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 [6]. 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 [7]. 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 % [11]. 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).

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 [36]. 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 [48]. 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].