(a) Tooth 36 is symptomatic after root canal treatment. (b) X-ray in distal angulation suggests the presence of an untreated mesial-lingual canal. (c) Reamer 02 ISO 15 in mesial-lingual canal verifies the likely reason for case being symptomatic
Cone beam computed tomography (CBCT) is a very useful tool in diagnosing periapical lesions and root canal anatomy. It has its disadvantages in already root-filled teeth due to the creation of artefacts from the root filling material and also if posts are present (Fig. 8.2). The European endodontic society made a position statement in 2014  regarding the use of CBCT in endodontics. “A request for a CBCT scan should only be considered if the additional information from reconstructed three-dimensional images will potentially aid formulating a diagnosis and/or enhance the management of a tooth with an endodontic problem(s)”.
Cone beam computed tomography (CBCT) has its disadvantages in root-filled teeth due to the creation of artefacts from the root filling material and also if posts are present
8.3 Magnification and Illumination
Magnification and illumination in endodontic practice is a good help within any step of the procedures. The possibility to visualize the tooth and root canal system will give diagnostic information, and during treatment the surgical microscope makes the procedures easier and allows them to be conducted in a controlled manner. The clinical effect of the use of the surgical microscope is difficult to evaluate from a strictly scientific point of view, and there are no studies available that specifically investigated the outcome of orthograde retreatment if or not using an operating microscope, although the possibility to enhance the quality of the access preparation in terms of locating root canals has been studied [6–8].
8.4 Assessment of the Coronal Restoration
A root-filled tooth has usually a large coronal restoration or an artificial crown. The status of the coronal restauration needs to be assessed for several reasons. Gaps between the filling material and the dentine or carious lesions may be the source of persistent apical periodontitis through the mechanisms of coronal leakage. Consequently, in such a situation, all filling materials and soft dentine have to be removed to create aseptic conditions during treatment and to allow for a successful outcome in the long term.
In some situations the quality of the crown or restoration is judged to be faultless, and it seems to be safe to carry out the treatment with the maintenance of the restoration from a microbiological point of view. However, a proper access to the root canal system may be jeopardized. Hence a complete removal of the previous filling or artificial crown may be indicated anyway.
After the complete removal of the restoration and caries, the ability of the remaining tooth substance to provide support for a new restoration must be assessed. Not surprisingly, it is common for this appraisal to turn out negatively and the tooth is deemed for extraction. Therefore, the patient should be informed that the decision to try to cure a tooth by retreatment might be due to change as treatment commences and progresses. Another common finding is that remaining tooth structure is compromised by cracks or fractures. In conjunction with a local periodontal pocket, it gives reasons to believe that a vertical root fracture is present. Also under such circumstances, the retreatment is pointless and the tooth should rather be extracted. When only minor cracks without any periodontal involvement or mobility between segments are present, the clinical decision is more difficult. There is only scarce scientific documentation on the long-term outcome. The prognosis should be considered doubtful. If the treatment is carried on, it must be after thorough deliberation and informed consent from the patient.
The coronal examination also involves an inspection of the pulp chamber floor. The possible presence of untreated root canals or isthmuses can hereby be diagnosed. For example, 65–98% of the upper first molars have two root canals in the mesiobuccal root [9, 10]. And the most palatal of the two (MB2) has frequently been overlooked in the primary treatment and may be the underlying reason to treatment failure. The presence of previously made perforation in the pulp chamber can be diagnosed. If diagnosed the prognosis is dependent on the created damage to the periodontium and the possibility to successfully seal the area of perforation [11, 12].
The type of root filling material previously used can also be assessed. There are many different types of root filling materials used. However, in most cases the clinician will find canals filled with some type of sealer together with a core of gutta-percha. But sometimes only sealers or cements have been used. Core materials other than gutta-percha such as silverpoints or plastic carriers may also have been used. Tooth colouring can sometimes give a hint about what materials have been administered. A pink-coloured tooth is usually root filled with resorcinol-formaldehyde resin, so-called Russian red .
8.5 Removal of Crowns, Cores and Posts
The removal of a crown will almost always also remove some dentine since the border between the crown and the underlying dentine is difficult to visualize. The crown is most easily removed by drilling an axial furrow on the buccal side in to the dentine. The cemental lock is then breakable by creating a bending force inside the furrow with, for example, a small and short screwdriver or a carver. Depending upon the core material of the crown, it will be more or less challenging. There are today burs available that are designed to cut through given materials such as titanium, zirconium, cobalt-chrome, etc. A high-speed hand-piece should be avoided due to its low momentum. An upregulated air-turbine-driven hand-piece is preferable.
