In self‐etch adhesive systems, demineralization and infiltration of the acidic resin blends occur simultaneously, thus no discrepancy between demineralization and infiltration (typical of the etch‐and‐rinse approach) has been described. Additionally, due to the lack of rinsing after etching, residual hydroxyapatite crystals remain available on the dentinal collagen. For this reason, the number of denuded collagen fibrils within the hybrid layer should be significantly less than in the etch‐and‐rinse system. However, some strong and aggressive one‐step self‐etch systems showed continuous etching after curing, leading to exposed collagen and accelerated bond degradation. 

Similar to etch‐and‐rinse adhesives, self‐etch systems (particularly the most simplified one‐step systems) exhibit high permeability correlated with a low degree of curing and increased water sorption. As previously described for two‐step etch‐and‐rinse systems, all these degrading phenomena seem to be strictly correlated with the high hydrophilicity of one‐step systems and the difficulties with getting them to properly polymerize.

Postoperative hypersensitivity

Postoperative hypersensitivities are mainly caused by fluid movement in dentinal tubules which irritates odontoblast processes. This is mainly attributed to an insufficient adhesive dentin seal and not to cavity depth because hypersensitivities also occur in shallow cavities. 

The main reasons for postoperative hypersensitivities with etch‐and‐rinse adhesives are:

  1. overetching of dentin with phosphoric acid
  2. overdrying of dentin after phosphoric acid etching 
  3. overwetting of previously phosphoric acid‐etched dentin
  4. no or insufficient light curing of the adhesive.

Also, self‐etch adhesives may fail, this is mainly due to:

  1. Too thick smear layers
  2. Application time is too short for smear layer removal
  3. Insufficient evaporation of the solvent.

Under strict adherence to the adhesive’s instructions for use and 
prevention of common treatment errors, enamel and dentin may be effectively pretreated, providing a tight and durable seal. The dentin seal is better than any cement linings use previously.

Technique sensitivity 

Etch-and-rinse adhesives 

In contrast to enamel bonding, dentin bonding is considerably more technique‐sensitive. Handling is a fundamental point in adhesive dentistry and is very important to guarantee a well‐functioning seal of enamel and dentin. Any deviation from the recommendations for use will decrease the clinical performance. These errors were:

  • overetching of dentin
  • overdrying of previously etched dentin
  • shortcuts in application protocols.
* Overetching of dentin results in deeper demineralization zones, and potentially insufficient impregnation by primers. With time, these areas of naked collagen are subjected to biodegradation and hydrolysis. The first hints of insufficient interface forming are postoperative hypersensitivities. During clinical use of etch‐and‐rinse adhesives, the problem becomes easily apparent: the whole cavity is etched with phosphoric acid. However, ideal etching times are 15 seconds for dentin and 30 seconds for enamel. When an uncontrolled flush of acid is applied many times a day, this idea is not achievable. 
Therefore, it makes sense to reduce the overall etching time to 15–20 s for both adhesive substrates without any decrease in performance.

* Overdrying
 of etched dentin 
should be avoided as it causes the collagen fibers to collapse. The 
drying process is necessary to reveal the frosty appearance of etched enamel, however, this always leads to some dried areas on adjacent dentin. 
Rewetting is the only solution to this particular problem. A microbrush is either sprayed with air/water mix from 25 cm away in order to generate some hoar frost or dipped into water or into a 2% chlorhexidine solution and dabbed on a paper tissue resulting in a moist but not dripping wet tuft of the microbrush. This microbrush is applied to the cavity, creating a slightly shiny surface in dentin. It can even be accepted that water also contaminates enamel margins. This is not crucial because primer solvents remove the water equally just like the interfibrillar spaces in dentin do. 
Water‐based primers are easier to handle. They consist of 50% water, which is able to re‐expand the collagen meshwork by itself. Therefore, water and water/ethanol‐based adhesives provide less technique sensitivity because rewetting is not as mandatory as with ethanol‐ and acetone‐based systems.  Also, with less‐technique‐sensitive adhesives, rewetting is not forbidden. However, as mentioned before, rewetting can also be managed by chlorhexidine, which supports the longevity of etch‐and‐rinse adhesives. When working with water‐containing components, it is fundamentally important to dry primers instead of gently blowing them because the water must be safely evaporated. In contrast, it is easier with ethanol and acetone due to their higher vapor pressures. 

* Air thinning of unfilled resins is also restricted due to technique sensitivity. On the other hand, thicker layers of unfilled resins should also be avoided because, due to their lack of radiopacity, these zones may be misinterpreted as gaps in bitewing radiographs.
The unfilled resin also needs some time to penetrate, and it should not be overly thinned by air due to the presence of an oxygen‐inhibition zone that may counteract light curing. 

General problems with wet bonding still result in a remarkable amount of postoperative hypersensitivity with this class of adhesives. The reason is not primarily the presence of some aggressive phosphoric acid, but improper moisture management after the actual etching process.

Self‐etching adhesives

All‐in‐one adhesives represent a revolutionary simplification because only one liquid needs to be applied. Nevertheless, technique sensitivity is not completely absent. For example, most of the products must be repeatedly applied to generate measurable bonding, which means that the timesaving aspect is almost gone. The major issue with all‐in‐one adhesives is their permeability against water. Several publications have shown that these adhesives are permeable membranes even after polymerization. 

