The Evolution of Adhesives
Understanding today’s universal capabilities
Stephen Poss, DDS
Based on an understanding of the structures, composition, and interaction of dentin, enamel, and resin materials, successful adhesive dentistry is predicated typically on first preparing the teeth with instrumentation, then conditioning enamel and etching the dentin surfaces, after which bonding resins can be placed.1 The etching process in most cases removes or modifies the smear layer and/or imparts irregularities in prepared enamel to enhance micromechanical and chemical retention.1 This protocol is intended to enhance adhesion in the wet oral environment by improving the attachment of restorative materials to tooth structures.
Over the years, demands for less technique sensitive and more predictable bonding has led to the ongoing development of new adhesive materials with different compositions, mechanisms of action, and techniques for their application. In fact, eight generations of adhesive bonding agents have evolved, along with three different etching protocols—total etch, self-etch, and selective etch.2,3
With each generation, manufacturers have sought to enhance adhesive predictability while simultaneously reducing the number of components/bottles required (separate etchant, primer, adhesive), simplifying application protocol, and improving material chemistry for stronger and more durable adhesion.
Etching Considerations
Although newer generations of adhesives have demonstrated effectiveness in clinical and laboratory studies, total-etch adhesives containing 30% to 40% phosphoric acid have long remained the most effective for etching. Total-etch adhesives comprise a two-bottle system requiring application of a separate acid and primer. The phosphoric acid is applied to enamel for up to 30 seconds and to dentin for 15 seconds, after which it is rinsed from the preparation. Typically this is accomplished by tracing the cavosurface of the preparation with acid first, and then filling in exposed dentin, taking care to avoid over-etching the dentin. The primer is then applied to promote the ideal bond between the resin, preparation, and restorative material.4 Despite their many benefits (superior enamel etching, greatest efficacy for etching compromised dentin), total-etch adhesives are more technique sensitive, which can result in over-etching/over-exposing dentin and gap formation between the adhesive and dentin. These challenges could lead to postoperative sensitivity and decreased bond strengths.
Self-etch systems typically contain a 10% maleic acid that is milder than phosphoric acid and a formula combining the primer and adhesive resin. This one bottle combination effectively modifies the smear layer, facilitating penetration of the resin into the dentin tubules and forming a consistent adhesive layer with decreased technique sensitivity. In addition, self-etch adhesives eliminate the need for rinsing and drying after application.5
In most cases self-etch adhesives can be used for selective etching techniques where only the enamel edges of preparations are carefully etched with phosphoric acid, after which dentin is either conditioned with a primer or the self-etching adhesive.
Universal Adhesives
Today, there are 8th generation universal adhesive bonding systems, such as Prime & Bond Elect (Dentsply Sirona, www.dentsplysirona.com), Adhese Universal (Ivoclar Vivadent, www.ivoclarvivadent.com), All Bond Universal (Bisco, www.bisco.com) and Scotchbond Universal (3M ESPE, www.3m.com), that are indicated by manufacturers for use as either a self-etching bonding agent or an etch-and-rinse bonding agent with phosphoric acid. They have also been reported to demonstrate enhanced material characteristics for adhesively bonding both enamel and dentin to a variety of restorations (Figure 1).3
However, regardless of the advanced material composition of universal adhesive bonding systems or the conditioning/placement protocol employed, certain characteristics ensure successful and predictable adhesive bonding,6,7 including high bond strength values regardless of etching modes,8,9 resistance to degradation,10,11 and low film thickness for passively fitting indirect restorations.12 They should also be compatible and/or function similarly with other restorative materials, such as light-cure and dual-cure cements, ceramics, composites, etc., in a variety of clinical situations to ensure durable predictability.5-7
Today’s universal adhesives can be applied across varied moisture levels. In particular, dipentaerythritol pentaacrylate monophosphate (PENTA) technology, found in Prime&Bond Elect, can be used in virtually all restorative bonding procedures, contains acetone to repel water, inhibits water uptake, and enables the material’s resins and fillers to penetrate deep into the dental tubules. It also reduces microleakage and postoperative sensitivity.
