The Ever-changing World of Dentinal Adhesives By: Karl F. Leinfelder, DDS, MS

The mechanism of retention for all direct restorative materials traditionally has been through preparation design. Specifically, amalgam, silicate cement and the early resins achieved retention by means of undercutting portions of the preparation as well as converging the walls towards the enamel surface. Furthermore, the traditional use of well-defined internal line angles was considered mandatory.

Bonding to enamel has been clinically employed since the beginning of the 1970s.^1-5 The first applications of this concept involved the restoration of fractured incisal edges and the use of pit and fissure sealants.⁶ Interestingly, the ability to restore incisal edges not only lead to a more conservative type of preparation design but also an abandonment of the many types of pins formerly necessary to retain the restoration. And on a more subtle note, the need for internal line angles also diminished.

Although enamel acid etching techniques have been part of the treatment plan for over three decades, the same is untrue for the adhesion to dentin. It is notable that the quest for an adhesive to dentinal structure began many years before anyone tried to find a means for bonding to enamel.^7-10 Ironically, the discovery that resin could be bonded to enamel put the search for a dentinal adhesive on the “back burner”. It was by serendipity that Michael Buonocore¹¹ discovered that zinc phosphate cement liquid blanched the surface of enamel. This in turn created a microporous surface, which permitted him to fasten acrylic splints to periodontally involved teeth without mechanical retention. Historically, it took nearly a decade and a half before his discovery became an accepted part of clinical procedures. With few modifications in technique, the principles of bonding to enamel have remained relatively constant over the years.

Considerably greater efforts have been associated with the quest for a dentin adhesive. The first endeavors towards finding this goal centered on the development of a chemical agent or radical that would bond directly to the collagenous structure.^12-13 While many attempts were made to reach this goal, none of them came to fruition. Instead, a mechanism of adhesion was found once again with phosphoric acid; and strangely enough, like its enamel counterpart, the mechanism was micromechanical.^14-18 Instead of dealing with porous defects on the ends of enamel rods, the one associated with dentin involved individual bundles of collagenous fibers. The commonality between the two was indeed micromechanical retention.

Mechanism of Bonding
Without going into the history of dentin bonding agents, it can be stated with no fear of contradiction that the fourth generation of dentin bonding agents made it possible to accomplish those clinical procedures always desired in association with bonding to both dentin and enamel. Conceptually, the hydroxyapatite component of the dentin is removed superficially by acid etching. The zones in which the inorganic component is removed averages less than 10 microns.¹⁹ The dimension is the same for both the intertubular dentin (on the surface of the preparation) and the peritubular dentin. Once the microscopic regions are evacuated, these zones are filled with the resin monomer associated with the dentin bonding agent.

Related to dentin adhesion Kanca^17-19 and Gwinnett²⁰ demonstrated that the degree of wetness or moisture on the surface of the dentin strongly influences final adhesion. Both demonstrated that the drier the surface of the dentin, the lower the bond strength. The reason primarily relates to the fact that the current bonding systems are hydrophilic thereby requiring the presence of moisture to facilitate the penetration of the bonding agent into the permeable surface of the dentin.

The mechanism of bonding then is simply the replacement of a natural occurring inorganic component within the dentin by a manufactured component consisting of various monomers (hydroxyapatite) and low viscosity agents. Anything that interferes with this process will produce a hybrid region (resin diffusion zone) that is for all practical purposes, defective. This, in turn, can readily lead to leakage, extended postoperative sensitivity, debonding, marginal discoloration and the potential for secondary caries.

The secret to successful bonding and formation of the hybrid zone on a routine basis lies in the ability of the clinician to generate total diffusion of bonding agent into the decalcified zone. Due to considerable differences in dentin of different ages, the dentin bonding agent and, locations within the tooth, it is appropriate for the clinician to employ a simple test that measures the efficiency or completeness of diffusion. This test relates to the surface reflectance of light prior to the application of the light source. Simply monitor the surface of the prepared dentin for 15 to 20 seconds beyond the last application and air dispersal of the bonding agent. Insufficient application of the bonding agent will result in a gradual “loss of gloss” or dulling of the surface. This of course indicates an insufficient application of the dentin bonding agent. If not corrected by an additional application, the region below the hybridized zone will consist of collagenous fibers that are completely protected. Subsequently, over a period of time, they will undergo biological degradation or deterioration. This clinically is manifested by leakage, latent postoperative sensitivity, debonding and possibly caries. A clinical example of a properly bonded surface is depicted in figure 1. The illustration on the left shows the surface appearance before placing the bonding agent. The one on the right demonstrates the appropriate reflectance of light once the agent has been applied. It is interesting to note that a highly deflective surface will become dull in appearance in about 15 seconds when an insufficient bonding agent has been applied. The bottom line here is that all dentin bonding agents, regardless of generation, complexity or simplicity, number of bottles or the specific manufacturer, are technique sensitive. Since the profession has had access to this type of material for nearly two decades, there is a tendency to discount the need for stringently following the entire litany of rules associated with their application.

