Breakage of endodontic files during treatment can result in serious complications and jeopardize the outcome of treatment. When using nickel-titanium (Ni-Ti) files, prevention of file breakage is complicated by the fact that these files rarely demonstrate visible evidence of cyclic fatigue and torsional stress and do not show wear before breakage.1 In the event that a file does break, removal can be difficult due to anatomical considerations, and the long-term prognosis of the tooth can become guarded.
The purpose of this article is to discuss the causes, prevention, and removal of separated Ni-Ti endodontic files. In addition, the prognosis when a broken file cannot be removed and a new file design that enhances the removal of the broken file from a canal will be discussed.
ADVANTAGES OF NI-TI FILES
With the introduction of Ni-Ti rotary file instrumentation in the field of endodontics, conventional endodontic treatment has become more predictable and efficient.2 Due to the flexibility of the alloy, Ni-Ti files allow for improved access along curved canals as compared to hand stainless steel files. Short and colleagues3 demonstrated that Ni-Ti rotary files were associated with significantly faster canal preparation as compared to hand stainless steel files. This study also observed that Ni-Ti files were better centered in the canal than stainless steel files. Another study determined that Ni-Ti files did not transport the apex as much as stainless steel files.4 Transportation of the apex is the moving of the canal’s physiologic portal of exit to a new, iatrogenic location on the external root surface. Reddy and Hicks5 demonstrated that rotary Ni-Ti instruments extrude significantly less debris apically than stainless steel hand instruments. Rotary files also help to reduce ledging and zipping of canals during preparation. Zipping is caused when a file creates an irregular area by opening up the apical region away from the curve of the canal.6
CAUSES OF ROTARY NI-TI FILE BREAKAGE
The 2 main causes of breakage of rotary Ni-Ti files are cyclic fatigue and torsional stress.1 Cyclic fatigue occurs when the file is freely rotating in a canal and flexes until fracture occurs. Usually the file fractures at the point of maximum flexure (clinically, this corresponds to the most curved portion of the root). Cyclic fatigue is similar to taking a piece of wire and bending it up and down until it breaks. 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. A study by Sattapan and colleagues1 noted that torsional stress occurred in 55.7% and cyclic fatigue occurred in 44.3% of the fractured files that were evaluated. Pruett and co-workers7 stated that the radius of curvature, angle of curvature, and instrument size play a role in cyclic fatigue. Rotational speed of the instrument has also been shown to contribute to cyclic fatigue. With higher rotational speed, the time to file failure decreases significantly.8,9
PREVENTION OF NI-TI FILE BREAKAGE
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| Figure 1. The difference between straight-line (glide path) access with only 1 maximum point of file flexure (blue line) as compared to lack of straight-line access and the presence of 2 maximum points of file flexure (red line). |
With the understanding that cyclic fatigue and torsional stress can cause file breakage, the goal is to place minimal force on a file during clinical use. This effectively will increase the time of usage before failure occurs. It is important to achieve straight-line access into a canal. This eliminates placing 2 points of maximum flexure on the file and provides a better glide path to the apical third of the canal (Figure 1). The use of stainless steel hand instruments to prepare the apical one third of curved canals (before introducing rotary files) significantly reduces the incidence of file breakage.10 The use of rotary files in a cyclic axial motion (up and down hand motion) also reduces the incidence of torsional stress on a file. Dederich and Zakariasen11 demonstrated that cyclical axial motion on a rotary endodontic file significantly extended the time before instrument fatigue occurred. Further, another study demonstrated that as the up and down distance increased to 3 mm (the maximum distance tested), the time to rotary file failure increased.8 It is important in the prevention of file breakage to use the speed (in revolutions per minute [rpm]) and torque control on the electric slow-speed handpiece that the file manufacturer recommends.
