"Root canal therapy is needed when the nerve of a tooth is affected by decay,infection or trauma. In order to save the tooth, the pulp (the living tissue inside the tooth), nerves, bacteria, and any decay are removed and the resulting space is filled with special, medicated, dental materials, which restore the tooth to its full function. Having a root canal done on a tooth is the treatment of choice to save a tooth that otherwise would die and have to be extracted."

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Root Canal Treatment

Endodontic therapy or root canal therapy, colloquially root canal, is a sequence of treatment for the pulp of a tooth which results in the elimination of infection and protection of the decontaminated tooth from future microbial invasion. This set of procedures is commonly referred to as a "root canal". Root canals and their associated pulp chamber are the physical hollows within a tooth that are naturally inhabited by nerve tissue, blood vessels and other cellular entities. Endodontic therapy involves the removal of these structures, the subsequent shaping, cleaning, and decontamination of the hollows with tiny files and irrigating solutions, and the obturation (filling) of the decontaminated canals with an inert filling such as gutta percha and typically a eugenol-based cement. Epoxy resin, which may or may not contain Bisphenol A is employed to bind gutta percha in some root canal procedures. 

After endodontic surgery the tooth will be "dead", and if an infection is spread at the apex, root end surgery is required. For more info: What is a root canal treatment?