Occasionally the nerve within a tooth can die either from a hard knock to the tooth or because the nerve has become infected with an abscess. If the tooth in question is otherwise sound, it is often advantageous to keep the tooth instead of extracting it by doing root canal therapy. This is usually a two-stage treatment whereby the perished nerve is meticulously removed from the tooth. The root canals are cleaned and then sealed to prevent re-infection. The head of the tooth then receives a dental crown to protect it from breaking under normal functioning as root treated teeth are more brittle than normal teeth.