Factors affecting intracanal instrument fracture
Skill, proficiency, judgment
Root canal anatomy
Manufacturing process and errors
Motors operating parameters
Reuse and sterilization
2.2 Operator-Related Factors
Just like many other dental procedures, RCT involves a series of delicate and meticulous manipulations requiring adequate training and dexterity; preparation of root canals is perhaps one of the most technically demanding phases, so it doesn’t come as a surprise that factors pertaining to the operator’s skill and proficiency have been ranked as the most important among those contributing to instrument fracture (Parashos et al. 2004; Cheung 2009).
Practitioners need to choose from a constantly expanding variety of instruments, each one having its own design and mechanical properties and being accompanied by its own guidelines for use; this process can already create some confusion. Once the choice has been made, the clinician needs to become familiar with the instruments, their specific mode of use, and the manufacturer’s recommendations. For example, switching from hand instrumentation by SS files to rotary instrumentation by NiTi files can be rather challenging; NiTi files provide less tactile feedback regarding the morphology of the canal, so a different kind of awareness is required.
Proper in vitro training is necessary in order to bridge this gap (Yared et al. 2001, 2002; McGuigan et al. 2013). Despite wide variability among clinicians, it appears that the handling of instruments is characteristic for each clinician (Regan et al. 2000) so it could be modified through training. Avoiding aggressive penetration in the root canal by applying too much apically directed force on the instrument (Saber 2008), sensing when a rotary instrument is about to bind inside a root canal so that it is withdrawn before torsional overload occurs, and recognizing the stress applied to the instruments during preparation of very curved root canals that could lead to a fatigue failure are skills that can be developed through practicing on extracted teeth and fine-tuned through the gradual accumulation of clinical experience. Even so, a clinician’s performance may still vary to some extent over time depending on workload and physical fatigue (Briseno et al. 1993). Finally, the operator has to develop his/her judgment in order to discard an instrument that shows a dubious defect or that has been used in a difficult-to-prepare root canal.
2.3 Anatomy-Related Factors
2.3.1 Access Cavity
The definition of an “adequate” access cavity has undergone several changes throughout the years. A completed access cavity should still allow unobstructed visual access to all root canals and act as a funnel to guide the instruments into the canal, straight to the apex, or to the point of first curvature (Peters 2008). Interference by the cavity walls or by unremoved dentin shoulders in the coronal third of the root canal can increase the stress imposed on the instruments during preparation by increasing the number and severity of curvatures that must be negotiated (iatrogenic S curve) (Roda and Gettleman 2016); this could lead to instrument failure (Figs. 2.1, 2.2, and 2.3). Conversely, expanding the access cavity beyond the confines of the pulp chamber could also hinder the entrance of files into the root canals and lead to accidental bending of the tips.
Fractured instrument in the mesial root of a mandibular molar due to inadequate access cavity preparation. Note the presence of pulp chamber roof (short arrow) and insufficient shoulder removal (long arrow) that impeded straight-line access to the coronal third of the canal
Improper access cavity through a mesial or distal carious lesion. Instruments penetrating through such lesions cannot follow a straight-line path to the apex, which may eventually result in a variety of iatrogenic errors, including instrument fracture
(a) Two fractured instruments, one in each maxillary central incisor, were identified in the preoperative radiograph. The instruments fractured possibly due to incorrect access cavity preparation through existing carious lesions. (b) Following conventional access cavity preparation, the fragments were retrieved and RCT was completed. (c, d) Three- and twelve-month recall radiographs revealed uneventful healing
Nowadays, the extensive use of the dental operating microscope that provides superior magnification and illumination has facilitated more conservative access cavities specifically designed for each case according to the pulp chamber morphology in an effort to conserve as much tooth structure as possible. The extensive occlusal tapering of the cavity wall circumferentially has been replaced by selective tapering of the cavity walls only where necessary, depending on the location of the root canal orifices and the direction and shape of the canals (Peters 2008). Taken to its extremes, this trend has led to the concept of minimally invasive access cavities which advocates removal of only a minimum amount of hard dental tissue (Gluskin et al. 2014; Krishan et al. 2014; Eaton et al. 2015; Moore et al. 2016), even if subsequent treatment procedures become far more challenging. Nevertheless, anecdotal evidence indicates that such miniature access cavities do not seem to increase the chance of instrument fracture, at least when treatments are performed under the microscope by experienced and skillful clinicians.
