The discomfort is dependent on the condition of the nerve at the time of the procedure, the amount of irritation to the area surrounding the root tip and the reaction of the patient to the procedure. The most obvious failure is the inability to complete the root canal. The anatomy of the root and in turn the nerve canals can exhibit anomalies that make it impossible to treat. These variations occur more frequently in the posterior teeth. Roots that twist and turn and contain angles too sharp to properly clean and fill are causes of failure.
Canals containing obstructions that are impossible to remove are potential problems.
Teeth that have had large fillings or large areas of decay may contain canals that have receded away from these irritants. The resulting canals may be too small to instrument or even impossible to find. Sometimes these compromises lead the dentist and patient to decide to remove the tooth rather than proceed with the root canal. The presence of a dental cyst will not allow the area around the root tip to heal properly. When a tooth abscesses it is a result of the nerve dying.