Adhesion

Resin Adhesion to Enamel and Dentin: A Review

Edmond R. Hewlett, DDS

Author
Edmond R. Hewlett, DDS, is an associate professor and vice chair of the Division of Restorative Dentistry at the University of California at Los Angeles.

Copyright 2003 Journal of the California Dental Association.

This article reviews the current knowledge base regarding resin adhesion to enamel and dentin. A descriptive classification system for adhesive resin products as well as clinical considerations derived from the review are also presented to assist the clinician in the selection and application of these products.

Introduction

Advances in dental material science continue to generate products with increasingly biomimetic and bioactive qualities, and restorative dentistry is a conspicuous beneficiary of this trend. Current restorative techniques are capable of producing lifelike function and appearance while maximizing both preservation and resistance to further destruction of tooth structure. Adhesive procedures, however, are typically associated with high technique-sensitivity due to the complexity of interactions between contemporary materials and enamel and dentin substrates. Furthermore, material properties are often mistakenly assigned greater significance than treatment planning and clinical technique in influencing the longevity of adhesive restorations. It is incumbent upon the clinician to be cognizant of these issues and to account for them to obtain consistently predictable outcomes.

This article reviews the current knowledge base regarding resin adhesion to enamel and dentin. A descriptive classification system for adhesive resin products as well as clinical considerations derived from the review are also presented to assist the clinician in the selection and application of these products.

The Substrates

Enamel

Enamel -- the hardest substance in the human body -- consists primarily of highly mineralized inorganic substance (95 percent to 98 percent by weight, mostly hydroxyapatite) arranged in a dense crystalline structure. Assuming normal tooth development, this composition allows for minimal inherent variability, rendering the effect of acid etching on enamel highly consistent from tooth to tooth and from patient to patient. It follows then that the fundamental technique for bonding resin to enamel has undergone minimal change since its introduction in 1955.1 Acid etching produces a complex three-dimensional microtopography at the enamel surface, increasing not only its surface area but also its surface free energy, which in turn increases its wettability and capacity for adhesion.2 Flow of adhesive resin into surface irregularities is thus facilitated, creating a durable, leakage-resistant interface upon polymerization.

Phosphoric acid of 30 percent to 40 percent concentration applied for 15 to 30 seconds2 produces optimal enamel etch patterns and resin retention. Alternative conditioners (e.g., oxalic acid, maleic acid, EDTA) associated with the so-called “third-generation” dentin adhesives produce suboptimal etching of and adhesion to enamel.3-5 These later disappeared from use, as the “total-etch” technique (simultaneous dentin and enamel etching with phosphoric acid) became the norm for resin bonding. Currently, however, self-etching primer products have emerged as alternatives to total-etch. The ability of these products to produce optimal resin-enamel adhesion is questionable,6,7 but recent reports are encouraging.8,9 Claims that cavity preparation with air abrasion or laser devices alters enamel such that acid etching is unnecessary for resin bonding have been countered by substantial evidence to the contrary.10-14

One commonly encountered exception to enamel’s structural consistency should be noted. Unprepared enamel at the outer surface of the clinical crown is aprismatic, requiring light grinding prior to etching to obtain an optimal etch pattern.15-19

Dentin

Just as the relatively uneventful evolution of resin-enamel bonding reflects the static nature and structural consistency of enamel, the dynamic, variable nature of dentin poses significant challenges to developing predictable resin-dentin bonding techniques. Dentin’s composition by weight -- 75 percent inorganic material (hydroxyapatite), 20 percent organic material (collagen, other noncollagenous compounds), and 5 percent water -- suggests a highly mineralized substance. The arrangement of these components (50 percent inorganic material and 50 percent organic material and water by volume),20 however, makes it an inherently problematic resin bonding substrate as compared to enamel.

Other factors further complicate the issue. Dentinal tubules of vital teeth communicate directly with the pulp and house cellular extensions of odontoblasts. Unbonded regions of dentin beneath a restoration can permit sufficient tubular fluid movement under functional or thermal stress to distort afferent nerves in the pulp and elicit pain.21 Dentinal fluid under positive pressure from the pulp may affect diffusion of monomers into etched dentin. This phenomenon will vary in the presence of vasoconstrictors from local anesthetics, however, which reduce intrapulpal pressure. The structural variability of dentin must also be considered. Peritubular dentin, the cylindrical lining of tubules, is more highly mineralized than intertubular dentin. Tubule density (tubules/mm2) and diameter vary significantly such that tubules make up 1 percent of the dentin surface at the dentinoenamel junction and 22 percent near the pulp.22 Intertubular dentin, rich in collagen fibrils and considered optimal for hybridization, occupies 96 percent of a cut dentin surface near the DEJ but only 12 percent near the pulp.23,24 Sclerotic dentin, formed by either reactive or aging processes, is characterized by tubular occlusion via peritubular dentin apposition, rendering this substrate hypermineralized25 and acid-resistant.26,27 The “smear layer”28 of debris formed during instrumentation of dentin acts as a barrier to the underlying substrate and must be modified or removed for resin bonding to occur. Thickness and tenacity of smear layer attachment to the underlying dentin surface is inconsistent.29

