Effective and safe removal of existing root canal fillings is an integral step of root canal retreatment as it allows access to the canal terminus and facilitates thorough disinfection of the root canal system. After careful radiographic analysis, each case should be approached with an individual treatment strategy. Where essential, additional information from cone-beam computed tomographic imaging will aid diagnosis and treatment planning. Missed canals are often associated with persistent periapical pathosis and are one of the main reasons for endodontic failure. Although gutta-percha along with a sealer are the most widely used root filling materials, several other materials such as carrier-based obturation systems, silver cones, pastes, and cements may be encountered during retreatment; these require a range of techniques and instruments for their removal. This chapter focuses on the most common clinical strategies for gutta-percha/sealer removal and also describes techniques for removal of other types of filling materials, which can be challenging and time-consuming. Despite the effectiveness of techniques to remove root fillings, there are risks and complications associated with retreatment procedures. It is often advisable to consider using a combination of different instruments for removal of the existing filling material. However, complete removal of the filling material is impossible to achieve. The use of a dental microscope improves precision during filling removal in order to preserve sound tooth structure.
20.1 Indications for Root Canal Retreatment
Retreatment of a previously root filled tooth is indicated in cases of persistent (Figure 20.1a) or secondary endodontic infections (Figure 20.1b, 20.1c) characterised by the presence of post-treatment apical periodontitis . The presence of microorganisms within the root canal system is the main reason for treatment failure. Intraradicular infection may be caused by missed canals, ledge formation, fractured instruments as well as anatomical complexities such as isthmuses and apical ramifications . Especially in molars, untreated root canals are the most common reason for treatment failure due to their complex anatomy . Therefore, elimination of intraradicular biofilms remains the goal for root canal retreatment and the key to long-term success . Nevertheless, retreatment may also be indicated in root filled teeth with lost coronal restorations and where technical deficiencies exist in the quality of canal preparation and/or filling prior to subsequent prosthodontic restorations with the aim of preventing apical periodontitis .
20.2 Objectives of Root Canal Retreatment Procedures
Retreatment usually consists of three phases: 1) revising the access cavity preparation, which may lead to identification of missed canal orifices; 2) gaining access to the root canal(s) followed by an attempt to completely remove filling materials from the canal system; and 3) instrumentation and disinfection of the canal system . After proper extension of the access cavity, removal of the root canal filling material is crucial to regain access to the root canal walls and the apical foramen, which are the prerequisite for effective disinfection. However, evidence from laboratory studies suggests that complete removal of filling materials is impossible [7–10]. During removal of the bulk of the root filling material, copious irrigation with sodium hypochlorite (NaOCl) and/or a chelator is mandatory to flush out debris and mechanically loosened gutta-percha and sealer [5, 6]. Care should be taken in order to avoid unnecessary alteration to the existing canal shape (such as curvature) or damage to root dentine (such as zipping, transportation, or perforation). However, maintaining the canal shape during retreatment is challenging and apical transportation may occur due to canal curvatures and/or the deflection of instruments away from the main canal . Moreover, the improper use of aggressive instruments for removal of root filling materials such as Gates-Glidden drills, ultrasonic tips, or large/rigid files with sharp tips may aggravate existing procedural errors such as zipping, ledging, or perforation or result in instrument fracture [12–14] (Figures 20.2–20.3).
During retreatment of teeth with clinical symptoms and/or radiographic signs of apical periodontitis, thorough disinfection of the root canal system is of utmost importance. Remnants of filling material that adhere to the canal walls and within irregularities of the canal systems may protect microbial biofilms from disinfecting agents. Therefore, inspection of root canal instruments for adherent material , identification of filling remnants with the use of an operating dental microscope  (Figure 20.4), and careful probing of the canal walls using small precurved stainless steel hand files to evaluate the effectiveness of removal is recommended . Instrumentation of the root canal should be continued carefully as long as filling remnants are detectable. In this regard, the careful and selective use of precurved files is recommended to detach filling residues from the canal walls in order to avoid overpreparation and excessive thinning of the root. A periapical radiographic image may be helpful to visualise residual filling material, especially in the apical root canal third beyond the curvature (Figure 20.5). However, some remnants usually remain undetected due to the limited sensitivity of radiographs in determining the presence of small volumes of materials that will have low radiopacity .