SS instruments possess several advantages regarding their placement in the root canal as compared to NiTi files that require considerably more attention to gaining straight-line access. SS files can be pre-bent enabling their introduction into difficult-to-access canals; with the exception of controlled-memory files (Coltene Endo 2014), NiTi instruments are very difficult to pre-bent accurately. In addition, stiff hand-operated SS files also provide superior tactile feedback regarding obstacles as opposed to the flexible NiTi files that are usually attached to a handpiece.
2.3.2 Root Canal Anatomy
The risk of instrument failure seems to increase in cases with complex root canal anatomy (Peters et al. 2003). Fractures appear more often in molars than premolars or anterior teeth (Iqbal et al. 2006; Wu et al. 2011; Ungerechts et al. 2014; Wang et al. 2014) and also in the mesiobuccal root canal of maxillary and mandibular molars (Iqbal et al. 2006; Wu et al. 2011) than in other root canals. These findings could be explained by the overall morphological complexity of the molar root canal system and the existence of multiple canals within each tooth, but the primary reason is most likely the curvature of these root canals.
The curvature of a root canal is described by its angle and radius (Pruett et al. 1997); the wider the angle and the smaller the radius, the more abrupt the curvature. These two parameters can vary independently of each other, so it is possible that two root canals may have the same angle but very different radii of curvature or the opposite (Fig. 2.4). In addition to the shear stress applied to the instrument during preparation of any root canal, a bending stress is concurrently applied inside a curved root canal. As the file rotates, it undergoes repeated cycles of tension and compression, with tension occurring near the outer curved surface and compression near the inner. This repeated cyclic loading may result in crack initiation and eventually in fracture (Pruett et al. 1997; Cheung 2009).
Ex vivo studies have suggested that root canal curvature may increase the failure rate of rotary NiTi instruments (Li et al. 2002, Zelada et al. 2002, Martin et al. 2003, Di Fiore et al. 2006, Kosti et al. 2011) due to both torsional overload and cyclic fatigue (Pruett et al. 1997; Zelada et al. 2002; Kosti et al. 2011), and clinical studies have corroborated these findings (Wu et al. 2011; Wang et al. 2014). The risk of fracture seems to increase as the angle increases, especially beyond 30° (Zelada et al. 2002, Martin et al. 2003, Kitchens et al. 2007), and also as the radius decreases (Haikel et al. 1999; Booth et al. 2003; Patino et al. 2005), and it appears that the radius has a more pronounced effect on this process.
Moreover, an early curvature in the coronal or middle third of the root canal is more likely to lead to failure compared to an apical curvature (Peters and Paque 2010; Lopes et al. 2013) because the diameter of the instrument at the area where flexural fatigue is concentrated (point of maximum flexure) is larger in the former two cases. This is consistent with the authors’ anecdotal observation that NiTi instruments seem to fracture more easily when the tip binds in an acutely curved root canal compared to a straight one. Therefore, it is widely recommended that instruments should not be held at a static position inside a curved root canal but should rather be moved continuously in an axial direction in order to avoid concentrating the flexural fatigue on a specific part of the instrument (Gambarra-Soares et al. 2013). Furthermore, instruments should be discarded after a single use in very complex, calcified, or sharply curved canals.
2.4 Instrument-Related Factors
Raw materials, design, and manufacturing process can have a significant impact on instrument fracture (Alapati et al. 2005; McSpadden 2007). A noteworthy example was described several decades ago when a large number of alarmed dentists complained about fracturing of SS reamers of a certain size manufactured by a single company. These incidents were attributed to manufacturing errors (Lilley and Smith 1966) that were subsequently corrected.
