Update on Seventh-Generation Bonding Agents
John M. Powers, PhD; Kathy L. O’Keefe, DDS, MS
Do we need four generations of dental adhesives to serve our patients? No. Is it time to stop using phosphoric acid as an etchant for bonding to tooth structure? Yes. Can seventh-generation bonding agents meet all of our clinical needs? Yes. If these questions are answered appropriately, the practice of adhesive dentistry could be greatly simplified. Seventh-generation bonding agents, introduced in the early 2000s, contain acidic primers and adhesive monomers in a single bottle, eliminating separate etching and mixing steps. Initially seventh-generation bonding agents were available only as light-cured formulations, but now several dual-cured products are offered. This article reviews the characteristics, properties, and use considerations of seventh-generation bonding agents.
Materials on the Market
Current bonding agents for bonding restorative materials to tooth structure can be classified according to four generations, primarily characterized by the type of etching (etch-and-rinse [total-etch] or self-etch) and the number of separate components or steps required for application.1-3 Table 1 compares the characteristics of current bonding agents. Bonding agents require a number of chemical components to achieve bonding to tooth structure (Table 2).2,3 Seventh-generation bonding agents were introduced in the early 2000s (Table 3). These bonding agents contain acidic primers and adhesive monomers in one bottle, so separate etching and mixing are not required.2,3 Dual-cured products may require mixing of the separate catalyst. To maintain stability on storage, some products require refrigeration. Initially, seventh-generation bonding agents were available only as light-cured formulations, but several years ago various dual-cured products entered the marketplace. Seventh-generation bonding agents offer some desirable characteristics.2,3
Mechanism of Bonding
Seventh-generation bonding agents use the smear layer as a bonding substrate.4 The acidic primer demineralizes the smear layer and the top layer of the underlying dentin surface. The acidic primer also infiltrates the exposed collagen along with hydrophilic monomers, which then copolymerize.4 Because the etched surface is not rinsed, the demineralized smear layer is incorporated into the hybrid layer. The hybrid layer ranges in thickness from 0.5 µm to 5 µm.4 The acidic primer and adhesive monomers also infiltrate collagen fibers as the primer decalcifies the inorganic component in dentin to the same depth, which should minimize voids, potential leakage, and postoperative sensitivity.4
Bond strengths of seventh-generation bonding agents were measured in tension. Extracted human third molars, previously stored in sodium azide solution in saline, were embedded in resin and abraded on the facial surface with 600-grit SiC paper to form a bonding substrate of enamel or superficial dentin. The bonding agent was applied as recommended by the manufacturer. Resin composite (TPH 3, DENTSPLY Caulk, Milford, DE) was placed in a polytetrafluoroethylene mold in the shape of an inverted truncated cone with a diameter of 3 mm at the bond interface and a diameter of 4 mm at a height of 4 mm and cured as recommended.5 Specimens were stored in water at 37°C for 24 hours. Specimens were debonded in tension on a testing machine (Model 8501, Instron, Canton, MA) at a crosshead speed of 0.5 mm/min. Bond strengths to enamel ranged from 19 MPa to 32 MPa (Table 3).1 Bond strengths to dentin ranged from 18 MPa to 28 MPa.1 These bond strengths are adequate and comparable with other current generations of bonding agents.
The uses of seventh-generation bonding agents are summarized in Table 1.1 Light-cured, seventh-generation bonding agents are recommended primarily for bonding of light-cured direct resin composites.1 Because of their acidic monomers, the light-cured products are not recommended for use with self-cured core materials and resin cements.1 The acidity of these bonding agents can deactivate catalysts associated with self-cured resin composites and inhibit polymerization. The dual-cured seventh-generation bonding agents (Table 3) can be used with indirect posterior restorations and self-cured resin composite core build-ups.
Questions and Answers
Q: Are seventh-generation bonding agents sensitive to the wetness (over dry, over wet) of tooth structure and other contaminants (saliva, blood)?
