When teeth are heavily broken down, a crown on its own may not be enough to retore the tooth. teeth which have been root filled commonly fit in this category. To help retain the crown a post is placed inside the root canal.
A special drill is used to remove the soft rubber root filling to a point around 3mm from the tip. This drill makes a large diameter hole down the middle of the tooth in which the post will be fixed.
Once the hole is prepared an impression is taken. A special plastic post is placed in the tooth and the impression is taken. This plastic post is matched to the special drill and sticks in the impression when it is removed.
The impression is cast up at the dental laboratory in stone and a metal post is made. This is returned to the dentist and then cemented into the canal.