Remedying and Preventing Endodontic Rotary Nickel-Titanium (NiTi) File Breakage

Rotary nickel-titanium (NiTi) files are commonly used instruments for performing endodontic procedures. However, while providing improved canal access, these types of flexible, mechanical files can be fragile and possibly break within the canal (Figure 1). In addition to posing an increased risk of post-endodontic complications, a broken file can cause both the clinician and patient added stress.

There are two primary factors that contribute to rotary NiTi file breakage: cyclic fatigue and torsional stress. Cyclic fatigue takes place when the file is freely rotating in a canal and flexes until fracture occurs. Usually the file fractures at the point of maximum flexure—typically at the most curved portion of the root. Cyclic fatigue is similar to taking a piece of wire and bending it back and forth until the wire breaks in two. Pruett stated that radius of curvature, angle of curvature, and instrument size play a role in placing cyclic fatigue on a file.¹ Rotational speed (RPM) of the instrument has also been shown to increase the effects from cyclic fatigue—the higher the rotational speed, the time to file failure decreases significantly.^2,3

Torsional stress occurs when the tip or any other part of the file is locked or bound within a canal while the shaft continues to rotate. In an in-vivo study by Sattapan,⁴ investigators found that torsional stress occurred 55.7% of the time, and cyclic fatigue occurred in 44.3% of the fractured files that were evaluated. They also noted that NiTi files rarely show any wear before breakage.

Prevention of NiTi File Breakage

Since cyclic fatigue and torsional stress can cause file breakage, placing minimal force on a file clinically will increase its time of usage before any failure occurs. Attaining straight-line access into a canal is critical, as this will eliminate the need to place two points of maximum flexure on the file and will provide a better glide path to the apical third. Patiño showed that the use of stainless-steel hand instruments to prepare the apical one third of curved canals before introducing rotary files significantly reduced the incidence of file breakage.⁵ The use of rotary files in a cyclic axial (“hen peck”) motion also reduces the incidence of tortional stress on a file. An in-vitro study by Dederich demonstrated that cyclical axial motion on a rotary endodontic file can significantly extend the time before instrument fatigue occurred.⁶ Li showed as pecking distance increased, the time to rotary file failure increased.² To prevent file breakage it is important to use the speed (RPM) and torque control on the electric slow-speed handpiece as recommended by the individual file manufacturer.

In a study by Roland, researchers demonstrated that by preflaring a canal—ie, opening up the coronal portion of the canal—prior to rotary file placement, there was a significant reduction in the breakage of 0.04 taper rotary NiTi files as compared to when preflaring a canal was not performed.⁷

After preflaring of the canal, a modified crown-down technique—smaller files to larger files at working length—should be used to prepare the apical third of the canal.⁸ Because clinicians are often apprehensive of file separation at this point in the canal preparation, they often make the mistake of not allowing the individual rotary files enough time to work in the canal. Thus, when they change to the next larger file size, the file does not extend to working length because the dentinal constriction has not been removed from the previously used smaller file. Subsequently, clinicians often compensate for this by applying more pressure on the handpiece. This added pressure can cause binding of the file while the shaft is still rotating (ie, torsional stress), thus increasing the likelihood of file breakage. Using the “hen peck” hand motion, the operator should work the file until it is passively moving in and out of the canal before changing to the next larger file size. A rotary file should not be forced beyond the point at which it meets resistance in a canal.

Another issue is proper instrumentation maintenance. Clinicians should properly clean a file after each use in a canal. Alapati reported that dentin chips wedged in a file appeared to propagate any original manufacturing flaws of a file during clinical usage and, thus, play a role in clinical failure of these instruments.⁹

Prior to use, a file can be tested on a cyclic fatigue endodontic rotary file testing instrument (Figure 2). Additionally, a torsiometer endodontic rotary file testing machine can be used to test for torsional stress (Figure 3).

Unfortunately, the endodontic literature does not provide clear clinical guidelines as to how many times a rotary NiTi file should be used. Due to the many factors that can contribute to NiTi file breakage—including canal anatomy, canal calcification, the difficulty of identifying metal wear of the instrument, and the size and taper of the rotary file—there is no definitive answer as to how many times a file can be used clinically. From strictly an empirical point of view, on average, a rotary NiTi file should be used between one and three times before discarding.