The New Dentin Bonding Agents
All manufacturers have made numerous modifications to their respective bonding agent through the course of time. Each subsequent generation (there are now seven, table 1) was designed for one of two purposes. These include increased simplicity and solutions for problems associated with the former product. For example, although the fourth generation dentin adhesives were quite successful (i.e. All Bond 2, Bisco and Bond-It, Pentron Clinical Technologies), they were characterized by complexity and technique sensitivity. Within a few short years, a so-called “fifth generation” adhesive was introduced to compensate for these perceived deficiencies. Unfortunately, however, while these systems were simpler to use and perhaps even faster to apply, they commonly were characterized by a pronounced increase in postoperative sensitivity. Ironically, this serious problem was so great that a large number of desensitizing agents were introduced to offset patient’s complaints.

After a short period of clinical use, it became apparent that the fifth-generation dentinal adhesives suffered from a number of major deficiencies. In addition to post operative sensitivity, it slowly became apparent that this new class of adhesives was generally incompatible with dual cure composite resins as well as dual cure composite resin luting agents. The relative bond strengths of several dentin bonding agents in conjunction with light-cured and chemically-cured composite resins are shown in figure 2. Note the appreciable differences in adhesion values for the two different types of resins.

Although there may be some clinicians who still may be unaware of this problem, more and more literature has been addressing this serious drawback. While the etiology of this undesirable trait may be somewhat complex, it generally is agreed that the primary factor is related to the higher acidity associated with this bonding system. In essence, the decreased pH of the bonding agent interferes with or protonates the tertiary amine, thereby rendering them useless. The result is that these materials do not polymerize appropriately. Incidentally, the lower pH associated with the fifth generation adhesives is necessary for putting the “primer” and “adhesive” in one container.

In addition to the above-described problems, these adhesives were more expensive for the clinician. In fact, the newer generation was nearly twice as expensive per milliliter as was its predecessor. A comparison of a number of fourth and fifth generation systems and their cost per milliliter is shown in figure 3. The average cost of them is summarized in figure 4. One can conclude then, that while the latter generation (fifth) may have been faster to use, it was anything but better than the fourth.

The annoying problem of postoperative sensitivity was so overwhelming that a new generation of bonding agents was introduced. Identified as a “sixth generation” adhesive, this new material was characterized as self-etching. This time, rather than putting the primer and adhesive in one container, the acid etching agent and the primer were included in one bottle. The introduction of this new product was immediately successful simply because the problem of postoperative sensitivity “went away”. The reason for success was directly related to the sequencing process during application of the bonding agent. All of its predecessors employed a technique by which the acid etching procedure was done first. This then was followed up by an application of the bonding agent, which sequentially filled in all of the voids or vacancies created by etching. In the case of the sixth generation bonding system, both the decalcification and deposition processes are essentially carried out almost simultaneously. Since this type of mechanism prevents the potential for inadequate diffusion of the bonding agent into the decalcified regions, postoperative sensitivity is commonly prevented.

The rapid and almost immediate acceptance of this modified bonding system is not surprising. However, it should be pointed out that this system, like its immediate predecessor, retained the same negative features; in fact, one or more news problems have been added to the list. First of all, due a low pH value at the time of application, the self-etching bonding agent have retained the problems of incompatibility with dual cured-core materials and dual-cured composite rein luting agents. Furthermore, this system of adhesives cannot be used in conjunction with indirect restorative procedures. While eliminating the postoperative sensitivity issue, this new agent brings along several undesirable characteristics. In addition to the ones already presented, many of the self-etching systems do a less than adequate job etching enamel. While the initial bond strength to enamel generally is reported to be adequate, these values drop appreciably over a relative short period of time. The dramatic reduction is bond strength has been reported by O’Nose and is presented in figure 5. In most cases the problem can be related to the inability of the agent to generate an etch pattern similar to that produced by a 37% solution of phosphoric acid.

Realizing the problem of relating less than adequately to enamel, the various manufactures have suggested etching the enamel first with H3PO4 before applying the self-etching agent to the cavity preparation. Unfortunately, however, accidental etching of the dentin (through washing and rinsing away the H3PO4) may offset the potential for eliminating postoperative sensitivity. The reason can be related to the fact that the H3PO4 has now removed some of the inorganic component of the dentin. The nature of the “etch and simultaneous add” of the self etching system is violated. This may result then in less than total diffusion of the bonding agent into all the inter-collagenous spaces; thereby resulting in postoperative sensitivity and less than effective bonding.

