Clinical radiograph demonstrating various instrument fractures referred to specialist endodontic practice. Note (a) instrument inadvertently pushed beyond the confines of the canal after the general dental practitioner attempted retrieval. (b) Two separated instruments were noted in the MB and DB canals (green arrows). The patient was a lawyer and had not been informed of the mishaps. (c) and (d) Instrument fractures present in the apical 1/3 of the canal system at and beyond the canal curvatures. (e) Separated Lentulo spiral filler likely due to minimal canal preparation prior to dressing the tooth. (f) A fractured rosehead bur. (g) Endodontically treated tooth with a separated stainless steel hand file in the MB canal
Within the endodontic literature, it appears that retained fractured instruments do not reduce the prognosis of endodontically treated teeth if apical periodontitis is absent. However, in cases where peri-radicular disease is present, healing is likely to be significantly compromised. The hindrance posed by the presence of an endodontic instrument is particularly evident in those cases where separation has occurred early before adequate canal disinfection has been achieved. Considering the risks associated with file removal, perhaps this should only be attempted if apical disease is present [1–3].
The majority of endodontic instruments are made of stainless steel, nickel-titanium (NiTi) alloy or carbon-steel under ISO/ANSI specifications [4, 5]. Fractured root canal instruments may include endodontic files, finger spreaders, spiral fillers and Gates Glidden burs.
Nickel-titanium was developed 40 years ago by Buehler and colleagues in the Naval Ordnance Laboratory . Walia and associates first proposed the use and development of NiTi alloy for the fabrication of endodontic instruments due to their special characteristics of super elasticity and shape memory . Super elasticity is associated with the occurrence of a phase transformation of the alloy upon the application of stress or temperature above a critical level. The low temperature phase is called martensitic and the high temperature phase is known as austenitic. The stress-induced martensitic transformation reverses spontaneously upon release of the stress returning to its original shape and size. This special property manifests as an enhanced elasticity of the NiTi alloy, allowing the material to recover after large strains. NiTi instruments appear highly flexible and elastic, hence the possibility of use in a continuous rotary fashion even in a curved canal. Despite these advantages, unexpected instrument fracture is not uncommon and represents a major concern in clinical use.
Two distinct fracture mechanisms responsible for file separation, acting alone or in combination, have been described in the literature – namely, torsional stress and cyclic fatigue [8–10]. The twisting of a file about its longitudinal axis at one end whilst the other end is fixed generates torsional stress. This can happen in straight or curved canals if the tip binds resulting in friction against the canal wall. When the elastic limit of the metal is exceeded, the rotary instrument undergoes plastic deformation (unwinding). The file will ultimately fracture if the load is sufficiently high. Cyclic fatigue results in failure of the file when repeated cycles of tension and compression occurring during bending are sufficient to cause structural breakdown and eventual fracture. Clinically, rotation of a file in a curved canal contributes to the tension/compression cycling as a file is rotating. In reality, both factors work together to weaken a rotary file and ultimately cause file fracture with little to no warning.
Many manufacturing design characteristics of rotary NiTi files can influence their resistance to fatigue and fracture. As a general rule, fine and flexible files are vulnerable to torsional stress but more resistant to cyclic fatigue. Conversely rigid, larger files can have a greater torque applied without torsional failure and a greater propensity towards failure by cyclic fatigue. Conventional stainless steel K-files have a total cutting length of 16 mm and a standardized increase in diameter by 0.02 mm per millimetre. This increase in diameter is termed a taper of 2 %. For example, an instrument designated as size 25 is 25/100 thick at the tip (0.25 mm). At the end of the cutting edge, it is 16 × 0.02 mm = 0.32 mm thicker (i.e. 0.25 + 0.32 mm =0.57 mm). 6 % tapered NiTi instruments have less resistance to fracture compared to 4 and 2 % . Moreover, an acute canal curvature coronally is more likely to lead to instrument fracture compared to a gradual apical curve.
