Summary

Instrument fracture during root canal preparation is frustrating. Instrument retrieval is even more frustrating and considered more challenging than other endodontic procedures. In general, when nickel titanium (NiTi) instruments fracture, they mostly fracture in the apical one-third or beyond a curve of the canal because of their superelasticity. In addition, the retained instrument fragment hinders further treatment, and thus the outcome of the treatment will be compromised. Although the success rates of instrument retrieval with ultrasonics alone are very high and in the range of 80 to 90 %, attempts at ultrasonic retrieval are deemed to be unpredictable in terms of time and dentine removal as there had been no standardized protocol for instrument retrieval in the past. Hence, in order to make removal of fractured instruments predictable, the instrument retrieval protocol should be performed in a two-step procedure consisting of an initial preparation phase followed by attempts at retrieval based on the individual treatment plan developed to address the specific diagnosis. It is essential to maintain as much tooth structure as possible to prevent root fracture and perforation during the preparation phase. Depending on the clinical scenario of each case, ultrasonics, loops, and NiTi rotary files can be used to remove fractured instruments during the retrieval phase. A recent study has indicated that instrument retrieval following this protocol was predictable and was significantly more successful as well as conservative in dentine removal than other traditional techniques. The prognosis of cases with retained fractured instruments in the canal may not be compromised when there is no preoperative periapical lesion associated with the root.

18.1 Aetiology of Instrument Fracture

18.1.1 Factors Affecting Instrument Fracture

Fracture of an endodontic instrument is often caused by improper use [1, 5]. An instrument is fatigued because of inadequate access in relation to the direction of the root canal, overuse of the instrument, too much apical pressure during instrumentation [6, 7], and the continuous rotation of a large diameter instrument at the same point in a curved canal [8, 9]. Other contributing factors to instrument fracture include operator experience [10, 11], rotational speed [12, 13], canal curvature (radius) [9, 14], instrument design and instrumentation technique used [15, 16], torque setting [17], manufacturing process [18], metallurgical properties of the instrument [19–22], the type of rotational motion (continuous rotation versus reciprocating motion) [23], the type of tooth [24–26], and absence of a glide path [27].

18.1.2 Incidence of Instrument Fracture

The factors mentioned previously make the incidence of instrument fracture very unique within a wide range of 0.4% to 23% [18, 28–32].

More instruments are reported to fracture in molars than anterior teeth, especially in mandibular molars. Several factors affect the high incidence of instrument fracture in molars including accessibility to the canal, diameter of the root canal, and root canal curvature [24–26]. Because of those multiple factors the majority of the practitioners doing root canal treatment (93.6% of endodontists and 79.5% of general practitioners) have been reported to have experienced an instrument fracture [33]. It has also been reported that the incidence of instrument fracture even for endodontists was as high as 5% [10, 18, 34]. The patient must be informed of not only treatment options but also potential complications that may occur when the fractured instrument is retained in the root canal as well as when instrument retrieval procedures are attempted.

18.1.3 Mechanisms for Instrument Fracture

There are two main mechanisms for instrument fracture: cyclic fatigue and torsional failure or a combination of both [35–39]. Cyclic fatigue occurs more frequently in curved canals without previous signs of plastic deformation due to repeated compressive and tensile stresses generated when the file rotates [10]. Cyclic fatigue failure is caused by crack initiation at the surface and transgranular crack growth of the instrument rotating within a curvature [40]. Torsional fatigue occurs when the file tip is locked but the shank of the file is still rotating (driven by the handpiece) [5, 35, 36]. Shear failure is the result of an applied shear moment exceeding the elastic limit of the material, which mainly arises from torsional fatigue [41–43]. Clinically, cyclic fatigue seems to be more prevalent with rotary instruments in curved root canals, whereas torsional failure might happen with hand instruments rotated with apical pressure even in straight canals [44, 45]. Instruments of a smaller dimension are generally more resistant to cyclic fatigue than larger ones [46], whereas instruments of a larger dimension tend to be more resistant to torsional fatigue. In addition, when a nickel titanium (NiTi) instrument is way below the austenitic finish (A_f) temperature, it has more fatigue resistance, whereas a NiTi instrument above the A_f temperature has a higher modulus of elasticity and less fatigue resistance [47, 48]. Hence, instruments of a smaller dimension or NiTi instruments with a high A_f temperature should be used to shape canals with severe curvatures [49]. Interestingly, the use of reciprocating motions is reported to extend the life span of an instrument [50, 51] and increase cyclic fatigue resistance, compared to continuously rotating motions [23]. Motorised instruments should be used in a reciprocating motion if they are expected to be placed under substantial torsional or cyclic stress in a severely curved canal, and that they should be discarded after the single use to prevent instrument fracture.

