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THE SCIENCE OF BONDING: FROM FIRST TO SIXTH GENERATION GERARD KUGEL and MARCO FERRARI J Am Dent Assoc 2000;131;20S-25S

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ARTICLE 2

THE SCIENCE OF BONDING:FROM FIRST TO SIXTH GENERATION
GERARD KUGEL, D.M.D., M.S.; MARCO FERRARI, M.D., D.D.S., PH.D.

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Background. Adhesive dentistry has revolutionized restorative dental practice during the past 30 years. Improved adhesive materials have made resin-based composite restorations more reliable and long-standing.

Clinical Implications. This article reviews the evolution of bonding from the first generation to current bonding materials. It discusses the composition and effectiveness of the various iterations. Current products are highlighted to improve clinical use and performance of the materials.

for us to examine the past. The principles of adhesive dentistry date back to 1955 when Buonocore, using techniques of industrial bonding, postulated that acids could be used as a surface treatment before application of the resins.1 He subsequently found that etching enamel with phosphoric acid increased the duration of adhesion under water. In 1963, Buonocore demonstrated his insight into adhesion dentistry when he discussed the difference in bonding to enamel and to dentin,2 particularly when he referred to Dr. Bowen’s attempts to investigate substances that will displace water from tooth surfaces3 with the idea that they could be used as pretreatment for enamel or dentin. Buonocore then stated that they could even be incorporated into the adhesives.2 In the late 1960s, Buonocore suggested that it was the formation of resin tags that caused the principal adhesion of the resins to acid-etched enamel.4 The idea that resin penetrates the microporosities of etched enamel and results in a micromechanical bond is well-accepted today. As time went on, variations in duration of the
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As we enter the new millennium, it is important

acid-etching procedure and concentration of the phosphoric acid, along with alternative acids, were tested for the etching of enamel.5-7 The current thinking is that a 30 to 40 percent phosphoric acid etch of 15 seconds is acceptable. The ability to bond reliably to enamel is now well-accepted, but as Buonocore suggested in 1963, adhesion of our restorative materials to dentin has proved to be more elusive.2 Early attempts to bond to dentin resulted in poor bond strengths.8 This is not surprising given the fact that while enamel contains little protein, dentin is 17 percent collagen by volume. This collagen is inaccessible due to surrounding hydroxyapatite crystals.9 The dentinal tubules are the only pores available for micromechanical retention. These tubules contain fluid, which would be an impediment to bonding. The number of tubules available for bond also varies depending on location, with deep dentin having more tubules than superficial dentin.9 Other factors such as age of teeth, direction of tubules and of enamel prisms, presence of cementum and type of dentin can affect dentin bonding.10,11

JADA, Vol. 131, June 2000 Copyright ©1998-2001 American Dental Association. All rights reserved.

Early dentin bonding was further complicated by the presence of the smear layer. (The smear layer is the organic debris that remains on the dentin surface after the preparation of the dentin during restoration of a tooth.) The smear layer blocks the dentinal tubules and acts as a “diffusion barrier.” This was originally thought of as an advantage in that it protected the pulp by decreasing the permeability of dentin.12 As dentin bonding improved, the removal of the smear layer became necessary, but not without controversy. Several factors characterize the change in bonding systems from the Buonocore era to today: detching enamel; detching-conditioning dentin; dsmear layer treatment; dhandling properties. This article reviews several aspects of the different generations of bonding systems.
FIRST GENERATION

were only 1 to 3 megapascals. The clinical results with these systems were poor.
SECOND GENERATION

As improvements were made in the adhesive coupling agents for composites, the adhesion to dentin increased. In the late 1970s, the second-generation systems were introduced. The majority of these incorporated halophosphorous esters of unfilled resins such as bisphenol-A glycidyl methacrylate, or bis-GMA, or hydroxyethyl methacrylate, or HEMA.15 The mechanism by which these second-generation systems bonded to dentin were postulated to be through an ionic bond to calcium by chlorophosphate groups. These were weak bonds (in compari-

much of the adhesion was due to bonding to the smear layer. Some of the second-generation systems were thought to soften the smear layer and thus improve resin penetration. However, these systems resulted in bond strengths to dentin that were weak and unreliable.
THIRD GENERATION

