# Removal of the Previous Root Canal Filling Material for Retreatment: Implications and Techniques
## 1 Introduction
### Supplementary Approaches  
These procedures are recommended for use after instrumentation with the main purpose of maximizing cleaning and disinfection before placing an intracanal medication or the final obturation. Supplementary approaches include mechanical, sonic, ultrasonic and laser means [56]. Mechanical effects are usually represented by the agitation of the irrigant in the canal by manually pumping instruments or gutta-percha points or by engine-driven instruments operated at low speed (e.g., lentulo spirals, plastic instruments, or NiTi finishing instruments). Finishing instruments have received a great deal of attention over the last years and studies have demonstrated that they can significantly improve disinfection after instrumentation [53,106]. One of the most commonly used is the XP-endo Finisher R instrument (FKG Dentaire, LaChaux-deFonds, Switzerland), which was specially designed for retreatment. This finishing instrument is a variant of the XP-endo Finisher (FKG Dentaire), with a larger core diameter (ISO 30). It exhibits a zero taper and, at the body temperature, undergoes morphological changes to acquire a spoon-like shape that expands its reach in the canal system. Both the XP-endo Finisher and the XP-endo Finisher R have been demonstrated to be highly effective in removing the residual filling material during retreatment [21,25,26,107,108,109,107,109]. The additional reduction in residual material provided by these finishing instruments may reach a mean of 40% to 69%, including the isthmus area [42,75]. A study showed that XP-endo Finisher R was more effective than sonic and ultrasonic devices in removing the previous filling material in oval canals [110]. A systematic review concluded that the supplementary approach with XP-Endo Finisher or XP-Endo Finisher R is effective in enhancing the removal of root canal filling materials during root canal retreatment [111]. The Self Adjusting File (SAF) (ReDent-Nova, Ra'anana, Israel) is a non-conventional hollow and flexible instrument for root canal preparation, which was devised to adapt itself to the canal morphology in cross-section. Because the instrument surface is lightly abrasive and can scrape dentin, there is a good potential for cleaning the canals free of residual filling material during or after preparation [112,113]. When used for canal preparation during retreatment, the amount of residual material after using SAF was not significantly different from conventional instruments [25,114]. Nonetheless, favorable results have been reported for the SAF instrument when used as an adjunctive step after the preparation of oval [113] or curved canals [115]. A commonly used approach to supplement disinfection after preparation is the agitation of sodium hypochlorite by sonic or ultrasonic devices. Sonic frequencies rangefrom 20 Hz to 20 kHz. The EndoActivator is a sonic handpiece that operates at 167 Hz or 10,000 cpm and uses non-cutting polymer tips to agitate the irrigant in the canal [116]. Studies have evaluated its effects on filling material removal. Some showed that EndoActivator might increase the removal of filling material [117], even though it was significantly less effective than ultrasonics [117] and XP-endo Finisher [110; 118]. However, another study observed no significant improvement in filling material removal [94]. Ultrasonic frequencies are greater than 20 kHz. Ultrasonic units for endodontic use usually oscillate at frequencies ranging from 25 to 30 kHz [119; 120]. Ultrasonic activation of irrigant solutions is another widely used supplementary approach. The main effects of ultrasonics result from the phenomena of cavitation and acoustic streaming and possibly warming of the irrigant [119; 121; 122; 123]. Ultrasonic effects can affect and disorganize bacterial biofilms and wash away detached bacterial cells from the canals [120; 124; 125]. Moreover, cavitation may weaken the bacterial membranes and potentiate the antimicrobial effectiveness of NaOCI [120; 126].