Roland and co-workers12 demonstrated that by preflaring a canal prior to rotary file placement to the full working length, there was a significant reduction in breakage of 0.04-taper Ni-Ti rotary files as compared to canals with no preflaring. (Although not specified in this study, preflaring is usually accomplished within the coronal two thirds of a canal.) The opening of the coronal portion of the canal reduces the stress and binding of files on the canal wall. After preflaring the canal, a modified crown-down technique (small-er files to larger files at working length) should be used to prepare the apical third of the canal.13 Due to the concern for file separation at this point in the canal preparation, the mistake most dentists make is not allowing the individual rotary files enough time to work within the canal. Therefore, when the next larger file is used, it does not go to the working length due to the dentinal constriction that the previous (smaller) file did not remove. The dentist then compensates for this by placing more pressure on the handpiece. This added pressure can cause binding of the file while the shaft is still rotating (torsional stress), thus increasing the likelihood of file breakage. By using an up and down hand motion the clinician should work the file until it is passively moving in and out of the canal before he or she changes to the next, larger file size. In reality, the additional time using the smaller file is relatively brief, amounting to a few up and down motions with the electric slow-speed handpiece.
The clinician should properly clean each file after it is used in a canal. It is not sufficient simply to insert the endodontic file into and out of a sponge as a cleaning method. This method only removes the superficial debris. Instead, alcohol gauze should be wrapped around the file and then twisted to remove the debris between the flutes of the file (avoiding the tip of the file to prevent piercing the clinician’s skin). Dentin chips wedged in a file may magnify any original manufacturing flaws in a file. This may play a role in the clinical failure of these instruments.14
A question that remains is how many times a rotary Ni-Ti file should be used before being discarded. Unfortunately, due to the many factors that can contribute to Ni-Ti file breakage, as well as the difficulty in identifying wear of the instrument, there is no definitive answer to this question. The manufacturers of Ni-Ti rotary files suggest single use, especially after treatment of a calcified or curved canal. However, Gambarini15 demonstrated that Ni-Ti engine-driven instruments were successfully used in as many as 10 clinical cases without intracanal fracture.
There are no defined American National Standard/American Dental Association (ANSI/ADA) specifications for rotary Ni-Ti files. It is for this reason manufacturing variations exist among files.
REMOVAL OF SEPARATED ROTARY FILES
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| Figure 2. File breakage in a curved canal. |
Even with the most appropriate preventive measures, endodontic rotary files may separate within a canal. Although many file removal systems are on the market today, the chance of successfully removing a broken file primarily depends on where it is located within a canal and the canal morphology (Figure 2). The removal of a fractured Ni-Ti instrument is influenced primarily by the anatomy of the tooth, degree of root canal curvature, and location of fragments rather than the specific removal technique employed.16 Souter and Messer17 concluded that removal of a broken file that is located beyond a canal curvature should not be routinely attempted due to limited success of file removal, increased risk of perforation, and reduction of root strength.
Ultrasonic instrumentation is often used in the initial attempt to remove a broken file. The clinician uses the ultrasonic instrumentation to expose circumferentially a minimum of 2 to 3 mm of the separated file, then with the aid of various file removal instrument kits on the market, the exposed portion of the file is engaged with the file removal instrument in an attempt to remove it from the canal.18 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 or in the event that preoperative pathology and/or symptoms do not improve as a result of pulp tissue that was not removed because of the broken file blocking the canal. The surgical procedure involves the resection of the root end that contains the separated instrument. Prior to the rootend surgery, the canal is obturated to the point of the file separation. This enables the canal to receive a proper guttapercha seal after the root end has been removed. A retrofill material is placed into the remaining root end, depending on the specific case circumstances. It is important that good endodontic surgical technique is implemented. This involves proper gingival tissue reflection, root isolation, hemostasis, and visualization.19
PROGNOSIS OF ENDODONTIC TREATMENT WHEN A BROKEN FILE CANNOT BE REMOVED
It is a common misconception that a broken file is the specific cause of conventional endodontic treatment failure. However, the basis of endodontic treatment failure after a file breaks is the inability to remove the remaining vital or nonvital pulp tissue due to the impediment that the separated file poses, which can lead to inflammation or infection.20 Crump and Natkin21 found that in most instances a broken file does not have an adverse effect on tooth prognosis. Saunders and colleagues22 demonstrated no significant difference in bacterial leakage comparing teeth obturated with gutta-percha and sealer versus teeth obturated with gutta-percha, sealer, and a separated instrument in the apical third of the canal.