In 1956, Buonocore and colleagues demonstrated that use of a glycerophosphoric acid dimethacrylate-containing resin would bond to acid-etched dentin.13 This bond was believed to be due to the interaction of this bifunctional resin molecule with the calcium ions of hydroxyapatite. Of course, immersion in water would greatly reduce this bond. Nine years later Bowen14 tried to address this issue using N-phenylglycine and glycidyl methacrylate, or NPG-GMA. NPG-GMA is a bifunctional molecule or coupling agent. This means that one end of this molecule bonds to dentin while the other bonds (polymerizes) to composite resin. The bond strengths of these early systems

As dentin bonding improved, removal of the smear layer became necessary, but not without controversy.
son to fifth- and sixth-generation systems) but they were a significant improvement over first-generation systems. One major concern with these systems was that the phosphate bond to calcium in the dentin was not strong enough to resist the hydrolysis resulting from water immersion. This hydrolysis, resulting from either saliva exposure or moisture from the dentin itself, could result in composite resin debonding from the dentin and causing microleakage. Since dentin was not etched in these early bonding systems,

With the third-generation systems, the acid etching of the dentin partially removes and/or modifies the smear layer.16 This effect is due to the pK of the primer solution. The acid opens dentinal tubules partially and increases their permeability. The acid must be rinsed completely before the primer is applied. The primer contains hydrophilic resin monomers which include hydroxyethyl trimellitate anhydride, or 4-META, and biphenyl dimethacrylate, or BPDM. The primers contain a hydrophilic group that infiltrates the smear layer, modifying it and promoting adhesion to dentin, and the hydrophilic group of the primer creates adhesion to the resin. Following primer application, an unfilled resin is placed on dentin and enamel. These third-generation adhesion systems usually use a hydrophilic dentin-resin primer. Dentin primers may be 6 percent phosphate penta-acrylate, or PENTA; 30 percent HEMA; and 64 percent ethanol. Following etching and primer application, the unfilled resin adhesive is applied to dentin and enamel. In most of these systems, the phosphate primer modifies the smear layer by softening it; after penetration, it cures, forming a hard surface. The adhesive is then applied, attaching the cured primer to
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JADA, Vol. 131, June 2000 Copyright ©1998-2001 American Dental Association. All rights reserved.

Figure 1. Collagen fibers exposed after etching dentin (scanning electron microscopy X5000; reprinted with permission of P.N. Mason).

Figure 2. Hybrid layer formed between etched dentin and a one-bottle system (scanning electron microscopy × 1,550).

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the composite resin. Bonding to smear-layer–covered dentin was not very successful before 1990, however, because the resins did not penetrate through the smear layer and the smear layer was very weak.17
FOURTH GENERATION

The complete removal of the smear layer is achieved with fourth-generation bonding systems. Fusayama and colleagues tried to simplify bonding to enamel and dentin by etching the preparation with 40 percent phosphoric acid.18 Unfortunately, it was not understood that this procedure overetched dentin and resulted in the collapse of exposed collagen fibers. In 1982, Nakabayashi and colleagues reported the formation of a hybrid layer resulting from the polymerized methacrylate and dentin.9 The hybrid layer is defined as “the structure formed in dental hard tissues (enamel, dentin, cementum) by demineralization of the surface and subsurface, followed by infiltration of monomers and subsequent polymerization.”9 The use of the total-etch technique is one of the main
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characteristics of fourth-generation bonding systems.19,20 The total-etch technique permits the etching of enamel and dentin simultaneously using phosphoric acid for 15 to 20 seconds. The surface must be Figure 3. Resin tags and adhesive lateral branches left moist (“wet contribute to micromechanical bonding formation bonding”), how- (scanning electron microscopy × 2,000). ever, in order to avoid collagen collapse (Figure dissolved by the acidic action; 1); the application of a the initial surface penetration hydrophilic primer solution can exposes the collagen fibers. In infiltrate the exposed collagen this area, for a depth of 2 to 4 network forming the hybrid micrometers, hybridization layer (Figure 2).21,22 Unfortakes place, and resin tags can seal the tubule orifices tunately, “moist dentin” is not firmly.16,26 easily defined clinically and may lead to less-than-ideal FIFTH GENERATION bonds if the dentin is excessively wet23 or dried.16 To simplify the clinical proceThe formation of resin tags dure by reducing the bonding and adhesive lateral branches steps and thus, the working complete the bonding mechatime, a better system was neednism between the adhesive ed. Also, clinicians needed a material and etched dentin sub- better way to prevent collagen strate (Figure 3).22,24,25 The mincollapse of demineralized eralized tissues of the peritubudentin. The fifth generation of lar and intertubular dentin are bonding systems was developed

JADA, Vol. 131, June 2000 Copyright ©1998-2001 American Dental Association. All rights reserved.

