The primary factor responsible for the failure of root canal treatment is incomplete cleaning and shaping of root canals.

- During root canal therapy, the main goal is to thoroughly remove all necrotic tissue, bacteria, and infected pulp remnants from the root canal system.
- Failure to adequately clean and shape the canals can leave behind infected tissue or microbial biofilms that continue to multiply, leading to persistent infection and inflammation.
- This residual infection is the most common cause of treatment failure and can result in periapical pathology or abscess formation.

- Although other factors such as incorrect rubber dam placement and use of inadequate obturation materials can influence the outcome, they usually contribute indirectly or are secondary in comparison.
- Post-restoration infection is also possible but generally occurs as a consequence of inadequate initial disinfection.
- Hence, the cornerstone of successful root canal therapy remains effective cleaning and shaping to ensure proper disinfection of the complex root canal anatomy.

Key points:
- Complete removal of infected pulp tissue is critical.
- Effective shaping facilitates irrigation and obturation.
- Residual bacteria lead to treatment failure.

Reference: Grossman’s Endodontic Practice, 13th Edition, Chapter 5, Page 220