Decay extends to the inner most layer of the tooth called the pulp(contains nerve and blood tissue) causing structural defect along with tenderness or pain in the tooth.
ROOT CANAL TREATMENT:
If tooth decay has spread to the pulp, the pulp may have to be removed and replaced with an artificial filling material called (gutta percha) that will keep the tooth in place followed with a process of artificial cap (crown) on the tooth to withstand chewing forces. This is known as root canal treatment. Root canal therapy has had a reputation of being painful, but improved dental techniques mean it is now comparatively painless.