The self etch bonding agents have come a long way and are certainly very useful, but I have taken 20 years working at Moody Terrace Dental Practice and Albert Place Dental Referral Centre to understand the nuances of 4th Generation (Sorry! Etch and rinse!) and it is still the “gold standard”. So I’m going to explain how I understand it works and some of the tricks (and pitfalls) of predictable bonding, which should allow confident use of composites in high stress areas with great aesthetics.
Resin bonding to tooth is largely a mechanical process. It involves getting resin to infiltrate the surface of the tooth to form a layer that is a mixture of tooth and resin, a “hybrid layer”. When we etch the surface of enamel with 32% phosphoric acid we create a spikey serrated surface which the prime + bond will infiltrate fairly easily to give a good bond. However, when we do the same to dentine we end up with a much more diffuse and fragile fibrous surface, a bit like the surface of a scouring pad. It is important to get to the base of this layer with the resin, otherwise you end up with your restoration held in place by a few collagen fibers with a space underneath which can cause “nanoleakage” (another day!)
So, in order to get your prime and bond to soak into this layer it need to be nice a “fluffy” and not too thick. If the dentine is over-etched it will create a hybrid layer which is just too deep for the resin to penetrate the dentine is over-dried after etching it becomes matted and crusty like a kitchen sponge when it dries out. This tends to repel the bonding agent when it is applied. If the dentine is moist, then the prime and bond will displace any moisture present and make its way to the base of the etched dentine.
I etch the enamel for 30 seconds and the dentine for 15. I rinse and dry them both very thoroughly to give a nice frosted appearance on the enamel. Then, I go back in with a damp fiber-tip and swab the dentine only. If you get this right, you can see the dentine absorb the water. If the surface looks wet then dry very gently until it has a slightly matt appearance. Then apply the prime and bond and rub it into the surface for at least 20 seconds. This allows time for it to penetrate. After that, air-dry the surface gently. What you’re trying to do is evaporate any solvents in the bonding agent and any water that has been displaced. There should be enough resin left to saturate the etched layer or you will end up with exposed collagen fibers and the bond to your first layer of composite will not be so good. This layer should then be light-cured. If you leave it uncured you will squash down the hybrid layer with your first increment of composite.
It is really simple. Obviously good moisture-control is essential. It’s really just a matter of taking your time and being methodical. My bonds have improved over the years and post-operative sensitivity is a thing of the past since I have been rewetting as part of my routine.
I hope this helps!