Early studies have provided some support to the widespread notion that rotary NiTi instruments seem to fracture more often than hand SS instruments during clinical use (Iqbal et al. 2006). Arguably, manufacturing of NiTi instruments is much more complicated compared to that of SS instruments (Thompson 2000), and manufacturers continuously explore metallurgical modifications to the NiTi alloy, new instrument designs and additional treatments in an ongoing effort to improve the material properties, minimize inherent defects, and increase the instrument resistance to permanent distortion or fracture; still, details about proprietary manufacturing methods are rarely revealed.
Owing to the shape memory of the NiTi alloy , most such instruments are milled rather than twisted (Shen et al. 2013a), a process that allows creation of complex shapes through computer-aided design and manufacturing (CAD-CAM) technology (Thompson 2000) but that can also result in surface imperfections such as milling grooves, cracks, pits, and regions of metal rollover (Fig. 2.5) (Marsicovetere et al. 1996; Eggert et al. 1999; Kuhn et al. 2001; Tripi et al. 2001; Martins et al. 2002; Alapati et al. 2003, 2004, 2005; Valois et al. 2005; Alexandrou et al. 2006a, b; Chianello et al. 2008). It has been hypothesized that these irregularities may render the instruments more prone to fracture (Alapati et al. 2003) because they could act as stress concentration points and enable the initiation of cracks; propagation of these cracks requires less stress and could eventually lead to failure (Sawaguchi et al. 2003; McSpadden 2007). Several methods, such as implantation of argon, boron, or nitrogen ions, thermal nitridation, plasma immersion, deep dry cryogenic treatment, and electropolishing, have been applied to reduce these surface imperfections and consequently improve the resistance of instruments to failure (Anderson et al. 2007; Cheung et al. 2007a; Condorelli et al. 2010; Praisarnti et al. 2010), but the results are inconclusive in most cases [for an extensive review, see Gutmann and Gao (2012)].
Metal rollover at the edge of an unused Profile NiTi instrument (Dentsply Maillefer, Ballaigues, Switzerland). (Magnification ×100) (Courtesy Dr. S. Zinelis)
Rather than applying surface modifications on the milled instruments, additional thermomechanical processing of either the raw NiTi alloy or the completed instruments (Gambarini et al. 2008; Johnson et al. 2008; Larsen et al. 2009; Gao et al. 2012; Shen et al. 2013a, b; Zhao et al. 2013, 2016; Plotino et al. 2014; Capar et al. 2015) seems to be more effective and appears to increase the flexibility of the files and their fatigue resistance (Zinelis et al. 2007; Plotino et al. 2014, 2017; Kaval et al. 2016). However, it should be noted that, in general, more flexible NiTi files are also considered less resistant to torsional loading (Peters and Paque 2010; Shen et al. 2013a).
Some issues may also arise from the quality of the raw material (NiTi alloy). Oxide particles may be incorporated into the alloy during production, and later, during stress application, they could serve as nucleating sites for micro-voids that may be related to the failure process (Alapati et al. 2005). The relative concentration of these particles may indicate the metallurgical quality of the alloy (Alapati et al. 2005).
The cross-sectional area of an instrument could also affect instrument fracture (McSpadden 2007). This area is determined by a number of other parameters, including the size and taper of the instrument and its specific design (Schäfer et al. 2003; Parashos et al. 2004). Increasing the cross-sectional area by either increasing the size or the taper will increase the resistance to torsional failure (Yared et al. 2003, Guilford et al. 2005, Ullmann and Peters 2005), but it will concurrently decrease the resistance to cyclic fatigue (Haikel et al. 1999, Gambarini 2001c, Hübscher et al. 2003, Ullmann and Peters 2005, Plotino et al. 2006, Kitchens et al. 2007, Peters and Paque 2010), although indications to the contrary have also been reported (Hilfer et al. 2011). In the absence of definite evidence about the primary cause of instrument fracture in vivo (torsional overload, flexural fatigue, or a combination of both), it is noteworthy that smaller files seem to fracture more frequently during clinical use (Inan and Gonulol 2009).
