January 07, 2024
Dental fillings have been used to replace the tooth structure lost due to trauma or caries. Earlier, the concept of a tooth filling material was such, that the material's retention on the tooth was completely dependent on the way the cavity is made. This cavity preparation is done by the burs (dental drills) that shape the walls of the tooth cavity accordingly. Such mechanical binding of the restorative material to the tooth structure often failed due to excessive biting forces, improper cavity preparation or overhanging of the filling material.
Bonding the material to a tooth through chemical means was first advocated in the 1950s. This was possible largely because of the advancements in dental materials. The idea behind chemically bonding a filling material was to create some roughening and microporosities on the enamel and dentin (hard tissues of the tooth) wherein the filling material could flow and adhere to. The porosites or the microtags are created by an acid and the process is referred as etching or conditioning. When the tooth is being drilled in the process of removing caries, then the residual material like debris, denatured collagen, minerals and fluids are all encompassed in a layer. This is called the smear layer. Earlier, it was believed that the smear layer had to be removed for better bonding of the filling material. However, newer generation of materials advocate that the smear layer contributes to the binding of the material.
Once the tooth surface is etched, it opens up small tubules that make the dentin. These tubules and the surrounding collagen (type of protein) is further coated with a primer. The primer acts as an adhesive between the etched tubules and the resin-based filling material. Once the resin is applied, it flows within the microtags and binds there chemically.
As mentioned above, the three main components of effective binding of the material to the surface of the tooth requires three components: an etchant, a primer and a bonding agent. When they were first introduced in the 1990s, all three components were available in different bottles. The etchant, after being applied needed to be washed off before the next two components could be applied. This concept was known as wet bonding, where adhesion was facilitated by the presence of some water that prevents the collapse of collagen. However, these initial systems of totally etching the enamel and dentin were difficult to control as excessive wettability could result in failure of restoration.
A single bottle system that showed the primer and bonding agent in one bottle was introduced later. Here, while etching the tooth surface, the primer and bonding agent (dispensed from a single bottle) were also applied simultaneously. The single bottle system was hydrophilic in nature; meaning that they had a high affinity towards water. This made them more susceptible to water degradation.
SELF ETCH PRIMERS
The total etch systems were clinically very technique sensitive. Hence, in the early 2000s, a major breakthrough was the formulation of self-etch primers. These new systems did not have a separate etching step and were instead combined with the primer and bonding agent. The self-etch primers incorporated the smear layer and also ensured that hydration of the dentin or enamel was not a necessary factor for achieving bond strength.
When it comes to systems utilizing self-etch primer systems, the bonding to enamel is often less effective. Hence, etching the enamel has often been recommended. However, it also opens up the possibility of over-etching the tooth surface which leads to Post-operative sensitivity.
Self-etching systems were introduced in the sixth generation of bonding systems. Now, in the seventh generation, all three agents, the etchant, primer and boding agent are available in a single bottle. In the eight generation (introduced in 2010), nanotechnology was incorporated. The particle size of the bonding agents are too small and fine, and hence they penetrate the enamel and dentin better. They are shown to perform better despite contamination of the tooth layers with saliva or moisture.
Self etch primers are available in different forms depending upon their acidic content. The ones that are mildly acidic are more commonly used as they have a relatively lesser chance of Post-operative pain. They are mainly important for reducing the clinical working time, improving bonding in humid conditions and incorporating smear layer for better adherence.