When a cavity is left untreated, it can become larger until it reaches the pulp of the tooth; this causes infection at the base of the root canal, and leads to the formation of an abscess. This process is generally painful and requires urgent attention for the sake of your tooth and comfort.
The process is decently simple; first, an opening is made through the crown of the tooth to access the pulp chamber. The pulp is then removed, and the root canals are cleaned, enlarged, and shaped. The infected area is medicated, and the root canals are filled. Finally, the crown opening is filled with a temporary filling.