It is well known that the nature of the alloy and the manufacturing process greatly affect the instruments’ mechanical behaviour. To improve fracture resistance of NiTi files, manufacturers have either introduced new alloys to manufacture NiTi files or developed new manufacturing processes. M wire (used in Vortex Blue and WaveOne) has been developed by a series of proprietary thermo-cycling processing procedures creating instruments that are more flexible and more resistant to failure compared to traditional NiTi alloy [12–14]. Another novel approach has been the development of the Twisted File using a combination of heat treatment and twisting of a ground blank nickel-titanium wire. A raw nickel-titanium wire is selected in its austenite crystalline structure, and then by means of heating and cooling procedures, a completely new crystalline phase (R phase) is created. The Twisted File is created by twisting the R phase wire into the desired shape, heating and cooling it to maintain the shape and then converting it back to its austenite crystalline structure. The instrument is deemed more resistant to cyclic fatigue, and the manufacturing process deems it less susceptible to microfracture points along the length of the file since it is not ground like conventional NiTi files [15, 16]. Recently, NiTi rotary instruments made from a NiTi-controlled memory wire (CM Wire; DS Dental, Johnson City, TN) have been introduced. The major advantage of the Typhoon CM File is that its shape memory has been removed or controlled by a special thermo-mechanical process. This unique file can be prebent or curved retaining its shape without any attempt at straightening. In curved root canals, the instrument will follow the natural root canal anatomy without undue disproportionate lateral forces resulting in transportation and straightening. The Typhoon CM File has increased torsional strength and increased resistance to cyclic fatigue resulting in a file that is more likely to unwind as opposed to unexpected fracture [17, 18].
Instrument fracture can be a serious iatrogenic mishap that can complicate or compromise the outcome of endodontic treatment. Several factors have been identified in the literature, which may be responsible for the ultimate demise of an instrument and methods that can help prevent nickel-titanium rotary fracture (Table 11.1).
Factors identified that can either contribute towards or prevent instrument fracture
File flexibility and taper
Inappropriate access design
Reproducible glide path
Angle of curvature and radius of curvature
Repeated use of instruments
Instrumentation technique and manipulation
Use of torque-controlled motors
Use of irrigation and/or lubrication
Reciprocation vs. continuous rotation
Studies have demonstrated that higher rates of NiTi rotary instrument fracture occur with less experienced operators whose tactile sensation may not be adept at recognizing when a file may undergo excessive torsional resistance, taper lock and ultimately fracture. Preclinical training in extracted teeth or blocks and hands-on courses allows the clinician time to acquire the necessary competence, proficiency and tactile skills required to avoid procedural mishaps. Examination of instruments prior to and after use is recommended whereby flutes are inspected for deformities and signs of unwinding due to excessive torsional stresses. Instruments should be used according to manufacturer’s instructions with appropriate torque settings and speed of rotations always avoiding the use of excessive forces, especially apical pressures [19, 20].
Inappropriate and inadequate access cavity design features during preparation procedures will result in complications during the cleaning and shaping process. Ideal access preparations should ensure straight-line, unimpeded access to the root canal orifices with all obstructions such as dentine or calcifications having been removed. Ideally direct access to the apical portion of the canal or to the point of the first curvature should be achieved avoiding overzealous preparation for fear of stripping or perforations. Endodontic instruments that are then passively introduced into the canal will follow the natural canal anatomy with less torsional and cyclic fatigue stresses on the instrument. Deviation from the original canal curvature, which can lead to procedural errors such as zipping, stripping and perforations, is also minimized. Correct access preparation and design is a crucial first step before the introduction of instruments necessary to avoid catastrophic mishaps and fractures [21, 22].
A smooth reproducible glide path extending from the canal orifice to the apex of the root in theory should be created before introducing NiTi rotary instruments into the canal. This can be achieved using either traditional stainless steel instruments or using the recently developed PathFile system (see Chap. 3). The premise is that unwanted anatomical interferences and canal obstructions are eliminated reducing the unexpected file separations during rotary preparation. The use of stainless steel instruments has the advantages of excellent tactile sensation especially with respect to torsional stresses and the enhanced ability to negotiate blockages and calcifications often encountered in root canal systems. The use of rotary NiTi files for creating such a path has been found to cause less canal aberration and less modification of the original canal anatomy in stimulated root canals [23–26]. The PathFile system consists of three instruments, with 21–25–31 mm length and 0.02 taper; they have a square cross section. The PathFile #1 (purple) has an ISO 13 tip size; the PathFile #2 (white) has an ISO 16 tip size; the PathFile #3 (yellow) has an ISO 19 tip size. The manufacturer suggests using the first PathFile immediately after a #10 hand K-file has been used to scout the root canal to full working length.
