|Year : 2017 | Volume
| Issue : 1 | Page : 37-44
Fracture of rotary nickel titanium instruments
Rajendra Kumar Tewari, Bhumika Kapoor, Ashok Kumar, Surendra Kumar Mishra, Syed Mukhtar-Un-Nisar-Andrabi
Department of Conservative Dentistry and Endodontics, Dr. Ziauddin Ahmad Dental College, AMU, Aligarh, Uttar Pradesh, India
|Date of Web Publication||2-Mar-2017|
Department of Conservative Dentistry and Endodontics, Dr. Ziauddin Ahmad Dental College, AMU, Aligarh - 202 002, Uttar Pradesh
Source of Support: None, Conflict of Interest: None
With the advent of rotary nickel titanium (NiTi) instruments, preparation of curved canals have become easy but chances of instrument separation have increased. Such an instrument separation can cause procedural problems in rendering endodontic therapy. There are various factors that cause separation of rotary NiTi instruments. These factors, various failure mechanisms and impact on the prognosis of instrument separation have to be well understood by the clinician.
Keywords: Nickel titanium instruments, prognosis, rotary, separation
|How to cite this article:|
Tewari RK, Kapoor B, Kumar A, Mishra SK, Mukhtar-Un-Nisar-Andrabi S. Fracture of rotary nickel titanium instruments. J Oral Res Rev 2017;9:37-44
|How to cite this URL:|
Tewari RK, Kapoor B, Kumar A, Mishra SK, Mukhtar-Un-Nisar-Andrabi S. Fracture of rotary nickel titanium instruments. J Oral Res Rev [serial online] 2017 [cited 2019 Sep 17];9:37-44. Available from: http://www.jorr.org/text.asp?2017/9/1/37/201408
| Introduction|| |
Preparation of curved canals is a challenge to an endodontist as it can lead to various iatrogenic problems such as zip formation, ledging or slit perforation. To overcome this problem nickel titanium (NiTi) instruments were discovered which proved to be beneficial for the preparation of curved canals due to its property of high flexibility and superior resistance to torsional fracture. Despite this flexibility, these instruments have a tendency to separate in the canal and can affect the treatment outcome. Many factors contribute to such instrument fracture. One of most common reasons is incorrect use or overuse of an endodontic instrument. It is observed that occurrence of instrument fracture is more with advent of rotary NiTi instruments. The stainless steel (SS) instruments show signs of distortion before fracture while NiTi instruments separate without warning.,, There are no visible defects of previous permanent deformation and most of file separation occurs in apical third of canal.,,, This review discusses various causes, prevalence, impact on prognosis and clinical recommendations regarding prevention and the management of instrument facture.
| Prevalence|| |
Separation rates of SS instruments have been reported to range between 0.25% and 6%.,, On the other hand, frequency of fracture of rotary NiTi instruments is approximately 1.0% with a range of 0.4%–3.7%., It was concluded by Rowan et al. that the force required to fracture both SS instruments and NiTi instruments in both clockwise and anticlockwise direction is same. Sattapan et al. reported that 21% from 378 discarded Quantec instruments were fractured. These discarded instruments were collected over 6-months period from various endodontists. A study examined used, discarded rotary NiTi instruments obtained from 14 endodontists in four countries and identified factors that influenced defects produced during clinical use. Among 7159 that were examined fractures were seen in 5% of cases (1.5% torsional, 3.5% flexural). Alapati et al. found 5.1% of 822 rotary NiTi instruments were discarded by graduates in endodontic clinical practice. The frequency of instrument fracture and its impact on treatment outcome were determined from an analysis of specialist endodontic practice records involving 8460 cases. The prevalence of retained fractured instruments was found to be 3.3% of treated teeth. The instruments that were once used by a different endodontists showed a fracture incidence of 0.9% of 786 teeth. Thus, for better treatment results, it is mandatory for the clinician to recognize the chances of instrument fracture.
| Nickel Titanium Metallurgy|| |
NiTi rotary instruments have been extensively used in endodontics since the physical properties of nitinol were defined in 1988. NiTi instruments have been introduced in dentistry due to its property of shape memory, super elasticity, and resistance to corrosion. These properties are due to its phase transition from austentic to martensite. The phase transition is due to alteration in type of atomic bonding. The martensitic transformation requires a reversible atomic process termed twinning that reduces strain during the transformation. Unlike SS, NiTi instruments have low ultimate tensile strength and yield strength which makes it susceptible to fracture.
