# Removal of the Previous Root Canal Filling Material for Retreatment: Implications and Techniques
## 1 Introduction
### Main Variables Influencing Filling Material Removal
This can be explained by the fact that the apical advance of root canal procedures during the retreatment of infected teeth can lead to better disinfection of the apical part of the canal, where the bacteria commonly associated with the post-treatment disease are usually located. 7. Irrigant agitation. It has been demonstrated that the removal of the filling material in the canal can be enhanced by supplementary approaches for agitation of sodium hypochlorite after canal preparation (see below). 8. Solvent. Its benefit is observed only in the initial stage of the retreatment as it may facilitate penetration of the instrument into the filling mass. As discussed above, solvents may impair proper cleaning because of the formation of a thin layer of soft gutta-percha, which can cover the canal wall [47; 74]. With regard to filling removal, some studies have not observed greater benefits when a solvent was used [47; 75; 76]. 9. Magnification. The use of magnification under abundant illumination is of utmost importance in many phases of endodontic treatment/retreatment and surgery. The impact of magnification on filling material removal is more evident in straight canals [23; 77; 78]. Furthermore, an improved retreatment outcome was observed when the operating microscope was used [15]. 10. Type of filling material. The different types of filling materials may cause diverse levels of difficulty for removal during treatment. Of the most currently used materials, the greatest challenge has been reported for biocearamic sealers [67; 79; 80; 81; 82]. 11. Age of filling. Although the effects of the root canal filling age and its impact on gutta-percha removal are not well known, a recent study using a tricalcium silicate sealer showed that filling removal was less effective in freshly filled canals than aged filled canals [83]. 1.4. Retreatment with Hand or Engine-Driven Instruments  
The results of different studies evaluating the removal of the filling material by hand and engine-driven instruments are conflicting. While some studies showed a better performance for hand instruments [51; 84], others have not reported significant differences [28; 85], and even others observed better results for engine-driven instruments [60; 86]. A systematic review concluded that, unlike engine-driven instruments, retreatment with hand instruments was not associated with iatrogenic errors [49]. Hand instrumentation usually spends more time for retreatment when compared to rotary or reciprocating systems [28; 84; 87]. However, speed should not be the main factor when selecting one system over another. The difference in time is usually limited to only a few minutes and a longer action time also results in a longer retention time for antimicrobial irrigants, which can improve disinfection [88]. 1.5. Retreatment with Rotary or Reciprocating Instruments  
In general, studies comparing rotary and reciprocating systems for filing material removal during retreatment have shown a similar performance [89; 89; 90; 88; 48; 67; 86; 87; 84; 88; 89; 91; 92; 93; 94]. Because these systems are often compared in teeth with similar anatomy, under standardized irrigation conditions, working length and apical preparation sizes, the isolated effects of the operation mode on filling removal may not be of great significance. These findings are in line with many other studies that showed similar shaping [95; 96; 97] and disinfection [98; 99; 100] effects when using NiTi instruments in continuous rotation or reciprocation.