Incidence of instrument fracture

The incidence of instrument fracture widely ranges from 0.4% to 23% (Alapati et al., 2005; Al-Fouzan, 2003; Iqbal et al., 2006; Pettiette et al., 2002; Ramirez-Salomon et al., 1997; Schafer et al., 2004; Spili et al., 2005). Notably, the incidence of instrument fracture amongst endodontists was as high as 5% (Alapati et al., 2005; Parashosh et al., 2004; Shen et al., 2009). Additionally, 93.6% and 79.5% of endodontists and general practitioners, respectively, have experienced instrument fracture (Madarati et al., 2008). This could be attributed to endodontists treating more challenging cases in molars compared with non-endodontists. Contrastingly, other studies have demonstrated that the incidence of instrument fracture amongst dentists with low experience is higher than that amongst dentists with higher experience, including specialists (Ehrhardt et al., 2012; Fiore et al., 2006; Iqbal et al., 2006; Knowles et al., 2006; Ramirez-Salomon et al., 1997; Shen et al., 2009; Tzanetakis et al., 2008; Wolcott et al., 2006).

More instruments are fractured in molars, especially mandibular molars, than in anterior teeth (Tzanetakis et al., 2008), which is due to several factors, including canal accessibility, root canal diameter and root canal curvature (Cujé et al., 2010; Nevares et al., 2012; Shen et al., 2004). The incidence of instrument fracture is significantly greater (almost thrice) in molars than in premolars (Iqbal et al., 2006). Furthermore, the prevalence of instrument fracture in the apical third (52.5%) was found to be significantly higher than that in the coronal third (12.5%) and middle third (27.5%) of the canal (Tzanetakis et al., 2008).

Instrument fracture is influenced by the canal anatomy; moreover, it is determined at the discretion of the treating clinician. There is a positive correlation between anatomical challenges in root canals and the incidence of instrument fracture since both the radius and angle of the canal curvature are the main determinants of the fatigue resistance of NiTi files (Alghamdi et al., 2020; Pruett et al., 1997; Shen et al., 2004). Moreover, a recent study reported decreased fatigue resistance for all instruments in canals with middle or coronally located curvatures than in canals with apical curvatures (Alghamdi et al., 2020).

Amongst specialists, reciprocating file systems were found to have a lower incidence of instrument fracture than rotary files systems (Ahn et al., 2016; Bueno et al., 2017; Cunha et al., 2014); moreover, compared with rotary motion, reciprocating motion has been proven to be safer against cyclic fatigue and torsion fracture (Ahn et al., 2016; Kim et al., 2012). Accordingly, instruments with reciprocating motions have an extended lifespan (Pirani et al., 2014).

Martensite-phase NiTi files at body temperature have a higher fatigue resistance than austenite-phase ones (Alghamdi et al., 2020; Shen et al., 2011; Vasconcelos et al., 2016). Increased temperature decreases the resistance to cyclic fatigue failure for all types of NiTi instruments (Shen et al., 2018). Additionally, Pedullà et al. reported that instruments with a 0.04 taper in the martensitic phase had a higher cyclic fatigue resistance than those with a 0.06 taper (Pedullà et al., 2020), which indicates that the extent of tapering is negatively correlated with the resistance to cyclic fatigue. From a dimensional perspective, the diameter of similarly designed files is negatively correlated with the fatigue life (Schneider, 1971). Therefore, a file with a large diameter and extensive taper is less resistant to cyclic fatigue than files with lesser values for these parameters, especially around the curve. Shen et al. reported similar torsional fatigue resistance of the NiTi instruments in both martensitic and austenitic phases; contrastingly, martensite-phase NiTi instruments showed significantly higher cyclic fatigue resistance than austenite-phase ones (Shen et al., 2018). Taken together, the incidence of instrument fracture can be greatly reduced by using a file with a small diameter and less taper in the martensitic phase at body temperature to shape the canal, especially around a severe curve (Hieawy et al., 2015; Pedullà et al., 2020).

Factors affecting instrument retrieval

The dental operating microscope (DOM) along with small-diameter ultrasonic tips allow minimally invasive root canal preparations and facilitates the safe removal of fractured instruments (Cujé et al., 2010; Fu et al., 2011; Nevares et al., 2012; Suter et al., 2005; Ward et al., 2003; Wong & Cho, 1997). Here, visualization and accessibility to the fractured instruments are crucial factors for successful instrument retrieval (Nevares et al., 2012). Accordingly, predictive factors for instrument retrieval include instrument retrieval protocols (Carr, 1992; Eleazer & O’Connor, 1999; Gettleman et al., 1991; Pierre & Reit, 2003; Roig-Greene, 1983; Ruddle, 2004; Wong & Cho, 1997); location, visibility, size, length, and type of the fractured instrument; root canal curvature; curvature radius; and operator experience and fatigue (Alomairy, 2009; Cujé et al., 2010; Gencoglu & Helvacioglu, 2009; Pruett et al., 1997; Rahimi & Parashos, 2009; Shen et al., 2004; Souter & Messer, 2005; Terauchi, 2012; Terauchi et al., 2021; Ward et al., 2003).