Figure 4. Enamel surface after etching with a selfetching primer solution; the enamel surface is less retentive than that obtained with phosphoric acid (scanning electron microscopy × 1,500).

Figure 5. Laminate veneers of maxillary central incisors; the porcelain restorations are luted thanks to the adhesive technique.

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to make the use of adhesive materials more reliable for practitioners. The fifth generation consists of two different types of adhesive materials: the so-called “one-bottle systems” and the self-etching primer bonding systems. One-bottle systems. To facilitate clinical use, “one-bottle” systems combined the primer and adhesives into one solution to be applied after etching enamel and dentin simultaneously (the total-etch wet-bonding technique) with 35 to 37 percent phosphoric acid for 15 to 20 seconds.27 These bonding systems create a mechanical interlocking with etched dentin by means of resin tags, adhesive lateral branches and hybrid layer formation and show high bond-strength values both to the etched enamel and dentin.28,29 Self-etching primer. Watanabe and Nakabayashi developed a self-etching primer that was an aqueous solution of 20 percent phenyl-P in 30 percent HEMA for bonding to enamel and dentin simultaneously.30

The combination of etching and priming steps reduce the working time, eliminate the washing out of the acidic gel and also eliminate the risk of collagen collapse. However, the self-etching primer solution also has some disadvantages. For example, the solution must be refreshed continuously because its liquid formulation cannot be controlled where it is placed,31 and often a residual smear layer remained in between adhesive material and dentin.16 Also the effectiveness of selfetching primer systems on properly etching the enamel was less predictable than the result obtained with phosphoric acid gel (Figure 4).31 Toida32 advised that removal of the smear layer by a separate etching step before bonding would produce a more reliable and durable bond to dentin. Bond strength tests made under laboratory conditions often did not demonstrate statistically significant differences between one-bottle systems and self-etching primer bonding systems.33 Leakage tests conducted under laboratory and clinical conditions showed that the seal

achieved at the enamel margins with one-bottle systems is superior to that resulting from selfetching primer.31
SIXTH GENERATION

Recently, several bonding systems were developed and proposed as the sixth generation of adhesive materials. These bonding systems are characterized by the possibility to achieve a proper bond to enamel and dentin using only one solution. These materials should really be a one-step bonding system. Unfortunately, the first evaluations of these new systems showed a sufficient bond to conditioned dentin while the bond with enamel was less effective. This may be due to the fact that the sixth-generation systems are composed of an acidic solution that cannot be kept in place, must be refreshed continuously and have a pK that is not enough to properly etch enamel.34 However, any improvement in the direction of clinical simplification of bonding procedures can bring us closer to achieving an ideal bonding system.
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JADA, Vol. 131, June 2000 Copyright ©1998-2001 American Dental Association. All rights reserved.

DISCUSSION

Bonding to etched enamel was considered a safe and reliable procedure for many years. Because of the inorganic composition of the enamel, the acid attack produces interprismatic and prismatic dissolution, creating irregularities into which the resin can flow and, after polymerization, create a mechanical interlocking. The demineralization of the enamel depends on the low pH of the acid and on the etching time. The pH and the etching time must be sufficient to provide adequate enamel retention without the need for additional steps. The morphological studies made on the first five generations of bonding systems, in which phosphoric acid was used to etch enamel, showed a uniform etch pattern. When phosphoric acid was not used or when self-etching primers (fifth and sixth generations) were applied, the bonding mechanism of adhesives to enamel was less effective. The bonding mechanism to dentin was effective and predictable when the smear layer was completely dissolved, intertubular and peritubular dentin were dissolved, collagen fibers exposed and, after infiltration of resin monomers, a hybrid layer formed. This bonding mechanism was evident from fourth to sixth generations of enamel dentin bonding systems. A wide range of clinical indications for bonding systems are commonly followed by practitioners. Bonding systems can be used as coupling resin sealants to enamel in young patients.35 Also, the sealing effect of bonding systems can be used on protecting dentinal surfaces after
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preparation36 or beneath amalgam restorations.37,38 Desensitization of root exposure is another clinical indication for bonding systems.39-41 Also, bonding systems are indicated in any direct esthetic restorations. In fact, the mechanical properties of the bonding mechanism achieved with hybrid layer and resin tag formation can be greater than the forces of polymerization contraction.16 Finally, bonding systems are essential for a proper bonding-luting procedure of any indirect restorations (Figure 5).42
CONCLUSIONS

