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In most fields of modern technology, the latest version of a product is usually an improvement over the previous ones. For example, smartphones, hybrid cars, the Blu-Ray disk, and high-definition television represented significant advancements over their predecessors. So why is this continual advancement apparently not the case when it comes to dental adhesives? Over the last 25 years, dentistry has seen significant generational changes, new materials categories, new chemistries, and new clinical protocols with dental adhesives—much of it driven by an effort to simplify or to shorten the bonding procedure. However, not all of the newer materials have necessarily offered improvements to the patient or for the long-term viability/prognosis of the restorations placed. How does the clinician make a rational choice from among more than 65 adhesives still on the market today? Among the so-called fourth-, fifth-, sixth-, and seventh-generation adhesives—alternatively known as the “etch-and-rinse” and “self-etch bonding agents”—which system gives the most consistent, long-term clinical results and has the longest viable bond strength over time? Which system resists oral degradation and allows for the integration of new methodology to treat the hybrid layer for long-term stability while addressing the inevitability of the presence of bacteria and composite polymerization and functional stress?
The current resin-dentin bonding mechanism, whether using etch-and-rinse or self-etch systems, relies on the formation of a hybridized layer that couples adhesives/resin-composites with the underlying mineralized dentin. With the exception of resin tags, which extend down into the dentinal tubules, only the collagen fibers offer physical continuity between the hybrid layer1 (as it is known after being infiltrated with resin) and the underlying mineralized dentin. The collagen fibers represent millions of fibril anchors, which emerge from the underlying mineralized dentin matrix into the demineralized layer.
In an excellent overview of factors that affect the bond strength of bonding agents, Powers et al2 point out that the type of substrate (ie, superficial dentin, deep dentin, permanent/primary dentition, carious dentin), phosphoric acid/acidic primers, preparation by air abrasion and laser, moisture, contaminants, desensitizing agents, and self-cured/light-cured restorative materials all affect the bond strength; bond strength is reduced by more than 50% when bonding conditions are not ideal. Further, when lasers are used to prepare hard tissues, studies show that bonding to these surfaces may be more problematic than bonding to conventionally bur-prepared preparations.3 Rushing to complete such procedures by reducing the priming time from 20 to 5 seconds can cause a 17% reduction in mean bond strength.4 In contrast, using a 20-second application time to agitate a self-etch adhesive significantly improves the shear bond strength to dentin.5 In addition to agitation, rather than applying a single coat of adhesive resin on dentin, up to four additional coatings increase the bond strength and decrease nanoleakage.6 Multiple research reports attest to the existence of material incompatibilities that depend on formulation, and that bond strengths can be reduced by 45% to 90% or more when incompatible combinations are applied clinically.7-9 Acetone-based adhesives show a high degree of technique sensitivity,10 and over- or under-drying the acid-etched dentin compromises the bond.11 Simplification of the bonding procedure does not necessarily lead to improved bonding performance, especially in the long term.12 Alex3 perhaps has stated it best: “The bottom line is, it is incumbent on every dentist to learn about their specific adhesive system, its idiosyncrasies, its strengths and weaknesses, and how to maximize its performance.”
Practitioners and their teams should be aware not only of the immediate bond strengths—whether shear or tensile (the immediate configuration factor [C-factor] polymerization shrinkage stress is about 24 MPa in a Class I cavity preparation)13 but also, the long-term performance or longevity (patency) of their bonding agent in actual clinical performance.14 Much of the bond testing has been done in a laboratory under ideal controlled conditions, which may not be possible to replicate in a clinical environment, and results may vary depending on the testing methods and devices used.15 Some studies test immediate or 24-hour bond strength only on dentin just below the enamel surface, where bonding is easiest and strongest; and most of these studies have been done in vitro on human or bovine teeth, without positive fluid flow or the positive pressure that exists clinically—both of which can drastically decrease long-term performance. A short, simplified overview of the systems and their clinical behavior follows.
