Last Friday I presented a webinar entitled “Minimizing Adhesive Failures”. When we think about how adhesive failures show up in our clinical practices we see open or leaking margins, recurrent decay, staining or perhaps the most severe is the loss of a restoration due to ineffective bonding. There are many common easy to overcome technique challenges that contribute to these clinical failures. One that I wanted to get the participants to think about was including retention form and resistance form in our preparations for bonded restorations. It is become more of a trend to rely exclusively on the micro-mechanical and chemical adhesion of dentin adhesives, and completely eliminate the macro-mechanical features of the preparation. Before the advent of adhesive dentistry retaining a restoration was dependent entirely on the features of the prep. So now that we have the ability to “bond” do we need retention form?
I think it is really more a question of what are the risks and benefits of adding retention form to our preps. One of the major reasons we have moved away from these design features is trying to minimize the stress on all ceramic restorative materials. When we think about all porcelain restorations, we need to be cautious to avoid sharp line angles, very intricate preparation features, and places where the porcelain will have insufficient thickness to be fracture resistant. The classic “retention” grooves I learned to prep in the corners of a proximal box with a 181L bur would be inappropriate. With the goal of having a final prep where the ceramic has adequate thickness and all of the internal line angles are rounded and smooth, and I have flat smooth margins, I can still add in retention features. These look more like semi-lunar variations in the depth of the prep but accomplish the intended outcome. Another reason to move away from these design features is in the name of staying conservative. If I don’t “need” them to keep the restoration in place and sealed why take away the tooth structure?
So in order to answer that question let’s talk about bond degradation. This is a phenomena that happens to all “bonds”. If you test the strength of the adhesion or bond over time what you will see is that it weakens. We can look at the alteration in bond strength at 24 hours. This is a common way that dentin adhesives are compared to one another. Maybe more importantly is to understand bond degradation after months or years int he oral environment. There is no avoiding bond degradation, however we can understand the factors that increase the speed at which it occurs in an attempt to increase the longevity of our restorative dentistry. One of the major factors in bond degradation is the stress the bond is under. So when the restoration/tooth interface is loaded that stress is delivered to the bond. When forces act upon the restoration to displace it from the prep that force is resisted by the bond. The less stress the bond is exposed to, the less degradation it undergoes. Therefore, if we add features to the preparation that resist displacement of the restoration, are anti-rotational in nature we effectively slow or minimize the bond degradation, increasing the clinical longevity of the restoration.
With this in mind when possible I add porcelain friendly retention features to my preparations. As an added benefit it makes seating easier when there is one and only one way the restoration fits on the prep.