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Recent Advances in Dental Adhesives

An Overview

Dental adhesives are used for a wide range of clinical applications in restorative dentistry. Direct composite resin restorations all require bonding, and indirect resin inlays, onlays, and veneers require bonding and--depending on their design--crowns, bridges, and endodontic posts and cores may require and/or benefit from the use of dental adhesives in conjunction with resin luting materials. Bonding to tooth structure is essential for in pit-and ­fissure sealants, some endodontic obturation materials, fixed orthodontic appliances, and dentinal hypersensitivity treatments.

Buonocore is widely considered to be the first person to propose the application of adhesion technology in dentistry.1 Hagger developed a material composed of glycerolphosphoric acid dimethacrylate in 1949;2 it was postulated by Kramer and McLean in 1952 that this material chemically bonded to tooth structure.3 By the mid-1960s, the first commercially available pit-and-fissure sealants and composite resin materials utilizing this new adhesive technology were used clinically. Since then, dental adhesives that provide numerically higher bond strengths and more substantive bonded interfaces to both enamel and dentin have been developed. By the 1980s, etch-and-rinse adhesives had gained widespread acceptability, and Nakabayashi in 1982 proposed a mechanism for bonding to dentin that described the infiltration of resin monomers into the tooth.4 By the 1990s, the concept of the "hybrid layer" was accepted, and both multi-step and single-step adhesives were available.5

Using dental adhesives can promote conservation of tooth structure and enable minimally invasive cavity preparation. For example, acid-etch retained “Maryland” bridges are based on the concept of minimal tooth preparation, by taking advantage of the adhesion of the "wings" of the bridge to abutment teeth. In intracoronal cavity preparations, direct composite resin restorations--unlike amalgam fillings--are believed to reinforce weakened dentin or enamel, reduce marginal staining, reduce microleakage, and may also reduce postoperative sensitivity.6

The resin-based adhesive systems developed to date can be categorized as one of two types: 1) etch-and-rinse (total-etch) adhesive systems, available as three-step and two-step systems, and 2) "self-etch" adhesive systems, available as two-step and one-step systems.7 All modern systems show some etching and demineralization of the tooth surface depending on the chemistry of the system selected. Both etch-and-rinse and self-etch adhesives produce an interface at the juncture of tooth surface and adhesive. Differences in the enamel bonding interface, as compared to the dentin bonding interface, is an important consideration in the selection of either an etch-and-rinse or a self-etching adhesive system.

 

Etch-and-Rinse Adhesives

Etch-and-rinse adhesives are available as three-step systems and two-step systems. Etch-and-rinse adhesives produce high and durable adhesion values for both enamel and in the bond to dentin. Self-etch adhesives generally demonstrate better adhesion to dentin than to enamel. Deficiencies in enamel bonding can be particularly critical in anterior aesthetic restorations. If enamel is the dominant substrate to be bonded to, an etch-and-rinse system is preferred.

Mechanisms of Action—Enamel and Dentin

The mechanism for enamel and dentin bonding using etch-and-rinse systems consists of these steps: demineralization of the surface by the acid, penetration of the adhesive monomers into the microscopic spaces created by the etchant, and curing of the adhesive monomers to form resin tags that microscopically provide a mechanical bond and seal to dentin and enamel.8 In enamel, macro resin tags are created peripherally around the demineralized surface of the hydroxyapatite crystals, and microtags are formed by resin penetration into demineralized crypts within the crystals. On dentin, the etchant creates a demineralized surface 3 µm to 5 µm in depth and, upon rinsing the acid, the smear layer and other debris are removed from the surface. Failure to remove the smear layer reduces dentin permeability, as that layer acts as a barrier and prevents adhesion to the underlying intact tooth structure. Following acid conditioning, collagen contained in the dentin is exposed. The adhesive system monomers then envelope the exposed collagen and remaining mineral, penetrating to the residual intact mineralized dentin within this microscopic etched surface. This interface is referred to as the "hybrid layer"' and is observed in three distinct areas: within the dentinal tubules, in the microscopic branches lateral to the tubules, and in the intertubular dentin.9

Three-step etch-and-rinse systems contain an etchant, a primer, and a bonding resin (ie, an adhesive). The primer is used to displace residual moisture, thus creating a surface to which the hydrophobic bonding resin can adhere. The hydrophilic solvent in the primer facilitates penetration of the resin monomers into the tooth surface. Applied bonding resin fills the residual space and seals the dentinal tubules. Two-step systems combine the primer and adhesive components, thus simplifying the application process into only one step and their clinical performance has been extensively documented. One key performance factor that influences the success of etch-and-rinse systems includes the thorough removal of the etchant prior to bonding.10 It is also critical to ensure that the dentin remains moist when the primer and adhesive (or primer-adhesive combination) are applied so that adequate space is retained between the exposed collagen to allow for penetration of the etched surface by the resin monomers. Larger posterior cavities and full-coverage crown preparations with extensive dentin surface areas are particularly subject to postoperative sensitivity if the dentin is dried, the collagen collapses, and inadequate resin penetration is developed.7

There is proven long-term bond stability using etch-and-rinse systems with both enamel and dentin. Most studies have shown bonding to enamel is more consistent, with higher initial bond-strength values and less breakdown at the enamel margin over time than with self-etching adhesive systems.11

 

(Continued from page 1 )
 

Self-etching Adhesives

Self-etch adhesives are available as one-step and two-step systems. These systems do not require an etch-and-rinse procedure. There is no need to address residual dentin moisture with the clinical application of these systems. The demineralizing process occurs simultaneously with the penetration of the adhesive, thus eliminating the potential for over-etching when treating dentin.7