A: Seventh-generation bonding agents have water as a solvent. Thus, the wetness or dryness of the tooth surface is less critical than with bonding agents with solvents of ethanol or acetone. Research on tooth substrates contaminated in vitro with human saliva or human blood showed that bonding agents based on acidic primers were less sensitive to these contaminants.6,7 It is recommended that heavily contaminated tooth surfaces be rinsed with water and the bonding agent reapplied.
Q: Should unground enamel be etched with phosphoric acid before the use of a seventh-generation bonding agent?
A: Instructions of most seventh-generation bonding agents recommend the use of phosphoric acid on unground enamel before application of the bonding agent. Some bonding agents may require more than one application of bonding agent to accommodate for the higher depth of etching caused by the phosphoric acid as compared with the depth if the self-etching adhesive were used alone. If dentin is exposed to etching by phosphoric acid, then reduced bond strength can be expected unless additional coats of the bonding agent are applied.8 An alternative to application of phosphoric acid to unground enamel is pumicing of the enamel.9 This technique provides a similar bond strength to that found with etching with phosphoric acid.
Q: Should seventh-generation bonding agents be used with anterior all-ceramic veneers?
A: A recent survey of clinical consultants for The Dental Advisor indicated that these dentists preferred etch-and-rinse (total-etch) bonding agents for bonding of all-ceramic veneers.1 The main concern is the perceived potential for marginal leakage with self-etching bonding agents.
Q: Do seventh-generation bonding agents maintain stability during storage?
A: The instructions for many seventh-generation bonding agents recommend refrigeration as shown in Table 3. Increased time of storage and increased temperature of storage can reduce stability, eventually leading to gelation of the bonding agent.
Seventh-generation bonding agents offer good bond strengths to tooth structure and less technique sensitivity than etch-and-rinse (total-etch) and sixth-generation bonding agents. They may be excellent choices for bonding direct and indirect resin and all-ceramic posterior restorations.
Dr. Powers is an owner/employee of Dental Consultants, Inc, publisher of The Dental Advisor. He has received grant/research support from Kuraray America, Inc, GC America, Inc, Tokuyama Dental, Kerr Corporation, DENTSPLY Caulk, and Ivoclar Vivadent, Inc.
Dr. O'Keefe is assistant editor of The Dental Advisor.
1. Farah JW, Powers JM, eds. Bonding agents. Dental Advisor. 2008;25(5):1-9. [www.dentaladvisor.com/publications/the-dental-advisor/index.shtml]
2. Powers JM, Sakaguchi RL, eds. Craig’s Restorative Dental Materials. 12th ed. St. Louis, MO: Mosby Elsevier; 2006.
3. Powers JM, Wataha JC. Dental Materials: Properties and Manipulation. 9th ed. St. Louis, MO: Mosby Elsevier; 2007.
4. Pinzon LM, Reis A, Saiz E, et al. Interfacial structure and nanomechanical properties of self-etch adhesive systems over-time [abstract]. J Dent Res. 2007;86(Spec Iss A): 116.
5. Barakat MM, Powers JM. In vitro bond strength of cements to treated teeth. Aust Dent J. 1986;31:415-9.
6. Pinzon LM, O’Keefe KL, Powers JM. Adhesion of composite with self-etching primer to saliva-contaminated moist and dry dentin. Trans Acad Dent Mater. 2002;16:336.
7. Pinzon LM, O’Keefe KL, Powers JM. Bond strength of composite with self-etching adhesives to blood-contaminated dentin [abstract]. J Dent Res. 2003;82(Spec Iss A):570.
8. Farah JW, Powers JM, eds. 6th- and 7th-generation bonding agents. Dental Advisor. 2006;23(8):5.[www.dentaladvisor.com/publications/the-dental-advisor/index.shtml]
9. O’Keefe KL, Uceda-Gomez N, Pinzon LM, et al. Bond strength of self-etching adhesives to pre-treated enamel. Dental Advisor Research Report. 2005;(2):1. [www.dentaladvisor.com/publications/research-reports/index.shtml]
About the Authors
John M. Powers, PhD
Professor of Oral Biomaterials and Senior Scientist
Houston Biomaterials Research Center
University of Texas Dental Branch at Houston
Kathy L. O’Keefe, DDS, MS