Dentin/Enamel Adhesives: Their Current Status

by Edward J. Swift, Jr., DMD, MS and Marcos A. Vargas, BDS, DDS, MS

Enamel bonding has been routinely and successfully used in dentistry for about thirty years, but reliable dentin bonding has been possible only during the latter half of that period. Dozens of dentin/enamel adhesives are available on the market today. Although the sheer number of materials can be confusing to clinicians, these adhesives can be easily classified by bonding strategy — i. e., etch-and-rinse or self-etch — and complexity. Because each of the two basic strategies can be accomplished using a more complex or a simplified approach, four types of modern dentin adhesives can be described. Each has advantages and disadvantages, with varying degrees of proof of clinical efficacy. The clinician should understand these advantages and disadvantages to choose appropriate products for use in the practice.

ENAMEL BONDING

The roots of today’s adhesive dentistry can be traced to 1955, when Dr. Michael Buonocore reported that phosphoric acid could alter the surface of enamel to “render it more receptive to adhesion.” 1 Inspired by the industrial use of phosphoric acid to improve adhesion of paints and acrylic coatings to metal, Buonocore discovered that acrylic resin could be bonded to human enamel that was etched with 85% phosphoric acid. He predicted that this “bonding” technique could be used in various dental procedures, including Class III and Class V restorations and pit and fissure sealants.

Enamel bonding did not become widely used until 20-25 years after Buonocore’s first publication on the subject. The technique has now proven successful and reliable over decades of clinical use and has revolutionized the practice of restorative dentistry and other disciplines such as esthetic, preventive, and pediatric dentistry and orthodontics. Currently, phosphoric acid in concentrations of 35-40% is used to etch enamel to provide micromechanical retention of resin-based materials (Fig. 1).

PROBLEMS IN BONDING TO DENTIN

Interest in adhesion of restorative materials to dentin actually predated Buonocore’s 1955 paper on enamel bonding. 2 However, bonding of resins to dentin is far more difficult and less predictable than bonding to enamel. Dentin not only has a more complex histologic structure than enamel, but and composition occur not only with differences in depth, but also from region to region of the tooth. The permeability characteristics of dentin clearly illustrate these regional variations. For example, the permeability of occlusal dentin is higher over the pulp horns than at the center of the occlusal surface, proximal dentin is more permeable than occlusal dentin, and coronal dentin is more permeable than root dentin. 7,8

When attempting to bond to dentin exposed during tooth preparation, the inherent complexity of the dentin morphology is further complicated by the formation also has more variation with location. Enamel is roughly 92% inorganic hydroxyapatite by volume, and dentin is only 45% inorganic. Dentinal hydroxyapatite crystals are not regularly arranged as they are in enamel, but are randomly arranged in an organic matrix. 3

Dentin contains numerous fluid-filled tubules that run from the pulp to the dentinoenamel junction (DEJ) (Fig. 2). The relative area of dentin occupied by tubules decreases towards the DEJ, from about 22-28% of the cross-sectional area near the pulp to only 1-4% near the enamel. 4 An odontoblastic process extends from the pulp into the inner portion of each tubule. 5 The plasma-like fluid in the tubules is under a slight, but constant, outward pressure from the pulp. 6

Variations in dentin structure of a “smear layer.” 9 The smear layer consists of debris (such as ground enamel and dentin) that is burnished against, and bound to, the dentin surface during instrumentation. Depending on factors such as the type of cutting instrument used, the smear layer is typically just 0.5-5.0m thick, but occludes the orifices of the dentinal tubules. Although the smear layer acts as a diffusion barrier that decreases dentinal permeability, it also can be considered an obstruction that prevents resin from reaching the underlying dentin substrate. 10

Alterations in the mineral content and structure of dentin — as in caries-affected or sclerotic areas — represent another source of difficulty in bonding resins to dentin. 11 Resin penetration into sclerotic dentin is less than in normal dentin, and this can compromise the outcome of bonding procedures.

