April 1, 2004
by Karl F. Leinfelder DDS, MS and Luiz C Teixeira DDS, MS
The first realistic promise for bonding resins to tooth structure came about through the efforts of Buonocore almost 50 years ago.1 Although the mechanisms may be different, his concept of etching the subsurface was taken from the pages of surface chemistry where phosphoric acid has been used to enhance the adhesion of paints and acrylic resin to metal substrates. Within 15 years of his discovery, bonding of resin through acid etching became a standard means for bonding composites to enamel surfaces. In more recent years the formulations for the phosphoric acid etchant has been standardized.2 Numerous investigators in the 1970s helped to establish the concentration of the etchant to between 30 and 40 percent.3-6
It is of interest to note that the same Buonocore (1956) also investigated the potential for bonding resins to dentinal surfaces.7 Unfortunately, bond strengths diminished when stored in water. However, in the early 1980s a new group of commercially available systems became available. Here the mechanism of bonding to the dentinal surface was accomplished through wetting and possible chelation of the calcium component of hydroxyapatite.8,9 General failure once again occurred due to bond strengths of 10 MPa and less.
Undoubtedly, the greatest move in the right direction came about with the introduction of the third generation of dentinal adhesives. By removing the smear layer and allowing the monomers to penetrate into the dentinal subsurface, the bond strengths not increased but were more stable.10-14 Adhesion to the dentin now, approached that of enamel bonding.
While these latter systems have been used by many clinicians for a number of years, the greatest improvements came about just prior to 1990. The introduction of the total etch15,16 which consisted of etching both the enamel and dentin with phosphoric acid, resulted in a redirection of the methods for bonding as well as the mechanism of adhesion. The introduction of this so-called “fourth generation” bonding system is the basis for the current formulations that have been shown to be so successful.
FOURTH GENERATION DENTIN ADHESIVE
In essence the dentinal adhesives consisted of three basic components. These included the acid etching agent, a primer and an adhesive. All three were applied separately in a carefully controlled sequence. The acid etchant removed the hydroxyapatite amongst the collagenous fibers both on the surface of the cut dentin as well as within the dentinal tubules. After washing, and minimal drying, the surface was treated with the primer.
The chemistry of the dentin primer was so designed that it could effectively remove the intercollagenous water (introduced by washing of the etched surface) and fill in all vacancies created by the etching process itself. The final stage required the application of the so-called “adhesive” agent. This component was necessary to affect a bonding of the overlying composite resin restorative material to the already primed and hybridized surface.
If the clinician carefully adhered to the specific recommendations made regarding the methods and times required for application of the bonding agent, the results were highly effective. Shear bond strengths to both enamel and dentin routinely approximated 25 MPas. Such a process has permitted forms of esthetic and functional treatment not previously conceived.
FIFTH GENERATION DENTINAL ADHESIVE
While the success of this so-called fourth generation dentin bonding agents were quite gratifying professionally, a number of manufacturers began to market a new generation of adhesives that could combine two of the steps normally included in the dentinal bonding procedure. Identified as a “fifth generation” dentin bonding agent, the steps for placing the primer and the adhesive were combined. Theoretically, the major advantage to this modified system was that it would save time for the clinician since one of the steps virtually had been eliminated.
While many clinicians reluctantly gave up their “tried and true” bonding material, others bought on to the concept immediately. Once the market began to weigh heavily in favor of the new generation dentin bonding agent, it unfortunately became apparent that the incidence of postoperative sensitivity was appreciably greater than in the past. Although the etiology of the annoying (and long lasting) sensitivity was yet to be determined, many clinicians began to return to their fourth generation system.
In the absence of well-defined mechanisms associated with the sensitivity problem, it probably relates to the lack of manipulative control. In the case of the fourth generation dentin bonding systems, the clinician could readily influence the rate of diffusion of the primer. In the case of the fifth generation systems, the combination of the primer and adhesive as a single component tended to obviate that potential.
An interesting analogy can be made to various forms of shampoos found in hotel room showers. In the more sophisticated locations, the shampoo commonly consists of two components: shampoo and conditioner (Fig. 1). In the less prestigious hotels there is only a single bottle or agent (Fig. 2). While the does clean your hair, only the two component systems offer you sufficient control for generating best outcome: clean but properly conditioned hair. Undoubtedly, the fifth generation bonding systems provide us less control in the diffusion process of the dentin bonding agent.
Interestingly, as the fifth generation dentin bonding agents began to replace their predecessors, other deficiencies became apparent. For example, the newer system offered less potential applications. In most cases, the bond strength for bonding to self or dual-cure composite resin core materials was appreciably less. Significant reductions in bond strengths commonly have been reported.
The same finding was associated with self or dual-cure composite resin luting agents. In addition, it was found that a chemical activating agent needed to be added to the dentin bonding agent to permit bonding to metal surfaces such as posts and base-metal alloy crowns. Finally, the cost of the fifth generation dentin adhesives was nearly twice as expensive to the operator as compared to those that belong to the fourth generation dentinal adhesives (Fig. 3).