Instrument retrieval protocol using ultrasonics

After completion of the preparation phase, it is important to confirm that the outer canal wall is smooth until the coronal extent with no overhangs blocking the fractured instrument from retrieval. In case there is an overhang on the outer wall, it should be removed using either a flexible NiTi rotary file or an ultrasonic tip. Before attempting to retrieve the fractured instrument with ultrasonics, the canal is dried using air from the Stropko irrigator (DCI International), which can lead to ejection of the fractured instrument. This is because the fractured instrument is already disengaged from the canal walls during the preparation phase and the blown air that bounces back from the bottom can cause the loosened fractured instrument to float from the canal walls and exit from the root canal.

First, the canal is filled with soybean oil or EDTA when the canal curvature is >30° or <30°, respectively, to facilitate instrument retrieval with ultrasonics. Most fractured instruments (94%) <4.6 mm can be removed solely with ultrasonics within 10 s in the retrieval phase (Terauchi et al., 2021).

Subsequently, instrument retrieval attempts are performed in wet conditions by filling the canal with an appropriate fluid, including EDTA and soybean oil, until the cavosurface (at least to half the pulp chamber level and ‘not just in the root canal’) in order to take advantage of cavitation and acoustic streaming in facilitating instrument removal using ultrasonics. If only the root canal is filled with fluid, the fractured instrument may not exit from the canal; instead, it may float within the root canal space during ultrasonic activation. Next, the ultrasonic tip is placed in the space and is continuously activated at a power setting 10%–20% higher than that used in the preparation phase, with short up-and-down strokes being performed on the inner curve until the fractured instrument is removed, which is usually achieved in 10 s (Terauchi et al., 2021; Figure 7). For ultrasonic removal attempts, the power setting can be safely increased since the ultrasonic tip is more resistant to fracture in wet conditions than in dry conditions. Moreover, the depth of ultrasonic-activated fluid increases proportionally to the applied power setting for removing debris apical to the ultrasonic tip in the canal (Malki et al., 2012). Moreover, the ultrasonic tip should be placed and activated on the inner wall as far away as possible from the fractured instrument, which provides space for ejection through the canal opening. Four reasons could contribute to unsuccessful instrument retrieval using ultrasonics within >10 s. The most typical reason is that the dancing fractured instrument might be stuck between the outer canal wall and the vibrating ultrasonic tip with a progressive taper or a large diameter (Figure 8). The second reason could be the space between the outer wall and ultrasonic tip being smaller than the fractured instrument diameter, which leads to no open space for the fractured instrument to escape (Figure 9). The third reason could be that the fractured instrument might relocate into a more radicular direction after being pushed by the vibrating ultrasonic tip from the top or outer wall (Figure 10). Finally, the fourth possible reason is the amount of fluid filled in the canal space being insufficient to create acoustic streaming and cavitation for removing the fractured instrument from the canal.

In case the fractured instrument is not retrieved within 10 s using the aforementioned protocols or it is initially longer than 5.7 mm, other retrieval techniques using the loop device or XP Shaper (XPS) rotary instrument (FKG Dentaire SA) should be attempted since it is difficult or time-consuming to keep using ultrasonics (Terauchi et al., 2021).

