An infected pulp can be caused by:
Deep decay that progresses quickly
By the time a patient feels a cavity in the tooth or has a toothache, the decay has progressed to infect the pulp. This is why regular surveillance (using dental x-rays) of early decay lesions is an essential part of your regular dental check-up, since these may not be visible using the naked eye.
Trauma
When a tooth is bumped, it may crack, chip or loosen. Bacteria can gain access to the pulp via these gaps and infect the pulp.
Severe bone loss caused by gum disease (perio-endo lesion)
Bacteria that live along the advancing front of periodontal bone loss can infect the pulp from the root end. A tooth with a perio-endo lesion can be very challenging to treat since both the infected pulp and the gum infection require treatment separately.
Whatever the cause, if an infected pulp is left untreated (by not performing a timely pulpotomy, root canal therapy or extraction), endotoxins (produced by the bacteria) leach out of the root apex and cause inflammation in the periapical tissues (the periodontal ligament, cementum and bone). Over time, this causes bone resorption and the formation of a periapical granuloma (reactionary inflammatory tissue) at the root apex.
In some cases, chronic inflammation (lasting over a long period of time) can lead to the formation of an inflammatory dental cyst (a fluid-filled sac formed by primordial cells lining the root surface) or root resorption (the root of the tooth dissolving). Both of these conditions may not respond to treatment apart from extraction.
Bacteria can also infect the internal surfaces of dentine and cementum. This makes it harder to thoroughly disinfect the root canal space when root canal therapy is eventually started. If root canal disinfection is not adequate, then the periapical inflammation may not fully heal. This can be felt as tenderness on chewing or a recurring swelling/abscess.
Ultimately, delaying treatment of an infected pulp leads to more complex problems that may only be resolved by extraction of the tooth. Please see your dentist for treatment as soon as possible to prevent further complications.