Retreatment of teeth without clinical or radiographic signs of post-treatment disease have been associated with approximately 25% higher success rates than teeth with preoperative apical periodontitis . Therefore, a more conservative filling removal may be justified in these cases; it is essential in these cases to avoid creating iatrogenic errors.
20.3 Removal of Crowns and Posts
After root canal treatment, teeth are often restored with post, cast core, crowns, or other prosthetic reconstructions. In many instances, the long axis of the tooth is altered, and anatomy of the crown has been modified by prosthetic crowns, which may complicate the location of root canal orifices during retreatment. In cases of persistent endodontic disease, access to the infected root canal system may be more difficult. Therefore, surgical procedures may be beneficial in order to avoid complications during orthograde retreatment.
Current instruments and procedures are presented to enable orthograde and minimally invasive removal of existing restorations.
Prior to root canal retreatment, removal of prosthetic restorations and posts is recommended . Crowns or posts should always be removed if a fracture or loosening of the restoration has occurred or if the coronal restoration is associated with defect margins or secondary caries. Moreover, aesthetic and functional reasons may justify the removal of an existing restoration. Complete removal of prosthetic restorations improves the assessment of residual tooth structure. Especially in roots treated with threaded posts or cast posts, vertical root fractures occur more frequently [20, 21]. After post removal, the use of a dental operating microscope facilitates the early detection of cracks, fractures, or untreated root canals and may improve the outcome root canal treatment in cases with anatomic complexity .
If the margins of a restoration appear to be satisfactory after clinical and radiological assessment, the restoration should also be evaluated from the intracoronal aspect after preparation of the access cavity.
20.3.2 Post Removal Techniques
184.108.40.206 Trephine Bur
The oldest/earliest techniques for removing casts and posts are the use of burs and trephine burs [23, 24]. Due to an excessive loss of dentine as well as an increased risk of perforation, which may result in a poor long-term prognosis, these techniques are currently no longer recommended .
220.127.116.11Post Removal Devices
18.104.22.168.1 Pulling Force
Common devices for post removal are the Gonon Post Remover , the Eggler Post Remover , and the Little Giant Post Remover (Ymazoe Dental C., Koshigaya, Saitma, Japan) . Complete post removal can be accomplished with these devices; however, their use requires additional removal of tooth structure resulting in an increased likelihood of dentine fracture. Especially in cases where alignment of the pulling forces with the long axis of the root cannot be achieved, dentinal fractures may occur . As the application of these tools is very time-consuming and is not suitable for minimally invasive root canal retreatment, their use has become much less popular.
22.214.171.124.2 Impulse Force
Crown Buttler and Coronalflex (KaVo, Bieberach, Germany) can be used to remove bridges and single crowns. These instruments are engaged under the margin and a subsequent impact delivered at this site will dislodge the restoration. The Coronaflex is operated with compressed air, which activates a firing pin breaking the bond between the cement and the restoration without irritating the periodontal ligament due to tensile stress. With this tool, fixed crowns can be loosened and removed nondestructively .
For removal of an individually fabricated cast core, a high-speed diamond bur may be used to generate a shape that can facilitate removal. The abutment is circumferentially reduced and perforated for the attachment of a wire loop. The wire loop is then aligned in the direction of the long axis of the post and is activated with an impulse (Figure 20.6). However, the lack of control regarding the optimal direction of the extraction force creates the risk of incorrect loading and fracture of root dentine.
126.96.36.199.3 Ultrasonics (Video 1)
Ultrasonic devices are used at frequencies between 25–40 kHz. The piezoelectric units appear to be superior compared to the magnetostrictive systems due to their better effectiveness in transferring energy to the file. Therefore, a stronger vibration with less heat development is achieved , which facilitates working under an operating microscope without water cooling for a minimally invasive approach [32, 33]. The prerequisite for using ultrasonics is the removal of the adjacent restorative material and the exposure of the post for visualising the cement at the root canal orifice (Figures 20.7–20.9). Ultrasound reduces the retention force of zinc phosphate and glass ionomer cement by 30% after just 10 minutes. In contrast, the adhesive force of the resin cement was not influenced by ultrasonics . The level of difficulty involved in removing posts depends on several factors such as type, length, diameter, design, and insertion depth of the post and the type of cement because these variables influence the adhesive strength of the post to dentine [34–36]. Conversely, it has been demonstrated that the type of cement has no significant influence on the removability of different posts [37, 38]. A reason for these conflicting results may be the use of different post removal devices and different ultrasonic settings [39, 40].
