F12. List three requirements of bonding to a substrate and discuss why the enamel surface is considered to be an extremely poor substrate for the bonding of sealants.

The composition of the substrate should be standardized, and its composition should be known in detail in order to make it possible to select a chemical component that reacts with a specific molecular group.

The surface should be easy to control during the bonding procedure. Other components present in the environment surrounding the surface should not interfere in the bonding process. In other words, it should be easy to wet the surface.

After a bond has been formed, a minimum of competing ions should be able to reach the bonded interface. The reason is that such ions could degrade the bond.

Based on the above considerations, we can conclude the following regarding enamel:

1.Enamel as a substrate is not well standardized.  

The composition of enamel depends to some extent on the environment where the patient grew up. Some ions that might have been built into the enamel structure may make the enamel more or less acid resistant. In addition to compositional variations, the etch pattern is also dependant of how the surface has been cut in relationship to the axes of the enamel rods.

2.The presence of moisture has a detrimental effect on enamel bonding, particularly when a hydrophobic resin is used.

In the oral cavity, saliva can easily contaminate the enamel surface. In addition, because of a pulpal pressure, tubuli fluid is forced through the tubules, and some of this fluid passes also through the enamel.

3.Because of the porous enamel structure (at least on an atomic level), ions can diffuse along the bonded interface. 

Some of these ions may interfere with bonds by either breaking the bond or loosening a surface ion present in the tooth surface and to which a chemical bond has formed (e.g. a -COO- group is bonded to Ca²⁺ and a H⁺ reaches this region).