DEVELOPMENT OF DENTIN ADHESIVES

Having begun in the 1950s, research on dentin bonding continued at a slow pace through the 1960s and 1970’s and culminated in the 1975 introduction of a commercial dentin adhesive system for restoring non-carious cervical lesions. However, this product had very poor clinical results when used to restore cervical lesions without mechanical retention. 12

A “second generation” of dentin bonding agents was introduced in the early 1980s. Most were halophosphorous esters of unfilled resins such as Bis-GMA (bisphenol A-glycidyl methacrylate) or HEMA (hydroxyethyl methacrylate). They bonded to dentin via surface wetting and interaction between their phosphate groups and calcium ions in the smear layer. 13 Shear dentin bond strengths were only about 1-10 MPa, 13,14 and were too weak to counteract the polymerization shrinkage of composite resin. 15 In clinical trials, fairly high percentages of cervical restoration were lost in just one or two years. 16 A major reason for the poor performance of these agents is that they bonded to the smear layer rather than to the dentin itself. Thus, bonding was limited by the cohesive strength of the smear layer or by the weak and unstable adhesion of the smear layer to the underlying dentin. 17

A third generation of dentin adhesives was introduced in the mid-to late 1980s. These either modified or removed the smear layer to permit resin penetration into the underlying dentin. Popular products included Scotchbond 2 (3M), Gluma (Bayer), Tenure (Den-Mat Corporation), Prisma Universal Bond 2 and 3 (Dentsply Caulk), and XR Bonding System (Kerr). Generally, their dentin bond strengths were greater than those of the second-generation agents. Clinical studies of cervical restorations reported that these systems also had considerably better clinical performance (e. g., retention and marginal integrity) than earlier adhesives. However, they did not nearly approach the ideal goal of 100% retention. 16

CURRENT STRATEGIES FOR RESIN-DENTIN BONDING

Total-etch adhesives

In North America, the modern era of resin-dentin bonding began in the late 1980s, with the introduction of the “total-etch” concept. Based on the earlier work of Fusayama and others in Japan, 18 Bertolotti and Kanca proposed a technique for phosphoric acid-etching of dentin as well as enamel, followed by the application of relatively hydrophilic resins that had recently become available. 19,20 The total-etch technique was considered quite controversial at the time, as earlier research had suggested that dentin etching could seriously damage the pulp. 21 Dentists in North America and most other regions of the world had been taught that dentin etching was contraindicated. Eventually, some of the early pulp studies were revisited, new research was performed, and the total-etch technique became widely accepted as both effective and safe. Today, total-etch materials are more commonly described as “etch-andrinse” adhesives.

THREE-STEP SYSTEMS

Many commercial products based on the total-etch technique were developed and marketed in the early 1990s. Several of those products, including All-Bond 2 (Bisco, Inc.), OptiBond FL (Kerr), PermaQuick (Ultradent Products), and Scotchbond Multi-Purpose (3M ESPE), remain available today.

Alth
ough their chemical compositions and clinical application techniques vary, these adhesive systems all include three fundamental steps for achieving a bond of resin to dentin. 22-24

The first step, acid-etching, removes the smear layer, opens the dentinal tubules, and demineralizes the intertubular and peritubular dentin (Fig. 3). The depth of demineralization is affected by the pH, concentration, viscosity, and application time of the etchant. The acid dissolves hydroxyapatite crystals, leaving a collagen meshwork that can collapse and shrink due to the loss of inorganic support. Preventing this collapse is an important consideration for etch-and-rinse adhesive systems, and will be discussed later in this section.

After the etchant is rinsed off, a primer containing a solvent such as acetone, ethanol, and/or water and one or more bifunctional resin monomers is applied. Primer resins such as HEMA contain two functional groups -a hydrophilic group and a hydrophobic group. The hydrophilic groups have an affinity for the dentin surface and the hydrophobic (methacrylate) groups have an affinity for resin. The primer wets and penetrates the collagen meshwork and increases the surface free energy, and therefore wettability, of the dentin.

The third of the three steps is the bonding agent, which is applied and penetrates into the primed dentin. The bonding agent typically contains a hydrophobic resin such as Bis-GMA, but many also contain a hydrophilic resin such as HEMA to improve wetting. Although most bonding agents are unfilled, specific products (e. g., OptiBond FL) contain filler particles, as some evidence suggests that filled resins provide stress relief at the tooth-restoration interface. The bonding agent copolymerizes with the primer to form an intermingled layer of collagen fibers and resin commonly called the “hybrid layer.” This hybrid layer, which was first described by Nakabayashi et al. in 1982,25 is considered the most important factor for ensuring a good bond between resin and dentin (Fig. 4).

High bond strengths have been reported for the three-step, etch-and-rinse adhesives; in fact, dentin bond strengths sometimes have exceeded enamel bond strengths. 26-28 Performance in microleakage tests has also been generally good. 29 In addition to laboratory studies, a number of clinical trials have now been reported on this group of adhesives. Retention rates of Class V restorations without mechanical retention have been in the range of nearly 90% in studies of up to 12 years. 30,31

ONE-BOTTLE SYSTEMS

Because the three-step etch-andrinse adhesives require multiple clinical steps, there are numerous opportunities for errors to occur. 32 Therefore, manufacturers attempted to simplify the systems, developing so-called “one-bottle” systems. While these still require etching as the first step, the primer and bonding functions are combined into a single solution; hence the term “one-bottle.” For several years, these products — including Prime & Bond NT (Dentsply Caulk), OptiBond Solo

Plus (Kerr), and Adper Single Bond Plus (3M ESPE) — were the most widely used adhesives and remain fairly popular today.