The instrument design can further reduce the cross-sectional area of an instrument by increasing the number or depth of the flutes (Schäfer and Tepel 2001, McSpadden 2007); deeper flutes seem to facilitate stress concentration (Xu et al. 2006), but the shank-to-flute ratio (Fig. 2.6) does not seem to be a contributing factor in the occurrence of fractures (Biz and Figueiredo 2004). Abrupt variations in the cross-sectional shape could also serve as stress concentration points and may promote crack initiation (Xu et al. 2006, McSpadden 2007). Finally, wide metal areas coming in contact with the dentinal wall (e.g., radial lands) (Fig. 2.7) increase the friction during use (Haikel et al. 1999, Xu et al. 2006) and could also increase the risk of failure.
Longitudinal sections of different files depicting the width of the shank (between the blue lines) in comparison with the flute depth (between the blue and red line on either side). (a) Smaller shank-to-flute ratio. (b) Larger shank-to-flute ratio (magnification ×110) (Courtesy Dr. S. Zinelis)
Cross section of a rotary NiTi file depicting wide metal areas that come in contact with the dentinal wall during instrumentation (radial lands)
Even if original files are manufactured according to the highest quality standards by well-established companies, it is prudent to examine all new instruments under magnification for gross manufacturing defects prior to the first use. This precaution is also required due to the circulation of counterfeit instruments resembling the original files only in macroscopic appearance (Figs. 2.8 and 2.9). Counterfeit instruments seem to have more variations in their design and shape and also more surface imperfections than original ones (Tsakiris 2016), and their use should be avoided.
Unused counterfeit (top) and original Protaper Universal F3 files (Dentsply Maillefer, Ballaigues, Switzerland) (bottom). Despite resemblance, differences in the design and diameter of the cutting part are noticeable (Courtesy Dr. G. Tsakiris)
Differences in design and surface smoothness between unused counterfeit (top row, a, b) and original Protaper Universal F2 files (Dentsply Maillefer, Ballaigues, Switzerland) (bottom row, c, d). A large amount of debris is visible on the counterfeit instrument. Its tip is larger and incorrectly manufactured as active (b), contrary to the original instrument’s tip which is smaller and non-cutting (d) (magnification ×20, ×100) (Courtesy Dr. G. Tsakiris)
2.5 Technique/Use-Related Factors
2.5.1 Motors-Operating Parameters
Nowadays electric motors are almost unanimously recommended over air-driven motors for rotary instrumentation mainly because they can maintain a constant rotational speed and also limit the maximum torque applied to the instruments; both parameters can be easily adjusted by the operator (Fig. 2.10). Air-driven motors lack such precise controls and may be also affected by air-pressure differences. Nevertheless, the instrument fracture rate may be similar for both types of motors (Bortnick et al. 2001).
(a) Electric motor featuring speed and torque control (X-Smart Plus, Courtesy Dentsply Maillefer, Ballaigues, Switzerland). (b) Gear-reduction contra-angle handpiece with predefined torque levels which can be attached to a conventional electric or air-driven motor (Mtwo Direct, VDW, Munich, Germany)
The widespread adoption of electric motors has occurred in parallel with the prevalence of the low-speed low-torque instrumentation concept (Gambarini 2001b). Manufacturers of rotary NiTi files recommend a specific rotational speed, usually in the range from 250 to 600 revolutions per minute (rpm), but its effect on instrument failure is controversial; several studies have found no influence on instrument fracture (Pruett et al. 1997, Yared et al. 2002, Zelada et al. 2002, Herold et al. 2007, Kitchens et al. 2007), while others have reported an increase in fractures with increasing speed (Li et al. 2002, Martın et al. 2003). In addition, fatigue failure seems to occur more often with motor-driven NiTi files compared with the same files used by hand, possibly because handheld files rotate at a much lower speed (Cheung et al. 2007b). Interestingly, even studies that found that cyclic fatigue is unaffected by rotational speed recognize that, since an instrument has a finite fatigue life (number of revolutions to failure), a higher rotational speed should consume this life span in a shorter time (Pruett et al. 1997), although it may also accelerate the preparation of the root canal. The rotational speed may also alter the tactile feedback provided by the instruments. Many canal irregularities can be felt through the instrument at low speed, but higher speed may result in almost total loss of any sensation, at least in vitro (Poulsen et al. 1995). In general, it is advisable to adhere to the manufacturer’s recommendations regarding the rotational speed.