Finally, there is another dentin bonding agent offered to the practitioner. Identified as a seventh generation adhesive, this system is a self-etching bonding agent that requires no mixing whatsoever. Currently only one or two products are available; (Heraeus-Kulzer and identified as iBond). The difference between this material and the sixth generation systems is that this is totally contained in one bottle.

An evaluation of the different generations’ ability to deal with specific situations is in table 2.

Clinical Techniques and Recommendations
The introduction of new materials and modifications of older systems many times results in a simplification of the clinical technique. Unfortunately, this is not necessarily true for the dentin bonding agents. These systems can perform successfully only if the clinician follows the rules to the nth degree. The most likely source of problems associated with the dentin bonding agent regardless of the generation is more associated with the practitioners failure to scrupulously follow the litany of procedures than it is with the material itself. The days of handling the bonding agent as one did with a copal varnish are gone forever.

While the fourth generation bonding agents have not been surpassed in terms of performance and applications, the newer ones require just as much attention to detail as their predecessor. There are no short cuts or special techniques that permit less chair time. Any failure associated with their use can be costly, clinically disruptive and frustrating.

In terms of general comments, the fourth generation adhesives continue to represent an excellent adhesive agent. Few of the newer generations possess the potential for clinical applications as this dental material. Of all the seven generations of adhesives, the fourth generation continues to provide the assurance of excellent long-term clinical performance. Furthermore, it can be used in conjunction with all self-curing resin chemistries including cores and luting agents. Also, it is entirely compatible with bonding indirect restorations including porcelain and polymers. Finally it is the material of choice for bonding full crowns in which the passive retention may be less than optimal.

If for example, the prepared crown is clinically short or if the taper is less than ideal, the fourth generation bonding agent can be used to achieve unusually high sheer bond strengths. The technique for achieving this type of bond strength is shown in figure 6. The procedure essentially consists of the following:

1. Acid etch, wash and dry
2. Hybridize the preparation with Primer A and B (mixed prior to application).; after the third application the surface is light-cured for 10 seconds.
3. A thin layer PreBond (chemical activator) is brushed on the surface and air dispersed.
4. Sand blast the casting
5. Apply thin layer of Primer B on casting surface and air disperse.
6. Use a composite resin luting agent to bond crown to preparation.

A comparison of the load necessary to separate full cast crowns using such a technique revealed excellent bond strengths. A comparison of the forces necessary remove cast crowns in conjunction with zinc phosphate cement, Panavia and composite resin are presented in figure 7. Note that the crowns using the fourth generation adhesive required 3.5 times the force for achieving separation as the zinc phosphate cement control. A review of the internal aspect of the crown after separation revealed that some of the debonded castings contained remnants of the prepared crown.

Self-etching systems
Due to the problem associated with developing a well-defined etched structure of the enamel, a special technique should be used with this system. When applying the self-etching bonding agent, it is important to initiate contact at the enamel margin and then slowly and gradually proceeding to the dentinal component of the preparation. This technique of course, permits the bonding agent to be in contact with the enamel for a longer period of time as compared to the dentinal surfaces. Failure to do so may result in under bonding of the enamel and the development of brown lines at the cavosurface angle.

Although the clinician for the first time can employ a dentin bonding agent that all but promises to avoid the issue of postoperative sensitivity, the sixth generation systems are far from perfect. As already pointed out, their major detraction lies in the fact that they lack the versatility of use characterized by the earlier fourth generation systems. Not all proprietary products falling into the self-etching category however, are self-restrained.

One exception to the “sixth generation rule” is an adhesive identified as Simplicity (Apex Dental). Officially categorized as a self-etching bonding agent, Simplicity (Fig. 8) etches the enamel in the same manner as phosphoric acid (37%), thereby eliminating the concern for a gradual reduction in bond strength commonly observed in most of those systems belonging to the sixth generation. Secondly, this material is very compatible with dual cure core materials as well as dual cure composite resin luting agents. Finally, it can be used with both direct and indirect restorative systems.

Conclusion
Dentin bonding systems as we know them today have dramatically changed the way that we conceptualize and practice restorative dentistry. By substituting superficially the hydroxyapatite component of dentin and replacing it with a polymer and sealing the dentinal tubules, we can routinely accomplish the following objectives:

1. Seal the dentin against microbial invasion into the pulpal chamber
2. Eliminate the potential for postoperative sensitivity, and
3. Provide a mechanism for the infinite adhesion of restorative materials to the dentinal structure.