Additionally, by promoting micromechanical and chemical bonding, this combined adhesive mechanism contributes to consistently high bond strengths when placing direct or indirect materials on dentin or enamel, regardless of whether total-etch, selective etch, or self-etch techniques are used (Figure 2).8,9 However, bond strengths become insignificant if they degrade over time due to masticatory function or simply normal service in the oral environment.11
Fortunately, universal adhesives are available that demonstrate lower degradation resulting from the material’s chemical and physical properties.10 In self-etch mode, Prime&Bond Elect has demonstrated the smallest decrease in bond strength after aging.10
Because indirect restorations are now fabricated and milled with increasing accuracy of fit, the film thickness of the adhesive layer has become more critical to passively seating restorations. Therefore, a universal adhesive that has been proven to demonstrate a lower film thickness of resin-dentin and resin-enamel interfaces can provide high bond strengths, contribute to an accurately fitting restoration (Figure 3 and Figure 4), and simultaneously minimize microleakage.12
Conclusion
Although the term “universal” suggests that bonding agents can be used for multiple substrates and for multiple restorative materials, some universal bonding agents recommend the use of additional primers for some substrates. Different manufacturers’ etchants, bonding agents, and cements may be similar chemically, but they may not perform ideally in all clinical situations with all other materials.5-7 Clinicians should carefully follow manufacturer’s instructions for use, as well as understand the capabilities and limitations of individual products, in order to make the best adhesive material selection on a case-by-case basis (Figure 5).
Disclosure
The author has no relevant financial relationships to disclose.
References
1. Pashley DH, Ciucchi B, Sano H, Horner JA. Permeability of dentin to adhesive agents. Quintessence Int. 1993;24(9):618-631.
2. Powers JM, O’Keefe KL. Update on seventh-generation bonding agents. Inside Dentistry. 2009;5(2);52-56.
3. Alex G. Universal adhesives: the next evolution in adhesive dentistry? Compend Contin Educ Dent. 2015;36 (1):15-26.
4. Strassler HE. Applications of total-etch adhesive bonding. Compend Contin Educ Dent. 2003;24(6): 427-430, 432, 434-436 passim; quiz 441.
5. Tay FR, Pashley DH, Yiu CK, et al. Factors contributing to the incompatibility between simplified-step adhesives and chemically-cured or dual-cured composites. Part I. Single-step self-etching adhesives. J Adhes Dent. 2003;5(1):27-40.
6. Miyazaki M, Tsujimoto A, Tsubota K, et al. Important compositional characteristics in the clinical use of adhesive systems. J Oral Sci. 2014;56(1):1-9.
7. Silva e Souza MH Jr, Carneiro KG, Lobato MF, et al. Adhesive systems: important aspects related to their composition and clinical use. J Appl Oral Sci. 2010;18 (3):207-214.
8. Takamizawa T, Barkmeier WW, Tsujimoto A, et al. Influence of different etching modes on bond strength and fatigue strength to dentin using universal adhesive systems. Dent Mater. 2016;32(2):e9-e21.
9. Walter R, Atlas A. Clinical evaluation of an experimental universal adhesive used with and without phosphoric acid pretreatment in posterior composite resin restorations. DENTSPLY CAULK. Internal research report. 2016.
10. Zhang ZY, Tian FC, Niu LN, et al. Defying ageing: an expectation for dentine bonding with universal adhesives? J Dent. 2016;45:43-52.
11. Hashimoto M. A review—micromorphological evidence of degradation in resin-dentin bonds and potential preventional solutions. J Biomed Mater Res B Appl Biomater. 2010;92(1):268-280.
12. Reis AF. Film thickness FE-SEM evaluation of resin-dentin interfaces produced by universal adhesives. DENTSPLY CAULK. Internal research report. 2016.
About the Author
Stephen Poss, DDS
Private Practice
Brentwood, Tennessee
For more information, contact:
Dentsply Sirona Restorative
800-532-2855
www.dentsplysirona.com