One-bottle adhesives contain mixtures of hydrophilic and hydrophobic resins in a solvent such as acetone or ethanol. Their bonding mechanism is the same as that of the three-step etch-and-rinse systems, and — like some of the three-step systems — many require a moist bonding technique. 33

When dentin is etched, the surface is depleted of the hydroxyapatite crystals that support the collagen framework. Thus, etching leaves a porous, collagen-rich surface that can collapse if dried, limiting the penetration of resins. In a moist bonding technique, the surface is not dried after etching and rinsing, and therefore the collagen remains in position and behaves almost as a sponge. The acetone or ethanol solvent displaces water and carries the resins into the collagen network. 34,35

If the surface must be dried — e. g., to check the enamel etch — it should be re-moistened. Various materials have been tested as re-wetting agents, including water, which does not re-wet the surface rapidly. Better alternatives are aqueous solutions of HEMA such as Aqua-Prep (Bisco, Inc.), Gluma Desensitizer (Heraeus Kulzer), or G5 (Clinician’s Choice).36,37 The latter two products also contain glutaraldehyde, which might stabilize the collagen layer, thus facilitating resin penetration. 37

As with the three-step etch-and-rinse systems, the one-bottle systems generally have demonstrated good performance in laboratory testing of dentin bond strengths and marginal seal. 38-40 Most also bond very well to either dry or moist enamel. 41 Unfortunately, only a few clinical trials have been reported on the one-bottle systems. However, the studies that have been published generally have reported good results. A recent study on two such adhesives reported an eight-year retention rate of about 90% for Class V restorations placed without mechanical retention. 42

As mentioned, the etch-and-rinse one-bottle adhesives remain relatively popular — and development of these materials has not ceased. New products in this category include XP Bond (Dentsply Caulk) and MPa Direct (Clin ician’s Choice). In a recent study, the dentin bond strength of MPa Direct was higher than that of four other systems, with the difference being statistically significant for three of those. 43 In addition, MPa Direct was very effective for bonding self-cure composite when the adhesive’s oxygen-inhibited layer was removed with alcohol.

Despite the good laboratory and clinical performance of the etch-and-rinse adhesives, some clinicians have reported problems with post-operative sensitivity. Once the dentin is etched, it must be sealed well, which is not always possible under clinical conditions. The problem of postoperative sensitivity is most common in situations that magnify the effects of composite polymerization shrinkage. An example of this is a simple Class I posterior composite restoration. The Class I has a configuration factor (or C-factor) of 5, which indicates that the ratio of bonded to unbonded walls is 5:1.44 When the composite shrinks during polymerization, some stress relief occurs at the occlusal (unbonded) surface, but inevitably some stress also occurs at the bonded interfaces. Furthermore, most of the dentin bonding occurs at a single location, the pulpal floor. The entire circumference of the restoration is bonded to enamel. If the bond of resin to the enamel periphery exceeds the bond to the dentin, the composite may partially debond from the pulpal floor, leaving a gap between resin and dentin. When the patient functions on the tooth, hydraulic forces within the fluid-filled gap and underlying tubules stimulate pulpal nerve endings, causing a sensation of sensitivity or pain. 45 Incremental placement of the composite can reduce post-operative sensitivity. 45 Resin-modified glass ionomer liners (e. g., Vitrebond Plus, 3M ESPE) are also effective for reducing early sensitivity after a restoration is placed. 46 Some clinicians also incorporate a HEMA/glutaraldehyde desensitizer into their bonding protocol, although evidence for its efficacy with composite restorations remains largely anecdotal. 47 If used, the desensitizer should be applied after etching and rinsing, prior to application of the combined primer/bonding agent.

SELF-ETCH SYSTEMS

Perhaps because of frustration over post-op sensitivity with etch-and-rinse adhesives, much of the current adhesive product development and clinician interest is focused on self-etching systems. These were popular for several years in Japan prior to their introduction in North America. One class of self-etch systems includes two steps — an acidic, self-etching primer (containing, for example, an acidic phosphate monomer) followed by a s
eparate bonding resin. Another class of adhesives is considered “all-in-one,” and contains etch, prime, and bond functions in a single solution. The former group of materials can be described as self-etch primer systems, and the latter can be called self-etch adhesives.

