A root canal can be done through an existing crown if the crown is in good condition, has closed margins (edges), and there is no decay underneath the old crown. After the root canal treatment is completed, the hole in the crown will be patched up with a filling material. However, if the crown needs to be replaced, then the crown will be removed, all decay will be cleaned off, and the root canal
treatment will be done. If insufficient tooth structure is left to support a crown, then a post will be necessary for the retention of a crown.