Bonding The journey to self-etch bonding agents

July 1, 2005
In virtually every product category, there is intense competition among companies marketing products to dentists.

In virtually every product category, there is intense competition among companies marketing products to dentists. Purveyors of everything from impression materials to curing lights, composite resins to prophy pastes, vie for our purchasing dollars. Practitioners are forced to wade through innumerable ads, mailings, faxes, etc. that try to influence our decisions. How is an individual to choose?

On the topic of bonding agents, I want to share the difficulties that I, and undoubtedly most dentists, have experienced in selecting a suitable product. For the last two years, I have used a product that addresses my bonding needs so successfully that I want to share my “secret.”

Not long ago, the choice of a bonding agent for composite resin restorations was uncomplicated. Only a handful of products existed. Most composite restorations were done on anterior teeth, where even products with questionable bond strengths (as well as other properties) seemed to do a reasonably acceptable job. Over time however, the body of knowledge about composite resin bonding has grown exponentially, as well as the use of the material. Patients’ desire for improved esthetics coupled with advances in materials has fueled a surge in the use of composite resins for posterior restorations as well as anterior. In fact, the use of composite resins has surpassed the use of all other materials for single-tooth restorations.

My own history of bonding-agent use, more than likely, resembles that of many practitioners. I was trained in an era when the protocol consisted of phosphoric acid etching of enamel only, followed by an unfilled resin. The etching of vital dentin was, in this country, considered “forbidden,” a misstep that was destined to keep our endodontic brethren busy with an endless supply of devitalized pulps. Despite the introduction of the Total Etch concept in 1977 by Fusayama in Japan, this approach was largely rejected in this country until 1990, when pioneers such as Bertilotti and Kanca convinced us that Fusayama was right.

The Total Etch concept spawned the developments of so-called fourth-generation bonding agents, which I used for several years with great success. These products required three steps - a phosphoric acid etch of enamel and dentin to remove the smear layer left on cut dentinal tubules, followed by copious rinsing with water, the placement of primer which was usually two separate bottles to be mixed (a bis-GMA resin diluted with acetone or alcohol to transport the resin to the etched dentinal tubules and chase out water), and then placement of a bis-GMA bonding resin. These products were reliable and contributed to the rapid increase in posterior composite use.

In their quest to speed and simplify the bonding process, manufacturers then introduced the so-called fifth-generation adhesives. They combined the primer and bonding resin into a one-bottle adhesive that was placed following the phosphoric acid etching of enamel and dentin and rinsing. These products never surpassed the properties of the fourth-generation agents; they simply reduced the number of steps involved. They flourished and became “state of the art” in this country, but users still grappled with occasional complaints of postop sensitivity. As any dentist who has meticulously placed, shaped, and polished such restorations can attest, even one such complaint is one too many. The satisfaction a dentist feels when showing the results of his or her efforts to an appreciative patient evaporates when the “sensitivity” phone call is received.

While American dentists were adopting these total-etch techniques, researchers in Japan began to explore their shortcomings and inconsistencies. They learned that the degree of wetness of etched dentin optimal for ideal dentin bonding varied from product to product, and that reproducing such ideal wetness was difficult. Over-drying or pooling of moisture in line angles could lead to imperfect bonding, micro-leakage, and sensitivity.

In a radical departure from the conventional bonding approach, Japanese manufacturers introduced so-called self-etching primers in the early 1990s. They contained monomers that were acidic enough to penetrate the smear layer without removing it, and became stabilized when followed by the adhesive layer. Keeping the smear layer intact was the key to eliminating postop sensitivity. The phosphoric acid etch step could be eliminated, as well as the subsequent copious rinsing.

Although I had become aware of the existence of these products, it seemed “too easy” and almost a sacrilege to bond without phosphoric acid etching. Could products that were so user-friendly live up to their claims of excellent adhesion without postop sensitivity?

Postop sensitivity had prompted me to try several different manufacturers’ fifth-generation bonding products. Their use of different solvents (alcohol vs. acetone) or special additives were the basis of their respective claims of superiority, but my experiences demonstrated too few differences and too many similarities - post-op sensitivity when you least expected it. This type of lateral change (from one fifth-generation product to another) had been easy; a paradigm shift seemed hard.

I asked myself, “If fourth- and fifth-generation adhesives have been the mainstays, and technique sensitivity has been their pitfall (which translates to postop sensitivity), then what criteria must the newer self-etch products satisfy to make me change my technique and purchasing habits?” Following is what my research revealed:

Efficacy - The adhesive agents must retain my restorations, prevent micro-leakage, and do so on a consistent, long-term basis. Self-etching products have been available in this country for 13 years and have been in constant use in Japan for at least that long. Certainly their use would have decreased, not increased, if they had proven ineffective. On the contrary, testing by Clinical Research Associates, (CRA vol. 24, issue II, Nov. 2000) and Reality (Reality Publishing 2001) showed that the bond strengths of leading self-etching products to enamel and dentin are among the highest recorded, exceeding the results of many total-etch products. Numerous studies since have supported these findings.