18.2 Diagnosis and Treatment Planning of Fractured Instruments

18.2.1 Factors Affecting the Success of Instrument Retrieval

The best treatment for instrument retrieval can be predictably provided following proper diagnosis and treatment planning including assessment of risk, prognosis, and expected treatment outcomes.

The success rate for removing fractured files varies widely from 33% to 95% [24, 28, 30, 52, 53] with instrument retrieval time using ultrasonic techniques extending from 3 minutes to over 60 minutes [11, 54, 55]. The variations in the success rates can be due to the range of techniques used for each case, the location of fractured instruments, the diameter of the root canal [26], the degree of canal curvature [54, 56, 57], the radius of canal curvature [9, 54], operator experience [11, 54], operator fatigue, and the length of the retained fractured instrument [58].

It has also been recommended that instrument retrieval attempts should not exceed 45 to 60 minutes because the success rates tends to drop with increased treatment time [59]. The reduced success rate could be related to operator fatigue or procedural accidents including perforation and over-enlargement of the canal, which may predispose the tooth to vertical root fracture.

Visualisation and accessibility to the fractured instrument play a very important role in instrument retrieval [24]. The success rate for removing fractured instruments was reported to be approximately two times greater when it was visible inside the root canal than when it was nonvisible [25]. In addition, the success rate for removal of the instrument located before the canal curvature is high, moderate for those located around the curvature, and low for those located beyond the curvature [14, 60] (Figure 18.3). The success rate is also more favourable when the canal curvature is less and the radius of curvature is greater (>4 mm) [54, 61]. The combination of ultrasonic techniques and microscopes typically improves success rates in instrument retrieval [24, 25, 53, 62]. One study reported that longer instruments took a significantly longer time and instruments in a more severe curvature required a significantly longer time compared to shorter instruments [63].

18.2.2 Diagnostic Examination Using CBCT for Instrument Retrieval

When it comes to accurate diagnosis and treatment planning, cone beam computed tomography (CBCT) is considered to be useful in terms of predictability and accuracy. A study on treatment planning using CBCT concluded that the examiners altered their treatment plans after viewing the CBCT scan in 49.8% of the cases and CBCT imaging directly influenced endodontic retreatment strategies among general dentists and endodontists, which resulted in more predictable outcomes [64]. CBCT also reveals significantly more apical lesions than conventional radiographic systems [65] (Figure 18.4). CBCT images are able to reveal root canal angles, and can define the location of the major foramen three-dimensionally, which is not identifiable with sufficient precision on periapical radiographs [61]. Because of the three-dimensional scan, CBCT can also provide the exact location of the fractured instrument and more accurate and reliable root canal length measurements compared with a gold standard (which was measured with a K-file for the distance between the same reference points as determined in the CBCT measurement). However, there was no significant difference among CBCT measurements obtained at four different voxel sizes (0.080 mm³, 0.125 mm³, 0.160 mm³, and 0.250 mm³ voxel sizes) even though accuracy increased with smaller voxel sizes without statistically significant differences [66]. There was a positive relationship in the use between the dental operating microscope (DOM) and CBCT as a diagnostic method when identifying smaller anatomical spaces without destroying the tooth [67]. Those are the reasons why CBCT is suggested as an auxiliary means of identifying the presence of small canals as well as fractured instruments and diagnosing them. Therefore, the use of CBCT with a smaller voxel size in making an accurate diagnosis is essential for more predictable outcomes in instrument retrieval. Knowing the exact location, the length of the fractured instrument, the canal curvature, and the surrounding anatomical structures before the instrument retrieval procedure is initiated is very time saving and minimally invasive compared to a lot of guess work without the accurate information, which would be time consuming (Figures 18.5 and 18.6).

The diagnostic examination using CBCT includes:

• Location of the fractured instrument
• Length measurement of the fractured portion
• Angles of the canal curvature
• Location of the inner wall curve contacting the fractured instrument
• Possibility of creating a straight-line access to the fractured instrument from the orifice without perforating the canal
• Expected amount of dentine sacrifice during the preparation phase
• Thickness of the canal wall
• Presence of perforation and root fracture
• Presence of periapical lesions associated with the root
• Location of the canal causing the periapical lesions in relation to the intracanal fractured instrument
• Possibilities of iatrogenic accidents when attempting to retrieve the fractured instrument
• Possibilities of nonhealing or healing when the fractured instrument is retained in the root canal in relation to the potential causes of the periapical lesions

Teeth with preoperative periapical lesions have been reported to be significantly less likely to heal than those without preoperative periapical lesions when the fractured instrument was retained [32] (Figure 18.7). The clinician needs to weigh up the advantages and disadvantages of retrieval or retention of the fractured instrument in each individual situation before the treatment is initiated (Figure 18.8).