Mechanisms of Action — Enamel and Dentin

The mechanism of adhesion is thought to be a combination of mechanical and/or chemical retention for both enamel and dentin. A hybrid layer that mimics the hybrid later formed with etch-and-rinse adhesives is produced with these systems, and deep resin tag penetration can also be seen. Self-etching adhesive formulations all contain water to facilitate the acid reaction on the tooth surface, but a great range of pH is observed depending on the specific adhesive system. Mild pH self-etch adhesives have shallower resin tags, and the hydroxyapatite with which they interact is not completely demineralized. This may be partly due to a chemical interaction between the monomer and the hydroxyapatite crystal, especially with the less acidic systems.7

Self-etch adhesives are moisture forgiving and are particularly useful in difficult-to-isolate areas or with difficult patients. Self-etch adhesives generally contain water in addition to monomers in their formulation. Water is more difficult to remove in the drying step after placement. Thus, it is more critical to ensure that the drying of the adhesive after application is effectively completed. Since self-etch adhesive systems demineralize and penetrate the dentin at the same time, there is no concern over incomplete penetration and residual gaps within demineralized dentin. Depending on the pH of the specific adhesive formula, the removal of the smear layer is often less complete, so the dentinal tubules may not be as open as in the case of the etch-and-rinse systems. This may reduce the risk of postoperative sensitivity, although the restorative technique is perhaps the key to reduced postoperative sensitivity.7

Newer Adhesive Developments

Most recent developments in adhesive materials have focused on the self-etching approach. One-bottle, self-etch systems have now evolved to where only one material is applied to the tooth, with no mixing required. Newer self-etching systems have developed compositions that have led to improvements in the products' shelf lives, with diminished hydrolysis in the package.

Most self-etching adhesives contain some phosphoric acid ester derivative in a mixture of water and other solvents. This acid component drives the demineralization reaction on enamel and dentin. A higher pH of the system usually promoted dentin adhesion while a lower component promotes enamel penetration. One newly developed formulation uses a carboxylic acid monomer to promote adhesion to dentin and also a phosphonic acid to promote adhesion to enamel. This approach has created a single-step, one-component adhesive system capable of generating a high and durable bond to dentin and enamel.12,13 Scanning electron microscopy (SEM) shows a durable hybrid layer formed to dentin that withstands thermomechanical stress as effectively as do multi-step, self-etching adhesive systems; SEMs also show a complete and stable infiltration of resin into the tooth structure.14

Summary

Numerous forms of adhesive systems are available and include etch-and-rinse systems and self-etch systems. The clinician’s selection of an adhesive has implications for the technique used as well as for the quality of the bond achieved. To date, enamel bonding is superior with etch-and-rinse adhesive systems, while self-etch systems may be more suitable for restorations with large areas of dentin. Newer formulas employing a combination of adhesive monomers, however, may facilitate a higher degree of performance for the self-etching approach. These newer developments, which have been introduced to simplify and improve the performance of adhesive systems for bonding of both enamel and dentin, may significantly advance the art and science of restorative dentistry.

*Associate Dean for Research, Professor of General Dentistry, Creighton University School of Dentistry, Omaha, NE.

References

  1. Buonocore MG. A simple method of increasing the adhesion of acrylicfilling materials to enamel surfaces.  J Dent Res. 1955:34: 549-53. 
  2. McLean JW. Historical Overview: The pioneers of enamel and dentin bonding. In: Roulet JF, DeGrange M, eds. Adhesion-the silent revolution in dentistry. Carol Stream, IL: Quintessence, 2000; 13-17. 
  3. Kramer IRH, McLean JW. The response of the human pulp to self-polymerising acrylic. Br Dent J 1952; 93:150-153.  
  4. Nakabayashi N, Kojima K, Masuhara E. The promotion of adhesion by the infiltration of monomers into tooth substrates. J Biomed Mater Res 1982; 16: 265-273.  
  5. Kanca J. Improving bond strength through acid etching of dentin and bonding to wet dentin surfaces. J Am Dent Assoc 1992;123(9);183-188.
  6. Perdigao J,Geradeli S, Hodges JS. Total-etch versus self-etch adhesive: Effect on postoperative sensitivity. J Am Dent Assoc 2003;134(12):1621-1629.
  7. Van Meerbeek B, De Munch J. Yoshida Y. et al Buonocore Memorial Lecture. Adhesion to enamel and dentin: current status and future challenges. Oper Dent 2003;28:215-235.
  8. Van Meerbeek B, Vargas MA Inoue S, et al. Adhesives and Ccements to promote preservation dentistry. Oper Dent 2001; Suppl 6; 119-143. 
  9. Van Meerbeek B, Inskoshi S, Braem M, et al. Morphological aspects of the resin-dentin interdiffusion zone observed with different adhesive systems. J Dent Res 1992;72(2):1530-1540.
  10. Perdigao J, Geradeli S. Hodges JS. Total-etch versus sell etch adhesive: Effect on postoperative sensitivity J Am Dent Assoc 2003;134 (12):1621-1629.
  11. Brackett WW, Ito S, Nishitani Y, et al The microtensile bond strength of self etching adhesives to ground enamel. Oper Dent 2006; 31(3):332-337.
  12. Kawamoto O, Nagano F, Yasumoto K, Sano H. Microtensile bond strength of one and two-step self-etching adhesives using a novel thermocycling method. Poster Presentation, 3rd IAD 2008 Special Issue: 126, Abstract PO85. 
  13. Shinno K, Ichizawa K, Nakatsuka T, et.al. Bonding ability of resin-bonding systems containing phosphoric acid adhesive monomer. J Dent Res 2008, Abstract 0394.
  14. Uno S, Morigami M, Sugizaki J, Yamada T. SEM and TEM observation of the bonding interface created with an experimental one-bottle one-step resin bonding agent. J Adhes Dent 2008; 26(1):30-35. 
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