The Future of Dentin Bonding
There is a good possibility that the approach to bonding composite resin to dentinal surfaces will change dramatically; and it is likely that this change will occur in the not too distant future. Ironically, the approach to adhesion will revert to that taken at the very beginning (over four decades ago). Essentially, the method will consist of chemically bonding the resin molecule to the collagenous structure of dentin rather than by means of micromechanical retention (as is done now).
The single reason is this: although the bond strengths are clinically adequate at the beginning, some of literature indicates that adhesion diminishes due to a deterioration of the collagenous structure once the hydroxyapatite has been removed through the acid etching process. Although the resin bonding agent forms an excellent interface with the collagenous structure, there is no chemical adhesion. Consequently, fluids that eventually come into contact with the unprotected collagenous surface cause a biologic degradation of this organic structure. At least one company (Pulpdent) has been making has been making major strides in achieving this goal. Incidentally, one other approach to dentinal adhesion could center on the chemical bonding of the resin to the hydroxyapatite
The history of dentin bonding is quite remarkable and productive to say the least. In this regard I thought that some of the readers might be interested in a “bird’s eye view” of the dentin bonding history. If not, then one cut stop reading any further.
One of the first works put together for the purpose of discussing the basic disciplines in the development of dentin adhesives was sponsored in 1961. Then another workshop was organized four years later in 1964. For a considerable time after that, most scientists tried to develop adhesion through intermolecular attraction between the adhesive and a substrate. Such biologic bonding would necessitate a close intermolecular approach. Most research centered on molecular structures that would bond to the collagen component of dentin.
The first step toward achieving bonding to dentin was a proposed application of coupling agent such as glycerol-phosphoric acid dimethacrylate as a cavity primer used for acrylic resins, and N-2-hydroxy-3-methacryloxypropyl and –N-phenyl phenyl glycine (NPG-GMA) and of silane coupling agents. The literature indicates however, that acidic glycerophosphoric acid dimethacrylate was developed by Hagger and was used to promote the adhesion of Sevriton (acrylic resin restorative material).
Rather than bonding to the collagenous component, the intention of this formulation was to bond to hydroxyapatite and to copolymerize to the restorative resin. While bond strengths to tooth structure were identified, the amount was not clinical significant.
Still another approach was based upon graft polymerization of suitable monomers to collagen. This concept was based upon graft polymerization of methacrylate monomers to collagen using ferric ammonium nitrate initiator.
Perhaps the first dentin bonding agent to appear on the market was Cervident (SS White Co, King of Prussia, PA). Essentially it was designed to bond restorative resin to the cervical portions of teeth. After etching the material actually penetrated the opened dentinal tubules for considerable distances. The mechanism of adhesion apparently could be related to deep penetration of the resin tags into the dentinal tubules. Furthermore, the Cervident formulation contained a chelating component that had the potential for bonding to the calcium component of dentin. Unfortunately, bond strengths of only two megapaschals were insufficient to retain the restorative material for extended periods of time. Long-term retention could only have been attained if some type of mechanical preparation was generated. Over the course of time this material became known as the first generation dentin bonding agent.
The succeeding generations of dentin bonding agents continued to deal with the impregnation of the dentin bonding agent into the dentinal tubules. In addition, the so-called “second generation” of dentin bonding agents also employed polymerizable phosphates added to Bis-GMA resins. As a consequence they commonly were classified as phosphate bonding systems. This also marked the introduction of HEMA (2-hydroxyethylmethacrylate) and phosphonated esters. The HEMA component served the purpose of rapidly driving the bonding agent into the prepared dentinal surface. Bond strengths for this new class of materials ranged from two to seven Megapaschals.
Within a relatively short period of time another generation (3rd) of dentin bonding agents was introduced to the profession. While the composition of the various proprietary materials varied considerably, they generally employed a conditioning step on dentin in conjunction with the bonding agent. The chemistry is a bit more diverse than its predecessor and also included various agents for modifying the dentin. One of the first materials to be developed (Bowen) used a dentin conditioner of 2.5% nitric acid in combination with a ferric (later aluminum) oxalate. The priming solution contained an adduct product of N glycine and glycidyl methacrylate, a dianhydride and HEMA.
It is interesting to note that glutaraldehyde and HEMA were introduced by Asmussen as a dentin primer. The objective was to graft the resin to collagen. EDTA was used as the dentin conditioner.
Generations (4th and 5th) Undoubtedly the greatest clinical improvements in dentin bonding systems came about through the efforts of the Japanese In essence the surface of dentin was etched with a 10 percent solution of citric acid. High-resolution microscopy revealed that the acid superficially dissolved away a portion of the inorganic phase, hydroxyapatite. Careful examination revealed that the acid had dissolved away the calcium-phosphorous component to a depth of five to ten microns. Specifically, the acid attacks first the inorganic component of the intertubular dentin. At this point then the dissolving acid enters into the dentinal tubule to a depth approximating 100 microns. Dissolution of the mineral phase then continues into the peritubular dentin. Like the intertubular dentin on the cut surface, the penetration of the acid in this location is only five to ten microns.
Removal of the hydroxyapatite results in countless vacancies amongst the collagenous fibers. After washing the acid etched surface, a low viscosity resin is subsequently applied. Based upon the chemistry of this agent, it permeates the porous dentin and filled into all the vacancies previously occupied by the mineral phase. In addition to filling all the spaces previously occupied by the mineral phase, the dentin bonding agent also fills and seals the open dentinal tubules.
The basic differences between the 4th and 5th generation dentin bonding agent relate to the number of basic components of bottles. The fourth generation bonding system contains two or more bottles. One consists of the primer and the other the adhesive. The fifth generation dentin bonding agents on the other hand, contain only one bottle. Although not a major advance in science, the concept was marketed on the basis that it would be simpler and faster than the fourth generation systems.
On the basis of bond strengths, the two generations exhibit essentially the same values. Furthermore, no apparent differences could be detected in development and effectiveness of the hybrid zone. The newest generation however exhibited a number of problems. First of all, since it contained no separate adhesive component, the operator is prevented from carrying out a number of procedures former considered routine with the previous generation.
Still another potential problem associated with the fifth generation dentin bonding agent is a higher incidence of reported postoperative sensitivity. Again, the exact reason for the problems has yet to be determined. However, it should be pointed out that as the number of ingredients (bottles) decreased, so also did the potential for controlling the variables by the clinician. Essentially, the individual application of the primer first permitted the operator to be responsible for the degree in which the monomer penetration or diffused into the decalcified dentin. Placing both the primer and the adhesive into a single container simply reduced the opportunity for what may be lack of critical control.
The self-etching adhesives (6th and 7th generation) involve a somewhat different mechanism. In this case, as soon as the decalcification process is initiated, the infusion of the evacuated spaces by dentin bonding agent is begun. As a result, the potential for residual vacancies amongst the collagenous fibers is dramatically reduced or eliminated altogether.
The self-etching systems serve a number of purposes. In addition to ensuring complete deposition of the adhesive and thereby eliminating potential for degradation due to clinician error, the incidence of postoperative sensitivity is for all practical purposes eliminated. The seventh generation (of which there are now at least five on the market) essentially accomplish the same objective as the 6th generation. However, all the ingredients are in one bottle.
Hopefully the next generation dentin bonding agent will be one that actually bonds chemically to either the collagenous structure (grafting) or to the hydroxyapatite component of the dentin.