The prognosis for a tooth with a separated instrument depends on the extent of undebrided and unobturated canal that remains below the broken instrument when the instrument cannot be removed or bypassed.23 Nevertheless, as noted earlier, avoidance of the problem is the best approach. The clinician should be proactive in instrumentation technique. The use of hand files prior to the use of rotary files will help to establish the glide path to the apical region and thereby help reduce file separation. Most file breakage occurs in canals that are curved,10 and this type of canal morphology is often observed in posterior teeth. Average foramen diameters in maxillary and mandibular molars are sized between Nos. 20 and 30 files.24 Therefore, manipulating a hand file at the working length to size Nos. 20 to 30 will enhance cleaning of the apical portion of the canal and provide a better transition to rotary files.
Preflaring the coronal portion of a canal before placing rotary files to their working length will not only reduce the chance of file breakage12 but will allow more of the irrigant (sodium hypochlorite, chlorhexidine, etc) to enter the canal. The combination of irrigants and hand filing of the apical region will aid in the removal of vital or nonvital pulp tissue.
Success in endodontic therapy depends upon the appropriate removal of vital or nonvital pulpal tissue.25 It is not possible to determine quantitatively for each patient the exact amount of tissue that should be removed to achieve success, because each individual responds differently to infection and/or inflammatory stimuli. Therefore, the need to debride and obturate a root canal system properly is still of paramount importance for long-term success.
By using the above technique, in the event that a rotary file breaks and cannot be removed, the amount of debris past the point of file separation will be reduced. This will provide a better long-term prognosis for endodontic treatment.
Although breaking an endodontic file in a canal is not outside the scope of normal practice, it is important to note that in the event a broken file cannot be removed, the dentist must inform the patient and document the incident in the patient’s record.
INTRODUCTION OF CS FILES
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| Figure 3. The predetermined fracture point on the CS file. | Figure 4. Breakage of the CS file at the hub. |
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| Figure 5. Removal of broken CS file with hemostat. |
Controlled separation (CS) endodontic rotary Ni-Ti files are a new type of file with a predetermined fracture point (Figure 3). This fracture point allows the files to separate at the hub rather than the tip (Figure 4). With separation at the hub, there is a better chance of removing the broken file with a specially designed hemostat (manufactured by CK Dental Specialties, Figure 5). The innovative design of the CS file provides a fail-safe contingency. When the file is not subject to cyclic fatigue or torsional stress, the chance of separation is minimal. When cyclic fatigue becomes a factor or torsional stress begins to occur, the file will separate at the hub before it breaks at the tip.
SUMMARY
Complications can occur during many dental procedures. The prepared clinician responds by either correcting the problem during treatment, or, ideally, preventing the problem from occurring in the first place.
In endodontic treatment separated rotary Ni-Ti files are a common procedural problem. Through understanding that the main causes of file breakage are cyclic fatigue and torsional stress, a dentist can best prevent this occurrence by using hand files before rotary files, creating a straight-line (glide path) access into a canal, and preflaring the coronal portion before using rotary files in the apical third of the canal. In addition, using an up and down motion with the electric slow-speed handpiece (not allowing the file to bind within the canal) will significantly reduce the incidence of file breakage.
