole depends on adhesion.
Lack of trust in adhesion as a full replacement for retention and resistance form in restorative dentistry has slowed the acceptance of supragingival partial-coverage, tooth-preserving, restorative dentistry. Lack of trust in adhesion while performing bonded restoration in fact decreases success with bonded restorations. Deep-rooted habits such as the need to have retention and resistance form and ferrule often leads restorative dentists to undertake axial wall preparation in planned bonded restorations. Margins are thus placed subgingivally, making adhesive cementation more difficult and more unpredictable (Figures 4.4, 4.5a,b). Supragingival margins with a good enamel periphery are ideal for achieving predictable success with adhesive dentistry , so there is a need for a supragingival protocol which depends fully on adhesive retention of the restoration. It is important to note that preserving as much enamel as possible increases durability and success. Nevertheless, this should not be understood to imply that a lack of a full enamel margin is a contraindication with bonded restorations. The lack of enamel margin in segments of the peripheral margin is not a contraindication to bonded onlays and veneers, as clinical experience and current literature provides evidence that, with some bonding systems like Clearfil™ SE Protect (Kuraray) adhesion to dentin may in fact be reaching similar success as adhesion to enamel [7,8,9].
Restorations retained purely by adhesion, such as porcelain veneers, have shown some of the best longevity of any restoration, especially when the preparations have been left mostly on enamel [10,11,12,13]. Direct bonded composite restorations have rapidly replaced traditional mechanically retained amalgam with great predictability and success [14,15]. Other types of largely adhesively retained restorations, such as class V and posterior-bonded inlays and onlays, have been successful but have also encountered some complications, caused mainly by improper technique. Nevertheless, evidence from the literature and from clinical experience has shown that adhesion retention itself is not the primary reason for the failure of such restorations. In a retrospective clinical evaluation, Ruiz and colleagues  reported 100% inlay/onlay restoration success at 54 months, with no retention failures and only two small reparable fractures in 57 restorations recalled. Understanding the limited value of anecdote, in my personal experience in full-time private practice, placing approximately 10,000 bonded veneers (Figure 4.6a–c) and onlays, and many more thousands of direct composite restorations over the past 24 years, restoration failure due to loss of retention has been extremely rare and limited to a handful of cases. Evidence from the literature shows that, with bonded restorations, failure has most often been related to restorative material fracture and marginal leakage. Both problems are related to the failure of restorative materials, improper management of occlusion, complications caused by subgingival margins and difficulty in isolation, and improper technique, but not to lack of adhesive retention [8,17,18,19].
Adhesion to Enamel
As previously discussed, adhesion to enamel has been considered to be the ideal interface to which to bond because it is reliable and durable. In fact, not much has changed in 50 years since enamel etching was introduced by Michael Buonocore in 1955  with a bond strength of 15–20 MPa. The only real change has been in etching time. While the original etching time with 85% acid was one full minute, currently enamel is only etched for 20–25 seconds with 35–37% phosphoric acid. It is now known that 37% acid is a more aggressive etchant than 85% acid. Even when using self-etch bonding systems, selective enamel etching enhances adhesion  and seal , both very important for long-term success (Figures 4.7a–c, 4.8a,b). Because enamel is the most predictable substrate on which to bond, success with bonded restorations improves greatly when enamel is preserved, which is one of the five principles of supragingival dentistry.
Adhesion to Dentin
Bonding to dentin has been less consistent or reliable, because dentin is a much more complex substrate to which to bond. It is 30% organic and 20% fluid (Figure 4.9). It was only after the introduction of hydrophilic resin infiltration  and the total etch concept , and subsequent improvements in the dentin bonding systems that true adhesion to dentin has become possible . To define clinical success with dentin bonding, multiple factors must be taken into consideration. What makes for predictable clinical success bond strength is very important, but not the only factor to consider. What good is having amazing bond strength if postoperative sensitivity makes the restoration a failure from the patient’s perspective or it ends up causing pulp damage? During early attempts to bond to dentin, the entire dental community suffered, as even the smallest direct composite restoration led to postoperative pain and much frustration. We now know that the shallowest class I restorations are the most prone to sensitivity to bite pressure. The reason for this is higher shrinkage stresses in these composites, described by configuration factors  (Figure 4.10). In addition to bond strength, other important characteristics are that adhesives should produce no postoperative sensitivity, should have a good marginal seal, should be easy to use, and should be durable. The issue of the durability of bond to dentin has been one of the major concerns, although newer bonding systems such as Clearfil SE Protect have improved this situation .
There have been different categories and names for the fourth to seventh generations of bonding systems. Regardless of the nomenclature, though, they can be divided into two families, the etch and rinse (also called “total etch”) and the self-etch (Table 4.1).
Table 4.1 The bonding families.
|2 Bottles (4th generation)||2 Bottles (6th generation)|
|1 Bottle (5th generation)||1 Bottle (7th generation)|
Etch and Rinse Systems (Fourth and Fifth Generations)
The first generation of bonding systems to actually achieve good adhesion to dentin is the so-called fourth-generation, two-component system. This system was first introduced by Fusayama in 1979 and Nakabayashi in 1982. These fourth-generation bonding systems have a long history of success.
Fourth-generation bonding systems work by acid etching the dentin and enamel with around 37% phosphoric or equivalent acid. Mineral components of both enamel and dentin dissolve. Enamel and dentin are then washed with water and the dissolved minerals are washed away, leaving the etched enamel, and the dentin with protein fibers and open exposed tubules (Figure 4.11a,b). On the now demineralized dentin and exposed tubules, a hydrophilic resin (such as hydroxyethyl methacrylate, HEMA), or dental primer, is applied. This resin goes down into the tubules, creates resin tags and infiltrates the demineralized dentin to create a hybrid layer (Figure 4.12). This mechanical/chemical adhesion is very strong. It is then reinforced by the third component, a slightly filled hydrophobic resin, making the entire complex thicker and less soluble.
Fifth-generation bonding systems are contained in one bottle, which combines primer and bond in one single component. Some have questioned the benefits of mixing primer and bond. These bonding systems suffer from several problems such as shelf-life stability and transudation (Figure 4.13) [24,25].
The problems associated with total-etch bonding systems are that they can be very technique sensitive or difficult to use. One complication is that the dentin can be easily over-etched. Research shows that more than 15 seconds of etching dentin can “demineralize dentin to a depth greater than resin monomers can penetrate” , which will leave a layer of demineralized dentin that has not been filled with resin. This can lead to a weak bond, because this demineralized but uninfiltrated layer is subject to degradation over time, probably by the action of matrix metalloproteinases (MMPs) .
A second problem with total etch is that it is moisture sensitive. When minerals are removed from dentin, weak dentin fibers are supported by water or moisture (Figure 4.14) . It is easy to over-dry the dentin after the phosphoric acid is washed off, allowing the fibers to collapse. It is then difficult to re-expand the collapsed fibers . Additional materials have been used to revive the collapsed fibers, but this procedure adds a clinical step. Wet bonding to overcome such problems has had some limited success, but also has serious disadvantages . Excessive humidity or water is undesirable, as hydrophobic resins (HEMA) only work with slight moisture. Too much is counterproductive, especially if not carefully evaporated, and remaining water and solvents can shorten the life of the adhesion in the long term . Minor errors during the total-etch procedure can also lead to postoperative sensitivity and weaker adhesion.