The flexibility and resistance to fracture of newer rotary NiTi instruments, can be increased by introduction of modified shape design and manufacturing processes. The instrument is subjected to a series of manufacturing processes such as R phase heat treatment, twisting of the metal, and special surface conditioning. These instruments have greater flexibility and resistance to cyclic fatigue, for example, Twisted Files (TF) (SybronEndo, Orange, CA, USA).
| Predisposing Factors for Nickel Titanium Fracture|| |
There are many factors but most common reason as mentioned is incorrect use or overuse.,
Number of uses
Cyclic flexural fatigue resistance decreases with prolonged use.,,,, Results of a clinical study that suggested that rotary instruments might be used to prepare four molar teeth, with no increase in the incidence of instrument fracture., Svec and Powers  found signs of deterioration of all instruments in their study after only one use. Parashos et al. concluded that there is no co relation between instrument fracture and frequency of use. However, there are only few studies that have focused on life of used rotary files.
Manufacturers have focused on number of use of rotary files to prevent inadvertent file fracture. However, the use of rotary file only once to prevent fracture is not supported in literature. Instrument fracture is multifactorial and number of use itself depends on various operator's skill, canal morphology and properties of the instrument. [Figure 1] shows various fractured rotary files.
Studies have shown that more the complicated canal anatomy more the chances of instrument separation. The chances of instrument fracture is more in molars,, especially in mesial canals of mandibular molars., Apical third is more subjected to instrument separation than coronal and middle third. Pronounced root curvatures lead to increase the chances of separation of instruments as there will be greater cyclic fatigue  [Figure 2].
|Figure 2: Two root canals of a molar have the same Shneider canal curvature, but measured different angles using the canal access angles (Adopted from Gunday M)|
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However, radius of canal curvature is more significant in instrument fracture than angle at which canal curves. When it decreases there are higher chances of file separation. A reduction in the radius of curvature reduces the instrument's ability to resist torsional forces.,
| Instrument Design (Size and Cross-Sectional Shape)|| |
The instrument design can affect the instrument resistance to fracture. Larger files have larger cross section, which permits them to cut more dentin. However, stress accumulation is more in such files which leads to cyclic fatigue. On the other side, large diameter files are more resistant to torsional force. A finite element analysis method showed that the model became more torque-resistant when the area of the inner core of the cross section increased. As, the diameter of file increases, the force needed to fracture also increases. While increase in taper decreases resistance to fracture., Instruments of square type cross section were more resistant to bending forces as compared to instruments with rhomboid type cross section. Similarly, triangular cross-sectional shape were more resistant to failure than S shaped files and H-type cross-section.
Triangular triple-helix design are not as flexible as U-flute design and smaller cross-sectional area files but more resistant when subjected to torsional stress., The triangular ProTaper instruments are stronger and they resist external forces effectively, this is because the stress distribution is lower and even in ProTaper instruments as compared to U-fluted ProFile instruments. Schäfer et al. reported the relationship between cross-sectional area and flexibility of NiTi instruments. They included FlexMaster, Hero 642, K3, ProFile, and RaCe, and found that the ProFile and RaCe instruments were most flexible, whereas K3 instruments were the stiffest as it had largest cross section.
The torque generated in small canals is higher than canals with large diameter. When a high torque is used, the instrument becomes very active and the chances of instrument locking increases. This leads to deformation of file and separation tends to increase. Small diameter files and files in acutely curved canals have more tendency to separate in the canal. When practitioners use low torque (<1 N/cm), the chances of fracture is less than when used at high torque values (>3 N/cm). When the diameter of file increases then torque required to unwind or to fracture also increases.
Yared et al., observed that high torque was safe for experienced operators and that most benefit from low-torque controlled motors. However, it must be kept in mind that the torque values indicated on various electric motors may be unreliable.,,
Light apical pressure and brief use of instruments causes less fracture of rotary NiTi instruments, as compared to continuous pecking motion.