Considering these factors, the term “normal dentin” seems an oxymoron. Each factor listed will influence the response of dentin to acid etching and resin bonding such that the response will vary not only from patient to patient and tooth to tooth, but between regions of the same tooth. Clinicians must avoid complacency toward resin-dentin bonding induced by manufacturer claims of speed and ease for current products. Only through methodical and meticulous manipulation of these products can dentin variability be neutralized and durable adhesion of restorative resins to dentin be realized.

The Products

This discussion will focus on current product strategies for dentin adhesion inasmuch as their interaction with etched enamel is relatively uncomplicated. All currently available resin adhesives utilize the same fundamental process to establish adhesion to dentin:

* Acid demineralization of the dentin surface that alters or removes the smear layer, exposes collagen fibrils, and renders the surface highly permeable;

* Infiltration of the demineralized dentin surface with a reactive hydrophilic resin primer to produce a resin/dentin interdiffusion zone30 and micromechanical attachment to dentin; and

* Stabilization of the hybrid layer with an overlay of low-viscosity, light-polymerizable resin that copolymerizes (chemically bonds) with both the primer and subsequently applied composite resin restoratives. Bis-GMA and HEMA are the predominant resins in these systems, but there are notable exceptions such as 4-META/MMA-TBB. These adhesives similarly share a common goal with respect to their clinical utilization: a completely “hybridized,” hermetically sealed resin/dentin interface with bond strength adequate to resist both immediate (composite polymerization shrinkage) and long-term (thermal expansion and contraction) stresses at the resin-dentin interface. Specific strategies for accomplishing dentin bonding vary widely within this framework, however, as indicated by the extensive and ever-changing array of products marketed for this purpose. In any event, discontinuities in the bonded interface can compromise results and lead to premature restoration failure, regardless of the properties of restorative material placed on the interface. Strict adherence to evidence-based protocols for utilization of adhesive systems is critical to predictability and longevity.

Adhesive resins are commonly classified according to their position in the chronology of historical development (e.g., “third-generation,” “fourth-generation,” “fifth-generation,” etc.).31,32 An objective, more descriptive classification by Van Meerbeek and colleagues groups current products according to the steps involved in their clinical use and their mode of interaction with the dentin substrate.33,34 A modified version of this classification is presented here; an adhesive product is assigned to one of three groups according to the number of clinical steps it requires (one, two, or three), then subdivided according to whether it employs total etching or self-etching (Figure 1).

Total-etch adhesive systems utilize 30 percent to 40 percent phosphoric acid to simultaneously produce the desired effects on enamel (etch pattern) and dentin (smear removal, collagen exposure, increased permeability), followed by application of primer (hydrophilic resin) and adhesive resin or “bonding agent.” Three-step (aka multiple-component or fourth-generation) total-etch systems consist of separate primer and resin components applied in consecutive steps following etching. Newer two-step (aka single-component, one-bottle, or fifth-generation) total-etch systems package primer and adhesive resin as one component for simultaneous application following etching. These simplified systems have been enthusiastically embraced by practitioners as evidenced by the sheer number of two-step products on the market. Unit-dose delivery of many two-step adhesives facilitates compositional consistency as compared to bottles, which are prone to solvent evaporation if left opened. In vitro and clinical studies generally indicate equivalent quality of adhesion to enamel and dentin for current two- and three-step total etch systems when used with directly placed light-polymerized composites. These studies also generally indicate higher technique-sensitivity for the two-step types.35-41 Additional differences between these systems with respect to other clinical applications bear further discussion.

Resin or ceramic indirect restorations are typically bonded to tooth structure using low-viscosity composite resin cements. Dual-cured resin cements, containing both chemical initiators (requiring mixing of base and catalyst components) and photoinitiators are indicated for resin bonding of translucent inlays, onlays, and (some) crowns where restoration thickness may prevent complete polymerization with light only. It is also commonly recommended that a dual-cured adhesive system be used in these cases for the same reason. Light-polymerization of the adhesive resin layer prior to placement of the resin cement produces optimal fixing/hardening of hybridized dentin and higher bond strengths to dentin, but the adhesive film thickness may interfere with complete seating of the restoration.42 Dual-cured adhesives, as with dual-cured cements, are not polymerized with light until the restoration is seated, with the chemical initiators again ensuring polymerization in deep areas. Many three-step total-etch systems include an optional dual-cure activator for this purpose, while classic two-step systems are light-polymerizable only, limiting their use to direct restorations.