If a file does break, successful removal primarily depends on the location of the file in the canal rather than the specific technique em-ployed for removal. A case does not necessarily fail if the separated file cannot be removed. The prognosis when file separation occurs can still be favorable, especially if care was taken to reduce the critical concentration of canal debris with hand instrumentation and chemical irrigation prior to rotary file insertion. In addition, the introduction of a new CS file design will help the dentist increase the chance of removing the file in the event of breakage.
References
1. Sattapan B, Nervo GJ, Palamara JE, et al. Defects in rotary nickel-titanium files after clinical use. J Endod. 2000;26:161-165.
2. Walia HM, Brantley WA, Gerstein H. An initial investigation of the bending and torsional properties of Ni-tinol root canal files. J Endod. 1988;14:346-351.
3. Short JA, Morgan LA, Baumgartner JC. A comparison of canal centering ability of four instrumentation techniques. J Endod. 1997;23:503-507.
4. Kuhn WG, Carnes DL Jr, Clement DJ, et al. Effect of tip design of nickel-titanium and stainless steel files on root canal preparation. J Endod. 1997;23:735-738.
5. Reddy SA, Hicks ML. Apical extrusion of debris using two hand and two rotary instrumentation techniques. J Endod. 1998;24:180-183.
6. Weine FS. Endodontic Therapy. 6th ed. St Louis, Mo: Mosby; 2004:193.
7. Pruett JP, Clement DJ, Carnes DL Jr. Cyclic fatigue testing of nickel-titanium endodontic instruments. J Endod. 1997;23:77-85.
8. Li UM, Lee BS, Shih CT, et al. Cyclic fatigue of endodontic nickel-titanium rotary instruments: static and dynamic tests. J Endod. 2002;28:448-451.
9. Martin B, Zelada G, Varela P, et al. Factors influencing the fracture of nickel-titanium rotary instruments. Int Endod J. 2003;36:262-266.
10. Patino PV, Biedma BM, Liebana CR, et al. The influence of a manual glide path on the separation rate of Ni-Ti rotary instruments. J Endod. 2005;31:114-116.
11. Dederich DN, Zakariasen KL. The effects of cyclic axial motion on rotary endodontic instrument fatigue. Oral Surg Oral Med Oral Pathol. 1986;61:192-196.
12. Roland DD, Andelin WE, Browning DF, et al. The effect of preflaring on the rates of separation for 0.04 taper nickel titanium rotary instruments. J Endod. 2002;28:543-545.
13. Bahcall JK. Everything I know about endodontics, I learned after dental school: part I. Dent Today. May 2003;22:84-89.
14. Alapati SB, Brantley WA, Svec TA, et al. Proposed role of embedded dentin chips for the clinical failure of nickel-titanium rotary instruments. J Endod. 2004;30:339-341.
15. Gambarini G. Cyclic fatigue of ProFile rotary instruments after prolonged clinical use. Int Endod J. 2001;34:386-389.
16. Shen Y, Peng B, Cheung GS. Factors associated with the removal of fractured Ni-Ti instruments from root canal systems. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2004;98:605-610.
17. Souter NJ, Messer HH. Complications associated with fractured file removal using an ultrasonic technique. J Endod. 2005;31:450-452.
18. Ruddle C. Nonsurgical endodontic retreatment. In: Cohen S, Burns RC, eds. Pathways of the Pulp. 8th ed. St Louis, Mo: Mosby; 2002:907.
19. Kim S. Color Atlas of Microsurgery in Endodontics. Philadelphia, Pa: Saunders; 2001.
20. Lin LM, Rosenberg PA, Lin J. Do procedural errors cause endodontic treatment failure? J Am Dent Assoc. 2005;136:187-193.
21. Crump MC, Natkin E. Relationship of broken root canal instruments to endodontic case prognosis: a clinical investigation. J Am Dent Assoc. 1970;80:1341-1347.
22. Saunders JL, Eleazer PD, Zhang P, et al. Effect of a separated instrument on bacterial penetration of obturated root canals. J Endod. 2004;30:177-179.