18.2.3 Treatment Planning for Instrument Retrieval

The treatment plan should be made following diagnostic examination with CBCT. The instrument retrieval procedures must be divided into two parts: the root canal preparation for instrument retrieval and the instrument retrieval attempts.

The goal of the preparation for instrument retrieval is to make it loose or ‘dance’ with ultrasonics. In order to loosen the fractured instrument, it must be pushed from the inner wall with ultrasonics, which will result in shifting the fractured instrument to a more coronal level. If the fractured instrument is pushed from the outer wall in a curve or pushed on the top of the fractured instrument, it will be shifted downwards in an apical direction (Figure 18.9). This is why the location of the inner wall must be identified in advance prior to the preparation. It is also important to know the largest size of the rotary instrument used to enlarge the canal to the fractured instrument by measuring the canal diameter to prevent perforation while preparing the canal. Wilcox [68] reported that canal enlargement of 40 to 50% of the root width increased susceptibility to vertical root fracture (VRF). Therefore, the canal enlargement should be performed within this range. If the fractured instrument is initially not visible under the DOM, you may be able to make it visible by creating straight-line access to it with flexible NiTi rotary instruments in an anticurvature direction. Because the maximum flute diameter of the conventional NiTi rotary instrument used for root canal preparations is 0.12 mm, the canal can be enlarged to this size and it will be in this size anyway in the end with those NiTi rotary instruments. If the canal size is already larger than this size, it does not need to be enlarged. If the straight-line access is expected to exceed 50% of the root width or to result in perforating the canal, the fractured instrument should be left nonvisible and should be managed as such.

The instrument retrieval attempts can be made to remove it only after the fractured instrument is observed completely loosened under the DOM. A retrospective study and the author’s empirical findings revealed that when the fractured instruments were shorter than 3 mm or between 3.1 mm and 4.4 mm with the canal curvature smaller than 30 degrees, they were able to be retrieved with ultrasonics alone (Figure 18.10), whereas it took a significantly longer time to retrieve them with ultrasonics alone or it required different techniques such as a loop and an XP-endo Shaper (FKG Dentaire, La Chaux-de-Fond, Switzerland) to retrieve them when the fractured instruments were longer than 4.5 mm or between 3.1 mm and 4.4 mm with the canal curvature greater than 30 degrees [58] (Figure 18.11). This study also revealed all the fractured instruments that were removed with ultrasonics alone actually came out of the canals within 10 seconds in the instrument retrieval attempts. Whenever the fractured instruments in those conditions are seen dancing, they will most likely come out of the canal with ultrasonic activation in 10 seconds.

In short, visible fractured instrument retrieval attempts should be initiated only after the preparation is completed according to the treatment plan that always includes surgical interventions and the retention of the fractured instrument as options. It is always in the best interest of both the patient and the clinician to retain the fractured instrument if the tooth is predisposed to VRF or expected to be weakened by attempting the instrument retrieval. The fractured file can be incorporated as part of the root filling and kept under observation if it is retained in the canal [30, 69–71].

18.3 Root Canal Preparation Techniques

18.3.1 Potential Accidents in Ultrasonic Activation

Small ultrasonic tips allow continuous and improved vision of the field of operation. The use of ultrasonics, especially when performed under the DOM, enhances both the root canal preparation and the instrument retrieval attempts, and can provide safety and accuracy throughout the procedures [25, 59, 72].

Root canal preparations for fractured instrument retrieval can be made in either dry or wet conditions. Dry conditions provide better visibility with the DOM, preventing additional procedural accidents [2, 57, 59, 72]. For this reason, the root canal preparations for visible instrument retrieval should be conducted in dry conditions. However, heat generated with ultrasonic activation is inevitable [26, 57, 59, 72–76], and a temperature rise above 10° C on the external root surface for more than 1 minute can damage the periodontal tissues [77, 78]. In addition, the fractured instrument will be susceptible to secondary fracture with ultrasonics generating both the heat and cyclic fatigue on it [72, 79]. There is a crystal disk installed in the piezoelectric handpiece and when it is activated, the crystal disk expands and shrinks more than 20,000 times per second, which results in building up heat and creating a linear movement of the ultrasonic tip in one cycle (Figure 18.12). When this linear movement is suppressed in a narrow space during a continuous ultrasonic activation, the ultrasonic tip will be fatigued and immediately become susceptible to breakage. At the same time, when the NiTi instrument is heated above the A_f temperature, it will lose flexibility and the fatigue resistance, resulting in fracture [80]. Therefore, it is recommended that pulsing and in/out (or pecking) motions be used to prevent secondary fracture and breakage of the ultrasonic tip as well as the temperature rise while activating ultrasonics in preparation (Figure 18.13). Besides this, ultrasonic tips should be activated at the lowest possible power setting and the power can gradually be increased within 30% of the maximum power if the cutting action with ultrasonics is not as effective as it should be [72, 81–83].

