Pathogenesis
Pathogenesis
Bacterial antigens (LPS, peptidoglycans, lipoteichoic acid) and metabolic by-products from the infected root canal system pass through apical foramina and lateral canals. Pattern recognition receptors (TLRs, NOD receptors) on resident macrophages, dendritic cells, and mast cells in the periodontal ligament initiate innate immune activation.
IL-1β, TNF-α, IL-6, and prostaglandin E2 (PGE2) are released, driving vasodilation, vascular permeability, neutrophil recruitment, and — critically — RANKL-mediated osteoclast activation that produces the periapical bone resorption visible radiographically.
In acute apical periodontitis without abscess, the inflammatory infiltrate remains within the periodontal ligament and adjacent bone. Rising tissue pressure within this limited space produces the intense percussion sensitivity — even slight occlusal contact causes exquisite pain because the inflamed ligament cannot accommodate any compression.
When bacterial products overwhelm local defenses or when virulent organisms predominate, the inflammatory exudate becomes purulent. Pus (neutrophils, necrotic debris, bacteria) accumulates in the periapical region, forming an acute apical abscess. As pressure builds, pus dissects through cancellous bone toward the cortical plate, then perforates (fenestrates) the cortex to form a subperiosteal abscess, followed by a soft tissue fluctuant swelling.