Brief Notes on Rationale of Adhesive Systems (Dental)

Rationale of Adhesive Systems (Dental)

Primers of total etch system are far less acidic, and consequently require a preliminary treatment with 32 percent phosphoric acid to change the dentin surface in a way that facilitates a strong dentin bond.

This phosphoric-acid treatment completely removes the smear layer formed during cavity preparation, a layer that otherwise would block the dentin tubules, thus resulting in increased dentin permeability and, within potential for postoperative sensitivity.

This smear layer removal is necessary for the fourth generation primer and the bond to penetrate the tubules and form the hybrid zone. The self-etch technique, on the other hand leaves smear layer in place.

The self-etch system is based on infiltration and modification of the smear layer by an acidic monomer.

Intact smear layer impedes the fluid movement within the dentinal tubules and thus reducing postoperative sensitivity.

All dentin bonding systems employ acids of one type or another type to facilitate adhesion to the tooth tissues.

Acidic treatment of enamel and/or dentin creates a zone of demineralization, which is subsequently (i.e. total-etch) or concurrently (i.e. self-etch) infiltrated with various bi-functional primers and resins.

While many adhesive systems are capable of providing acceptable clinical results if used in a knowledgeable fashion with attention to detail, all have their particular idiosyncrasies.

The fourth generation, or three step total-etch systems generally have good long-term clinical track records and perhaps the most versatile of all the adhesive categories because they can be employed for virtually any bonding protocol (i.e. direct, indirect, self cure, dual cure).

These systems are still the “gold standard” by which the newer systems are judged. Indeed, none of the newer systems in the marketplace today perform any better, and often perform worse, than the original multiple component total etch systems of 15 years ago, if bond strength to dentin/enamel, micro leakage and long-term durability are used as the evaluation criteria.

The fifth generation, or two step total-etch system, evolved from the desire to simplify the three step total-etch system protocol. As a group, these are among the most popular systems presently being utilized in dentistry.

They have generally proven to be highly effective, simpler and faster than the multiple component predecessors.

On the down side, many in this category, with some exception, are not as predictable as the three step total- etch systems when it comes to bonding to self- and dual-cure composites.

It has been suggested that a small amount of residual acidic resin monomer can deactivate the tertiary amine that promotes the dark-cure polymerization of the resin cement or restorative.

It is for this reason that it is suggested that, unless specifically designed for use with dual- and/or self-cure resin cement or restorative, self-etching adhesives should be avoided.

In fact, even with etch-and-rinse adhesive systems, mixing different manufacturers’ materials should be avoided to prevent incompatibility issues between the interface of the adhesive and the dual- and/or self-cure restorative or cement material.

In addition, the two-step total-etch systems may be more susceptible to water degradation over time than three-step total-etch systems.

This is because the polymerized primer of the two-step systems tends to be hydrophilic in nature.

When using a three- step system, the hydrophilic primer is covered by a more hydrophobic resin, making it less susceptible to water sorption.

Majority of studies show that fourth generation and fifth generation total-etch system in the placement of a direct composite perform best when placed on moist dentin.

This has been termed as “wet” bonding, although the moist bonding may be a more accurate description of the phenomena.

Dentin exposed to phosphoric acid results in dissolution of the inorganic hydroxyapatite matrix. As the matrix dissolves, the collagen fibrils, which are inherent in dentin, become exposed as they are no longer supported and surrounded by their inorganic scaffolding.

The phosphoric acid treatment leaves the protein component of dentin completely unsupported, literally floating in the rinse water.

If the surface is too much dried, collapsed collagen fiber network creates an impermeable organic barrier for primer penetration.

This leads to gaps formation and bond strength decreases. On another side, if excessive water left on the substrate creates a different problem in that the primers do not remain dissolved in their solvents.

Consequently, resin globules and water trees can form, interfering with bond strength and creating fluid movement within the tubules, promoting postoperative sensitivity.