18.3.2 Refinement of the Damaged Ultrasonic Tip

The ultrasonic tip can be sharpened with a bullet-shaped Brownie polisher (Shofu Inc., Kyoto, Japan) to maintain the cutting function if it is broken or becomes blunt resulting in reduced cutting efficiency instead of increasing the power setting. The author’s empirical findings show that the increased power setting beyond 30% of the maximum power, frequently to improve the cutting action, often resulted in breakage of the ultrasonic tip or secondary fracture of the fractured instrument in many of the cases treated for the same purpose. A blunt tip or a broken tip can be sharpened by reducing the diameter 180 degrees around the tip or can be converted into a Katana/sword-shaped tip by flattening two sides (the upper and the lower sides or the right and the left sides) (Figures 18.14 and 18.15).

18.3.3 Root Canal Preparation Techniques for Visible Instrument Retrieval

18.3.3.1 Canal Enlargement to the Visible fractured Instrument

Root canal preparation for instrument retrieval should be initiated with the enlargement of the canal to the fractured instrument if the instrument retrieval is considered feasible without significantly weakening the tooth structure based on the treatment plan whether ultrasonics or other techniques are used to retrieve the fractured instrument in the instrument retrieval phase. Visualising the fractured instrument is essential to make it predictable and minimally invasive to remove it.

The majority of fractured instruments found in root canals were reported to be NiTi [32], with the mean length of fragments being 3 mm [84]. Many of those rotary instruments have a .04 to .08 taper range with a tip size of 15 to 30, resulting in a coronal diameter of about 0.27 mm to 0.54 mm. When the smallest diameter ultrasonic tip (0.1 mm) such as a TFRK-S (SAYA Dent, Wilmington, DE, USA) is used to cut dentine on the fractured instrument, the space between the ultrasonic tip placed on the inner wall and the outer wall in the curve should be at least 0.1 mm larger than the coronal diameter of the fractured instrument. For example, if the coronal diameter of the fractured instrument is 0.54 mm, the space needed to allow the fractured instrument to come through should be wider than at least 0.64 mm to stay minimally invasive with the 0.1 mm diameter tip placed and activated in the root canal next to the fractured instrument. In this case, the rotary instrument to be used to enlarge the canal should be a size 70 flexible NiTi rotary instrument for a curved canal or a no.2 Gates Glidden (GG; Patterson Dental, St. Paul, MN, USA) drill (0.7 mm maximum diameter) for a straight canal to be minimally invasive. This shows that the smaller diameter tip is used, the more minimally invasive it becomes when preparing the canal. In general, a no.3 GG drill (0.9 mm maximum diameter) can be frequently used for more visualisation to enlarge the canal to the fractured instrument with a brushing motion in an anticurvature direction (on the outer wall), followed by the micro-trephine bur (MT bur; SAYA Dent, Wilmington, DE, USA) for exposing the coronal 1 mm portion of the fractured instrument, with less risk than ultrasonics for secondary fracture of the fractured instrument if the canal curvature is less than 15 degrees because the canal size will be increased up to 1.2 mm in diameter by the time the conventional root canal preparation is initiated after instrument retrieval [11, 79] (Figure 18.16). The MT bur can only be used when the coronal diameter of the fractured instrument is smaller than 0.45 mm because the inner diameter of the MT bur is 0.45 mm. The MT bur is turned in a counter clockwise rotation at 600 rpm to possibly loosen the fractured instrument designed to rotate clockwise. If the curvature is more than 15 degrees or a straight-line access is not considered feasible, a large flexible NiTi rotary file (ideally three sizes larger than the fractured instrument size to provide sufficient space and visualization for instrument retrieval) should be used to prevent ledge formation or perforation.

18.3.3.2 Visible Space Creation on the Inner Wall with Ultrasonics

A thin ultrasonic tip (0.1 mm tip ideally with a taper less than 2%) is used to create a thin semicircular space on the inner wall of the curve under the DOM to loosen the fractured instrument. The ultrasonic tip must be applied to the space between the inner wall and the fractured instrument, behind which the outer dentine wall contacts the fractured instrument. When ultrasonics were activated on the fractured instrument where there was no dentine wall on the opposite side of the ultrasonic activation, secondary fracture occurred significantly earlier than when there was a dentine wall contacting the fractured instrument on the outer wall [79