Anatomical constraints and variation in root canal anatomy within a tooth increase the likelihood of instrument fracture in certain cases. Hidden challenges such as merging canals, curvatures (S shaped, abrupt angles of curvature, double curvature, abrupt radius of curvatures), dilacerating or dividing canals, isthmuses, fins and aberrations may all result in unexpected instrument fracture. The clinician must be aware that both the angle of curvature and radius of curvature in combination with instrument size selected will influence the possibility of failure. As the angle of curvature increases, there is an increased possibility of instrument fracture using larger less flexible instruments. With regard to the radius of curvature as the radius decreases, the instrument stress and strain increases with the likelihood of instrument fracture [9, 27–29].
The repeated use of endodontic NiTi or stainless steel instruments results in a higher risk of failure and fracture compared to single-use instruments. The more cycles of rotation an endodontic instrument endures, the greater the working stress that is created within the file resulting in deformation and failure [9, 30, 31]. Recently, the emergence of variant Creutzfeldt–Jakob disease (vCJD) has highlighted issues regarding the decontamination protocols commonly used for reuse of surgical and dental instruments. The resistance of prion agents to inactivation and the notorious difficulty associated with cleaning endodontic instruments has led to the emergence of single-use endodontic NiTi rotary instruments in recent times [32–35].
Endodontic instruments have been designed with tips that are described as passive or active. An active instrument has non-landed active cutting blades, which are effective at removing dentine. Passive instruments on the other hand have a radial land between cutting edge and flute, which reduces the cutting ability. In general, active instruments cut more efficiently and aggressively compared to passive instruments, which have less tendency to straighten the canal that can result in transportation and ledges . During instrumentation, the rotary file should always passively follow the canal pathway, and no attempt should be made to apply any apical pressure. The more apical pressure applied, the greater the risk that the NiTi instrument will encounter ‘taper lock’ undergoing structural fatigue and failure [10, 37, 38]. The term ‘taper lock’ describes the situation when an instrument dimension closely approaches the canal’s size and taper.
The manufacturing process of NiTi instruments during the machining and milling of the alloy during the production phase can lead to lattice structure distortion, microhardness inconsistency and surface microcracks. These could be the prelude to an imminent, further deterioration of the instrument and could explain the reported increased breakage of these instruments under normal operative conditions [39, 40].
Rotational speed of the instrument has been cited as a cause of fatigue fracture of NiTi instruments and that this is less likely to occur with lower rates of speed [41, 42]. Manufacturers advocate predetermined rotational speeds according to the instrument selected for use, and clinicians are recommended to adhere to these guidelines to minimize the risk of instrument fracture accordingly.
Torque-controlled motors were introduced with a built in feedback mechanism that limits the maximum torque delivered to a handpiece to reduce the risk of torsional fatigue of an endodontic NiTi instrument. Instruments will show differing torsional strengths according to file geometry (cross-sectional shape and area). Once the preset torque (preprogrammed into the circuitry of the machine according to manufacturer’s recommendations) is reached, the motor either stops or automatically reverses. This is to prevent excessive build up of stress in the instrument whereby limiting the applied torque to below the ultimate strength of the material reducing the risk of torsional failure. Note frequent engagement of the autoreverse function also carries a risk of torsional fatigue of the instrument. The use of such motors has been demonstrated to be beneficial in canals with limited accessibility [19, 38, 43, 44].
The use of irrigation and lubrication during the cleaning and shaping procedures has been shown to reduce root canal clogging of dentinal mud, bacteria and inorganic debris that can accumulate as a result. Irrigant and lubricant use has been shown to act synergistically to reduce frictional resistance and undue torsional stresses placed on the instrument reducing the likelihood of torsional failure. Canals should be copiously irrigated using sodium hypochlorite solution, and lubricants such as ethylenediaminetetraacetic acid (EDTA) can be used as adjuncts during the preparation phase [45, 46].
Rotary instrument manipulation has been shown to be one of the most important factors contributing to instrument fatigue and failure. Instruments should be used in a smooth, light apical motion and continuous pecking motion without allowing the instrument to rotate in any one particular area for an extending period of time. It has also been shown that as the length of this pecking motion increased, the number of rotations to fracture also increased [11, 37, 47].
NiTi root canal instruments have been traditionally used in a continuous clockwise rotation within the tooth overlaying a preparatory shape on the original anatomy of the root canal. Two single file systems – RECIPROC (VDW, Munich, Germany) and WaveOne (Dentsply Maillefer, Ballaigues, Switzerland) – have been recently introduced following a new reciprocating concept. The reciprocating sequence consists of a counterclockwise movement and a clockwise movement to prepare the root canal system achieving adequate shapes with efficient cleaning ability . The reciprocating motion of the file theoretically reduces the cyclic fatigue placed on the instrument, thereby reducing the propensity to separate and increasing the lifespan of the instrument. Theoretically, this ‘balanced force’ type of technique also allows for more centred canal preparations which are helpful when managing curved canal systems where repeated compression and flexion can lead to instrument fracture [49–53].