It is this friable “collagen network” that must be infiltrated by subsequently placed primers and resins to ensure good bonding.

Air-drying of acid-etched dentin causes collapse of collagen network and interferes with subsequent primer/resin infiltration.

In dentin that is left moist (after acid etching) the collagen fibrils remain in a relatively “open” state and appear to be more permeable to subsequently placed primers and resins.

The recommended technique when utilizing a total-etch protocol on unlined dentin is not air dry the dentin once the phosphoric acid conditioner is washed off.

The excess water is simply blotted out with moist cotton pellets prior to placing the primer. This leaves a visibly moist dentin surface.

It warrants noting that some total-etch systems, usually those that are acetone-based, appear to be more sensitive to this wet/dry dentin issue than others (e.g. alcohol/water-based systems).

It is mandatory to follow moist bonding with acetone containing adhesives. This highlights an inherent ambiguity many have with the concept of wet bonding, namely exactly how wet is?

Perhaps the biggest advantage of the sixth generation or two component, self-etching systems is that their efficacy appears to be less dependent on the hydration state of dentin that total etch systems.

Since dentin is not pretreated with phosphoric acid as in the case with total-etch systems- no exposed collagen layer is present to collapse on air drying prior to placement of self etching primers.

Clinically, this means that “wet” bonding is not a concern and the tooth surface can be briefly air-dried prior to placing self-etching primer.

This is not to say that self-etch systems perform any better than total etch system, but they need to be less technique sensitive in this regard.

One could also argue that a possible advantage of self-etching system is that demineralization of the dentin occurs concurrently with primer infiltration.

If the tooth surface to be etched consists mainly of dentin, self- etching systems may be a better choice in order to avoid the need to rein filtrate the dentin after exposing the dentinal tubules with the etchant of the etch-and-rinse system thus eliminating the potential for over-etching when treating dentin.

In principle, this helps ensure that the entire zone of demineralization is saturated with primer where it can then be polymerized in situ.

On the down side, many products in this category do not etch enamel as well as their total-etch cousins and many are not compatible with self- and dual-cure composites.

A common clinical technique that can be used is to first etch the enamel with traditional phosphoric acid prior to using self- etching system.

This ensures good bond strength to enamel but it does require an additional step in the bonding protocol.

Those utilizing this technique should take care to confine the phosphoric acid solely to enamel.

Additional etching of the dentin with phosphoric acid could, in principle create an “over etch” situation where the demineralization zone is too deep for subsequently placed primers to completely penetrate.

The seventh generation or one bottle self-etching systems represents the latest simplification of adhesive systems.

With these systems all the ingredients required for bonding are placed in and delivered from, a single bottle.

This greatly simplifies the bonding protocol. Incorporating and placing all of the chemistry required for a viable adhesive system into a single bottle and having it remain stable over a reasonable period of time, poses a significant challenge.

These inherently acidic systems tend to have a significant amount of water in their formulations and may be prone to hydrolysis and chemical breakdown.

In addition, once placed and polymerized, they are generally more hydrophilic than two-step self-etching systems, which make them more prone to water sorption.

This could contribute to hydrolysis and degradation of the adhesive interface, as well as a reduction in mechanical properties of the composite restoration.

The acidic nature of the polymerized primers in seventh generation adhesives generally makes them unsuitable for use with self-cure composites since their acidic nature degrades the tertiary aromatic amines required for chemical polymerization of self-cure composites.

According to some author’s opinion, while offering ease and simplicity, seventh generation adhesive systems should be used cautiously until more independent research clearly demonstrate their short- and long-term effectiveness.

In principle, the “ideal” adhesive system would be one that is hydrophilic when first placed in order to interact with dentin, which inherently has high water content, but then becomes completely hydrophobic once polymerized in order to discourage water sorption and hydrolysis.

Unfortunately, no such chemistry currently exists. A new recently introduced total-etch system among the first to address this issue by utilizing chemistries that are less hydrophilic in nature.