An important criterion for the removability of the various types of post systems is the method of attachment [38, 43]. Basically, posts can either be screwed into dentine or passively cemented. According to a survey in Germany in 2002, 47% of the dentists still used active screwed posts .
It is advisable to expose the interface between the metallic or ceramic post and the dentine completely until the cement line is visible. This procedure allows dentists to assess how deeply a minimally invasive exposure of the post can be achieved. Especially in oval root canals, a deeper intracanal exposure of the post is feasible without further dentine removal. Optionally, ultrasonic K-files of sizes 15 to 25 can be used (IrriK, VDW, Munich, Germany).
Regardless of the type and material of the post or the cement, the bond between the post and cement can be loosened. For this purpose, the post is ultrasonically activated with a circular motion under constant water cooling. The ultrasonic vibrations are transferred through the post and disrupt the cement structure along the interface of the post and dentinal wall, thereby decreasing post retention. The use of ultrasonic instruments does not allow for removal of fibre posts. However, mechanical shredding and removal of the fibre post can be achieved with abrasive ultrasonic tips.
20.3.3 Complications of Post Placement and Removal
With the use of prefabricated post systems, drills are often used to remove root dentine to create a post space. This reduced residual dentine thickness may result in the formation or propagation of microcracks [45, 46]. Moreover, actively screwed post systems may also produce dentinal cracks as soon as they are inserted. The use of ultrasonics for removal of posts may increase the expansion of existing dentinal cracks . In particular, screwed posts should only be removed by counter-clockwise rotation with an appropriate key when the post is loosened and moves visibly after ultrasonic activation in order to avoid additional stress on root dentine (Figures 20.10–20.13).
20.3.4 Custom Cast Core Posts
Indvidually made gold cast core posts are often characterised by an insertion depth of the post until two thirds of the root length and a deep intraradicular retention cavity. Depending on the residual coronal tooth structure, the clinician is faced with the decision whether post removal is possible or surgical retreatment is indicated (Figures 20.14–20.16).
The removal of metallic cast core posts is associated with the production of metal chips. Therefore, removal should be carried out under rubber dam isolation to avoid aspiration or swallowing of metallic fragments. The aim is to expose the post in such a way that, if possible, further dentine removal can be prevented. The core structure is divided with delicate carbide and diamond burs and should be removed from the inner preparation cavity (Video 2). The exposure is complete when the cement line can be visualised. The behaviour of the cement can be checked by application of the tip of an ultrasonic instrument. If the cement does not loosen after 30 seconds, the space between the cement layer and the root surface should be enlarged with a small ultrasonic size 25 file. Depending on root anatomy, it may be sufficient to expose the post on one side for improved transmission of the ultrasonic vibration. Subsequently, the post can be loosened and removed by activation with an ultrasonic scaler in circumferential motion under constant water cooling (Figures 20.17–20.20).
20.3.5 Ceramic Posts
The modulus of elasticity of zirconia-based dental ceramics is about 10 times higher than that of dentine . The hardness of the material reduces the possibility of post fracture but increases the risk of a root fracture. Currently, it is very difficult to reduce the diameter of a ceramic post without removing root dentine due to the lack of suitable instruments. Grinding and activation with ultrasonics can also damage dentine and the periodontal tissues due to heat generation. After removal of ceramic post with ultrasonics, dentinal cracks could be detected ; therefore, root-end surgical procedures may be indicated as an alternative to orthograde removal of long ceramic posts (Figure 20.21).
The standard procedure for removing relatively short ceramic posts is to use ultrasonic instruments under water cooling. For this purpose, surrounding resin composite materials should be completely removed. An ultrasonic scaler can be used to detach the composite bond at the post/dentine interface by circular movements under temporary water cooling (Figure 20.22).