Torque is a less straight forward parameter than rotational speed. It is a measure of the turning force applied to the instrument in order for the instrument to overcome friction and continue rotating. Since electric motors strive to maintain a constant rotational speed, the torque applied to the instrument can vary continuously depending on friction, which is, in turn, determined by the contact area between the instrument blades and dentin (Fig. 2.11) and the handling of the instrument. The contact area is mainly affected by the size, taper, and cross-sectional shape of both the instrument and the root canal; a wider contact area increases friction, so higher torque is necessary in order for a larger instrument to rotate inside a narrow root canal (Kobayashi et al. 1997, Sattapan et al. 2000a). For instance, the contact area increases considerably when instruments of the same taper but of progressively larger size are used consecutively in the same root canal; every subsequent instrument after the first one is subjected to excessive friction and requires much higher driving torque to rotate (a situation called “taper lock ”) (Fig. 2.12) that could lead to a torsional failure. Erroneous handling of instruments such as aggressive insertion of the instrument inside the root canal also increases friction and the required torque. The maximum torque that can be applied is limited by the instrument’s ability to withstand the applied stress without undergoing plastic deformation or fracture (Gambarini 2000, 2001a, b).
Cross section of rotary NiTi files having a large (a, b) or small (c, d) contact area with the root canal wall, which affects friction and the torque needed to drive the instrument
(a) Using instruments of the same taper but of progressively larger size to prepare a root canal results in excessive friction due to the wider contact area with the dentinal wall (“taper lock”) and requires higher driving torque that could lead to a torsional failure. (b) Taper lock can be prevented when sequentially used instruments have different tapers
The maximum torque at failure differs among instruments (Kobayashi et al. 1997, Gambarini 2001a, b), and it increases together with the cross-sectional area of the instrument (Yared et al. 2003; Guilford et al. 2005; Ullmann and Peters 2005); larger files can withstand higher torque without fracturing. Therefore, the applied torque should be always maintained within the narrow range that allows the instrument to rotate and cut dentin without exceeding its own plastic deformation or fracture limit (Gambarini 2000); this range is difficult to determine clinically. Manufacturers typically provide the proper maximum torque value for each instrument (Gambarini 2001a). This value is usually lower for the smaller and less tapered instruments and higher for the bigger and more tapered ones (Gambarini 2001a), which means that smaller instruments should be used taking special care not to force them aggressively inside the root canal. In addition, the recommended values refer to unused instruments and may need to be reduced for reused instruments (Gambarini 2001a).
Torque control electric motors allow the operator to determine a maximum torque value to be applied to the instrument during rotation; upon exceeding this value, the motor stops and usually reverses the rotation (auto-reverse) to disengage the instrument from dentin. Obviously, different torque limits should be used for each instrument, according to the manufacturer’s recommendations (Kobayashi et al. 1997, Gambarini 2001a, b). Nevertheless, it remains unclear whether low-torque motors are able to prevent or even reduce instrument fractures. Some studies have reported benefits for both experienced (Gambarini 2001b) and inexperienced operators such as students and dentists at their initial learning phase (Yared and Kulkarni 2002), while others found no improvement compared to high-torque air-driven motors (Bortnick et al. 2001). Just like lowering the speed, low-torque instrumentation may also improve the tactile feedback, but it could also reduce the instrument’s cutting efficiency to some extent and hinder its advance in the root canal; this might occasionally mislead an inexperienced operator to force the instrument which could result in locking, deformation, or even fracture (Yared et al. 2002).
