An indirect pulp cap, if done properly, and in the right cases, can help avoid the need for a root canal in severely decayed teeth.
An Indirect pulp cap is where, in a permanent tooth, most of the decay is removed. As a dentist, you find that the decay is extensive and very close to the pulp (nerve) of the tooth. In fact, it may be likely that if you did remove all of the decay, the pulp would be exposed by the infected decay thus resulting in the need for a root canal. So, in order to avoid a root canal, the last little bit of decay is left in there and a sedative temporary filling is placed, usually IRM. After a few months, the temporary filling is removed and the remaining decay is removed and the tooth restored. How does that work? In teeth that have no signs or symptoms of pulpal necrosis or abscess, and that seem to have a good blood supply (ie, the younger the patient, the more likely it will work), by removing the bulk of the nasty decay and sealing off the remaining decay from its nutrient source (the mouth), the pulp has a chance to repair itself and what I call, "scar away" from the decayed area. Once this secondary dentin layer is in place, the remaining decay can be removed without pulp exposure. It doesn't always work, as many teeth are too far along and cannot recover, but sometimes it is worth a try.
A direct pulp cap is where there is an actual exposure of the pulp and a medicament like Calcium hydroxide or Mineral Trioxide Aggregate (MTA) is placed to stimulate the secondary dentin formation. A direct pulp cap is usually done with small mechanical or traumatic exposures with no evident decay in the area.