If a post fracture occurs, it is still feasible to attempt preparation of a path alongside the post. The gap can be minimally extended with ultrasonic files in order to activate the ceramic post with ultrasonics and luxate it laterally . However, such a procedure should only be applied in root canals with an oval diameter or with thick dentinal walls.
20.3.6 Removal of Fibre Posts (Tooth 26 Case with Video 3)
Ultrasonic technology cannot be effectively used to remove fibre posts as vibrations are damped and the post material is slightly affected. Some manufacturers offer removal kits for fibre posts including drills with a cutting tip in combination with a pilot drill, e.g. DT Post Removal Kit (VDW). However, if there is no optimum access and visualisation, an apical or lateral perforation of the root may occur.
Long shaft drills, e.g. Endo Access Burs (Meisinger, Neuss, Germany) in sizes 005 to 012, are valuable instruments for the removal of fibre posts. During dry preparation, the transparent cross-section of the post can be differentiated well from the surrounding dentine using magnification (Figures 20.23–20.27). For minimally invasive preparation, the use of a dental microscope is recommended [50, 51]. With the use of cone beam computed tomography (CBCT), an optimum drilling pathway can be defined and transferred using 3D-printed guides for axis-appropriate removal of fibre posts [52, 53].
20.3.7 Prognostic Assessment of Post Removal
Orthograde retreatment seems to be superior in outcome compared to surgical retreatment in long-term investigations [54, 55]. Untreated isthmi and root canals as well as intraradicular infections can be adequately cleaned and disinfected in an orthograde approach rather than during apicoectomy (Figures 20.28, 20.29).
The outcome of post removal comprises the clinical assessment , as well as the long-term success of the treated tooth. Superior short-term results related to a rapid and invasive post removal technique may jeopardize long-term success due to weakening of tooth structure resulting in vertical root fracture . Failure of treatment that is clearly associated with post removal is difficult to prove because several factors influence the long-term preservation of a compromised tooth. As dentine ages, the modulus of elasticity increases, thus reducing the dissipation of energy, causing it to become more fragile and susceptible to micro-cracks . Particularly sclerotic dentine reacts by cracking upon drying or alkaline attacks, e.g. by NaOCl or calcium hydroxide, upon bending stresses or mechanical effects during post removal .
Teeth with deep and massive cast core build-ups should be critically assessed if persistent microbial infection requires retreatment. Especially when used as abutment teeth in a large prosthetic restoration, root canal treated teeth with posts are a risk factor for long-term success  (Figure 20.30).
The early detection of dentine fractures at the time of post removal facilitates assessment of the long-term prognosis. In laboratory research, dentinal microcracks can be detected using micro-CT imaging [58, 60, 61] or infrared thermography . The accuracy and reliability of root crack and fracture detection in teeth was investigated with acceptable results by using magnetic resonance imaging (MRI) . Until now, MRI is not practicable in dental practice due to high costs and large size of the device. Another technique for detection of dentinal cracks is transillumination, which utilises a light-emitting diode . However, this technique is limited to teeth without prosthetic restorations and the examination is restricted to the coronal part of the root .
In clinical practice, it is currently advisable to detect dentinal cracks with the aid of an operating dental microscope . Alternating the moistening and drying of root dentine, vertically extending intracanal microcracks can be differentiated, although they cannot be detected radiologically (Figure 20.31). Horizontally located microcracks or cracks in the apical part of a root canal system cannot be diagnosed at the moment.
20.4 Methods for Removal of Gutta-percha
During retreatment, the most commonly encountered root canal filling material is gutta-percha and sealer. Its removal can be accomplished using hand files [66, 67], rotary and reciprocating nickel-titanium (NiTi) instruments [68–70], heat , or ultrasonics  with or without the use of a solvent (see section 188.8.131.52 on solvents). The most appropiate removal technique for a specific case is selected after assessing the radiographic appearance of the root filling and after clinically exploring the quality of gutta-percha compaction using a small hand file (Figure 20.32) or by probing with a DG 16 explorer (Video 4). Intraoral radiographic views are useful for evaluation of length and density of the existing root filling (Figure 20.33) as well as for detection of missed canals (Figures 20.34, 20.35). Moreover, radiographic analysis should aim to assess whether root canal morphology has been respected or altered during the previous root canal treatment (Figures 20.36, 20.37). Alterations of the original anatomy such as transportation, ledging, or perforation influence the treatment outcome. It has been reported that the clinical success for retreatments was significantly lower for teeth with altered canal morphology (47%) when compared to teeth in which root canal anatomy was adequately appreciated (86.8%) . This indicates that the main problem during root canal retreatment is not only resistant micro-organisms but also the anatomic irregularities that cannot be penetrated by disinfectants .