Removal of Separated Rotary Files

Even with the best preventive measures, endodontic rotary files can still separate within a canal (Figure 4). Although there are many file removal systems on the market today, the chance of successfully removing a broken file depends mainly on where it is located within a canal and the canal morphology. A study by Shen showed that the removal of a fractured NiTi instrument is more influenced by tooth anatomy, degree of root canal curvature, and fragment location than a specific technique.¹⁰ A study by Souter concluded that the limited success of file removal, increased risk of perforation, and reduced root strength suggest that file removal beyond the curved portion of the root should not be routinely attempted.¹¹

A common method for the initial attempt of removing a broken file is the use of ultrasonic instrumentation. A surgical approach can be considered if a file cannot be removed by intracanal techniques. Endodontic surgery usually can eliminate any pathology and/or symptoms that develop after file breakage. Surgery can also be a successful approach if no improvement occurs in existing preoperative pathology and/or symptoms due to the un-instrumented pulp tissue that was unable to be removed because of the canal blockage by the broken file.

Prognosis of Endodontic Treatment When  Broken File Cannot Be Removed

The etiology of endodontic treatment failure after a file breaks lies in the inability to remove the remaining undebrided vital or non-vital pulp tissue due to the impediment of the separated file.¹² Crump found that, in most instances, a broken file does not have an adverse effect on the prognosis of the tooth.¹³ Saunders demonstrated that there was no significant difference in bacterial leakage between teeth obturated with gutta-percha and sealer compared to teeth obturated with gutta-percha, sealer, and a separated instrument in the apical third of the canal.¹⁴ Torabinejad stated that the prognosis of a tooth with a separated instrument depends on the amount of undebrided and unobturated canal that remains below the broken instrument when the instrument cannot be removed or bypassed.¹⁵

In order to maintain or improve the prognosis of a conventionally endodontically treated tooth with a broken file that cannot be removed, clinicians should be proactive in their instrumentation technique. As mentioned above, the use of hand files prior to using rotary files will help establish the glide path to the apical region and, hence, help reduce file separation. Most file breakage occurs in canals that are curved,⁵ a type of canal morphology that is observed, for the most part, in posterior teeth. In a study by Marroquin, it was reported that the average foramen diameters in maxillary and mandibular molars are between #20 and #30 file sizes.¹⁶ Therefore, using a hand file at working length to size #20 to #30 will enhance the cleaning of the canal tissue in the apical portion of the canal and provide a better transition to rotary files.

Another advantageous approach is to preflare the coronal portion of a canal before placing rotary files at working length. This will not only reduce the chance of file breakage,⁷ but it will also allow more of the irrigant, such as sodium hypochlorite, chlorhexidine, etc., to enter into the canal. The combination of the irrigants and hand filing of the apical region will aid in the removal of vital or non-vital pulp tissue.

Endodontic therapy success depends upon the removal of the critical concentration of vital or non-vital pulpal tissue.¹⁷ It is virtually impossible to determine quantitatively for each patient what this exact amount of critical tissue would be because each individual patient’s body responds to infection and/or inflammation differently. Therefore, proper debridement and obturation of a root canal system in conjunction with restorative dentistry remains a necessity for long-term success.

By adhering to these approaches when a rotary file does break and cannot be removed, the amount of debris past the point of file separation will be reduced, and, thus, a better long-term prognosis for the endodontic treatment can be expected.

It is important to note that when a broken file cannot be removed, the clinician must inform the patient and document the incident in the patient’s record. Although breaking an endodontic file in a canal is not malpractice, if a clinician neglects to tell the patient, the statute of limitations for a malpractice claim in most states can exist for many years from the time of the file breakage.¹⁸

Summary

Separated rotary nickel-titanium (NiTi) files are a common endodontic procedural problem. Understanding that the main causes of file breakage are cyclic fatigue and torsional stress, a clinician can best prevent this occurrence by using hand files prior to employing rotary files, creating a straight line (glide path) of access into a canal, and preflaring the coronal portion before using a rotary file in the apical third. In addition, using a “hen peck” motion with an electric slow-speed handpiece and not allowing the file to bind within the canal will significantly reduce the incidence of file breakage.

In the event that a file does break, the success of removal is more dependent on its location in the canal than the specific technique used. The case prognosis of a separated file can still be favorable, especially if care has been taken to reduce the critical concentration of canal debris with hand instrumentation and chemical irrigation prior to rotary file insertion.

About the Author

James K. Bahcall, DMD, MS
Associate Professor
Midwestern University College of Dental Medicine
Downers Grove, Illinois
Diplomate, American Board of Endodontics

References

1. Pruett JP, Clement DJ, Carnes DL Jr. Cyclic fatigue testing of nickel-titanium endodontic instruments. J Endod. 1997;23(2):77-85.

2. Li UM, Lee BS, Shih CT, et al. Cyclic fatigue of endodontic nickel-titanium rotary instruments: static and dynamic tests. J Endod. 2002;28(6):448-451.

3. Martin B, Zelada G, Varela P, et al. Factors influencing the fracture of nickel-titanium rotary instruments. Int Endod J. 2003;36(4):262-266.