The more hydrophilic the adhesive is, the less clinically promising its prognosis.

Conventional adhesives with separate primers and separate hydrophobic bonding agents still prevail. Two‐step self‐etch (2 SE) adhesives also avoid phosphoric acid etching; however, they have separate liquids for priming and bonding. Today, this class of adhesives is estimated to be the most promising for durable dentin bonds with low rates of postoperative hypersensitivity. Where dentin bonding is favorable with self‐etch systems, effective enamel adhesion is still questionable. 
Selective enamel etching with conventional phosphoric acid, followed by a self‐etch adhesive. 

Another clinically relevant problem is application time. This may be interesting from the marketing point of view. The resin-dentin interface improves considerably with multiple applications of adhesive when compared to a single coating. It has to be taken into account that not only does dentin need to be treated during that period, but so does a thick smear layer of dentin after rotary bur preparation. Therefore, the active application under continuous rubbing is recommended. Altogether it can be stated that easier handling does not automatically mean less or no technique sensitivity.

The clinical advantages reported for the self-etch adhesives include a simplified protocol and reduced postoperative hypersensitivity. However, there is only limited clinical data on the longevity of the bond.
Reasons for less technique sensitivity of SE adhesives:

  • The technique sensitivity associated with substrate hydration is eliminated because water is a fundamental component of these SE adhesives. 
  • SE adhesives don't allow a discrepancy between the depth of deminarlization and the depth of resin infiltration because both occur simulteneously.
  • Smear plugs are not completely removed during adhesive applicat

When using adhesives in a clinical setting the following steps should be followed:

  • read the manufacturer’s manual
  • shake bottles before use
  • all steps must be carried out under proper isolation, ideally with a rubber dam.

Recommendations for using etch-and-rinse adhesives:

According to the results of a huge number of in vitro and in vivo tests the following products can be recommended as the so‐called gold standard OptiBond FL, Syntac, and Scotchbond MP. Nevertheless, correct handling has an important impact on long‐term success in adhesive techniques:

  • It is important to control the penetrability of the cannula of the etch gel prior to intraoral use. Otherwise uncontrolled flushes occur.
  • The application of phosphoric acid always starts at the enamel margins and ends on dentin. When this is not possible, the whole cavity should be etched for 15–20 s.
  • The acid, the precipitates, and dissolved tooth substrate must be rinsed off thoroughly for 15 s. 
  • The cavity‐drying process has to be carried out with caution to prevent collagen collapse. Short air streams help to visualize the frosty appearance of etched enamel and prevent overdrying of adjacent dentin. Rewetting is mandatory for adhesives without water addition. 
  • Primers need time (30 s) to act on dentin. Rubbing accelerates chemical processes by continually supporting its activity with a fresh monomer. Etched enamel should not be excessively rubbed. The solvent has to be evaporated but not overly dried. 
  • Unfilled resins should get some time (10 s) to penetrate into enamel and dentin. Also, this layer should not be overly thinned.
  • The adhesive has to be light‐cured for 20 s with a sufficient light‐curing unit.

Recommendations for using self-etching adhesives:

According to the results of a huge number of in vitro and in vivo tests the following products can be recommended as the so‐called gold standard: Clearfil SE Bond, and OptiBond XTR. Nevertheless, self‐etching adhesives also need to be applied carefully in order to improve their performance:

  • Systems that involve mixing need to be mixed sufficiently. 
  • The applied primers should be agitated (30 s) on enamel and dentin to enhance chemical reactions. 
  • The solvent has to be evaporated accordingly. Then the cavity has to provide a glossy appearance, indicating that no dry spots with insufficient wetting are present. 
  • In systems with separate unfilled resins, the bonding agent should not be overly thinned. 
  • The adhesive needs to be light-cured for 20 s with a sufficient light‐curing unit. 

Clinical application

Various clinical procedures were proposed to increase the immediate bond strength and reduce aging:

  • Use of a hydrophobic coating:
    Since hydrophilicity of the simplified adhesives (2 E&R  and 1 SE adhesives) has been shown to increase water sorption and bond instability, the use of non-simplified bonding systems (3 E&R and 2 SE adhesives) characterized by a hydrophobic coating with a non-solvated bonding layer, should be preferred.
  • Increased application time:
    Prolonged application times (beyond the manufacturer’s recommendations) increase monomer penetration and favor solvent evaporation before light curing; this increases the immediate and long‐term bonding of most adhesives.
  • Active application:
    Continuous and active rubbing motion has shown higher strength and improved stability of the bond, particularly for 2 E&R adhesives. 

  • Enhanced solvent evaporation:
    The adhesive layer must be 
    carefully air‐thinned because if residual solvent remains within the polymer network it could plasticize the polymer, further affecting the adhesive’s properties. 

  • Extended polymerization time:
    Resin permeability and 
    monomer elution are related to suboptimal polymerization of the bonding. When curing times (particularly of simplified adhesives) are extended beyond those recommended by the manufacturer, improved polymerization can be achieved with reduced permeability. This contributes to stabilizing the bond over time. 

  • Use of MMP inhibitors:
    Matrix metalloproteinase inhibitors, such as chlorhexidine, 
    used as a rewetting solution (e.g. 2%), stabilize the bond over time, this inhibiting the activation of endogenous dentin enzymes especially when using etch‐and‐rinse adhesives. 

To be continued..