23. Torabinejad M, Lemon RR. Procedural accidents. In: Walton RE, Torabinejad M, eds. Principles and Practice of Endodontics. 3rd ed. Philadelphia, Pa: Saunders; 2002:323-324.
24. Marroquin BB, El-Sayed MA, Willershausen-Zonnchen B. Morphology of the physiological foramen: I. Maxillary and mandibular molars. J Endod. 2004;30:321-328.
25. Bram SM, Fleisher R. Endodontic therapy in a mandibular second bicuspid with four canals. J Endod. 1991;17:513-515.
Dr. Bahcall is assistant professor and chairman, department of surgical sciences, and director, postgraduate endodontic program, at the Marquette University School of Dentistry. He is a diplomate of the American Board of Endodontics. He can be reached at JKBMU@ aol.com.
Dr. Carp is a second-year endodontic resident at Marquette University School of Dentistry. He can be reached at stucarp@ yahoo.com.
Dr. Miner is a second-year endodontic resident at Marquette University School of Dentistry. He can be reached at marcusminer@yahoo.com.
Dr. Skidmore is a second-year endodontic resident at Marquette University School of Dentistry. He can be reached at lance.skidmore@marquette.edu.
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Continuing Education Test No. 71.1
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After reading this article, the individual will learn:
• the causes and prevention of endodontic Ni-Ti rotary file breakage, and
• how to improve the case prognosis in the event of file breakage.
1. Cyclic fatigue occurs when ______.
a. the file is locked or bound in the canal
b. the file is freely rotating in a canal
c. Ni-Ti files are resistant to cyclic fatigue
d. none of the above
2. Torsional stress occurs when ______.
a. the file is locked or bound in the canal
b. the file is freely rotating in a canal
c. Ni-Ti files are resistant to torsional stress
d. none of the above
3. The best preventive measure(s) to avoid Ni-Ti File breakage is to ______.
a. use hand files before rotary files and create straight-line (glide path) access into the canal(s)
b. use cyclic axial motion when using rotary
Ni-Ti files
c. preflare the coronal portion of the canal prior to rotary file usage in the apical third of the canal
d. all of the above
4. The removal of a separated rotary Ni-Ti file ______.
a. depends on the type of instrument(s) used to remove the file
b. becomes easier when the separated file is past the curvature (toward the apex) of the canal
c. is influenced primarily by the anatomy of the tooth, degree of root curvature, and the location of the separated file
d. all of the above
5. The prognosis for a tooth with a separated instrument depends on ______.
a. whether the separated instrument was a hand versus a rotary Ni-Ti file
b. whether the tooth is obturated with vertical compaction of gutta-percha
c. which type of handpiece was used during the procedure
d. the amount of undebrided and unobturated canal that remains below the broken instrument
6. Although rotary Ni-Ti files may separate in a canal, the advantage(s) of a Ni-Ti file is (are) ______.
a. flexibility of the alloy
b. it stays better centered in a canal
c. it is demonstrated to be significantly faster in canal preparation as compared to hand instruments
d. all of the above
7. In the event that a file does separate during an endodontic procedure, ______.
a. the dentist does not need to inform the patient
b. the dentist must inform the patient but does not need to document the incident in the patient’s record
c. the dentist must inform the patient and document the incident in the patient’s record
d. the dentist can either inform the patient or document the incidence in the patient’s record
8. Controlled separation (CS) endodontic rotary Ni-Ti files ______.
a. are a new type of file design with a predetermined fracture point
b. will resist breakage
c. will break at the tip of the file the majority of the time
d. none of the above
To submit Continuing Education answers, download the answer sheet in PDF format (click Download Now button below). Print the answer sheet, identify the article (this one is Test 71.1), place an X in the box corresponding to the answer you believe is correct, and mail to Dentistry Today Department of Continuing Education (complete address is on the answer sheet).