Several manufacturers have in recent years marketed two-step total etch adhesives claiming a film thickness (less than 20 microns) that allows light-polymerization prior to seating an indirect restoration. These claims raise questions as to the likelihood of consistently obtaining such minimal film thickness in the clinical setting as well as the potential for oxygen inhibition to interfere with complete polymerization of such thin layers.

A recent study43 indicates that the bond of two-step total-etch adhesives to dentin is prone to water degradation when the resin-dentin interface is exposed to the oral cavity, and that these products are more susceptible than their three-step counterparts in this regard. The study further demonstrates that a bonded resin-enamel margin completely surrounding the resin-dentin interface provides effective protection from this water degradation.

Two-step total-etch adhesives have been shown to bond inconsistently to autopolymerized (“self-cured”) composite resins such as those used for foundation restorations (“cores”) under fixed prostheses.44 The incompatibilities appear to be material-specific regarding the adhesive/composite combination, but have been ubiquitous enough that many manufacturers now provide an optional dual-cure activator for use with their single-component products. These activators are intended to render the adhesives compatible with autopolymerized composites, but some isolated incompatibilities remain.45 The activators also provide the option for a dual-cured adhesive under an indirect restoration. Many single-component products have been further reformulated to include filler particles, purportedly eliminating the need to place the more than one layer for optimal dentin sealing. These developments reflect an interesting trend: the two-step total-etch products, originally promoted as simpler and faster, have been incrementally reformatted to emulate more qualities of their versatile three-step predecessors.

An additional type of three-step total-etch system utilizes a unique combination of chemical compounds. Nakabayashi30 first reported the hybrid layer phenomenon in 1982 using this system. A solution of 10 percent citric acid and 3 percent ferric chloride etches enamel and dentin, followed by 4-META (4-methacryloyloxyethyl trimellitate anhydride) dissolved in MMA (methyl methacrylate) initiated by TBB (tri-n-butyl borane) - 4-META/MMA-TBB. The ferric sulfate is thought to cross-link proteins in the collagen matrix, immobilizing them and thus preventing collagen collapse. Maintaining a moist dentin surface is therefore unnecessary.46,47 A literature review of 4-META adhesives describes them as producing excellent results, being easy to use, not being technique-sensitive, and (unlike other systems) essentially retaining the same ingredients used since their introduction.46

Self-etching adhesive systems utilize an acidic primer to detach or dissolve the dentinal smear layer and demineralize the dentin surface to simultaneously expose and hybridize collagen fibers. A separate etching step is not used, nor is the self-etching primer rinsed off, thus streamlining the process. Two-step self-etching systems include a separate light-polymerized adhesive resin component placed after applying and drying the primer. One-step (aka “all-in-one”) versions accomplish all three steps of resin-dentin bonding (etch/prime/bond) simultaneously with a single liquid. In addition to genuinely simplifying dentin bonding, self-etching is arguably a less technique-sensitive process than total etching. Rinsing and drying of etched dentin prior to hybridization are eliminated, neutralizing the issue of dentin wetness/dryness (see below). Additionally, depth of etching does not exceed depth of primer infiltration, preventing overetching and unhybridized dentin. Depth of demineralization in enamel and dentin is shallower with self-etching primers than with phosphoric acid. Microscopic features of the hybrid layer and bond strengths to dentin are nonetheless similar to those seen with total-etch systems.48 An exception to this trend is sclerotic dentin, where a self-etching primer was shown not to etch beyond the hypermineralized surface layer.49

Ability of self-etching primers to produce optimal enamel adhesion was doubted initially,6,7 prompting recommendations to selectively treat enamel with phosphoric acid when using these systems. More-recent in vitro studies report tensile and shear bond strength values on enamel similar to, albeit less consistent than, those obtained with phosphoric acid.8,9 This difference is perhaps explained by the deeper interprismatic etch pattern produced by phosphoric acid as opposed to self-etching primers and the tendency of the latter to bond less tenaciously to unprepared vs. roughened enamel.50 Use of a self-etching system following phosphoric acid etching was shown to significantly increase enamel bond strength, but bond strength to dentin was significantly decreased.9