Fluid types used for ultrasonic instrument retrieval

The ability of a fluid to lubricate the fractured instrument in the root canal space and flush it away with the aid of ultrasonic-generated acoustic streaming and cavitation generated is mainly dependent on the fluid characteristics, including lubricity, viscosity and surface tension. Viscosity is defined as a fluid's resistance to flow and shear. The hydrodynamic force in high-viscosity fluids necessitates a high ultrasonic power setting to generate high pressure for dynamic impact on the fractured instrument since high-viscosity fluids absorb weak oscillatory shockwaves. In addition, aggressive oscillatory shockwaves generated by the high ultrasonic activation with high-viscosity fluids can create more powerful hydrodynamic pressure for pushing the fractured instrument compared with those with low-viscosity fluids. However, ultrasonic activation at a high power setting can cause ultrasonic tip breakage. Generally, fluids with high viscosity and surface tension reduce both acoustic streaming and cavitation, but increase lubricity, and vice versa. Water is a low-viscosity fluid with high surface tension (72 mN/m at 20°C), vegetable oil (e.g., soybean or olive oil) is a medium-viscosity fluid with low surface tension (23–32 mN/m at 20°C), syrup is a high-viscosity fluid with high surface tension, and ethanol is a low-viscosity fluid with low surface tension. Corn or soybean oil has a lower viscosity than olive oil (Sahasrabudhe et al., 2017). Generally, the fluid temperature is negatively correlated with the surface tension. Taking these factors into consideration, fluid with medium viscosity and low surface tension, including soybean/corn oil, may be ideal for use with a high ultrasonic power setting for fractured instrument retrieval, especially for canal curvature >30°; contrastingly, EDTA can be used with a medium power setting for canal curvature <30°. Additionally, the size of ultrasonic tips is negatively correlated with the generated acoustic streaming, with the resulting velocity being positively correlated with the power setting (Ahmad et al., 1987). Cavitation refers to the formation of ultrasonic-generated vapor-containing bubbles within a fluid. Forced bubble collapse causes implosions that cause an impact on surfaces, which produce shear forces that can dislodge the fractured instrument from the canal. Fluids with low surface tension result in more ultrasonic-generated oscillatory shockwaves on the fractured instrument, which make it more vulnerable to dislodgement and ejection by the continuous pressure waves of acoustic streaming and cavitation from the trapped space.

Accordingly, EDTA, which has a low surface tension, is expected to facilitate immediate instrument removal from a straight canal given that it also removes hard-tissue debris produced during ultrasonic activation to clear out the way. Contrastingly, the loosened fractured instrument may not be ejected from a severe curve (>30°) in the presence of low-viscous EDTA, which may insufficiently lubricate the fractured instrument. The use of soybean oil, which has medium viscosity and low surface tension, for instrument removal from a severe curve using ultrasonics could enhance lubrication. However, compared with EDTA, soybean oil results in slower instrument removal from a curve <30° given its medium viscosity. Nonetheless, both fluid types can allow cavitation and acoustic streaming as well as decrease friction generated between the fractured instrument and the canal wall to facilitate instrument retrieval. Therefore, using a mixture of EDTA and soybean oil may be an alternative for instrument removal from a moderate curve (30°–20°).

Instrument retrieval protocol using the loop system

The XPS should be used to retrieve fractured instruments leaning against the outer wall with no space for loop placement over the fractured instrument. Otherwise, the loop is used to remove fractured instruments >4.5 mm or in case of unsuccessful instrument retrieval using ultrasonics within >10 s. Unlike ultrasonics and the XP-Endo Shaper, which are used in wet conditions, the loop is used in dry conditions since it should be visualized throughout the retrieval procedure. Additionally, loop systems, including EndoCowboy (Köhrer Medical Engineering), BTR Pen (CERKAMED) and Yoshi Loop (DELabs), can be used to engage and remove long loosened fractured instruments through forces directed more coronally.

Loop placement in the canal requires a space with a diameter ≥0.4 mm; moreover, the coronal portion of the fractured instrument should be peripherally exposed by a depth ≥0.7 mm deep for grasping by the loop system. A #40 plugger is placed into the same space to measure the area created adjacent to the fractured instrument. This allows the introduction of the loop, which has a slightly smaller diameter than the #40 plugger, into the canal for engagement. Next, the loop size should be adjusted to fit the coronal diameter of the fractured instrument using an endodontic explorer, including a DG16 endodontic explorer (Hu-Friedy). Specifically, the tip of the DG16 endodontic explorer is placed in the loop, followed by tightening the loop around it. The apical portion of the DG16 is used to decrease and increase the loop size and coronal portion, respectively. Next, the loop is bent to 45° instead of 90° to facilitate loop placement over the fractured instrument since it occupies more room. Subsequently, the loop is inserted into the canal and pushed back to 90° upon the fractured instrument. The loop is then tightened around the loosened fractured instrument and pulled to smoothly retrieve it from the canal. In case any resistance is felt, the loop is slowly and gently pulled in different directions with a swaying motion until it is lifted from the canal. It should never be forced in a vertical direction to prevent breakage since the instrument retrieval is solely dependent on the pulling direction.

