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Bonding Agents 101
Understanding the different types of bonding agents is essential to obtaining successful restorative outcomes.
In the modern clinician’s operatory, if a tooth-colored material is being placed on the teeth, then some type of bonding agent is being used. Adhesives have come a long way since 1955, when Buonocore introduced the possibility of acid-etching a tooth and bonding a tooth-colored material.
It is important for dental assistants to understand the many types of bonding agents that are available, as different types may be used for specific procedures. Types of bonding agents are labeled by generation; there are currently four different generations available (fourth through seventh). These four generations can be divided into easily understood components and features.
Currently available bonding agents can be organized into two major categories: 1) total-etch and 2) self-etch systems. In a total-etch system, the bonding process starts with a phosphoric-acid etch. Typically blue or green in color, the etch is placed on the part of the tooth that is to receive the bonded restoration. In contrast, self-etch systems do not have a separate acid-etching step, but rather contain a mildly acidic additive to the bonding solution that will allow for etching process to occur.
Total-etch systems can be further divided into two different categories: 1) three-step etch-and-rinse (fourth generation), and 2) one-bottle etch-and-rinse (fifth generation). The three-step, fourth-generation systems require a phosphoric-acid–etch, followed by applying a primer that is air-thinned by expressing a stream of air from the air water syringe, then placing the bonding resin, which is air-thinned and light-cured. If two bottles of a solution are applied separately, then it is a fourth-generation system.
The fifth-generation, one-bottle systems also require a phosphoric-acid etch. This newer generation of bonding agents may save time by combining the primer and bonding resin into a single bottle. First the tooth is etched, then a single bottle of primer and bonding resin is used.
The self-etch systems may also be subdivided into one-bottle and two-bottle systems. In the two-bottle systems (sixth generation), acidic primer is applied to the prepared area first, then air-thinned, and a second solution of bonding resin (which includes acidic additive) is then applied to the primed tooth surface.
The one-bottle systems (seventh generation) have combined the acidic primer solution into the bonding agent solution.
Recently, some manufacturers have started to introduce an “eighth generation” of adhesives to the dental market. These adhesives are one-bottle systems that may be used in either the total- or self-etch mode, on either dentin or enamel. While its potential could be very interesting, because this category is so new, independently published data needs to be reviewed and evaluated before this author can recommend their use.
Indications and Issues
The most significant advantage of the total-etch system is that, historically, it has provided the strongest bond strength. If bond strength was the most important criteria for a restoration, then a fourth-generation bonding system was generally the first choice.
Total-etch systems are excellent at etching enamel. If the clinician is bonding composite or porcelain to a preparation that has a significant amount of prepared enamel, this system is a good choice. A total-etch system is also preferred when bonding a veneer (composite or porcelain). One negative association with these systems is postoperative sensitivity. Total-etch systems are affected by technique: An error in the technique may result in sensitivity.
There are simple ways to minimize the chance for technique error. First, etch only for 15 seconds. Then thoroughly rinse, but do not excessively air-dry the preparation. If the dentin is desiccated, then the chance of postoperative sensitivity is dramatically increased. With this in mind, during the next step use a quick blast of air to eliminate most of the water on the prepared tooth—however, allow the tooth to remain moist. Finally, apply the primer and resins for the exact amount of time recommend by the manufacturer. If not enough time is allowed for interaction, the chance of sensitivity will be increased.
The popularity of self-etch bonding systems is due to their reputation for low postoperative sensitivity. Additionally, they can be slightly faster to apply. Although there is an increased use of the self-etch systems worldwide, some caution should be used with these systems. Historically, self-etch systems are good at bonding to dentin but have not performed as well when bonding to enamel because there is not a strong etching component to maximize the enamel etch. Therefore, when bonding a restoration to a preparation that is all or mostly enamel, a self-etch system may not be the best choice. Self-etching systems are improving in adherence. However, some researchers are concerned that seventh-generation systems involve complex chemistry and, thus, potential risk variables may be present.
The Bottom Line
When applying adhesive systems, it is critical to follow the manufacturers’ instructions exactly. Complex chemistry is being used to bond important restorations. The directions are designed by the chemists to maximize the bond strengths of the materials, so it is essential to take advantage of their research and specific products.
Bonding agents will continue to change and improve—therefore, it is important to remain up-to-date on their clinical applications and mechanisms. Understanding the different types of bonding agents is important in order to know which ones to use for which types of restorations.
Dr. Radz has no financial relationships with the manufacturers mentioned in this article.
About the Author
Gary M. Radz, DDS
Associate Clinical Professor
University of Colorado School of Dentistry
Cosmetic Dentistry of Colorado