It has been suggested that the initial (as opposed to 24-hour) tensile bond strength of a resin adhesive to dentin is an important factor in preventing gap formation at the dentin/restorative interface.51 A recent study52 reports significantly lower immediate microtensile bond strengths for several self-etching, single-step adhesives as compared to values obtained for a three-step total-etch control. Bond strength values for the single-step adhesives were in some cases only slightly higher than the 20 MPa51 considered necessary for a gap-free interface. The study raised additional concerns regarding single-step adhesives in situations where polymerization of the composite resin restorative is delayed for two to three minutes after placement. Permeability of these adhesives was found to allow water diffusion from the underlying dentin into the interface between the adhesive and the uncured composite. Bond strengths were significantly lower than those produced with the three-step control under the same conditions. While this phenomenon should not affect a typical direct restoration where composite is placed and immediately polymerized, the authors of this study recommend that multiple direct or indirect restorations be individually light-activated immediately after application of composite restorative or cement. The authors further recommend avoiding the use of self-etching single-step adhesives when luting indirect restorations or posts with a dual-cure resin cement. Delay of light activation while removing excess cement may allow diffusion of dentinal fluid into the adhesive/cement interface and result in a diminished bond between these layers.

Self-etching systems show promise for routine bonding to tooth structure in a simplified manner. Additional clinical evidence of their ability to consistently produce durable adhesion to enamel and dentin, however, is clearly needed.

A resin-modified glass-ionomer adhesive (Fuji Bond LC, GC) for use in direct composite resin restorations is available in addition to the adhesive resins. As with self-etching primers, this strategy utilizes a less aggressive approach than phosphoric acid for attachment to tooth substrates. Conditioning of the prepared dentin surface with a polyalkenoic acid (25 percent polyacrylic acid) removes smear layer debris, permitting chemical adhesion of the resin-modified glass-ionomer cement adhesive to the underlying dentin substrate. In general, combined used of glass-ionomer cements and composite restoratives has demonstrated improved microleakage resistance at dentin margins (see “sandwich technique” below). A clinical trial of adhesively retained Class V composite restorations bonded with Fuji LC Bond reports an overall retention rate of 96 percent, with 20 percent of restorations available at five years displaying margin discoloration.53 More longitudinal clinical data is needed.

The array of currently available dental adhesive products reflects a dynamic area of research and development aimed at technique simplification and clinical permanence. While newer product types hold promise for achieving these goals, it is the author’s opinion that the three-step total-etch systems presently possess the most favorable levels of technique-sensitivity, clinical predictability, and range of application.

Clinical Technique Considerations

The foregoing information clearly illustrates the complex nature of resin adhesion in dentistry. The following techniques are provided to mollify the effects of high substrate and product variability inherent in resin bonding.

Overetching

Overetching of dentin can potentially occur with prolonged phosphoric acid contact. Denatured collagen resulting from excessive etching may compromise bond longevity.47,54 Primers may additionally be unable infiltrate the full depth of a deep demineralized zone, leaving an unhybridized collagen band, which may also give rise to premature bond failure.55-57 Dentin should typically be etched for no more than 15 seconds. Self-etching primer systems eliminate this consideration, as previously mentioned.

Sclerotic Dentin

Sclerotic dentin is atypically dense and hypermineralized and as such is resistant to acid etching. Self-etching primers are relatively ineffective on this substrate.49 Removal of the surface layers with rotary instrumentation or use of extended etching times are common strategies for improving bonding to highly sclerotic dentin. However, even these approaches cannot guarantee improvement given sclerotic dentin’s unique qualities.58

Moist vs. Dry Dentin

Drying of acid-etched dentin allows collagen fibers to collapse into a dense layer that resists penetration of primer resins.59,60 The desired effect of acid etching -- increased permeability -- is thus potentially lost. Sensitivity to this issue varies with respect to the solvent type used for the primer resin. Acetone-based primers are critically dependant on a moist dentin surface for hybridization. Acetone displaces water in the interfibrillar spaces of the collagen network, carrying with it the hydrophilic resin needed for hybridization. Water-based primers, on the other hand, are the least sensitive to dentin dryness, demonstrating the ability to self-wet a dried dentin surface, separating the collapsed collagen fibers, and enabling resin diffusion into the network.61 Ethanol-based primers display intermediate dependency on moist dentin.