Instrument retrieval protocol using the XP-endo Shaper

The XP-endo Shaper (FKG) is a unique rotary system made of a novel heat-treated MaxWire alloy with a size 15 booster tip that allows expansion of its core from size 27/.01 to a larger taper of ≥30/.04 at body temperature (Azim et al., 2017). The XP-endo Shaper allows instrument retrieval given the expandability of the file whilst being rotated clockwise inside the canal. It is straight in the martensitic phase at room temperature and expands to a snake-like shape with a taper of ≥.04 upon exposure to intracanal (body) temperature due to transformation to the austenitic phase. This allows the instrument to extensively touch and scrape the canal walls or possibly scratch and spin the loosened fractured instrument if there is a thin space along the fractured instrument inside the root canal. Clockwise rotation of the XP-endo Shaper in a swirling motion on the side of the disengaged fractured instrument causes counterclockwise rotation of the fractured instrument. This could eventually facilitate an unscrewing movement of the fractured instrument in the coronal direction (Figure 11). This movement can be facilitated by using vegetable or silicone oil as a lubricant, which also prevents breakage of the XP-endo Shaper. A limitation of the XP-endo Shaper is its low resistance to torsional fatigue failure compared with conventional NiTi rotary instruments due to its smaller taper (Silva et al., 2018). Accordingly, the XP-endo Shaper should be rotated at a relatively high speed (>2000 rpm) with 3–5 mm up/down strokes along the fractured instrument to mitigate torsional and cyclic fatigue. Moreover, the short up/down strokes yield vertical movement to facilitate instrument retrieval. Recent studies on the effect of rotational speed have demonstrated that the XP-endo Shaper allows more efficient and safer advancement of the instrumentation within the small canal space when rotated at 3000 rpm than when rotated at 1000 rpm, where it follows the small root canal space around the curve into the space created between the fractured instrument and the inner wall without major adverse effects (Azim et al., 2018; Webber et al., 2020). Taken together, the XP-endo Shaper can be used for instrument removal with rotation at a high speed in the presence of an appropriate oil, especially for >5.7 mm fractured instruments leaning against the outer canal wall without space for loop placement.

Instrument retrieval protocol using the XP-endo Finisher

The XP-endo Finisher (FKG Dentaire) is a size #25 nontapered instrument that does not damage or alter the original root canal anatomy. It is mainly used to clean the root canal by scraping irregularities on the canal walls given its high flexibility as well as its ability to expand up to a 6-mm diameter and adapt to the root canal three-dimensionally (Azim et al., 2016; Leoni et al., 2017). This is facilitated by its change to a spoon shape after exposure to intracanal temperature following transformation to the austenitic phase, as previously described for the XP-endo Shaper. The XP-endo Finisher can effectively remove accumulated hard tissue debris and the smear layer from the root canal system (Elnaghy et al., 2017; Leoni et al., 2017). Moreover, a recent study demonstrated the efficacy of the XP file in removing calcium hydroxide paste from artificial grooves within the apical third of a root canal system (Wigler et al., 2017).

Therefore, the XP-endo Finisher can be used at a high-speed clockwise rotation speed to remove a loosened fractured instrument from a wider canal, especially when the instrument is extruded beyond the large apical foramen into the apical tissues, by braiding it and creating a swirling fluid flow in the coronal direction (Figures 12 and 13). Additionally, the braiding technique can be performed using a Hedström or K-type file to retrieve fractured instruments located deep in the canal that cannot be observed on a DOM. However, successful retrieval using these files is solely dependent on tactile sensation, which makes them unpredictable for instrument retrieval (Shen et al., 2004; Suter et al., 2005). Moreover, braided Hedström files or K-files often detach during removal attempts (Ahmad et al., 1987).

Prognosis of the retained fractured instrument

The presence of fractured instruments in the root canal before completing the cleaning and shaping procedures negatively affects the prognosis since it hinders the cleaning of the canal space apical to the fractured instrument. Moreover, aggressive attempts to remove a fractured instrument from a curved canal without proper diagnosis may cause complications, including excessive dentine sacrifice, secondary instrument fracture, root perforation, or even root fracture (Hashem, 2007; Saunders et al., 2004; Souter & Messer, 2005; Spili et al., 2005). The fractured instrument within or outside the root canal does not necessarily induce postoperative disease. Instead, postoperative conditions result from an increase in the number of surviving bacteria, which become virulent due to favourable environmental changes to promote postoperative periapical disease (Moller et al., 2004). The closer the root canal preparation is to the terminus upon instrument fracture, the better the outcome (Torabinejad & Lemon, 2002). Instrument fracture that occurs in the later stage of canal instrumentation, especially if it is within the apex, has the best prognosis since the canal can be well debrided and is relatively free from microbes (Torabinejad & Lemon, 2002). In case there is no preoperative periapical lesion associated with the root, the fractured instrument does not affect prognosis (Crump & Natkin, 1970) and can be incorporated into the root canal filling materials (Figure 14). If the fractured file can be predictably removed without excessive dentine sacrifice, the prognosis should not be reduced. Taken together, the treatment prognosis is more dependent on removing the microbes rather than the fractured instrument itself. Failure to clean the space apical to the fractured instrument may negatively affect the prognosis similarly to endodontically treated cases with missed canals or ledge formation blocking the debridement of the root canal space apical to the ledge (Cohen & Burns, 2002; Cohen & Hargreaves, 2006; Gutmann & Lovdahl, 2010; Harty et al., 1970; Kapalas & Lambrianidis, 2000; Namazikhah et al., 2000; Nevares et al., 2012; Roig-Greene, 1983). Although cases with a retained instrument in the canal could have a poor prognosis, a systematic review found a favourable prognosis in most cases involving fractured instruments (Panitvisai et al., 2010). The most important prognostic factor for teeth with retained fractured instruments is the presence of preoperative periapical lesion. Specifically, endodontically retreated teeth with preoperative lesions have a poor prognosis when the fractured instruments are retained (Panitvisai et al., 2010; Spili et al., 2005).

