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April 13, 2008

By Sarita Arteaga, DMD, MAGD

In the age of adhesive dentistry, restorative techniques are now based on retention of tooth-colored materials with conservative preparations. The goal has been to develop durable and reliable adhesion between the restoration and the tooth. Thus, bonding methods have been examined and developed to be simpler, faster, and less technique-sensitive for clinical uses.

Clinical longevity of the restoration includes both physical and chemical factors. The physical factors take into account the occlusal chewing forces and repetitive expansion and contraction stresses due to temperature changes. The chemical factors affecting adhesive dentistry might include the acidic agents found in dentinal fluid, saliva, food, beverages, bacterial products, and residual resin monomers from suboptimal polymerization or degradation of the resin composite.(1)

The principles of bonding have not changed over the years and still rely on the close and intimate relationship of the materials to the tooth structure. Modifications have been made to simplify the adhesion process as we have gained a more thorough understanding of the composition of enamel and dentin. For example, our current knowledge of enamel rod and prism anatomy enables us to use acidic materials on the enamel surfaces to remove calcium phosphates, thereby creating microporosities that allow for a micromechanical interlocking of adhesive monomers. Dentin contains a higher percentage of water and less hydroxyapatite than enamel, which presents a more challenging configuration for bonding agents to infiltrate.

During tooth preparation, the cut debris adheres to the enamel and dentin surfaces and produces the smear layer. This introduces another issue: What do we do with the smear layer before placing our adhesive? "Current adhesive systems relate to the enamel and dentin by using two strategies: removal of the smear layer (etch and rinse technique) or maintaining it (self-etch technique)."(1)

The classification of modern adhesives is based on the generation (time frame developed) or the mechanism of adhesion, and the number of clinical steps needed for the procedure. In the mid-1950s, Buonocore determined that treating the enamel surface with an acidic solution resulted in the formation of resin tags that formed a micromechanical bond with the etched enamel. These materials were considered the first generation of bonding agents.

Second-generation adhesives were based on phosphorous esters of methacrylate derivatives that formed an ionic interaction between the negative phosphate groups and the positively charged calcium.

The removal of the smear layer with chelating agents or acids, after acceptance of the Japanese philosophy in the late 1980s, gave rise to the third-generation materials. These agents opened up dentinal tubules and allowed for microretention within the etched dentinal surfaces.

Multistep procedures began to develop in the mid-1990s, known as fourth-generation adhesives. These include the three-step etch, prime, and bond technique that is still used today.

The fifth-generation agents adjusted the three-step technique to include a total-etch phase of both the enamel and dentin, followed by the primer and adhesive in a combined step.

Self-etch adhesives comprise the sixth- and seventh-generation materials, differing in the number of steps required for bonding.(2)

Dental adhesives may also be grouped based on their adhesion approach and the number of clinical steps required.

Etch and rinse adhesives

Etch and rinse adhesives may be applied in two or three steps. This conventional and most effective three-step approach uses an acid (usually 30% to 40% phosphoric acid) to dissolve calcium substrate on the enamel surface. As the concentration of acid increases, the depth of decalcification into the prisms of enamel increases with an exposure time of 15 seconds or greater. The critical element of this technique is to adequately rinse all of the acid away from the tooth. Contact with the dentin by this acid will expose the collagen fibril scaffold to permit diffusion of adhesive resin after the use of a dentin primer. The primer contains acetone, ethanol, or water and will remove the smear layer to guarantee wetting of these exposed collagen fibrils, displacing excess moisture, and carries monomer into the channels.(3)

The final step consists primarily of hydrophobic monomers such as bis-GMA and UDMA. The application of the adhesive will result in hybridization — the penetration of the resin into the dentin. Three zones of dentin in the hybrid layer are observed prior to the resin-impregnation stage. The top layer is loosely arranged collagen fibrils and the middle layer is the typical "shag carpet" with collagen fibrils separated by spaces and residual mineral crystals. The base layer has unaltered peritubular and intertubular dentin.(2) Two-step etch and rinse adhesives combine the primer and adhesive into one solution. This creates a potential for a thin bonding layer and is considered technique-sensitive due to the need for moist dentin.(4)