In those cases in which two-dimensional periapical radiographs do not provide sufficient information for accurate diagnosis, a small field-of-view (i.e. <5 cm) CBCT imaging should be considered if the additional information from reconstructed three‐dimensional images is likely to aid diagnosis and treatment planning and/or improve clinical management [74–76] (Figures 20.38–20.40). Indeed, CBCT imaging may be indicated in teeth with possible untreated root canals (Figures 20.41, 20.42) and/or previous treatment complications (e.g. perforations) .
20.4.1 Hand Instruments
Poorly compacted gutta-percha or single-cone root canal fillings can often be removed easily with a Hedström file that is inserted between the filling material and the canal wall (Figures 20.43, 20.44). It is frequently possible to bypass and engage the existing gutta-percha cones with Hedström files using a gentle clockwise rotation until an engagement is felt and to remove the loosened root canal filling (Video 5). Usually, Hedström files sizes 20–45 are recommended for this procedure [78–80]; files of a smaller diameter are more prone to fracture when they are threaded into the gutta-percha because they may also engage with the canal wall . This technique may also be successful even when the gutta-percha cone has been extruded into the periapical tissues (Figures 20.45). In such cases, use of a solvent is not necessary as this reduces the ability of the Hedström file to engage with and lock onto the gutta-percha.
20.4.2 Softening of Gutta-percha
Prior to removal of the filling material from root canals, modification of an inadequate access cavity is necessary to establish proper access and to enable visualisation of the pulp chamber floor. Residual gutta-percha and sealer must be removed from the pulp chamber to identify any missed canals (Figures 20.46–20.48), and to accurately locate the orifices of filled canals because perforations using Gates-Glidden drills may occur due to improper visualisation and location of the orifices. Subsequently, well-compacted gutta-percha can be removed from the coronal third of the root canal using a Gates-Glidden drill of the appropriate size (smaller than the canal) in order to generate frictional heat to soften the gutta-percha (Video 6). Depending on the diameter of the root canal, normally, a size 3–4 Gates-Glidden drill can be used in maxillary incisors whereas a size 2 Gates-Glidden drill might be appropriate in mesial root canals of mandibular molars.
Heated pluggers or heat carriers from obturation devices may also be used in the coronal part of the gutta-percha to soften and remove the material [71, 81]. However, caution is essential to prevent irreversible damage to the periodontal ligament and bone by the heat generated, especially in roots with thin walls [82–84]. Thus, the electrically heated plugger should be inserted into the gutta-percha with a short burst of heat followed by cooling during which the filling material will adhere to the heat carrier. Then, a quick burst of heat activation allows removal of the heat carrier with any softened gutta-percha on it. In contrast, heated hand pluggers will cool immediately after removal from the flame resulting in inefficient heat transmission to the gutta-percha. Moreover, another disadvantage of these hand pluggers is that retransmission of heat is impossible while the instrument is located inside the root canal.
Removal of gutta-percha is usually accomplished by mechanical instrumentation; however, anatomic irregularities such as curvatures, ramifications, or isthmuses impede complete cleaning of these hard-to-reach areas of the root canal system . Moreover, heat-softened root canal filling materials may easily flow into complex anatomies from which removal is almost impossible [88, 89]. If the existing filling material is well-compacted and cannot be removed or bypassed, the use of a solvent to soften the gutta-percha is recommended to assist its removal and to reduce the risk for altering the original canal shape, straightening, or perforation [72, 90, 91] (Figures 20.49, 20.50).
