Adhesive and Bonding Systems

History of Bonding Agents

Dr. Michael Buonocore’s advances in adhesive dentistry in the 1950s in using acid to etch enamel laid the groundwork for those that followed.⁴ The first three generations of bonding agents had bond strengths of only 2 to 15 MPa, exhibiting extensive microleakage and staining at the margins. The fourth generation comprised the first formulations that removed the smear layer, leading to greater dentin adhesion.⁵ Both the fourth and fifth generations of bonding agents introduced the total-etch or etch-and-rinse technique, which increased tensile bond strengths in enamel to 33 MPa and in dentin to 28 MPa.⁶ When using previous generations, the dentin had been dried after etching, causing the collagen fibers to collapse. Kanca’s studies in the 1990s paved the way to wet bonding techniques, showing that wet dentin possessed greater bond strengths than the dry surfaces.⁴ With the emergence of the sixth and seventh generations, total-etch led the way to self-etching primers and adhesives, thereby eliminating several bottles and steps (Table 1), with tensile bond strength to dentin increasing to 35 MPa.⁷

Universal Bonding Systems

Bonding to enamel is easily accomplished with a gel etchant of 30% to 40% phosphoric acid, dissolving the hydroxyapatite crystals and establishing micromechanical retention.⁸ However, bonding to dentin has proven to be more difficult and technique sensitive. Practitioners demanded a material that was less technique sensitive with reduced application time and less postoperative sensitivity. Manufacturers responded by developing what is now classified as the universal bonding systems. They can be used with total-etch to self-etch to selective-etch techniques and exhibit similar bond strength values regardless of their application mode.⁹ They can be classified as “strong” (pH <1), “intermediately strong” (pH = 1.5), “mild” (pH = 2), and “ultramild” (pH ≥2.5).¹⁰

Even though universal systems say that they can be used in the self-etch mode, a more favorable approach is etch-and-rinse treatment on enamel with a mild self-etch approach, which appears to provide better long-term outcomes on dentin.¹¹ Since the smear layer is not removed when using the self-etch mode, techniques and instruments that produce a thinner smear layer are preferred during cavity preparation. Finishing the cavity walls with extra-fine diamond burs results in thinner smear layers, which consequently may promote a better interaction between mild and ultramild self-etch adhesives.¹²

The Importance of Technique

Whether using total-etch or self-etching primers and adhesives, the success of the restoration is dependent on meticulous technique. To support clinical decision-making that leads to optimal outcomes, a review of factors essential to excellent technique is presented here.

Use of Additional Agents

During tooth preparation, many outside agents can adversely or positively affect the bonding procedure. Hemostatic agents are routinely used in many dental procedures. Bernades and colleagues showed that both dentin and enamel bonding are adversely affected by hemostatic agents, with the bond strength of self-etching adhesive systems affected more negatively than total-etch systems. However, a 60-second application of ethylenediaminetetraacetic acid (EDTA) or a 15-second application of phosphoric acid followed by a water spray restored the bond strength of a self-etching adhesive to dentin.¹³

Disinfecting agents can have an effect on shear bond strength values. Hassan and colleagues found that dentin treated with EDTA had the highest bond strength, followed by sodium hypochlorite and chlorhexidine. The teeth that were not disinfected showed the lowest shear bond strength. They concluded that treating teeth with a disinfecting agent before bonding had a significant effect on the bond between the composite and dentin surface.¹⁴ Disinfecting agents in dental cooling water containers may also influence dentin bonding, depending on which adhesive system is used.¹⁵

Desensitizing agents are commonly used in many operative procedures. In a study by Sabatini and Wu, pretreating the teeth with three different agents prior to bonding with self-etching adhesives showed that these agents can be used to control hypersensitivity without negatively affecting their bond strength to dentin.¹⁶ Another study concluded that a desensitizer along with self-etch adhesive systems can be used without interfering with the bonding system.¹⁷

The use of glycol caries disclosing solutions in dentin have been shown in preliminary results shown to not negatively impact shear bond strengths of total-etch or self-etching, non-rinsing primer.¹⁸

Contamination

Contamination by blood and saliva can also be a problem in all restorative procedures. Santschi and colleagues studied the effect that salivary contamination and decontamination had on shear bond strength in both primary and permanent teeth. They noted that after the dentin was exposed to saliva, rinsing with water, air drying, and reapplication of the adhesive restored the original bond strength.¹⁹ In a study using one-bottle dentin adhesives and a three-step bonding system as a control, it was concluded that blood contamination significantly decreased the bond strength of all materials used.²⁰

With the use of a dental dam, many complications can be avoided. Plasmans and colleagues concluded that a “dry” working field in the oral cavity could not be achieved without the use of a rubber dam. They noted that “without rubber dam, the intraoral environment in which bonding procedures are to be performed is comparable with a high humidity climate such as that of Central Africa or the South Pacific.”²¹

Contamination from handpiece maintenance spray is also a significant issue. A test of high-speed, air-driven handpieces showed that oil was discharged for at least 40 minutes in the direction of the bur, and oil continued to be expelled from the handpiece for up to 240 minutes. The common practice of running the handpiece for 1 to 2 minutes was not effective in preventing cut-surface contamination, which may jeopardize bonding procedures.²²

Application Technique

A working knowledge of all materials the prepared tooth may be exposed to and their effect on the bond strength is critical, but the dentist also needs to know the correct method of application for each product. Amaral and colleagues compared methods of application of three one-step self-etch adhesives. The first group employed an inactive application technique. The second group used the active application technique, in which the adhesive was rigorously agitated on the entire dentin surface for 15 to 20 seconds. It was shown that by using the active application technique on the tested one-step self-etch adhesives, the bonding performance was improved and remained so over time.²³

Another application technique, single versus double, was studied by Nakaoki and colleagues. They compared the results of a single application of all-in-one adhesives on dental bonding to a double application and discovered there was no statistically significant difference on micro-shear bond strengths.²⁴ A later study by Nagpal and colleagues tested three single-step self-etch adhesives to determine if employing the double application technique would have any effect on microleakage scores. This technique improved the marginal sealing ability in dentin, although it was product dependent.²⁵

Final Thoughts

With the overwhelming majority of direct restorations being placed today as composite resins, the success of these restorations lies solely in the hands of the practitioner, the assistant, and a thorough knowledge of the materials and proper technique. Many dentists thought that the only dental materials course they needed was in dental school; however, as quickly as materials and techniques are changing, they now must be continual students. It is imperative that the practitioner stays current with the intricacies of these materials as new ones are continually brought to market by the manufacturers.