11.2 Outcome and Prognosis
The impact of a retained fractured instrument on endodontic treatment prognosis has been assessed by two retrospective case control studies. The conclusion drawn from both studies was that a retained fractured instrument per se generally did not adversely affect endodontic case prognosis. In addition, the presence of a preoperative radiolucency was significantly associated with a reduced chance of healing [54, 55]. On the basis of the current best available evidence, the prognosis for endodontic treatment when a fractured instrument fragment is left within the root canal is not significantly reduced . The success of removing a fractured instrument is closely related to whether the fragment can be visible using a dental operating microscope. In those cases where the fragment is not visible, the probability of successful removal is reduced significantly .
Several methods have been proposed for the removal of fractured instruments within the root canal system with varying limitations and success. Techniques include the use of the Masserann Kit , Endo Extractor , Canal Finder System , ultrasonic devices [61–63], staging platforms  and the Instrument Removal System (IRS) . The inherent problems associated with all these devices include the risk of excessive removal of root canal dentine, ledging, perforation, limited application in narrow and curved canals and the possibility of extrusion of the separated instrument through the apex [60, 62, 66] (see fig. 11.1).
Successful removal of any retained fragment is dependant on factors such as the position of the instrument in relation to canal curvature, depth within the canal (apical, middle or coronal) (Fig. 11.2), whether the instrument is visible using the dental operating microscope and type of instrument that has separated (NiTi or stainless steel files). The more apically positioned the instrument, the greater the risk of iatrogenic damage, including root perforation or fracture. Conservation of root dentine is paramount to long-term longevity of the tooth, and this must be balanced against the proviso of straight-line access necessary to visualize the instrument. In some cases, it would be advisable to consider leaving the instrument in situ especially if the instrument can be bypassed or the instrument is at or beyond the curvature where visibility is less than ideal or not possible .
Diagrammatic representation of location of instrument and ease of removal. Note (a) coronal fragment, (b) middle 1/3 and (c) apical at level of curvature. (d) Presence of an instrument beyond the curvature where retrieval attempt is the most difficult and adverse risk is greatest
Another consideration when determining whether to bypass/leave a separated instrument as opposed to removal is when instrument breakage has occurred. Early instrument breakage increases the likelihood of inadequate chemomechanical canal enlargement, debridement and ineffective microbial load reduction. If the instrument cannot be bypassed and then provided, it is not at the apex or beyond the curvature instrument removal could be recommended, particularly if a peri-radicular preoperative radiolucent lesion is present.
Careful adherence to recommended principles when using rotary NiTi instrumentation would minimize the unfortunate occurrence of instrument fracture. Where instrument fracture has occurred, the patient should be informed, and appropriate referral sought to an endodontist for case assessment. The decision to bypass or leave the instrument in situ must be weighed up with the benefits of removal. Intraoperative factors such as timing of separation, type of instrument fractured, length of instrument fracture, position of fractured instrument (apical, middle and coronal), level of apical curvature in relation to fractured instrument, presence or absence of apical periodontitis, possibility of merged canals, residual dentine thickness and visibility must all be considered before making a decision.
11.3 Retrieval Methods
Manual Retrieval Techniques
The use of hand files (usually Hedstrom files), excavators and various gripping devices such as a fine haemostat or Stieglitz forceps has been advocated for the removal of separated instruments. Hedstrom files can be inserted to varying depths within the root canal system, whereas the other instruments are only useful when the metal object requiring removal extends into the pulp chamber. The use of these instruments is only beneficial when attempting removal of silver points or metal objects that are not bound within the root canal and easily accessible from within the pulp chamber itself (see chapter).
The Masserann Kit
The Masserann Kit (Micro-Mega, Besancon, France) has been used for removal of broken metal objects in the root canal including separated endodontic files. The technique requires removal of excessive amounts of dentine in order to accommodate the various trephines used to expose the metal fragment. This method is not suitable in narrow and curved canals where there is an increased risk of perforation. The kit is ideal for fragments located in the coronal aspect of the root canal where the trephine drill is least likely to result in perforation of the root. The trephine drill is used to expose and accommodate the metal object in its centre whilst cutting a circumferential trough around the object. The smallest tubular extractors have diameters of about 1.20 and 1.50 mm, which limit their safe use to generally larger canals in the anterior teeth.
