Bacteria Present in the Root Canal after Obturation Are the Result of Primary Endodontic Failure, Regardless of the Radiographic Appearance of the Completed Root Canal Filling
Objective: The technical, radiographic appearance of a completed root canal filling is widely used by clinicians as a proxy for treatment success. This narrative review examines whether the presence of viable bacteria within the root canal system at the time of obturation constitutes the principal determinant of endodontic failure, independent of how the completed root filling appears on subsequent radiographs. Methodology: A narrative literature review was conducted using PubMed, Scopus and Google Scholar, combining terms relating to endodontic microbiology, persistent and secondary infection, obturation, radiographic outcome, and histobacteriologic and molecular detection methods. Landmark prospective clinical studies, histobacteriologic investigations, molecular microbiology studies and imaging-accuracy studies published between 1990 and 2026 were synthesised narratively. Major findings: Prospective clinical cohorts demonstrate markedly lower healing rates when canals yield positive bacterial cultures immediately before obturation, irrespective of the technical standard of the subsequent filling. Histobacteriologic studies of extracted and surgically retrieved root-filled teeth show viable bacteria and organised biofilm in the apical root canal system in the great majority of teeth with post-treatment apical periodontitis, even when the root filling appears dense and well-condensed radiographically. Culture-independent molecular techniques repeatedly detect bacterial DNA in canals that are bacteriologically “silent” by culture and radiographically unremarkable. Conventional periapical radiography under-detects periapical disease relative to histopathology and cone-beam computed tomography, meaning an apparently successful radiograph does not reliably indicate bacteriological cure. Conclusion: The weight of clinical, histobacteriologic and molecular evidence indicates that bacteria persisting in the canal at obturation are the primary determinant of endodontic failure. Radiographic appearance of the completed filling, being a surrogate marker of osseous host response rather than of canal sterility, cannot be relied upon in isolation to confirm elimination of infection.
Keywords: Endodontic failure; Root canal bacteria; Persistent infection; Apical periodontitis; Obturation; Enterococcus faecalis
