Depending on the preparation design that we have, the need for higher bond strengths becomes more and more important.  A traditional preparation design where you have parallel walls and lots of wall length, bond strength is not as critical.  Think of old gold crowns that were held in place with zinc phosphate.  The bond strength of zinc phosphate was literally nothing yet those crowns stayed in place for ever.

Today with CEREC, our preparation designs are much different.  They are designed to hold the restoration in place permanently with a high strength bonding agent but they are not designed to hold provisionals because of the non traditional preparation design. So the prep design tends to be more conservative and less retentive in an  effort to save as much tooth structure as possible.

Fast forward to today where we have these non retentive preparations and we have a rise in popularity of self etching bonding agents.  Bonding agents such as Multilink, Scotchbond Universal are just a few examples of bonding agents that are designed to be used without phosphoric acid etch.  The idea is that the total etch technique with phosphoric acid, while giving you high bond strengths, the etching and rinsing can potentially lead to sensitivity which is why the self etching bonding agents have become so popular.

With the self etching bonding agents, you are not to use etch at all because as mentioned, it may lead to sensitivity.  My recommendation however is that you always selectively etch when you have a large amount of enamel.  The one downside to most self etching bonding agents is that their chemistry is usually not acidic enough to do a proper etch on enamel.  They are especially poor at etching uncut enamel. I think we can all agree that we are trying to bond beyond our margins for a proper seal of the tooth.

For that reason, I recommend a quick 10 second etch on the enamel margins, if you have a lot of enamel available on the prep. You will have higher bond strengths, less micro-leakage and a better seal of the enamel.  Try to minimize the etch on the dentin but definitely etch the enamel margin. Your recalls will look much better and you will have a better seal on the tooth structure.

At Ivoclar we were told that selective etching the enamel actually weakens the bond strength with Multilink. We did not test it to confirm it though.

If you get the etch on the dentin, you are removing the smear layer- Self etching bonding agents bond to the smear layer. So you have to be careful to limit your etch only on enamel. Having said that, the negative of trying to bond to uncut enamel is worse than accidentally removing a bit of the smear layer.

Doug,
There was some testing done by Ivoclar and etching does not effect the bond strength with multi link in either way. The big thing with multi link is to scrub the enamel for 30 seconds instead of 15 like is recommended. Then clean off the excess on the facial and lingual before tack curing. IF you do this you won't get the break down or staining at the margin.

Here's a question for you: can you OVER-ETCH enamel? I've heard plenty about doing so with Dentin, but never about Enamel. I suppose the biggest reason to be cautious is so that any incidental dentinal coverage does not go overboard. But still something I've wondered for a while.