As moist dentin is compatible with all primer types, it is recommended that a moist dentin surface be routinely established for resin-dentin bonding with total-etch products. Avoid drying with compressed air after rinsing away etchant. Use high-volume evacuation to remove gross excess water, then blot remaining pooled water on the dentin surface with gauze or sponge applicators to leave dentin optimally moist.62 Overwet dentin (pooling of water) must also be avoided as excess water will dilute resin primer and out-compete it for sites in the collagen network, preventing hybridization.63 Overdried dentin should be rewet and blot-dried before applying primer. Rewetting etched, dried dentin for 30 seconds with water or 35 percent HEMA is effective for hydrating and re-expanding the collapsed collagen network.64,65

The “10-3” etching solution used in conjunction with 4-META adhesives, as mentioned previously, prevents collagen collapse and permits resin diffusion without moistening the dentin surface.46,47 The 3 percent ferric chloride component of this solution is also believed to provide protection against the bond-reducing effects of denatured collagen produced by acid-etching of dentin.54

Primer application to etched dentin must be performed with care. Primer or primer/adhesive solutions should only be dispensed immediately prior to application to avoid solvent evaporation. Bottle containers must be recapped immediately after dispensing for the same reason, with acetone-based products being particularly susceptible to evaporation. Primers generally benefit from extended application time as diffusion of monomers into dentin is time-dependant.66 Acetone-based three-step total-etch systems are thus placed with multiple applications of primer (four to five) without drying between applications. This technique provides more diffusion time, prevents evaporation of acetone before diffusion is completed, and accounts for the high dilution factor of the primer. Regarding two-step total-etch systems, many manufacturers indicate that a single coat of the primer/adhesive is adequate, but some studies report significant improvement of bond strength for these products when the manufacturer’s protocol is doubled.67 Light agitation of primers or primer/adhesives can enhance diffusion into demineralized dentin, particularly with higher-viscosity filled adhesives. Forceful scrubbing, however, should be avoided. Finally, residual solvent may act as a contaminant and adversely effect bonding. Solvent must be thoroughly evaporated with a gentle stream of compressed air.68 Avoid a forceful air blast as it may displace resin. Acetone- and ethanol-based primers dry readily whereas drying water-based products may require a few more seconds. Hybridized dentin will appear shiny -- additional primer or primer/resin should be applied if dull areas are evident after drying.

Minimizing interfacial stresses during the restoration placement as well as over the long term is essential to preserving the bonded interface so painstakingly produced up to this point. Disruption of (as well as discontinuities in) the interface can allow fluid movement in unbonded tubules, producing pain under functional load temperature extremes. Suboptimal bonding at margins allows microleakage, causing sensitivity and secondary caries.

Stresses produced by polymerization shrinkage are of immediate concern. Incremental placement of composite resin has long been recognized as an effective strategy in this regard, using an initial increment of minimal thickness. Additionally, the concept of reducing interfacial stresses either with thicker, partially filled layers of bonding resin69,70 or by adding an intermediary layer of low elastic modulus71,72 has generated considerable interest in recent years. Low-viscosity (flowable) composite resin is frequently used for this purpose, as it potentially provides immediate as well as lasting stress absorption to prevent interfacial rupture. Additionally, manufacturers seek to impart adhesive systems with high immediate bond strength levels such that they can withstand composite shrinkage stress.

An entirely different strategy avoids resin-dentin bonding in areas at higher risk for postoperative sensitivity (medium-to-large Class I and II composite restorations) or microleakage (margins lacking enamel). The “sandwich technique” employs resin-modified glass-ionomer restorative as a base (dentin replacement) laminated with composite resin (enamel replacement). Glass ionomer produces a chemical bond to dentin following a non-invasive conditioning of the surface and displays superior resistance to caries at dentin margins as compared to resin bonding. Elimination of etching dramatically reduces the likelihood of postoperative sensitivity, and restoration stiffness is reduced with a concomitant increase in stress absorption capacity.34 The “open sandwich” option leaves glass ionomer exposed as the cervical portion of the final restoration when cervical margins terminate on dentin.

Other techniques such as directed polymerization shrinkage73 and use of light-reflecting wedges, both intended to reduce cervical margin gaps by inducing shrinkage toward the margin, are no longer considered valid in light of current understanding of resin shrinkage dynamics.74-76

Summary

Resin adhesion to tooth structure is a complex entity demanding thoughtful utilization of available systems to fully exploit their capabilities. The extraordinary range of materials and techniques available for resin-dentin bonding in particular is indicative of this complexity and of the degree to which questions regarding resin-dentin interface remain to be answered.

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To request a printed copy of this article, please contact/Edmond R. Hewlett, DDS, UCLA School of Dentistry, Box 951668, Los Angeles, CA 90095-1668.

Figure Legend:

Figure 1. Classification of adhesives based on the number of clinical application steps and the type of substrate conditioning (self-etch or total-etch) employed. GI = glass ionomer, PAA = polyalkenoic acid (adapted from Van Meerbeek and colleagues33).