have mechanical properties close to those of tooth structures, be resistant to degradation in the oral environment and easy to use for the clinician. Although important improvements in bonding have been made in the last 30 years, note that the requirements of an ideal bonding system are quite similar to those indicated by Buonocore.2 Apparently, the future has a sound background in the past. s
Dr. Kugel is professor and assistant dean for research, Tufts University School of Dental Medicine, Boston, Mass. 02111 Address reprint requests to Dr. Kugel. Dr. Ferrari is a research professor at Tufts University School of Dental Medicine, Boston, and professor, University of Siena, Italy. The authors dedicate this paper to the memory of Gaia Gotti. They also express their appreciation to Jennifer Towers for her help in the preparation of this manuscript. 1. Buonocore MG. A simple method of increasing the adhesion of acrylic filling materials to enamel surfaces. J Dent Res 1955;34:849-53. 2. Buonocore MG. Principles of adhesive retention and adhesive restorative materials. JADA 1963;67:382-91. 3. Bowen RL. Investigations of the surface of hard tooth tissues by a surface cavity test. In: Adhesive restorative dental materials. Phillips RW, Ryge G, eds. Spencer, Ind.: Owen Litho Service; 1961:177. 4. Buonocore MG, Matsui A, Gwinnett AJ. Penetration of resin dental materials into enamel surfaces with reference to bonding. Arch Oral Biol 1968;13(1):61-70. 5. Silverstone LM. Fissure sealants: laboratory studies. Caries Res 1974;8:2-26. 6. Barkmeier WW, Gwinnett AJ, Shaffer SE. Effects of enamel etching time on bond strength and morphology. J Clin Orthod 1985;19(1):36-8. 7. Kugel G, Habib C, Zammitti S. Enamel and dentin surfaces after treatment with adhesion conditioners using the environmental SEM (abstract 2260). J Dent Res 1993;72:386. 8. McLean JW, Kramer IRH. A clinical and pathological evaluation of a sulphinic acid activated resin for use in restorative dentistry. Br Dent J 1952;93:255-69, 291-3. 9. Nakabayashi N, Kojima K, Masuhara E. The promotion of adhesion by the infiltration of monomers into tooth states. J Biomed Mat Res 1982;16:265-73. 10. Duke ES, Lindemuth J. Variability of clinical dentin substrates. Am J Dent 1991;4:241-6. 11. Cagidiaco MC. Bonding to dentin (Ph.D. thesis). Amsterdam, The Netherlands: Acta University, 1995. 12. Pashley DH, Michelich V, Kehl T. Dentin permeability: effects of smear layer

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To satisfy the growing esthetic demands of today’s dental patients, improvements in materials and procedures have been made to make it possible

The mechanical properties of the bonding mechanism achieved with hybrid layer and resin tag formation can be greater than the forces of polymerization contraction.
to reproduce the natural appearance of natural teeth with direct and/or indirect esthetic restorations. Esthetic techniques involve a bonding step to ensure durability and reliability. Thus, the ideal bonding system should be biocompatible, bond indifferently to enamel and dentin, have sufficient strength to resist failure as a result of masticatory forces,

JADA, Vol. 131, June 2000 Copyright ©1998-2001 American Dental Association. All rights reserved.

removal. J Prosthet Dent 1981;46:531-7. 13. Buonocore M, Wileman W, Brudevold F. A report on a resin composition capable of bonding to human dentin surfaces. J Dent Res 1956;35:846-51. 14. Bowen RL. Adhesive bonding of various materials to hard tooth tissues II. Bonding to dentin promoted by a surface-active comonomer. J Dent Res 1965;44:895-902. 15. American Dental Association Council on Dental Materials. Instruments and equipment. Dentin bonding systems: an update. JADA 1987;114:91-5. 16. Nakabayashi N, Pashley DH. Hybridization of dental hard tissues. Tokyo: Quintessence; 1998. 17. Tao L, Pashley DH, Boyd L. The effect of different types of smear layers on dentin and enamel bond strengths. Dent Mater 1988;4:208-16. 18. Fusayama T, Nakamura M, Kurosaki N, Iwaku M. Non-pressure adhesion of a new adhesive restorative resin. J Dent Res 1979;58:1364-72. 19. Kanca J. A method for bonding to tooth structure using phosphoric acid as a dentinenamel conditioner. Quintessence Int 1991;22:285-90. 20. Gwinnett AJ. Quantitative contribution of resin infiltration/hybridization to dentin bonding. Am J Dent 1993;6(1):7-9. 21. Kanca J. Wet bonding: effect of drying time and distance. Am J Dent 1996;9:273-6. 22. Gwinnett AJ, Tay FR, Wei SHY. Bridging the gap between overly dry and overwet bonding phenomenon of dentin hybridization and tubular seal. In: Shimono M, Maeda T, Suda H, Takayashi K, eds. Dentin/pulp complex. Tokyo: Quintessence; 1996:359-63. 23. Tay FR, Gwinnett AJ, Wei SH. The over

