Inflammation of the periapical tissue is sustained by:
Stagnant tissue fluid
Necrotic tissue
Microorganisms
Pus cells
Apical periodontitis is a chronic inflammatory disorder of periradicular tissues caused by aetiological agents of endodontic origin. 
Persistent apical periodontitis occurs when root canal treatment of apical periodontitis has not adequately eliminated the intraradicular infection (microbes).