Motor-driven NiTi instruments were initially used only in continuous rotation, contrary to the earlier reciprocating SS instruments that were introduced more than 60 years ago (Frank 1967, Klayman and Brilliant 1975, Hülsmann et al. 2005). The idea of reciprocation was reintroduced by Yared (2008) who proposed root canal preparation using only a very small hand instrument and a single reciprocating NiTi file. Evidently, reciprocation has evolved a lot since its reintroduction. Nowadays, elaborate electric motors allow for precise and independent setting of the clockwise and counterclockwise angles of reciprocation, and, contrary to the earlier reciprocating SS files, the rotation angle of modern NiTi files in the cutting direction is larger than in the opposite direction, enabling the so-called partial or asymmetrical reciprocation with a rotary effect (Plotino et al. 2015). This motion is believed to prolong the life span of NiTi instruments and their resistance to cyclic fatigue compared to continuous rotation (De-Deus et al. 2010, Varela-Patino et al. 2010, Gavini et al. 2012, Pedulla et al. 2013, Ahn et al. 2016), although the method used to quantify the resistance to cyclic fatigue is markedly different in continuous rotation and in reciprocation and the results may not be directly comparable. The difference between the nominal and actual rotation speed could also affect these results (Fidler 2014).
2.5.2 Instrumentation Technique
The instrumentation technique has an influence on instrument failure (Roland et al. 2002). For instance, hand-operated NiTi files used clinically in a modified balanced force movement seem to fail mainly due to torsional overload, while motor-driven files of the same type appear to fracture mostly because of cyclic fatigue (Cheung et al. 2007b). The crown-down approach has been recommended for the vast majority of rotary NiTi instruments in order to reduce friction and minimize the fracture risk (Peters 2004), even though this may not be necessary for other types of NiTi files that are advocated as “single-length” instruments and should be advanced to working length irrespective of size (Plotino et al. 2007; Ehrhardt et al. 2012). Most currently available reciprocating files are also used in a single-file single-length manner (De-Deus et al. 2013; Rodrigues et al. 2016).
Regarding the technique, light apical pressure, continuous axial movement (pecking motion), and brief use inside the root canal are almost unanimously recommended (Parashos and Messer 2006) in order to prevent torsional overload and prolong the fatigue life (Sattapan et al. 2000a; Li et al. 2002; Rodrigues et al. 2011; Gambarra-Soares et al. 2013). Moreover, the handpiece should not be tilted away from the root canal axis at the orifice in order to avoid increasing friction. In general it is advisable for inexperienced users of a particular system to adhere to the recommended instrument sequence, but files from different systems can be combined in hybrid protocols to cope with individual clinical needs; the latter requires a certain level of expertise.
Due to the non-cutting tip of most NiTi files, it is of particular importance to ensure that the root canal will allow free rotation of the tip even at its narrowest point in order to avoid locking and eventual torsional failure (Sattapan et al. 2000b; Peters 2004). This almost uniform requirement can be met by creating a continuous smooth pathway to the apical terminus of the root canal (glide path) before using the main series of rotary NiTi instruments. A glide path can be prepared by small-size hand SS instruments (Blum et al. 1999; Patino et al. 2005; Lopes et al. 2011) or by specially designed rotary NiTi instruments (Fig. 2.13) (Alves et al. 2012; Lopes et al. 2012; De-Deus et al. 2016; Alovisi et al. 2017). The latter may present some advantages according to some studies (Paleker and van der Vyver 2016; Alovisi et al. 2017) but not according to others (Alves et al. 2012) and still suffer from the typical limitations of NiTi instruments. It has been claimed that reciprocating NiTi files are able to reach working length safely without a previously established glide path and without increasing the instrument failure rate during both in vitro (De-Deus et al. 2013) and clinical use (Rodrigues et al. 2016). However, operators are advised to follow the manufacturers’ recommendations regarding the need for a glide path.