Cost - I am fortunate to practice fee-for-service dentistry, free from participation in any insurance plans. I choose dental materials, labs, etc., based on quality, not strictly on cost. Dentists nationwide differ widely, however, with practice types ranging from “boutique” cosmetic practices to inner-city or rural clinics treating indigent patients. Therefore, self-etching products should appeal to a broad spectrum of practitioners. Price is in the same range as total-etch products, yet there are hidden cost savings. For example, introductory kits of total-etch products often include a small sample of phosphoric acid etchant. When it is depleted, you must add the cost of the etchant to the cost of the total-etch adhesive to fairly compare the cost of this technique. Self-etching products need no phosphoric acid etchant, essentially reducing their cost per application. Also, we should not ignore the cost of replacing a restoration when patients suffer postop sensitivity. That alone may have pushed some practitioners back to the safety of amalgams.

Ease of use - The development of adhesive agents in the competitive environment of dental marketing and sales has led to an interesting phenomenon. It is said that the creation of manufacturers’ recommended instructions is often left to the marketing department - seldom to the scientists and researchers who developed the product. For example, instructions may tout the need for only a single application of primer/resin after etching. Why? Because they fear recommending two coats may “turn off” potential buyers who may choose a competing product offering an easier and faster protocol. If bond strength or reduced postop sensitivity are sacrificed, then so be it. As a result, I can scarcely recall a continuing-education seminar where alternative bonding protocols - involving multiple applications - were not recommended.

In contrast, the use of self-etching products is simple and straightforward. While I never considered it too inconvenient, skipping the phosphoric acid etching step makes things so simple that it seems we must be somehow cheating or cutting corners. For all of you who have ever placed a composite without a rubber dam (I’ll confess that I have done so on occasion), you’ll notice that, without the rinsing which must follow your acid etch, it is no longer necessary to replace a flock of wet cotton rolls to re-isolate before adhesive placement. Also, the degree of wetness of our preparation prior to the placement of self-etch adhesives is not significant, because water is the solvent for these products.

And how about flowable composites? Their use mushroomed as dentists tried to eliminate postop sensitivity. Their ability to wet the hybridized dentinal surface was thought to reduce the microgaps under hybrid composites purported to be responsible for much of the postop sensitivity, but their diminished filler content may result in diminished physical properties. Do we want this weak link at the gingival cavo-surface interface of our proximal box? And although some newer flowable composites are radiopaque, most are not. Have you ever examined a radiograph of a posterior composite and wondered whether the radiolucency between restoration and tooth was recurrent decay, or a layer of flowable composite?

Postop sensitivity - This may be the most important criterion. The elimination of sensitivity through self-etching products enables you to reap the benefits of satisfying the aforementioned criteria. Without a sacrifice in efficacy, we can enjoy the cost savings from eliminating several adjunct products, such as etchant and flowable composites, realize time-savings from fewer steps, and gain peace of mind that comes with predictably pain-free results.

Nay-sayers abound whenever a new approach is developed, and the practitioner must be cautious before embracing new ideas. I, too, was reluctant to switch from my tried-and-true total etch concept. But our memories may be short. The total etch concept to which many still cling was itself considered heresy not so long ago.

The early adoption of new products may be exciting in other endeavors, such as computers, but those of us who are loathe to use our patients as guinea pigs may have memories of various products that have calamitously failed to live up to their claims, and were precipitously pulled from the suppliers’ shelves. Self-etching bonding agents, however, were introduced in 1992 (Clearfil Liner Bond 2, Kuraray America). Their sales and use grew slowly at first, as dentists wrestled with this paradigm change, but have grown tremendously in recent years. Their track record by now is proven, and perhaps we need not be so cautious.

For several years now, I have been using self-etching adhesives for all of my posterior light-cured applications including core build-ups, bonding to deep dentin, and even adhesive pulp caps, with amazingly consistent results. The change has been liberating. I can now confidently and unequivocally recommend posterior composites to my patients. That niggling uncertainty about postop sensitivity is a thing of the past. This has enabled me to all but eliminate the flowable composite I was accustomed to placing under my hybrid material, with no consequences. The “weakest link” is no longer a necessity. I prevent voids by careful placement of hybrid composite using nonstick instruments moistened with a dab of ethanol or unfilled resin to prevent pullback.

If all of this is not evidence enough that the self-etch concept is viable, one need only look at the explosion of self-etch products that have entered the marketplace - now termed the sixth- and seventh-generation bonding agents. Moreover, in a nod to the viability of this “new” approach, the biggest players in the fifth-generation market have introduced many of the new products. Perhaps it is time for you to experience the benefits the self-etch approach may offer your practice.

Dr. Robert Rosenfeld is a member of the ADA and the American Academy of Cosmetic Dentistry. He maintains a private general practice in Westwood, N.J., with an emphasis on esthetics and reconstruction. He has authored several articles and has lectured to numerous dental groups. Reach him at (201) 666-8080 or e-mail [email protected].

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