The root filling removal is preferably done using a stepwise “crown-down” strategy. It is strongly recommended not to push or advance any rotary instrument (burs, drills or root canal instruments) beyond the length that has been first accessed by a K-file 15 (corresponding to creating a “glide path” in primary treatments).

A low-speed bur can be used to remove the 1–2 mm coronal part. In the next step, Gates Glidden drills can be used to advance further 3–5 mm down the root canal. Rotary or reciprocating instruments may now be the perfect choice to start to create a predetermined shape of the root canal [16]. The file should be working in the centre of the material to avoid iatrogenic damages. Many of the rotary file systems have special retreatment files that are usually stiffer and with non-cutting tip and also design to be driven at a higher speed. However the procedure must be done with caution because of the risk creating a ledge. By careful widening of the coronal part of the root canal, a better access to the apical part is created. Hand files, preferably Hedstrom files, can be used but will be more time consuming. Studies have shown that rotary files remove root filling material and prepare the root canal more quickly comparing to hand-instrumentation. Rotary files will leave more root filling materials behind inside the root canal compared to hand files [17, 18]. Using rotary files often needs finishing off with hand files to remove the middle and apical part of the remaining gutta-percha. Ultrasonic files may also be used for this purpose. Remnants of sealer and cement are also easier removed by ultrasonic without risk of removing more root canal dentine.

Carriers covered by gutta-percha are easiest removed by creating a space between the material and root canal wall by inserting a rotary file. The operator must be aware of the risk that the pressure of the wall will control the movements of the file and consequently use only light pressure in order to avoid creating a ledge or even perforation if canal is curved.

Solvents of gutta-percha and some sealers may be a valuable adjunct in the retreatment procedure when the root canal is densely packed or if the root-filled canals are severely curved. Guttasolv™ , Endosolv™ and chloroform are substances aimed for this purpose. Entering a few drops of the solvent into the canal will soften the gutta-percha, and the file can lodge in to the material and follow its path. Since many of these solvents contain chemicals potentially allergenic or even carcinogenic, they should be considered a working environmental hazard. The solvent also creates a layer of gutta-percha on the root canal walls that can be difficult to remove. Therefore they should be used with caution and only when considered necessary and not on a routine basis.

Often a ledge has been formed at the end of the previous filling in the coronal, middle or apical part of the root canal. Often the ledge is the result of an inadequate angle of access to the root canal during primary treatment. The ledge can be passed and removed if access to the root canal can be recreated. But, it may be very difficult or even impossible to pass a ledge. However, the attempt should commence with a pre-flaring of the coronal portion of the canal giving the operator a chance to move the file in the right direction. Usually the coronal part of the canal needs to be widened even more but in the opposite way of ledge. A K-10 file pre-bent in its apical portion can be used to probe the actual pathway. First, the file should be inserted in the canal with the tip directed toward the canal curvature. With very short strokes, the clinician must search for a catch. If unsuccessful, the file tip must be bended in a slightly other way and the procedure is repeated until a catch is felt. Then the file should be wiggled back and forth maintaining a light apical pressure. By moving the file in an up-and-down motion, the ledge is smoothed. A Hedstrom file size 15–20 can also cautiously be used to establish a good glide path. When the block is bypassed, copious sodium hypochlorite irrigation should be used to remove debris.

During the primary treatment, a root canal instrument may have been fractured and left inside the root canal. Instrument fracture occurs and it is mostly due to a procedural error. The frequency of instrument fracture during root canal treatment has been reported to be 1–5% [21]. In retreatment cases the root filling material can act as a blockage of the file that will then not rotate freely and as a consequence instrument fracture may also occur during retreatment procedures. Removal of fractured instruments can be difficult and is depending on the location within the canal. If in the coronal part before the apical curvature, the instrument is more likely to be managed and removed. The fractures occurring in the coronal part are often due to excessive amount of apical pressure, and if in the apical curvature, cyclic fatigue is more likely to be the reason. Since the file has been rotated and screwed inside the root canal wall or root filling material, the principle is that it has to be rotated out. The possibility to access the coronal 1–2 mm of the broken instrument needs to be assessed without major risk of root perforation. A careful monitored preoperative radiographic examination is therefore mandatory. The access is made preferably by a blunt instrument such as a bevelled Gates Glidden drill which will create space for an ultrasonic thin tip to reach in between the file and the root canal wall. The ultrasonic tip is rotated around the broken instrument in a counterclockwise direction removing small amounts of dentine and vibrating the file until it comes loose. The procedure can be very time consuming, and the necessity of its removal and cost-benefit is to be considered [22] (Fig. 8.3).