A phoenix abscess is a dental abscess that can occur immediately following root canal treatment. Another cause is due to untreated necrotic pulp (chronic apical periodontitis). It is also the result of inadequate debridement during the endodontic procedure. Risk of occurrence of a phoenix abscess is minimised by correct identification and instrumentation of the entire root canal, ensuring no missed anatomy.
Treatment involves repeating the endodontic treatment with improved debridement, or tooth extraction. Antibiotics might be indicated to control a spreading or systemic infection.
References
Phoenix abscess Wikipedia(Text) CC BY-SA