The Canal Finder System
This system connects to an air motor providing a simultaneous watch winding and reciprocation movement. The amplitude of reciprocal movement is up to 1 mm and increasing the speed decreases the amplitude of motion. The system has been used to bypass and remove fractured instruments within the root canal with varying degrees of success.
The Cancellier Extractor Kit (SybronEndo; Orange, California) contains four different-sized microtubes with diameters of approximately 0.50, 0.60, 0.70 and 0.80 mm. An ultrasonic instrument is typically used to trephine around and expose the coronal 3 mm of an obstruction allowing the internal diameter of the microtube to fit over the coronally exposed obstruction. The pre-fit microtube may now be bonded onto the obstruction with an adhesive, such as cyanoacrylic glue. This removal method is effective for retrieving a non-fluted broken instrument or when there is difficulty retrieving a separated file that is already loose within the root canal. Caution should be exercised to not use too much adhesive, which could inadvertently block a canal.
Instrument Retrieval System (IRS)
The Instrument Removal System (IRS) is a mechanical method for the removal of intra-canal obstructions such as silver points, carrier-based obturators or broken endodontic files. The IRS is indicated for the removal of broken instruments that are lodged in the straight portions of the root or partially around the canal curvature. Three instruments with diameters ranging from 0.60 (yellow), 0.80 (red) and 1.00 mm (black) are available comprising of a microtube and screw wedge. Each microtube has a small-sized plastic handle to enhance vision during placement, a side window to improve mechanics and a 45° bevelled end to “scoop up” the coronal end of a broken instrument. The clinician must utilize ultrasonic instrumentation to circumferentially expose 2–3 mm of the separated file. An IRS microtube is then selected that can passively slide through the pre-enlarged canal and drop over the exposed broken instrument. Once the microtube has been positioned, the same colour-coded screw wedge is inserted and slid internally through the microtube’s length until it contacts the obstruction. The obstruction is engaged by gently turning the screw wedge handle counterclockwise (CCW). A few degrees of rotation will serve to tighten, wedge and, oftentimes, displace the head of the obstruction through the microtube window. If any given colour-coded screw wedge is unable to achieve a strong hold on the obstruction, then another colour-coded screw wedge may be chosen to improve engagement and successful removal.
The most widely used device to remove a broken instrument is to utilize piezoelectric ultrasonic technology and specific ultrasonic instruments. An ultrasonic unit should be set within the lower power settings, and ultrasonic instruments should have a contra-angled design to provide good access and visualization when using the dental operating microscope. Various working tips and dimensions are available and selected whereby the length of the ultrasonic instrument will reach the broken obstruction and its diameter will passively fit affording a favourable line of sight within the canal. Troughing tips such as diamond-coated CPR 3D (15 mm), 4D (20 mm) and 5D (25 mm) (Spartan CPR instruments, Fenton, Missouri), BUC3 (Spartan instruments) and titanium CPR 6 (red 20 mm), CPR 7 (blue 24 mm) and CPR 8 (green 27 mm) are recommended. CPR 3D–5D are active along the sides of the instrument, whereas CPR 6–8 are end cutting and only active at the tips.
To facilitate this straight-line access to the head of the broken instrument, preparation of a ‘staging platform’ may be necessary. The tip of this ultrasonic instrument is placed in close contact against the obstruction and typically activated within the lower power settings. The clinician should always work at the lowest power setting that will allow for efficiency and safety without the risk of further mishap. The main risks when using ultrasonic instruments deep in the canal are excessive heat generation leading to damage to the attachment apparatus, guttering of root dentine resulting in an increased perforation risk and further separation of the fragment reducing the possibility of retrieval or ultimately fracture of the ultrasonic instrument itself. Ultrasonic preparation below the orifice is conducted dry so the clinician has constant visualization of the energized tip against the broken instrument. Ultrasonic tips have been manufactured with a coating of zirconium nitride (ProUltra ultrasonic instruments; Dentsply, Tulsa, Oklahoma) specifically to function dry. To maintain vision, the dental assistant can direct a continuous stream of air to blow out dentinal dust and use irrigation from time to time. Irrigation not only allows removal of dentine dust that is created thereby improving visibility but also ensures that any potential heat that is generated is dissipated preventing any harmful effects to the surrounding attachment apparatus.