When a cavity gets too deep, bacteria can reach the nerve inside the tooth. Once the nerve becomes infected, there are two options for treating it. The tooth may be extracted thereby removing the source of infection. Or we can treat the infection inside the tooth, without removing the tooth, with Root Canal Therapy (RCT).
The first step is getting the tooth numb. Despite popular opinion, dentists do NOT like to cause pain, and, with modern anaesthetics, RCT can be a completely painless procedure. We then clean the cavity by removing the soft and infected pieces of the tooth (very similar to a regular filling).
Once the infected nerve, already anaesthetized, is uncovered, we use very fine files to remove the entire nerve all the way to the tip of the root. Then we place medication into the now empty canal space and place a temporary filling over the top. This is left for a period of time to help kill any remaining bacteria inside the canal. Sometimes we prescribe antibiotics to help fight any bacteria that have escaped out of the tooth root into the surrounding bone.
At the second appointment, we remove the temporary filling and use the fine files again to remove the medication and shape the canal. Then we can use small cones of a rubber-like material called gutta percha to plug and seal the canal. Once this is done, a regular filling can be placed over the top but a full-coverage crown is usually recommended for long term strength of the tooth as RCT is usually performed after the cavity has gotten quite large.
The best way to avoid needing Root Canal Therapy is to brush, floss, and have regular check-ups to catch any cavities early on.