Periapical periodontitis or apical periodontitis[9] (AP) is an acute or chronic inflammatory lesion around the apex of a tooth root, most commonly caused by bacterial invasion of the pulp of the tooth.

[10] It is a likely outcome of untreated dental caries (tooth decay), and in such cases it can be considered a sequela in the natural history of tooth decay, irreversible pulpitis and pulpal necrosis.

Other causes can include occlusal trauma due to 'high spots' after restoration work, extrusion from the tooth of root filling material, or bacterial invasion and infection from the gums.

Etymologically, the name refers to inflammation (Latin, -itis) around (peri- ) the root tip or apex (-apical) of the tooth (-odont-).

While millions of root canal treatments are carried out in the United States alone each year, total numbers of such cases do not provide reliable indicators of frequency, even for symptomatic periapical periodontitis (given that root canal treatment is not always indicated or complied with, and may also be performed in the absence of periapacial periodontitis).