The best way to stick stuff successfully to teeth!

Faster, easier. Compatible with all composites and with all modes of curing (auto, dual, and light). Will cure with all types of curing lights (laser, PAC, QTH, and LED) in 10 seconds to a depth of 7.0 mm for all shades.

Does this sound familiar? We read this type of advertising copy regularly, especially in those free publications that arrive in our mailboxes. There is no scientific data, no peer-reviewed articles, mainly because products change so rapidly there isn’t enough time to evaluate them through properly designed scientific studies and publication in peer-reviewed journals. Remember, some dental manufacturing companies test materials before they get to your office, and some test them in your office.

Adhesives are the agents which bond various materials to tooth structure. How can we determine which ones can be trusted? Is it the product that has been successful for years from the same company, or products from companies that market a “new generation” adhesive annually with ads that say they are faster and easier and will make you more profitable?

There are some basic principles for all adhesives. Very simply, they are all about the same. They consist of three components - etchant, primer, and adhesive. When these three components are separate and in three distinct steps, they can be described as Type I agents. They are also called total-etch adhesives because a strong etchant - phosphoric acid - is placed on tooth structure for 15 seconds and rinsed off. This completely removes the dentin smear layer present following rotary-instrument action, opening up tubules and leaving unsupported collagen which can result in postoperative sensitivity if product directions and procedures aren’t followed to perfection. “Faster, easier” also means that technique sensitivity increases! Dentin shouldn’t be completely dried and shouldn’t be left too wet. The next step is placement of the primer and very unique moiety.

Primers are unique resins that have some hydrophilic properties, which is unusual since resins, in general, hate water. Therefore, most resins - adhesives, composites, and luting agents - begin hydrolytic breakdown the minute they’re in the mouth, and some last longer than others. The primer displaces the water that temporarily supports the etched dentin and demineralized collagen fibers. The hydrophilic portion of the primer permeates the wet collagen layer more efficiently due to the presence of a “water chaser,” which is included. The most common water chasers are acetone and ethanol. Both work very well, assisting the primer in chasing off the remaining water and replacing the inorganic material removed by etching. Acetone has a more narrow window of opportunity for success due to its high volatility and sensitivity to water.

If the tooth is too wet or too dry, bond strength may be critically deficient. If an acetone containing adhesive sits out exposed to air too long, the acetone evaporates, leading to poor results, and may contribute to postoperative sensitivity and a bad reputation for the agent. Ethanol, on the other hand, is more tolerant of etched dentin being too wet or too dry and has a lower vapor pressure.Agents with ethanol may be tolerant of greater variations in clinical technique. A word of advice: shake all bottles, keep the cap on, and use immediately upon dispensing to maximize the potential for success. Some Type I adhesives are listed in Table 1.

To simplify clinical procedures and decrease the time of application, Type II adhesives combine the primer and adhesive in one bottle and leave etching with phosphoric acid as a separate preliminary step. These have been clinically successful for the most part, but in general, have not demonstrated the bond strengths or longevity of the Type I agents. The chemistry of this type is very similar to the first type. This class of bonding agents is the most commonly used for direct composite restorations. Examples of Type II agents are listed in Table 2.

Type III adhesives were developed to save time, simplify technique and eliminate or minimize the problem of postoperative sensitivity. In this category, etchant and primer are combined in one bottle and placing the adhesive is a second, separate step. By etching and priming simultaneously, time is saved because no rinsing of the etchant is required. This assures the presence of primer in all etched areas, so no unsupported collagen is left behind. Finally, since a weaker acid must be used, the extent of dentin etching is decreased and selective removal of the top portions of the smear layer are removed, leaving the tubule smear plugs intact. This minimizes postoperative sensitivity.

Are Type III agents the answer to our problems with postop pain? Adhesion to dentin is satisfactory, at least initially. Unfortunately, the weak acid in the etchant doesn’t adequately etch enamel, so this category of adhesives instructs clinicians to grind all enamel margins to be adhered to or, alternatively, to etch all enamel margins with phosphoric acid. Since the bond to enamel isn’t optimal, yellow margins may appear in a matter of months with self-etch adhesives. In addition, since the acid etchant isn’t rinsed off and remains on the tooth, there are reports that continued etching occurs in the deeper dentin layers, leaving pockets of tooth that are not filled with cured resin primer. These spaces are filled with water, and hydrolytic breakdown can begin right away.

One other problem with the chemistry of Type III adhesives is that all acids have water as a vehicle and solution, and this water isn’t removed in the process of placement. This water solution may accelerate hydrolytic deterioration. Many Type III agents demonstrate clinical longevity for approximately two years in vivo, while many of the Type I and II agents show clinical success for five years and more. Some Type III adhesives are listed in Table 3.

Finally, Type IV bonding agents (Table 4) are the ultimate in simplicity. They combine etchant, primer, and adhesive in one single bottle. This is a hard process to achieve chemically and still deliver clinical success! These agents possess the same clinical advantages and disadvantages as Type III agents, yet save even more time. This group is very new and has yet to demonstrate long-term clinical success comparable to the previous three types. Although single bottles may be the wave of the future (this would be nice!), this author would not use them on family or friends yet. They are marketed as being compatible with composites of all modes of cure, but this has not been proven yet. And the acid left on tooth structure in Types III and IV is incompatible with auto- and dual-cured composites, so these adhesives should only be used for direct bonding of light-cured composites.

Confused yet? There are other issues such as filler in adhesives which, in general, increases physical properties, film thickness, and bond strength. Film thickness is an important consideration when having to light-cure the adhesive before cementing a porcelain inlay, onlay, or crown with a resin-luting system. Obviously, a successful clinical practice doesn’t exist on one type of adhesive. A type of light-cure bonding agent is most commonly used for direct composites. Total-etch systems adhere to enamel best. Dual-cure adhesives are preferred when cementing posts in endodontically-treated teeth. Research shows the three-step, total-etch, filled, and ethanol-containing agents possess the widest universal compatibility for all direct composites with all types of curing mechanisms.

Adhesives are rapidly evolving, and clinicians must get through the marketing hype and gather evidence-based information which truly can lead to the highest quality practice. This is the best way to be faster, easier, and more productive and will lead to the enjoyment of quality dentistry with a minimum of patient complaints and remakes required. Faster, easier, yes ... but each generation is more technique sensitive, with success dependent upon proper isolation (yes, the rubber dam is important), a properly maintained curing light, and a working knowledge of the potential of bioincompatibility of resin materials. These are all great topics for future discussion. Have you bonded today?

Charles W. Wakefield, DDS, MAGD, ABGD, FICD, FACD, is a Diplomate of both the Federal Services Board of General Dentistry and the American Board of General Dentistry. He is currently a consultant to the Commission on Dental Accreditation of the American Dental Association. He also is a tenured professor in the Department of General Dentistry and director of the Advanced Education in General Dentistry Residency Program at Baylor College of Dentistry, Texas A&M Health Science Center, at Dallas. He can be contacted by e-mail at [email protected].