Some of the reasons for persistent infections are:
- Bacteria that is resistant to treatment
- Narrow or curved canals were not initially treated.
- Complicated canal anatomy.
- The placement of the final restoration was delayed which allowed salivary contamination to the inside of the tooth.
In other cases, a successfully treated tooth may become infected due to:
- New decay that causes exposure of the root canal filling material to bacteria
- A loose, cracked or broken filling.