SIXTH GENERATION DENTINAL ADHESIVES
As some of the inherent problems of the fifth generation dentin bonding agents became known to the clinician, another type of dentin bonding agent was introduced. Capable of etching the dentin (and enamel) and diffusing in primer into the surface almost simultaneously, the problem of sensitivity decreased appreciably. This major change in composition and technique resulted in the application or designation of a new term: sixth generation.
While both of the two preceding generation families were different in techniques as well as components, they both consisted of two major procedures. The first consisted of partially decalcifying the dentin as the first stage followed by a separated and individual resin uploading stage. This concept is illustrated in Fig. 4. The first stage (red curve) depicts the dissolution of the hydroxyapatite. The second stage (blue line) depicts the actual uptake of the bonding resin into the interfacial regions created during the first stage.
The potential problem here (both scenarios) is that the resin uploading may not go to completion (Fig. 4). As a result, some of the evacuated intercollagenous spaces remain unfilled. This then can result in biodegradation of the exposed collagenous fibers, potential debonding, nanoleakage and eventually, sensitivity.
In the case of the sixth generation dentinal adhesives, the mineral extraction and resin (dentin bonding agent) diffusion are carried out almost simultaneously (Fig. 5). Since the etching and bonding procedures are carried out together, the potential for under diffusing and postoperative sensitivity are all but eliminated. The greater potential for filling in all evacuated spaces created by the etching process will automatically reduce th
e potential for postoperative sensitivity, leakage and debonding. Theoretically at least, the potential for secondary caries should also be minimized. It is little wonder then, why so many clinicians have embraced the concept of the sixth generation dentin bonding agents.
While a large number of clinicians began converting to this new system of dentinal adhesives, another but different type of problem surfaced. Specifically, while the retention of restorations in Class V cavity preparations (without mechanical retention) remained in tact, the margins sometimes exhibited a zone of discoloration. The apparent reason for this type of clinical failure was related to the etching process itself. As it turned out, this new class of dentinal adhesives generally was not as effective in etching the enamel surface (less pronounced etching pattern) as compared to phosphoric acid.
For this reason then, that some manufacturers of the sixth generation dentinal adhesives recommend etching the enamel separately with phosphoric acid prior to the application of the self-etching adhesive. In this way, there is a certainty that the enamel will be properly responsive to the bonding agent itself.
Therein however, lies a potential problem. According to Torii and his colleagues17 phosphoric acid etching prior to the application of self-etching primers decreases the adhesion of resin composite to dentin, although it does increase enamel adhesion.
In essence, the pretreatment of the preparation with phosphoric acid prior to the application of a self-etching adhesive can cause two problems. While a thick hybridized layer is formed, it may be quite porous in nature. Secondly, there is a distinct possibility that the bonding agent, which is subsequently applied may not infiltrate the entire dentinal layer decalcified by the etchant.18 This in turn can readily result in eventual collagenous degradation, postoperative sensitivity, nanoleakage and debonding.
It is interesting to note that the pH of 10 percent phosphoric acid is 0.8 while that of a 37 percent concentration is 0.1. By comparison, many of the self-etching systems exhibit a pH of only 2.0 or even higher (Fig 6). This of course would explain the differences in the microstructure of the etched enamel surfaces.
It can be argued that the pronounced etch pattern generated by phosphoric acid on enamel may not be necessary for the retention of the restoration. While this may be true at least in the initial stages of clinical duration, there is ample evidence that the adhesion to enamel gradually reduced over a period of time. Incidentally, all of this would explain the etiology for the occurrence of superficial discolorations along the prepared cavo surface angles. Incidentally, to offset this potential for marginal imperfections and diminishing bond strengths, some of the manufacturers have reduced the pH of their self-etching dentinal adhesives to values approaching that of phosphoric acid gels commonly used in practice.
Another interesting side light to the self etching bonding systems is that they, unlike the fifth generation adhesives, require a minimum of two components: a) self-etching primer and b) adhesive. Without the adhesive component, it is not possible to bond the overlying composite to the hybridized surface. While “time saving” cannot be declared a positive feature of this generation of dentinal adhesives, the basic advantage lays in the promise of completely filling the evacuated intercollagenous spaces and the absence of post-operative sensitivity.
SEVENTH GENERATION DENTIN BONDING AGENT
Most recently an additional adhesive has been introduced to the market. As compared to other systems, the uniqueness of this agent is that it is truly a single step. It simply is dispensed and then applied to the surface of the preparation. Identified as iBond this seventh generation formulation was designed to simplify the application procedure and reduce the potential for being technique sensitive. The acid etching component, primer and adhesive are all contained within a single bottle. Like most self-etching systems, iBond requires attention be given to the application of the liquid to the prepared surface. And like all SE bonding agents, the application procedure should involve contact with the enamel somewhat longer than the dentinal component of the preparation. This can be done simply by initiating contact of the liquid at the cavosurface margin and then working inwardly to the dentinal surfaces. Presently, this system is the only one on the market that is actually a single step and requires no mixing of components whatsoever.