As all solvents exibit cytotoxic effects to a certain degree, their proper handling is recommended. Adhesive resin composite restorations for build-up of proximal walls and careful isolation of the tooth with rubber dam are important to avoid damage to the gingival tissues and to protect the patient from ingestion or aspiration of solvents (Figure 20.51).
The most popular solvent is chloroform because it has been demonstrated to dissolve gutta-percha effectively [92–94]. However, concerns have been raised regarding its cytotoxic and carcinogenic effects on the periapical tissues [95–98]. Extrusion of chloroform through an existing perforation to the surrounding periodontal ligament space and subsequent necrosis of supporting bone and tissues has been described . Although some reports indicated no negative health effects to the dental team, and the air vapour levels were well below mandated maximum levels if used during common retreatment procedures [100, 101]. Despite that, its sale to dentists has been prohibited in Germany , and several alternative solvents are available for gutta-percha removal. In general, the choice of an ideal solvent for root canal retreatment requires a balance between acceptable biocompatibility and the chemical capacity for dissolution [103, 104]. Nevertheless, results from the literature regarding the dissolution ability of different solvents may be controversial due to the variation in composition of gutta-percha brands .
Xylene, an aromatic organic solvent, has a similar effectiveness compared to chloroform [105–107] but is also considered a potential carcinogen [108, 109]. Although halothane has been proved to be an effective solvent , the potential risk to induce a host-dependent and dose-independent idiosyncratic hepatic necrosis is a major biological drawback [111, 112] that limits its use in endodontics . Besides biological considerations, solvents such as chloroform, xylene, and halothane are reported to reduce the microhardness of enamel and dentine [113, 114]; however, the clinical relevance of this effect remains unclear.
Rectified white turpentine may also be a suitable alternative to chloroform for softening gutta-percha because it is not carcinogenic and has adequate biocompatibility . Turpentine oil dissolves gutta-percha at body temperature  but warming it to 160°F (71°C) will increase its efficiency .
Essential oils, such as orange oil and eucalyptus oil, have been shown to be clinically effective solvents although they do not soften gutta-percha as rapidly as chloroform [110, 118–120]. Moreover, they do not alter the histochemical composition of root dentine ; however, eucalyptol may affect the bond strength of some adhesive systems to dentine . The favourable biocompatibility of these solvents and their effectiveness make them a suitable alternative to chloroform [106, 110, 123, 124]. One randomised clinical trial reported that the removal of root canal fillings with or without the use of a eucalyptol-based solvent resulted in equivalent postoperative pain intensity and analgesic intake .
The dissolution of root canal sealers by eucalyptus oil and orange oil is reported to be moderate and depends on the type of sealer [104, 126]. Whereas sealers based on calcium hydroxide, zinc oxide-eugenol, and glass ionomer had only a limited solubility in eucalyptus oil , orange oil effectively dissolved zinc oxide-eugenol-based root canal sealers . Silicone-based sealers such as RoekoSeal (Coltène/Whaledent, Langenau, Germany) are virtually insoluble in eucalyptol and orange oil . The widely used epoxy resin-based sealer AH Plus (Dentsply Sirona, Baillaigues, Switzerland) can be dissolved to some extent by using either eucalyptus oil  or orange oil .
In summary, because all solvents are toxic to some degree and exert adverse effects, their use in retreatments should be limited or avoided whenever possible [95, 130]. Moreover, the use of a solvent creates a thin layer of gutta-percha on root canal walls and inside dentinal tubules that impedes the antibacterial action of irrigating solutions . In addition, it is impossible to completely dissolve root canal filling materials and their removal often requires a combination of hand and/or rotary files and utrasonically activated instruments [13, 15, 32]. Therefore, the use of solvents should not be standard practice during root canal retreatment and their application should only be considered if working length cannot be reached without their use . In teeth with a single cone or poorly compacted root canal filling, the gutta-percha can be easily removed with Hedström files making the use of a solvent unecessary (Figure 20.52).
Studies using micro-CT imaging also demonstrated that the use of a solvent did not significantly improve the removal of filling material from isthmus‐containing root canals [132, 133]. However, the use of a solvent in the early stages of retreatment may reduce the time required to reach working length [69, 88, 134].