Specially-designed rotary NiTi files for preparation of a glide path (Pathfiles, Dentsply Maillefer, Ballaigues, Switzerland) (Courtesy Dentsply Maillefer)
2.5.3 Reuse and Sterilization
Due to the increased cost of root canal instruments and especially of NiTi files, the question of whether they can be reused is always pertinent. The number of times that a file can be safely used is still a topic of ongoing debate. Manufacturers claim that the only predictable way to prevent failure is by discarding rotary instruments on a regular basis; in some cases, special features are embedded in the NiTi instrument handle to prevent their reuse after sterilization and enforce a single-use policy. However, these recommendations and policies may be influenced to some extent by the commercial interest involved.
Grossman (1981) recommended using small hand SS instruments no more than twice. More recently, single use of all rotary NiTi instruments has been suggested as a precaution (Pruett et al. 1997; Arens et al. 2003), while others advocate this strict rule only concerning the smaller files (Haapasalo and Shen 2013), possibly because any defects may be more difficult to detect. A survey found that discarding after a certain number of uses is a common practice among both general dentists and endodontists (Madarati et al. 2008), and the type of alloy, the design and size of the instrument, and the case difficulty are parameters frequently taken into account in order to decide when to discard an instrument (Cheung et al. 2005).
The evidence behind these recommendations is conflicting. Prolonged clinical use of NiTi rotary files seems to reduce their resistance to cyclic fatigue during subsequent in vitro tests (Gambarini 2001b, c; Bahia and Buono 2005; Plotino et al. 2006), so larger files should be discarded earlier than smaller ones when preparing curved root canals because their resistance to cyclic fatigue is lower (Bahia and Buono 2005). However, instrument failure is a complex and multifactorial problem, and it seems impossible to predict when an instrument will fracture during clinical use based on simplified in vitro tests. The number of uses before failure varies widely (Parashos et al. 2004; Kosti et al. 2011), and fracture can occur even during the first use in the hands of experienced clinicians (Arens et al. 2003). In addition, instruments may fracture following clinical use for fewer times than identical instruments that present no defects or fracture. Therefore, it appears that other variables such as the operator proficiency and the root canal anatomy may be far more significant determinants of the instrument fracture rate (Parashos et al. 2004).
This apparent discrepancy could be explained by the fact that NiTi instrument failure during clinical use seems to occur because of a single overloading event (e.g., inadvertent locking in the root canal) rather than a fatigue accumulation process (Spanaki-Voreadi et al. 2006); in vitro failures during preparation of root canals seem to occur by a similar mechanism (Kosti et al. 2011). Interestingly, even authors concluding that files should not be reused because their resistance to cyclic fatigue is reduced actually managed to prepare up to ten clinical cases using the same set of instruments without any intracanal fracture (Gambarini 2001c). Furthermore, contrary to earlier views about the effect of repetitive loading on the NiTi instrument fatigue life (Sattapan et al. 2000b), more recent studies found that mild torsional preloading (not causing permanent deformation) can actually improve both the torsional strength (Oh et al. 2017) and the resistance to cyclic fatigue during subsequent loading (Cheung et al. 2013); this effect could reduce the fracture risk during clinical use, but the result may differ among various types of files (Ha et al. 2015). Therefore, taking all evidence into account, multiple uses of NiTi instruments are clinically acceptable from a mechanical point of view (Parashos et al. 2004), but it is impossible to recommend a safe number of uses. These findings are at variance to the failure of SS instruments that seems to occur mostly because of fatigue accumulation, during both in vitro (Kosti et al. 2004) and clinical use (Zinelis and Margelos 2002), and justifies frequent discarding.