# Removal of the Previous Root Canal Filling Material for Retreatment: Implications and Techniques
## 1 Introduction
### Main Variables Influencing Filling Material Removal  
1. Anatomy. The outcome of non-surgical endodontic retreatment can be affected by the tooth type; studies revealed that multi-rooted teeth have a significantly lower percentage of success than premolars and anterior teeth [58; 59]. Certainly, the complex internal anatomy, including isthmuses and ramifications, as well as the occurrence of curvatures can make the retreatment of infected teeth more difficult. Root canals that are oval or flattened in cross-section also pose additional difficulties to proper cleaning and disinfection [60]. Rotary or reciprocating instrumentation often produces a circular and centered preparation, with limited lateral action on the recess areas of oval /flattened canals [61; 62]. Although brushing or circumferential filling might help overcome this problem, a high percentage of the canal surface still remains unprepared in these teeth [63; 64]. 2. Coronal access. Adequate coronal access is expected to influence the filling material removal. Studies reported that minimally invasive access cavities resulted in a greater amount of filling remnants compared to conventional cavities [65; 66]. 3. Instrument. A systematic review concluded that NiTi instruments specially designed for retreatment do not have advantages over conventional techniques with regard to filling material removal [49]. Although these instruments may more easily penetrate into the filling mass and reduce the procedure time, they have been shown not to be essential for better material removal [41; 49]. On the other hand, the use of hybrid techniques and larger preparation diameters are associated with greater cleaning [49]. Studies have shown that differences in taper, tip and cross-sectional shape, as well as the operation mode and the number of instruments used, usually failed to promote significant differences in filling removal [28; 41; 48; 50; 67]. 4. Quality of filling. A prospective study with long-term follow-up showed that the outcome of endodontic retreatment was significantly better in teeth, with previous obturation ranked as inadequate [15]. Conceivably, thorough removal of the previous filling material in these cases is more predictable. This increases the possibility of further antimicrobial procedures to reach the residual infection. 5. Canal enlargement. Larger preparation sizes are associated with greater filling material removal [42]. The final size of the preparation in retreatment should incorporate the diameter of the previous preparation along the entire canal length. This is justified not only for maximum filling material removal, but also to promote proper cleaning and disinfection [68; 69]. In addition, a prospective cohort study [70] reported a high success rate (89%) for the retreatment of lower molars using contemporary techniques and preparation sizes of 0.35-0.40 mm, with tapers varying from 0.04-0.06 mm/mm. Nevertheless, it is important to emphasize that excessive  
Figure 2: Image obtained in a stereomicroscope (16\(\times\)) after root canal retreatment in a lower incisor. The system used was the Universal ProTaper (final instrument F4). enlargement should be avoided, especially in the pericervical area, in order not to weaken the root, which could predispose the tooth to fracture. 6. Patency. Foraminal patency and the extent of root canal cleaning as close as possible to the apical terminus were identified as factors that positively influence the root canal treatment/retreatment outcome [71; 72; 73]. This is because, in some infected teeth, the most advanced frontline of infection may be located close to or at the apical foramen. In addition, increasing the apical level of the root canal filling during retreatment also positively affected the outcome when compared to teeth in which such an increase was not observed [18].