wet phenomenon: an optical, micromorphological study of surface moisture in acid-conditioned, resin-dentin interface. Am J Dent 1996;9(1):43-8. 24. Chappel RP, Cobb CM, Spencer P, Eick JD. Dentinal tubule anastomosis: a potential factor in adhesive bonding? J Prosthet Dent 1994:72:183-8. 25. Mjor IA, Nordhal I. The density and branching of dentinal tubules in human teeth. Arch Oral Biol 1996;41:401-12. 26. Titley K, Chercnecky R, Chan A, Smith DC. The composition and ultrastructure of resin tags in etched dentin. Am J Dent 1995;8:224-30. 27. Ferrari M, Goracci G, Garcia-Godoy F. Bonding mechanism of three “one-bottle” systems to conditioned and unconditioned enamel and dentin. Am J Dent 1997;10:224-30. 28. Tay FR, Gwinnett AJ, Wei SHY. Structural evidence of a sealed tissue interface with total etch wet bonding technique, in vivo. J Dent Res 1994;73:629-36. 29. Mason PN, Calabrese M, Graif L. Modified extrusion shear bond strength of the new 3M adhesive (abstract 256). J Dent Res 1998;77:1239. 30. Watanabe I, Nakabayashi N. Bonding durability of photocured Phenyl-P in TEGDMA to smear layer-retained bovine dentin. Quintessence Int 1993;24:335-42. 31. Ferrari M, Mannocci F, Vichi A, Davidson CL. Effect of two etching times on the sealing ability of Clearfil Liner Bond 2 in Class V restorations. Am J Dent 1997;10(2):66-70. 32. Toida K, Watanabe A, Nakabayashi N. Effect of smear layer on bonding to dentin prepared with bur. J Jpn Dent Mater 1995;14:109-16.

33. Yoshiyama M, Sano H, Carvalho RM, Pashley DH. Adhesive mechanism of a selfetching/self-priming adhesive resin to enamel and dentin. J Hard Tiss Biol 1996;5:31-5. 34. Fabianelli A, Vichi A, Kugel G, Ferrari M. Influence of self-etching-priming bonding systems on sealing ability of Class II restorations: leakage and SEM evaluation. Paper presented at annual meeting of the International Association for Dental Research; April 6, 2000; Washington, D.C. 35. Swift EJ. The effect of sealants on dental caries: a review. JADA 1988;116:700-4. 36. Cagidiaco MC, Ferrari M, Garberoglio R, Davidson CL. Dentin contamination protection after mechanical preparation for veneering. Am J Dent 1996;9(2):57-60. 37. Eakle WS, Staninec M. Effect of bonded amalgam on fracture resistance of teeth. J Prosthet Dent 1992;68:257-60. 38. Belcher MA, Stewart GP. Two-years clinical evaluation of an amalgam adhesive. JADA 1997;128:309-14. 39. Dondi dall’Orologio G, Malferrari S. Desensitizing effects of Gluma and Gluma 2000 on hypersensitive dentin. Am J Dent 1993;6:283-6. 40. Tagami J, Hosoda H, Burrow MF, Nakajima M. Effect of aging and caries on dentin permeability. Proc Finn Dent Soc 1992;88(suppl):146S. 41. Ferrari M, Cagidiaco MC, Kugel G, Davidson CL. Clinical evaluation of a “onebottle” bonding system for desensitizing exposed roots with and without a prior acid etch step. Am J Dent (in press). 42. Rosenstiel SF, Land MF, Crispin BJ. Dental luting agents: a review of the current literature. J Prosthet Dent 1998;80:280-301.

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