SELF-ETCH PRIMERS

Examples of current two-step, or self-etch primer, systems include Clearfil SE Bond (Kuraray), Peak SE (Ultradent), and Adper Scotchbond SE (3M ESPE). These materials are simple to use and, at least anecdotally, are associated with very little post-operative sensitivity. However, controlled clinical trials generally have reported no difference in the incidence or severity of post-operative sensitivity using etch-and-rinse and self-etch adhesives. Of course, one could argue that clinical trials are conducted under conditions that do not precisely duplicate “real world” dentistry. 48-50

The chief concern about the self-etch systems is that they might not etch enamel effectively. 51 Bonding to uninstrumented enamel is particularly challenging, so enamel should be instrumented in some way before etching with these systems. 52 Some manufacturers even recommend that if a restoration will involve uninstrumented enamel, it should be etched with phosphoric acid first. Bonding of self-etch systems to sclerotic or caries-affected dentin also might be problematic. 53,54 Regardless, the most popular product in this category — Clearfil SE Bond — has performed extremely well in a five-year clinical study. 55 There is some evidence that the bond durability of this material is partially due to chemical bonding of its adhesive monomer, 10-MDP, with residual hydroxyapatite crystals in the hybrid layer. 56

SELF-ETCH ADHESIVES

The most recent developments in dentin bonding have been in the area of the self-etch adhesives, or “all-in-one” systems such as Adper Easy Bond and Adper Prompt L-Pop (3M ESPE), Bond Force (Tokuyama), Brush & Bond (Parkell), and OptiBond All-in-One (Kerr). These materials deliver the etching, priming, and bonding functions in a single solution and are the most hydrophilic type of adhesive. 57 The hydrophilicity of these materials is not particularly advantageous, and some evidence even suggests that their performance could be improved by application of an overlying hydrophobic resin. 58,59 Regardless, they have rapidly gained popularity because of their apparent simplicity of use and perceived lack of post-operative sensitivity.

Because this is the newest category of adhesives, less independent research has been reported on these than on the other categories. The first all-in-one adhesive, Prompt L-Pop (ESPE)* performed poorly in short-term clinical trials of Class V restorations. One study reported a 35% retention loss at just one year. 60 As a group, the self-etch all-in-one adhesives tend to have the lowest enamel and dentin bond strengths and the least proven clinical performance. 61-63 That said, some improvements do seem to be occurring.

CONCLUSIONS

Two primary strategies are currently available for bonding resin to dentin: etch-and-rinse and self-etch. For each strategy, simplified approaches are available, so that four distinct categories of dentin adhesives can be identified: (1) three-step etch-and-rinse; (2) one-bottle etch-and-rinse; (3) self-etch primers; and (4) self-etch, or “all-in-one,” adhesives. Each category has advantages and disadvantages, but more laboratory and clinical data are available concerning the etch-and-rinse systems. At present, the profession seems to be moving in the direction of the self-etch, all-in-one adhesives, but the clinical performance of these materials is not yet proven.

Regardless of the bonding approach the clinician selects, he or she must be aware that proper technique and attention to detail are critical to success. 64-66 In addition, dentin is a highly variable substrate, and this variability may lead to failures in specific cases.

OH

Edward J. Swift, Jr., DMD, MS, is Professor and Chair, Department of Operative Dentistry, University of North Carolina. ed_swift@dentistry.unc.edu

Marcos A. Vargas, BDS, DDS, MS, is Professor, Department of Family Dentistry, College of Dentistry, The University of Iowa, Iowa City, IA.

* The current Adper Prompt is an improved version of this original material.

Oral Health welcomes this original article.

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———

ABSTRACT

This paper briefly describes the development of dentin/enamel bonding systems, and describes the current strategies for bonding resin-based materials to tooth structure.

———

Buonocore discovered that acrylic resin could be bonded to human enamel that was etched with 85% phosphoric acid

———

Resin penetration into sclerotic dentin is less than in normal dentin, and this can compromise the outcome of bonding procedures

———

In North America, the modern era of resin-dentin bonding began in the late 1980s, with the introduction of the “total-etch” concept

———

When dentin is etched, the surface is depleted of the hydroxyapatite crystals that support the collagen framework

———

In a recent study, the dentin bond strength of MPa Direct was higher than that of four other systems, with the difference being statistically significant for three of those

———

Of course, one could argue that clinical trials are conducted under conditions that do not precise
ly duplicate “real world” dentistry

———

Regardless of the bonding approach the clinician selects, he or she must be aware that proper technique and attention to detail are critical to success

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