[Figure 3] demonstrates failures in TF and profile.
|Figure 3: Fractographic analysis of rotary nickel titanium instruments. For overloading fractures of Twisted files (A) and ProFile (B) samples, peripheral smooth (black arrows) and central dimpled (white arrow) areas are visible. For fatigue failures of PF samples (C), multiple striations (a) and surface pattern with dimples and cones (b) are observed in the same fracture plane (courtesy Dr. Haw-Ming Huang)|
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Speed of rotation
There is a general consensus that instrument separation is more frequent at high speed of rotation as compared to low speed of rotation. A higher rate of fracture was reported when high speed 300–350 rpm was used. Other studies have demonstrated that there is no effect of speed rotation on file separation., However, time required for the failure decreases as speed of rotation increases.,,, Most rotary instruments work at a speed of 150–350 rpm and it is important for the clinicians to use different files under the recommended speed provided by the manufacturers.
The properties of various instruments are different due different metal composition. SS instruments are preferable over carbon steel instruments. NiTi instruments have an advantage of flexibility and superior resistance to torsional fracture than SS.
Visual examination of files should be done before using them into canals because sometimes file fractures due to inherent manufacturing defect. During manufacturing process, various inclusion particles like oxide particles get incorporated on metal surface. Such particles act as a nucleating site for propagation of various microcracks. Sometimes microvoids are also formed on the surface due to inclusion oxygen, nitrogen, hydrogen, and carbon , [Figure 4]. Further cracks may propagate along these defects. Surface irregularities are found more on files that are manufactured using complex procedures such as greater taper files. In a study conducted by Alapati et al., it was concluded that cracks propagate perpendicular to the long direction of instrument. He also observed in another study that the dentinal debris that gets lodged into files can cause widening of cracks. Various manufacturing procedures such as heat treatment and cold working can cause brittleness in some areas that can develop cracks. Such areas when exposed to loading and unloading of the instrument results in phase transition between austentic and martensitic. This further effects the mechanical properties of the instrument.
|Figure 4: Scanning electron microscope image of the fracture surface of a ProTaper rotary instrument, showing elongated dimples indicative of ductile fracture and secondary phase particles which may be nickeltitanium oxides (original magnification, 2500; scale bar length, 6 m)|
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There are certain methods which can increase the strength of the instrument. Such methods include electropolishing, deposition of titanium nitride by chemical vapors  and physical vapor deposition, ion implantation, and cryogenic treatment.
Such treatments may have a promising result but chances of instrument separation may be still present.
| Instrumentation Technique|| |
It is recommended to use different designs, techniques and taper files to reduce stresses on individual file. Using a number of different taper files is safer than a single taper file. However, the number of files increases in this case but the lifespan of each file increases.
Preflaring of the canal creates a glide path which causes ease of instrumentation. Preflaring of canal before the use of rotary files increases the lifespan of the files. It helps to understand the anatomy of the canal. Furthermore, it reduces the torsional stresses on the file. Another way of reducing the fracture is by using rotary files in wet canals only. Use of chelating agents like ethylenediaminetetraacetic acid not only removes the smear layer but also provides lubrication which reduces the chances of fracture. The use of different irrigants can cause corrosion on surface of file and hence weakening of the file. Such areas which may become site for microcrack propagation.
| Sterilization|| |
The effect of sterilization on fracture resistance of file is not well understood. Some studies show that there is no effect of sterilization , but others suggest that there are significant changes. The thermocycling procedures results in metal fatigue. However, such studies do not simulate clinical conditions. Files that are immersed in 5.25% sodium hypochlorite have less fracture resistance when they underwent cyclic fatigue. Sodium hypochlorite removes organic layer from files. It has broad antimicrobial activity. For proper cleaning strength of sodium hypochlorite should be balanced against its potential damage to corrode the files. Haïkel et al. demonstrated using scanning electron microscope that no corrosion products were seen after immersing files for 12 h and 48 h in 2.5% NaOCl. Busslinger et al. showed that only slight amount of titanium was released from NiTi instruments after immersion in ultrasonicated 1% NaOCl for 1 h.
| Types of Handpieces|| |
Air driven and electric handpieces are used in endodontics for instrumentation. The use of electric handpieces enable the clinician to control the torque according to individual case.
