In the field of dentistry, etching gel is used to increase the area of the tooth surfaces used for bonding. In contrast to a cementing procedure, the surface to be bonded undergoes special preparation, which includes etching. During etching, the bonding surfaces of the prepared tooth cavity or ceramic inlay are roughened to create what is known as a microretentive pattern. The surface is initially smooth, but after a certain period of time, the etching gel penetrates into the tooth enamel and creates small lacunae (depressions) and caves. The etching gel is rinsed off after a few seconds (as specified by the manufacturer), leaving behind the surface irregularites. Now the bonding adhesive (green) being applied to the prepared cavity surface. This bonding adhesive flows over the newly created irregularities, provided the cavity is dry and the adhesive fluid is sufficient. If the bonding surface of the inlay is subjected to a similar procedure and placed in the etched cavity until bonding is complete, the surface area for bonding will increase dramatically.
Let us consider the original area of the tooth cavity surface with the inlay as X. When one examines the bonding surface after the etching procedure, the area will have increased by a factor of Y. Because the bonding surface is not two- but three-dimensional, it has actually expanded more than what is seen. The etching procedure also conditions the surface, thus facilitating more stable bonding.
Good results can only be achieved if the tooth is maintained dry throughout the bonding procedure. Therefore, a rubber dam should always be used for isolating the tooth. In the animation, one can see what happens if saliva for example, comes in contact with the bonding surface. The porous surface closes immediately and the bonding adhesive can no longer penetrate the enamel. Bonding—and consequently the imperviousness of the filling/inlay—is compromised. This results in the development of secondary caries several months or years after the procedure.
However, not all parts of the tooth cavity surface can be etched to an equal degree. In the animation, one can see a tooth in which the enamel has been removed by a drill. The dentine layer lies below the enamel layer. Dentine comprises protein threads and small canals. Drilling can destroy these structures, leading to the development of a smear layer.
This smear layer prevents the bonding adhesive from penetrating the enamel. If the etching gel is applied over the enamel and dentine for a similar period of time, the proteins released by the dentine will start to clump by a process known as denaturation. This process is similar to the grayish film that develops on the surface of chicken soup, which is caused by denaturated proteins. Because of this, the bonding adhesive cannot penetrate the dentine. Therefore, the gel should initially be applied to the enamel, and after a while, to the dentine, where it should be allowed to remain only for a few seconds. This allows the smear layer to be removed without causing protein denaturation, and it also allows the development of the necessary microretentive pattern. The bonding adhesive can now penetrate the dentine and enamel, which is the basis for optimal bonding.
The risk of complications during the etching procedure is negligible. However, in individual cases, complications requiring further treatment are possible. Every additional procedure carries its own risks, which may eventually lead to tooth loss. Complications specific to etching include the following:
Temporary tooth sensitivity to hot/cold sensations
Damage to the tooth nerve (pulp), necessitating root canal treatment in some cases