D ental Adhesive Systems
Typically, total-etch adhesives produce high and substantive adhesion values for both enamel and dentin. In comparison, self-etch adhesives generally demonstrate better adhesion to dentin than to enamel.
These are key considerations for anterior restorations. If the surface to which the adhesive will be applied consists of significantly more enamel than dentin and particularly if the surface consists of intact enamel, such as with an anterior diatom, an unprepared veneer, or a minimally prepared adhesive bridge, an etch-and- rinse system is preferred.
The main disadvantage of self-etch adhesive systems is that the enamel must be instrumented to provide an effective etch.
Self-etch adhesives typically do not provide as high an enamel bond compared to etch-and-rinse adhesives.
In some cases, self-etch systems demonstrated a higher incidence of marginal staining on enamel margins, while the dentin margins were acceptable (e.g. posterior/ more extensive preparations).
Self-etch adhesives thus may be preferred over the etch-and-rinse adhesives if the remaining enamel has been instrumented and if the surface area to be bonded consists mainly of dentin.
They may also be a better choice if sustained isolation from oral fluids is likely to be a problem.
Mechanisms of Action (enamel and dentin)
The basic mechanism for enamel and dentin bonding using etch-and-rinse systems consists of the following steps
Demineralization of the surface by the acid (etchant)
Penetration of the adhesive monomers into the microscopic spaces created by the etchant
Curing of the adhesive monomers to form resin tags that microscopically provide a mechanical bond and seal to dentin and enamel.
The etchant creates a dematerialized surface 3 mm to 5 mm in depth that is available for resin penetration while also removing the smear layer debris created during instrumentation.
Failure to remove the smear layer reduces the dentin permeability, as that layer acts as a barrier and prevents adhesion to the underlying intact tooth structure.
Demineralization results in the exposure of the collagen contained in the dentin. Macro resin tags are created peripherally around the dematerialized surface of the hydroxyapatite crystals, and microtags are formed by resin penetration into dematerialized crypts within the crystals.
The primer and adhesive (or combined primer-adhesive, in the case of two-step systems) then interpenetrates the exposed collagen and remaining mineral, penetrating to the residual intact mineralized dentin within this microscopic etched surface and providing retention upon curing of the adhesive.
This newly created interface is referred to as the “hybrid layer” and is observed in three distinct areas:
(i) Within the dentinal tubules
(ii) In the microscopic branches lateral to the tubules
(iii) In the intertubular dentin.
Accordingly, they treat cut dentin surface in two fundamentally different ways. The clinical picture stands in stark contrast to the dematerialized, highly permeable condition of the dentin that exists after the removal of smear layer by phosphoric acid needed for fourth- generation bonding systems.
One could conclude that the significantly reduced outcome than what is generally produced by the more technique sensitive total etch adhesives.
One key performance factor that Influences the success of etch-and-rinse systems includes the thorough removal of the etchant prior to bonding.
An important consideration when selecting between etch-and-rinse and self-etch systems is the condition of the enamel that will be bonded.
If the enamel comprises mostly intact or uninstrumented enamel, the choice of an etch-and-rinse system is preferred. 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.
Self-etch adhesives modify the layer and then penetrate through it to further dematerialize the superficial layer of the dentin and combine with collagen and the remaining hydroxyapatite to form a hybrid layer of not more than 0.5-1 micron.
This depth of 1 micron is much lesser than 4-5 micron depths of total etch.
Three-step etch-and-rinse systems contain an etchant, a primer and a bonding resin (adhesive). The primer is used to displace residual moisture, thus creating a surface upon which the hydrophobic bonding resin can adhere.
The primer assists the adhesive to flow into and penetrate the etched tooth surface. The primer often contains hydrophilic portion that interacts with the moisture present in the tooth structure, as well as a hydrophobic end that provides bonding sites for the methacrylate monomers in the bonding resin.
The subsequently applied bonding resin (adhesive) fills the residual space and seals the dentinal tubules.
Polymerization of the bonding resin stabilizes the hybrid layer and also provides a polymerized surface layer upon which the final composite resin is bonded.
Two-step systems combine the primer and adhesive materials into one component, thus condensing that application process into only one step.
Several authors have suggested that the two- step systems may introduce greater variability into bonding results. However, these systems have exhibited excellent clinical performance over the years.
While it has been suggested that self-etch adhesive systems result in less postoperative sensitivity, a recent study shows that there is no differences in postoperative sensitivity between patients treated with self-etch adhesives and those treated with etch-and-rinse adhesives nor have any differences been discovered in marginal integrity. It was concluded that restorative technique influences postoperative sensitivity.