WHAT’S THE BOTTOM LINE?
Undoubtedly, the profession has come a long way in the development and improvement of dentinal bonding agents. At the beginning, bond strengths to dentin were only three or four MPa. The advent of the 4th and 5th generation systems brought this level to 20 to 25 MPa. Interestingly, this value has remained relatively constant even to the present. In this regard, these newer adhesive systems have been quite successful.
In an effort to make a good system (4th generation) even better, efforts have been made to improve the ease for use, save time and to standardize the technique of application. Although their successor (the single bottle formulations or 5th generation dentinal adhesive) may have achieved this goal, a number of different problems such as postoperative sensitivity began to surface. The development of the self-etching systems (6th generation) may have resolved the latest annoyances, but they themselves generated their own set of problems.
Most significant among these is a general inability to properly etch the enamel surfaces. Although not characteristic of all the self-etching systems, pretreatment of the enamel commonly is recommended prior to application of the dentin bonding agent. It seems then that each new generation of dentinal adhesives resolves the problems of their predecessors but they also tends to produce new ones. As one considers the picture in its entirety, most changes were generated to make the technique of dentin bonding and hybridization simpler, faster and less complicated.
FOURTH GENERATION REVISITED
It is ironic in some ways, that many of the fourth generation adhesives offered the solutions for optimum success. All Bond II (Bisco) for example, provided all the appropriate parameters for excellent clinical success. Unfortunately however, suitable performance mandated that the clinician adhere closely to details of application as recommended by the respective manufacturer. Furthermore, by comparison to more recently marketed formulations, the litany of procedural steps was somewhat lengthy. Thus the move to reduce the number of components and to simplify the procedure seemed justified.
As already pointed out, the newer generation dentinal adhesives actually reduced the potential for achieving certain objectives. For example, the bonding potential of the fifth generation agents to dual-cure core materials generally are appreciably diminished. The same can be said for the dual-cure luting agents.
The reason for the minimal bond strengths to self-cure or dual cure materials can be related to the reduced pH associated with the bonding agents. In essence the acidic nature of the 5th generation materials ties up the tertiary amines of the self-cured systems. As a result there is an interference with the interfacial chemical linkage between the bonding agent and the luting agent (Fig.7).
It is interesting to note that One-step (Bisco), which is a 5th generation adhesive, does not exhibit reduced bond strengths with dual cured materials. The reason can be related to the fact that the pH of One-Step is considerably higher than most of the other 5th generation adhesives.
Values for pH values of several systems are presented in Fig. 8. Note that with the exception of One-Step (OS), the bond strengths associated with the self or dual-cured composite resins is appreciably less than those associated with the light-cured composite resins. Note also that there is a corresponding relat
ionship between pH values and bond strength (self or dual-cure).
Depicted in Fig. 9 is a simple relationship between pH of the dentinal adhesive and bond strength. Note that the highest bond strength (OS) can be related to the pH value. The reduced bond strengths exhibited by many of the 5th generation adhesives can be related to an expenditure of the tertiary amine (self-cure component) by the acidic monomer. It is important to remember then that the lower the pH of the dentinal adhesive, the greater the potential for a reduced bond strength.
Related to much of this discussion has been the development of a novel system designed to eliminate the problems of appropriately etching enamel without appreciably modifying the pH of the dentin bonding agent itself. Identified as Tyrian (an organo-sulphonic acid and organo-phosphate self priming etchant, Bisco), this agent effectively etches both cut and unground enamel surfaces equally and as effectively as Phosphoric acid. It also removes the smear layer and decalcifies the underlying dentin but retains water in dentinal subsurface so as to prevent potential collapse of the collagenous fibers.
It also ensures excellent penetration of the monomer of the dentin bonding agent. While it is highly effective with One-Step, it also is effective and compatible with many other systems. The ability to provide bonding to ground and unground enamel without the use of phosphoric acid pretreatment is depicted in Fig. 10. Finally, the ability to be effective with a number of other bonding systems is shown in Fig. 11.
Dentinal (enamel and dentin) bonding agents have dramatically changed the way that clinical dentistry is conducted. Undoubtedly, the combination of the dentinal adhesives and the composite resin represent the greatest advancement experienced by the restorative dentist in the last half century. While the bonding agents have made remarkable progress, each new generation has been characterized by new problems not previously exhibited by their predecessors. Each new modification does involve new chemical formulations and sometimes major influences on the mechanism of bonding. Unfortunately, sometimes the new problems associated with each subsequent formulation take months and sometimes years to discover. The resolution then takes even longer…
Dr. Leinfelder is adjunct professor, University of North Carolina, and is Professor Emeritus, University of Alabama.
Oral Health welcomes this original article.
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