F02. Describe the proper technique for acid etching enamel, emphasizing etchant concentration, application techniques, time of application, removal of acid, role of contamination, and potential harm to the pulp.

During etching, it is important to select an acid that forms an intimate contact with the enamel surface and etches the enamel efficiently. For example, during placement of the etchant, particularly if a gel rather than a liquid is used, it is always possible that air-bubbles are trapped between the gel and the enamel surface. To avoid getting unetched spots, low-viscous gels should be preferred, and these gels should be lightly rubbed against the enamel surface during the early stages of the etching procedure. Such a rubbing can be conducted with different brushes or pellets. A suitable brush is the "Benda Brush" shown below.

Through the years, different acids have been used (phosphoric, citric, maleic, etc.) and acid concentrations (10 to 40%) for enamel and dentin etching (conditioning). The most popular and reliable acids are phosphoric acids with a concentration of 27 - 37%. Regarding etch time for these phosphoric acids, an etch time of somewhere between 15 to 30 s should be used, which means that after the acid has covered the surface to be etched, one starts timing the acid for 15 s.  

Acid are available either as liquids or gels. The gels are easier to control, which explains their popularity. Adding different fillers to the acid (e.g. pyrogenic silica or polymer beads) makes the gels. Depending of the reactivity between the acid and the filler, the acidity of an acidic gel may change over time. The viscosity may also change. The ideal gel should have as low viscosity as possible, but still remain controllable.

It was first during the 1990th as it became acceptable to etch both enamel and dentin because of a concern that dentin acid would cause pulp irritations. However, today it is documented that as long as the dentin thickness exceeds 0.1 mm, the dentin with its hydroxyapatite has a sufficient buffering capacity to neutralize the acid before it reaches the pulp. Therefore, nowadays we recommend that both enamel and dentin be etched simultaneously. Dentin etching and bonding will be discussed latter.  

It is important to rinse the etched enamel carefully. This is done by rinsing the etched surface for 20 s to remove the acid and precipitates that might have formed during the etching procedure (see the above figure). Such precipitate form because of the presence of PO₄-ions in both the acid and the hydroxyapatite. During etching, the Ca- and PO₄-ion concentration increase at the gel/tooth-interface, which results in a supersaturation and precipitate formation. This is also the explanation why the 27% phosphoric acid is the most efficient concentration, because as the concentration increases, the precipitation formation accelerates and slows down additional etching.

A few years ago, dentists were told that after enamel etching, the surface should be air-dried until the surface looked frosty. The reason for that was that the resins being used to bond to enamel were hydrophobic. However, today dentists are told that the dentin surface should be slightly moist before dentin bonding with is done, which makes complete enamel drying an impossibility in combination with dentin bonding. We issue will be discussed in connection with dentin bonding.

If the etched tooth surfaces are contaminated with saliva or blood, they need to be reetched. Such a reetching procedure requires only 5 s. If the contaminations are not removed, these contaminations will interfere with the resins and reduce the bond strength. Therefore, to achieve the best bonding conditions, rubberdam should be used if possible.