Impact on prognosis
In 2005, Spili et al., did a study to find out the influence of retained fractured rotary NiTi instruments on the prognosis of endodontic treatment. One hundred and forty-six teeth were selected that had retained instrument. One hundred and forty-six matched controls were selected. The clinical and radiographic follow-up of 1 year was performed. Healing rates were very high in both cases with a fractured instrument (91.8%) and for the matched controls (94.5%). Healing was low in both fractured instrument (86.7%) and control groups (92.9%) when a preoperative periapical radiolucency was present. Thus, it was concluded that the presence of a preoperative periapical radiolucency rather than the fractured instrument has an impact on prognosis.
Fox et al. conducted a study to evaluate the intentional obturation of root canals with separated files. Two years follow up of 304 molars was done in which 33% of cases files were accidentally separated and 67% of cases intentionally separated. It was concluded that presence of periapical radiolucency rather than file separation had a negative factor on healing after completion of endodontic treatment. The author also hypothesized that broken instrument itself could serve as an adequate root canal filling.
Crump and Natkin  conducted a well-controlled clinical study. One hundred and seventy-eight cases out of 8500 endodontic cases that were predominantly obturated with silver cone were found to have a retained fractured hand instrument. These instruments were separated within 1 mm of radiographic apex and only 2 were bypassed. The 178 fractured-instrument cases were matched with nonfractured-instrument controls for four variables. One of the variable that was included was presence or absence of a preoperative periapical lesion. Fifty-three controls were selected and 2 years follow up was done. After 2 years, clinical and radiographic assessment it was concluded that no statistically significant difference in failure rates between the fractured-instrument and control groups was found. In addition, it was concluded that inadequate size of apical preparation and apical extent of the fractured instrument, had no impact on prognosis.
Molyvdas et al. followed 46 root canal filled teeth over 32 months. Sixty-six canals were inspected having 70 retained instruments. It was concluded that there was negative prognostic influence of preexisting periapical pathology in such cases. In this study, healing in vital and necrotic teeth were also distinguished and it was seen to be 100% and 75%, respectively. It was concluded that prognosis was better in those cases where file was bypassed. Ingle at al concluded in their study that treatment outcome was unaffected by a retained fractured instrument. 1229 cases were recalled for 2 years, of these 104 showed failure. Only one case could be attributed to failure due to instrument separation.
The removal of retained instruments is very difficult procedure. Various techniques and devices are available such as Masserann Kit, Endo Extractor,, wire loop technique, the Canal Finder System, long-shank burs and ophthalmic needle-holders, and ultrasonic devices.,,
However, use of these techniques has certain limitations. The chances of perforations, ledging, extrusion of fractured segment beyond apex, and chances of excess dentin removal increases. Hence, these techniques should be used when only fractures segment is in coronal or middle third of canal, fractured segment is not at or beyond the canal curvature. More the apical position of instrument more is the chance of root perforation.
With the advent of dental operating microscope, ultrasonic tips and instrument removal system the chances of instrumental retrieval have increased.,
It should be kept in mind that straight line access is mandatory for instrument retrieval.
Retrieving the fractured segment should be first preference [Figure 5], however if that cannot be achieved then the instrument should be bypassed. If the instrument is separated in initial stages of biomechanical preparation then prognosis may be poor because microbial load is not reduced. However, if instrument is separated at a later stage when disinfection is almost complete then prognosis is good as microbial load is sufficiently reduced.
|Figure 5: Radiographs demonstrating removal of a fractured rotary nickel titanium instrument with minimal dentin sacrifice. (a) At time of fracture, (b) immediately after removal, (c) immediately after completion of endodontic treatment (courtesy of Dr. Jeff Ward)|
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Fracture of instrument is not a rare finding. However, frequency is not much. If such an incident occurs then the patient should be well informed about this and it should be mentioned in the patient card. Good communication with the patient should be maintained and records should be kept. Patient should be kept on follow up in such cases.
| Conclusion|| |
Rotary files should be used according to manufacturer's instructions. Overzealous preparations and forceful application of apical pressure should be avoided. The preoperative radiographs should be evaluated for knowing the canal anatomy. The presence of preoperative apical periodontitis is confounding factor in prognosis rather than fractured instrument. Every attempt should be made for retrieval of separated instrument. The risk versus benefit ratio should be measured. Bypassing the instrument and leaving the instrument are other available options in case of disinfected canal. The patient must always be informed of the presence of the separated fragment and proposed management, if any.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]