Sometimes a core and post might be present inside the prosthodontic construction or even when there is a composite resaturation. The core is removed with burs. If a post is present, it can be of metal or a fibre material. Depending on the fit and retention of the post the removal might be quick or time consuming. The aim is to dislodge the lock of the cement surrounding the post after which it will become loose and easy to passively remove. This is preferably done by ultrasonic equipment even though other techniques are available. The coronal part of the post, above the orifice of the root canal, must be exposed. It is important to make sure that the core material is removed from the post. After choosing an ultrasound tip, designed for the purpose, this is placed against the post, and the ultrasonic unit is set on a high frequency. Then, the clinician moves the tip around the post in order to allow the vibrations to be transported along the post and break the cement. Thin ultrasonic files can be used to remove the cement between the root canal wall and the post. If the post does not come loose, the possibility of drilling away the post can be considered. There are specially designed burs to at a low speed remove fibre posts. The risk of at the same time removing dentine and increasing the risk of root fracture or perforation has to be taken into account. Magnification, visibility, acquaintance of the root canal anatomy and skill is obviously crucial for a successful result.
8.6 The Access Preparation
The access preparation should give visibility and easy access to the root canals and also allow a complete eradication of residues of pulp tissue and microorganisms. At the same time care should be taken to save tooth substance.
A number of different access burs are available that have in common to cut the surface effectively with as little damage to the tooth and resaturation as possible. Before starting drilling through a crown, its outline and its position in relation to the root must be examined and considered. For example, the root might be rotated but the crown is placed in line with the tooth arch.
The access cavity needs to be large enough to give the operator a possibility to see and introduce the instruments into root canal with a “straight-line access”. Rotary or reciprocating instruments should be allowed to act freely without touching the cavity walls. If touching the cavity walls during instrumentation the tapered instruments will be transported within the root canal, and there will be a risk of creating a ledge or instrument fracture. The judicious clinician also considers to reduce cusps not only for good access but also to avoid fracture of undermined tooth substance. The most commonly needed cusps to be reduced are the mesiobuccal on the upper molar and the buccal on the lower molar.
8.7 Rubber Dam and Aseptic Working Field
Next a rubber dam is placed to seal off the tooth and to create an operating field with good aseptic properties. The rubber dam will render an obstacle for saliva and microbes to enter into the cavity and thereby give the operating dentist a possibility to concentrate on eradicating the root filling material and microorganisms within the root canal . This basic endodontic practice also enhances a good field of view and comfort both for the patient and the operator. Furthermore patient safety considerations require the use of rubber dam in order to prevent inhalation or ingestion. The isolated tooth and the clamp and rubber dam fabric that surrounds itshould preferably be also be disinfected in order to further minimize risk of contamination during the treatment. For example, Möller suggests that the tooth is firstly cleaned with 30% hydrogen peroxide and then after disinfected with 10% iodine tincture .
8.8 Removing the Root Filling Material
The aim is to remove the root filling material together with necrotic pulp material and/or embedded microorganisms in order to create access to and enable a chemomechanical debridement of the persisting biofilm. At the same time the clinician must be prudent not to remove an excess of root dentine that in turn can jeopardize tooth survival in the long term. Depending on the type and quality of the root filling, it will be differences in difficulty and time for its removal. A tooth that has been root filled with sealer and has had excessive amount of leakage into the root canal will be more easy to treat than a tooth root filled with a densely compacted gutta-percha and a hard-set sealer or one filled with a core material such Thermafil™ or similar. It is a good clinical practice to probe the root-filled canal with a file, preferably a K-file 15, to feel to what extent it is possible to bring the file down in to the root filling material. This will give an immediate indication on the quality of the seal. If the quality is poor, the instrument will with ease penetrate into the root canal.
8.8.1 Removing Gutta-Percha and Sealer
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 . 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.
8.8.2 Removing Plastic Carriers
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.
8.9 Instrumentation of the Apical Part
When the previous root filling has been successfully removed the root canal instrumentation, I made the same way as at a primary treatment. The rotary or reciprocating instrument sequence should be followed and working length measured as recommended by the manufacturers. A conceivable crown-root length is estimated by studying the preoperative x-rays. An apex locator is preferably used together with an intraoral x-ray. Since the natural taper and possibly the constriction of the root canal is damaged by previous instrumentations, the apex locator will most likely only show if inside or outside of the root canal and not if the constriction is approaching as normally is shown. The intraoral x-ray will add information about the position and direction of the file in the root canal. It will also give an idea of the amount of root filling material still remaining. After working length determination, the root canal preparation continues accordingly to the manual of the selected system and with adjustments selected by the clinician for the individual case. Preferably the apical dimension is above ISO 20 to enable an effective removal of the microorganisms to working length [19, 20]. If the apical dimension needs to be enlarged, be aware of that further enlargement gives higher risk of perforation, zipping or transportation of the root canal. Overinstrumentation has a negative impact on the prognosis of endodontic retreatment and should always be avoided .
8.10 Obstacles and Previous Mishaps
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.
8.10.2 Instrument Fractures
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% . 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  (Fig. 8.3).
(a) Tooth 34 is diagnosed with a fractured instrument that is judged to be removable without major tooth loss. (b) The instrument was removed by using a thin ultrasonic tip in an anticlockwise motion. (c) Case completed with a root filling. (d) Tooth 36 with an instrument fracture. Excessive removal of dentine and the risk of root perforation that are evident in the attempt to remove the instrument are obvious
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