Compared to current adhesives or bonding agents, those still considered to be the “gold standard” in long-term durability are fourth-generation adhesives (etch-rinse-prime-adhesive resin) or multi-bottle systems (eg, 3M™ ESPE™ Scotchbond™ MP Multi-Purpose Plus, 3M ESPE, www.3mespe.com; ALL-BOND 2® and ALL-BOND 3®, Bisco, Inc., www.bisco.com; Perma Quick, Ultradent Products, Inc., www.ultradent.com; and OptiBond FL®, Kerr Corporation, www.kerrdental.com).16,17 These etch-and-rinse materials still provide the deepest, strongest, most predictable, and long-term bond to enamel.18 This strong layer resists microleakage and protects the dentin bond from water degradation, which may contribute to better long-term clinical performance.19-23 Arrais24 has reported that these three-step adhesives also form the thickest hybrid layer, followed by one-bottle adhesives, with the self-etching adhesives forming the thinnest hybrid layers. It must be noted that strict adherence to etching times is critical, as prolonged phosphoric acid-etching can form a deep demineralized dentin zone that may not be fully impregnated by the primer and adhesive, resulting in low bond strength.25 These products, which bond well to dentinal substrates, have lost some of their popularity due to the fact that they are multi-bottle, multi-step systems, which require etch, rinse, and application of hydrophilic primers followed by a hydrophobic adhesive layer. As such, these systems can be very technique-sensitive, if the sequencing and timing is not exactly adhered to, with some having complicated instructions on how to bond to tooth structure and other dental materials. The so-called fourth-generation bonding agents have the major benefit of being effectively used with self-cure, dual-cure, and light-cured composites as well as indirect restorations without concern. Representative bonding strengths for products in this category are: OptiBond FL, 39 MPa to dentin,26 and Scotchbond MP, 45.6 MPa to dentin.19
Fifth-generation adhesives (etch-and-rinse plus [primer and bond]) or the single-bottle systems, such as MPa™ (CLINICIAN’S CHOICE®, www.clinicianschoice.com); Sealbond Ultima (RTD, Clinical Research Dental, www.clinicalresearchdental.com); OptiBond® Solo Plus (Kerr); Adper™ Single Bond Plus (3M ESPE); Prime & Bond® NT™ (DENTSPLY Caulk, www.caulk.com); PQ1 and Peak™ (Ultradent Products); ExciTE® (Vivadent, www.ivoclarvivadent.com); and One Step Plus and ACE® ALL-BOND TE™ (Bisco), contain primers and adhesive in a single bottle. This simplifies the bonding technique by eliminating some of the variables regarding the number of bottles and steps required. This single-bottle, etch-and-rinse adhesive type shows the same effectiveness as the fourth-generation systems in terms of microleakage,27 and shows good dentin bond strengths with excellent marginal seal in enamel.28-30 However, not all fifth-generation adhesives are compatible with dual- and self-cure composites or core materials. The lower pH of the oxygen-inhibited layer, or the acidic monomers in some simplified products, are too acidic and thereby de-activate the tertiary amine in chemical-cured composites. This results in no bonding, or weak bonding, with a material such as Prime & Bond NT in its light-cured version showing a bond strength of 0 MPa.31 Some adhesive systems such as OptiBond Solo Plus require a dual-cure activator, which is a resin-free benzene sulphinic-acid/sodium salt solution, which will raise the acidic pH of the oxygen-inhibited layer in order to bond to self-cured/dual-cured products. However, this additional step lowers bond strengths because of the dilution of the adhesive and the inherent permeability of the polymerized adhesive.32 Other products, such as MPa, Sealbond Ultima, and One Step Plus, do not require the use of a dual-cure activator. With a self-cure composite, One Step Plus shows bond strength of 21.4 MPa33 in one study and 19 MPa in another.34 In the later study, MPa showed a bond strength of 18.5 MPa to the self- cure composite; however, when the oxygen-inhibited layer was removed with alcohol, the bond strength more than doubled to 38.9 MPa to the level obtained using light-cured composite.34
The solvents in bonding agents can be either ethanol-based, a nonvolatile solvent, or acetone-based. Products such as Sealbond Ultima and One Step Plus contain acetone, a very volatile solvent that should never be pre-dispensed into a dappen dish before the actual clinical bonding procedure, because the evaporation of the solvent will drastically reduce the patency of the bonding agent. Products that use ethanol or acetone need the proper amount of moisture (a moist bonding protocol), to prevent the collapse of the exposed collagen network.35 The amount of impregnation of the hybrid layer can be reduced by 50% if the dentin is over dried.36 Ethanol-based adhesives contained in products such as MPa™, Peak™, and OptiBond Solo Plus generally are more forgiving in clinical application and show the highest microtensile bond strengths.35,37,38 It must be noted here that the 3 to 5 seconds recommended by some manufacturers for the evaporation of solvents is generally not enough to remove even half of the solvent, and therefore extension of this time is recommended.39