Future directions in the removal of fractured instruments

Currently, using a DOM in combination with ultrasonic instruments is highly effective for removing fractured instruments (Cujé et al., 2010; Fu et al., 2011; Nagai et al., 1986; Nehme, 1999; Terauchi et al., 2021; Ward et al., 2003; Wu et al., 2011). However, the use of loops is only effective when the fractured instrument is visible under the DOM. Nonvisible fractured instruments can only be removed if a thin ultrasonic tip can be placed in the space between the inner wall and the nonvisible fractured instrument (length: <4.6 mm). The preparation phase is the most important factor for successful instrument removal since most fractured instruments (94%) shorter than 4.6 mm can be removed using only ultrasonics within 10 s during the retrieval phase after the preparation phase (Terauchi et al., 2021). Nonvisible fractured instruments that are longer than 4.5 mm can be removed using rotary instruments such as the XP-endo Shaper only if it is possible to create a thin space between the canal wall and the fractured instrument using ultrasonics or bypass the fractured instrument. Additionally, a longer fractured instrument requires a longer preparation time, with extensive dentin removal along the inner canal wall for loosening (Terauchi et al., 2021). Moreover, Suter reported that the duration of the retrieval procedure was negatively correlated with the success rate of instrument retrieval (Suter et al., 2005).

Two future directions for instrument removal might mitigate the challenge of removing nonvisible fractured instruments longer than 4.5 mm that are beyond the curve. First, instrument fractures within the root canal might be eliminated due to significant changes in the root canal preparation technique. This may render rotary instruments unnecessary since future irrigation systems, including the GentleWave System (Sonendo, Inc), may play more crucial roles in root canal preparation than conventional instrumentation techniques. The GentleWave System is a multisonic irrigation technology that uses advanced fluid dynamics, acoustic energy and tissue dissolution chemistry to facilitate the cleaning of minimally instrumented canals or those without instruments (Haapasalo et al., 2014). Studies have extensively described this innovative technology and its mechanism of action (Haapasalo et al., 2014; Molina et al., 2015); moreover, it has shown initial treatment success rates as high as 97% at 6- and 12-month follow-ups (Sigurdsson et al., 2018). However, in retreatment, none of the commercially available efficient irrigation techniques, including the GentleWave System, has been shown to completely remove debris and residual obturation materials from the canals; furthermore, there were no significant differences in debridement between the GentleWave System and other irrigation techniques (Wright et al., 2019). Therefore, even the novel systems have a higher debridement efficacy, and further improvements are required until no instrumentation is necessary.

The second possibility is the future development of a novel instrument-removal file system comprised of several rotary files for canal preparation and removal of all types of fractured instruments, including nonvisible long instruments beyond the curve. The initial files could be used for canal preparation to create a small space for fractured instrument removal whilst the secondary files could be introduced into the created small space to actually remove the fractured instrument with clockwise rotation of the instrument-removal file. Ultimately, fractured instrument removal may become as easy as conventional root canal instrumentation, regardless of the visibility. The development of such instrument-removal files may render the use of the DOM and ultrasonics unnecessary. A fractured instrument in a straight root canal is easier to remove than one in a curved curve, which is consistent with the fact that a straight root canal is easier to prepare using rotary instruments than a curved root canal. Nonetheless, ultrasonic may remain necessary for some cases to facilitate the creation of a thin space along the fractured instrument to prevent ledge formation or canal transportation; additionally, they may be used in combination with rotary instruments to remove nonvisible long fractured instruments beyond a severe curve (Figure 15).