Most root canal treatments are successful. But in some rare cases, a second root canal treatment is needed. Sometimes this is caused by new trauma, deep decay, or a loose, cracked or broken filling which can cause a new infection in your tooth. In some other cases, your dentist or endodontist may discover very narrow or curved canals that could not be treated during the initial visit. This is called retreatment. When retreating a tooth, the root canal filling material is taken out, and the canal is recleaned, reshaped and refilled.