The clinician also needs to know whether they are using a filled or unfilled adhesive, as the inclusion of fillers changes the viscosity and thickness of the applied layer. As well, evidence in the literature supports the concept that filled resins provide stress relief at the tooth–restoration interface.40 Not all fifth-generation adhesives are the same in regards to the number of applications (unfilled need more applications), so it is critical to follow the manufacturer’s directions. Some products, such as One Step Plus and Adper Single Bond Plus, require two coats of the adhesive, while others like MPa™ and PQ1 require only one.41 Of course a single coat saves material and time, because less of each is required for bonding the restoration. Representative dentin bond strengths reported in this category are 42 MPa to dentin for Adper Single Bond Plus25 and 41 MPa for MPa™.34
Postoperative sensitivity can be induced with the etch-and-rinse technique when exacting protocols are not followed. This is more prevalent in deeper dentin preparations and in high C-factor cavity preparations.42 Simplification by combining the primer and adhesive into one bottle, which also applies to the so-called seventh-generation or self-etch all-in-one adhesives, increases the hydrophilicity and permeability of the bond, making them more likely to undergo hydrolysis. This is especially true for the unfilled bonding agents, as they create a thinner hybrid layer.43
The self-etch systems, or sixth-generation adhesives, are classified into two types. Type I adhesives, which are self-etch priming systems such as Clearfil™ SE Bond (Kuraray America, Inc. Dental, www.kuraraydental.com), Prelude SE (Danville Materials), and Ultradent Peak™ SE (Ultradent Products Inc.), use a self-etching primer followed by the application of an adhesive. These systems are gaining in popularity due to the fact they are not as clinically technique-sensitive, and are easy to use,44 with less reported post-operative sensitivity. However, controlled clinical studies show no difference in postoperative sensitivity as compared with etch-and-rinse systems when proper clinical protocol is used.45,46
With most of the self-etch systems, the clinician may sacrifice marginal seal and longevity.47,48 In order for a self-etch system to provide a good enamel marginal seal, the pH must be lower than 1.5 to effectively condition uncut enamel, and prevent marginal leakage; however, some of these products are mildly acidic, having a pH as high as 3.3. The research literature is replete with studies showing that these self-etch adhesives have more marginal defects,49 poor penetration with less adhesion,50 and increased microleakage51 in enamel when compared to etch-and-rinse systems, with a somewhat better result with instrumented enamel.52,53 This is worrisome because the enamel bond helps to protect the dentin bond from long-term degradation. Some clinicians still incorporate the use of a phosphoric acid on the enamel margins when using a self-etch product to enable increased bonding to the enamel.54 Using this technique, the clinician achieves a more predictable bond on the enamel margin; however, depending on control of the phosphoric acid, this technique can lead to clinical variability, and/or more permeability of the bond and sensitivity. The reason self-etch systems tend to minimize postoperative sensitivity is that the smear layer on the dentin surface and smear plugs in the dentinal tubules are left intact. If the phosphoric acid migrates onto the dentin surface during placement or the washing step, it can remove the smear layer and plugs, which allows fluid movement within the tubule; this leads to decreased tensile bond strength and sensitivity.55
If this is a preferred technique, the clinician may want to consider a phosphoric acid such as Ultra-Etch (Ultradent), which has the ideal viscosity for placement on vertical surfaces without migration. A recognized variable in the self-etch systems is the thickness of the smear layer, with diamond burs creating a thicker smear layer than carbide burs, resulting in less adhesion to dentin when the thick smear layer cannot be fully penetrated by the self-etch system.56,57 The self-etching primers, which are not rinsed off, are an acidic monomer, which may not be fully neutralized; this creates a demineralized zone below the hybrid layer that can compromise bond strength.58 A concern with self-etch systems is that they may not have resin penetration into or obturate the dentin tubules, thus allowing pulpal fluids to penetrate past the smear plug and into the hybrid layer.59 “The higher the micropermeability, the higher the risk of defects at the resin–dentin interface, which may represent the pathway for hydrolytic and enzymatic degradation of the collagen and resin dentin bonds over time.”60 This causes what is known as a “hydrolytic” or “water-tree” effect, which breaks downs the bond from the inside out, reducing both in-vivo and in-vitro bond strengths.61
As a general rule, self-etching systems are more ideally used in posterior restorations for prevention of sensitivity. In addition, due to a less predictable bond to enamel, which can lead to additional staining secondary to microleakage, these self-etch systems are not ideal for anterior restorations. When the level of enamel etching is of concern (ie, the pH is above 1.5), phosphoric-acid–etching may be indicated. However, some of these Type I adhesives, such as Peak SE, which has the highest bond strength in this category of 50 MPa to dentin,26 has an adhesive, Peak™ LC Bond Resin (Ultradent), that can be used in the anterior dentition with an etch-and-rinse phosphoric-acid technique. Many of these products do have catalysts for use with dual-cured composites and can be compatible with self-cure.