Self-etch adhesives

Self-etching adhesives have been introduced that combine the etchant, primer, and adhesive into one or two steps and can be applied simultaneously on the enamel and dentin without rinsing. The basis for bonding is to enlarge the surface area for bonding by micromechanical retention within the enamel and interlocking within the dentin scaffold. Dissolved hydroxyapatite crystals and residual smear layer remnants are used as a substrate to bond. The two-step, self-etch system uses acidic monomers at varying pH strengths to act as a primer in the initial step and adhesive resin in the second step, without any rinsing. The one-step, self-etch system combines it all in one application.(5)

Lower pH (= 1) values for strong self-etch adhesives create deep demineralization effects for good enamel bonding, but adversely affect the hybrid layer. These liquids may also have a reduced shelf life. Mild self-etch adhesives with a higher pH (= 2), may have insufficient bonding effectiveness to enamel as they may not sufficiently demineralize the prism anatomy. Self-etch materials with intermediary pH levels demonstrate the most promise with the ability to adequately etch enamel and dentin without detrimental effects on the dentin, and allowing for penetration of the adhesive into both structures.

Conclusion

Attention must be taken to follow the directions appropriately with each step of enamel and dentin bonding to allow the chemistry of the materials to be effective. Studies indicate that the etch and rinse adhesive systems are effective for enamel bonding, yet degradation of the hybrid layer at the dentin bonded interface can affect the bond stability. Recent research recommends the use of a 2% chlorhexidine application to prevent degradation of the collagen matrix caused by metalloproteinases (MMPs). This would be applied after the etchant and rinsing for the etch and rinse adhesive technique, and prior to primer/adhesive application. For the self-etch adhesives, the CHX would be used prior to any other liquid.(6) Adhesives in dentistry continue to be a dynamic field of interest, particularly with the surge of esthetics.

Sarita Arteaga, DMD, MAGD, graduated from New York University's Washington Square University College with a B.A. degree in biology in 1986. After attending the University of Connecticut School of Dental Medicine, she received a D.M.D. degree in 1990 and completed a General Practice Residency at Bronx Municipal/Albert Einstein Hospital in the Bronx, N.Y. Returning to Connecticut, she was an associate in private dental practice. After an appointment as an assistant clinical professor at the University of Connecticut, she has been teaching and seeing patients at the Health Center since 1995. In the Department of Reconstructive Sciences, she is a course director for the Operative Dentistry I course and teaches in Operative Dentistry II, III, and IV, as well as the Fixed Prosthodontics preclinical course and clinic coverage for students in the third- and fourth-year dental school classes. She attained a Mastership from the Academy of General Dentistry, and is a member of numerous dental associations. She is currently president of the Hispanic Dental Association. She is a faculty advisor for the Student National Dental Association/Hispanic Student Dental Association at the University of Connecticut School Dental Medicine. She also serves on the Admissions Committee. Dr. Arteaga takes an active community role, performing screenings at the Head Start program in Waterbury and Meriden, the Special Olympics, and South Park Inn Homeless Shelter in Hartford. You may contact Dr. Arteaga by e-mail at Arteaga@nso2.uchc.edu.

References

1. Breschi L, Mazzoni A, Ruggeri A, Cadenaro M, Di Lenarda R, De Stefano Dorigo E. Dental adhesion review: Aging and stability of the bonded interface. Dent Materials 2008; 24:90-101.

2. Summitt JB, Robbins JW, Hilton TJ, Schwartz RS. Fundamentals of Dentistry: A Contemporary Approach. Third Ed., Quintessence Pub Co Inc, 2006.

3. Van Lunduyt KL, Snauwaert J, DeMunck J, Peumans M, Yoshida Y, Poitevin A, Coutinho E, Suzuki K, Lambrechts P, Van Meerbeek B. Systematic review of the chemical composition of contemporary dental adhesives. Biomaterials 2007; 28:3757-3785.

4. Tseng K, Weinberg GA, Woodlock D. Dental Adhesives: The foundation of esthetic dentistry. Gen Dent 2007; Special issue, pp. 698-708.

5. Knobloch LA, Gailey D, Azer S, Johnston W, Chelland N, Kerby RE. Bond strengths of one- and two-step self-etch adhesive systems. J Pros Dent 2007; 97(4):216-222.

6. Carrilho MRO, Carvalho RM, de Goes MF, di Hipólito V, Geraldeli S, Tay FR, Pashley DH, Tjäderhane L. Chlorhexidine preserves dentin bonding in vitro. J Dent Res 2007; 86(1):90-94.

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