# Removal of the Previous Root Canal Filling Material for Retreatment: Implications and Techniques
## 1 Introduction  
The ultimate goal of root canal treatment is to prevent or treat apical periodontitis (AP) [1]. Consequently, the development or persistence of clinical signs and symptoms of AP following treatment indicates an unsuccessful outcome. The disease associated with root canal-treated teeth can be termed post-treatment AP and is managed by nonsurgical root canal retreatment, perizicular surgery or extraction. Because both primary and post-treatment AP share the same etiology, i.e., intraracidular infection, the same basic antimicrobial treatment principles and practice can be applied to approach both conditions.  
Numerous epidemiological studies revealed that one of the most important risk factors for post-treatment AP is the technical quality of the previous treatment [2, 3, 4, 5, 6, 7]. Teeth with low-quality treatment exhibit normal periradicular tissues in less than 40-50% of the cases, while a successful outcome is observed in about 85-95% of the adequately treated teeth [8, 9].  
It is apparent that even some well-treated teeth can fail. Teeth with persistent lesions after root canal treatment are eligible for retreatment or surgery [8]. The primary cause of post-treatment disease in well-treated teeth is usually bacteria located in areas of difficult access to intracranial antimicrobial procedures [10].  
The success rate of retreatment in teeth with post-treatment AP, in turn, ranges from about 60% to 85% [11, 12, 13, 14, 15]. The lower rate of healing when compared to the initial treatment is likely related to the following: limited access to residual bacteria, which can be more complicated because of the need to completely remove the previous filling material; difficult to access to residual bacteria located in areas distant from the main canal lumen; and resistance of residual bacteria [8].  
Because of the intraracidular location of most persistent/secondary bacterial infections [16, 17], intracanal management by retreatment has great potential to promote healing. For this, disinfection of the entire canal system should be one of the most important goals of retreatment. A favorable outcome of non-surgical endodontic retreatment in teeth withpost-treatment disease will depend on proper infection control [18], including not only the main root canal but also difficult-to-reach areas, including lateral and apical ramifications, isthmuses, recesses and dentinal tubules. In this context, adequate removal of the previous filling material is of fundamental importance for disinfection during retreatment, as it permits the access of irrigants and medicaments to residual bacteria [19].  
Removal of the previous filling material is the first intracranial phase during retreatment, and should ideally be complete to expose areas of the root canal system where infection foci may exist. However, even with recent technological advances, which included the development of different instruments specially designed for retreatment, no technique has been shown to predictably promote thorough filling removal. This problem is even more critical in the apical canal region, especially in the presence of curvatures and anatomic irregularities [20; 21].  
Understanding the importance of attaining complete filling removal during retreatment as well as the variables and technical issues that may influence it is of paramount relevance for clinicians to improve the outcome. The purpose of this narrative review is to address diverse aspects of root canal filling material removal during retreatment.