Type II self-etch adhesive systems such as All-Bond SE® (BISCO, Inc.), Brush & Bond® (Parkell, www.parkell.com), Futurabond® NR (VOCO America, Inc., www.vocoamerica.com), Touch & Bond® (Parkell), Adper™ Prompt™ L Pop™ (3M ESPE), and Xeno III® (DENTSPLY Caulk), use a self-etch primer and adhesive that are mixed together prior to placement. These self-etching primer/adhesives should not be confused with the self-etching primers of Type I systems. Both Type I and Type II systems leave the smear layer intact before placement and thereby limit postoperative sensitivity. However, Type II systems have all of the same drawbacks as Type I, and can have 35% lower bond strength, because no hydrophobic adhesive layer is placed. As mentioned above, the hydrophobic resin layer helps to prevent water-induced interfacial changes.62 It is important to note that thermocycling, which is used to more closely mimic oral conditions, leads to further lowering of the bond strength of these hydrophilic systems.63 The remaining acidity and permeability of these Type II products make them incompatible with self- or dual-cured composites and core materials. Touch & Bond, for instance, demonstrates extremely low bond strengths of 12 MPa to enamel and only 2 MPa to dentin.64
Among the self-etch adhesives—also called “all-in-one” or seventh-generation adhesive systems—are G-Bond™ (GC America, www.gcamerica.com), iBond® (Heraeus Kulzer, www.heraeus-dental-us.com), Xeno® IV (DENTSPLY Caulk), Adper™ Easy Bond (3M ESPE), AdheSE® One (Ivoclar Vivadent), and Clearfil® S3 Bond (Kuraray). They are very similar to sixth-generation systems, with the biggest difference being that all the components are pre-mixed in one bottle. This one-bottle solution etches, primes, and bonds all at the same time. These self-etch adhesives are permeable membranes and are the most hydrophilic of the adhesive categories.65As such, water sorption by polymers (composites) causes plasticization and lowers mechanical properties.66 Residual acidity causes an adverse reaction with chemically cured composites and core materials, creating a reduction in bond strength with permeability, also a further cause of bond strength reduction.67 Swift states that for these single-step self-etch adhesives, the bond strength to chemical-cured composites can be enhanced with the adjunctive use of co-initiators, based on sodium salts of aromatic sulphinic acids, but he notes that it might not apply to all simplified adhesives.68 This generation of self-etch adhesives also has the highest pH values which results in one-fifth the bond strength to enamel versus the three-step etch-and-rinse system,69 and a higher percentage of leakage at the margins.70 In general, the all-in-one seventh-generation bonding agents show lower bond strengths, with G-Bond having a bond strength of 12.5 MPa to dentin and 15.4 MPa to enamel.26
Making a decision on a suitable dental adhesive is not easy, as there are many factors to consider. The driving factor should be choosing the product and/or procedure that will offer the best quality of care for patients, and deliver the most predictable long term-results. The predictability of fourth- and fifth-generation adhesives has been unmatched. Ongoing research may lead to advances in dental chemistry that result in stable, long-term attachment to both enamel and dentin.
Dr. Boksman was previously a part-time paid consultant to Clinician’s Choice and Clinical Research Dental, holding the title of Director of Clinical Affairs.
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About the Authors
Leendert (Len) Boksman, DDS, BSc, FADI, FICD
Retired from Private Practice
London, Ontario, Canada
Gregg Tousignant, CDT
Technical Support Manager
Clinical Research Dental
London, Ontario, Canada
Lee W. Boushell, DMD, MS
Department of Operative Dentistry
University of North Carolina School of Dentistry
Chapel Hill, North Carolina
Gildo Coelho Santos, Jr., DDS, MSc, PhD
Assistant Professor and Chair Division of Restorative Dentistry
Schulich School of Medicine and Dentistry
University of Western Ontario
London, Ontario, Canada
London, Ontario, Canada