So, truth or fiction, we can bond to old composite that is already in the mouth? If it works, it would have some direct benefits. We could repair composite restorations without replacing the entire old restoration. If it doesn’t work however we set ourselves and the patient up for a restorative failure. The challenge of bonding to old resin is the availability of uncured resin to bond to. Once resin has been fully cured and the air inhibited or uncured layer completely removed “bonding” to it becomes difficult if not impossible.
We worry about fabricating resin based provisionals over a freshly placed composite core. If we don’t place a separating medium they can bond together. This occurs because of the presence of unpolymerized resin. The completion of polymerization occurs over time. Studies show that the longer resin has been cured, the lower our ability to bond to it. In one study the cohesive strength of the restoration when repaired one week after the original increment of composite was cured was only 27% of the original when placed without using a bonding agent. The use of a bonding agent between the layers of composite showed cohesive strength of the repaired restoration at 48% of the original. additionally other studies have shown that preparing in undercuts does not improve these results.
So what does that mean in clinical practice. The greatest reason to only “repair” an existing composite is to be “conservative”, potentially not removing more tooth structure and adding insult to the pulp. However, the repair is destined to fail far sooner then had we replaced the entire restoration. Thereby meaning the tooth will now be traumatized again by recurrent decay, bacterial infiltration and another restorative procedure. So which is more conservative?