Overview
Acute apical periodontitis (AAP) is an acute inflammatory reaction of the periradicular tissues — the periodontal ligament, alveolar bone, and surrounding soft tissues — in response to microbial antigens or irritants egressing from the root canal system through the apical foramen. It represents the periapical response to pulp necrosis or, iatrogenically, to overextension of endodontic instruments or irrigants.
AAP exists on a spectrum from simple periapical inflammation with no bone destruction (apical periodontitis without abscess) to acute apical abscess with frank pus formation, cortical bone perforation, and soft tissue spread. This spectrum determines clinical severity, treatment approach, and prognosis.
The condition is one of the most common sources of acute orofacial pain and dental emergencies worldwide, with population-based studies showing radiographic evidence of periapical pathology in 20–50% of root-filled teeth.