Etiology
Etiology
Pulp necrosis with infected root canals is the predominant cause. Polymicrobial communities dominated by anaerobic gram-negative bacteria (Fusobacterium nucleatum, Prevotella intermedia, Porphyromonas endodontalis) produce endotoxins, volatile sulfur compounds, and enzymes that egress through the apical foramen and trigger periapical inflammation.
Root canal-treated teeth may develop acute apical periodontitis due to persistent intracanalicular infection (Enterococcus faecalis is the major organism in persistent infections), coronal leakage through failed restorations, or missed canals harbouring untreated bacteria.
Iatrogenic causes include: over-instrumentation pushing debris through the apex, extrusion of irrigants (NaOCl extrusion causing severe tissue damage), overfilling with gutta-percha or sealer beyond the apex, and perforation repairs. These cases present as 'flare-ups' — acute exacerbations during or after endodontic treatment.
Occlusal trauma from a high restoration or from bruxism can cause acute apical periodontitis even in a vital tooth through periapical vascular compromise — termed 'traumatic occlusal periodontitis.'