F01. Describe the structural changes which occur in enamel early in the acid etching cycle and recognize the effect of over-etching on the enamel structure.

The ameloblasts produce enamel rods. These rods have a cross-sectional shape reminding about a keyhole.

This shape relates to how the individual hydroxyapatite crystals are precipitated within the rod structure. The crystallites form perpendicular to the surface of Thome's process. Because of the orientation of Thome's process in relationship to the rod, the length axes of the crystallites at the top of the keyhole shape are oriented parallel to the axis of the rod, while the crystallites at the bottom are oriented perpendicular to the axis of the rod. 

This difference in orientation is important, because etching of hydroxyapatite crystals occurs along the axis of the crystallites.  

In other words, if the enamel rod is cut perpendicular to its axis and then etched, the crystallites at the top and upper half periphery will etch more than the differently oriented crystallites at the bottom of the keyhole structure. Thus, dependent on the way the cut is located in relationship to the axis of the rod, different etch patterns can be produced.

During etching, the enamel structure becomes more porous. The preferential etching (described above) allows the resin that is used to bond to enamel to form so called "resin tags." In addition, resin can also penetrate into the porous structure and enhance the mechanical retention. If, however, the enamel is overetched, all porosities may not be completely infiltrated by the resin during the time from resin placement until resin cure. Under such conditions, the uninfiltrated porous enamel structure may crack when the resin polymerizes and shrinks.  

Type 1 Etch Pattern

This slide is a SEM picture of an acid etched enamel surface. As seen from this picture, the top of the keyhole is preserved, suggesting that the central and bottom of the keyhole have been dissolved. In other words, the cut should have been oriented at an angle to the rods.  

Type 2 Etch Pattern

The etch pattern shown above is characterized by a localized etch pattern at the top of the keyhole shaped rod. In this case, the cut is perpendicular to the axis of the rods.  

Type 3 Etch Pattern

The etch pattern of this picture suggest that most etching occurred at the top of the keyhole shaped rod. That would suggest that this cut is perpendicular to the orientation of the rods, and close to a Type 2 pattern. However, because of the rather amorphous look this pattern represents, it is not classified as a typical Type 2 pattern. Thus, patterns that are not "clean" Type 1 or Type 2 patterns are referred to as Type 3 patterns.

One can imagine that Type 3 patterns are more frequently seen in regions where the rods are more disoriented (cusps and primary teeth).

Even though we know today that it is more difficult to achieve successful enamel bonding to primary teeth, there is no scientific support available that suggests that any of the three etch patterns is superior or inferior to the others.

After the enamel has been etched for 15 s, the surface is rinsed with water for 20 s. If only enamel is used for the bonding, and the bonding resin is hydrophobic, the surface should be dried with oil-free air for a few seconds until it looks "frosty". If any contamination (saliva or oil) is introduced during this step, the surface needs to be re-etched.

The bonding resin is then placed on the dry enamel surface. Because of the high surface energy of a clean dry and newly etched enamel surface, the bonding resin is drawn into the porosities of the enamel surface. Preferential etching along the rods result in tag formation shown to the right.

As should be clear now, enamel bonding is achieved due to the following reasons. The acid demineralizes and removes approximately 10 microns of the surface. When that happens, the surface becomes clean and more active, making it easier to be wet with a resin. In addition, the etchant also makes the surface also porous, which results in micro mechanical retention

With newer bonding system (use of hydrophilic resins), the need for a "frosty" looking surface is no longer an absolute requirement.