How to prevent fracture of NiTi files

SUMMARY More certain therapeutic and significantly facilitated solution of everyday endodontic problems are provided by the introduction of NiTi rotary files in endodontic practice and their proper application. However, their widespread use has caused more and more frequent fractures and has become a significant frustrating factor that diminishes their numerous benefits. Torsional stress and cyclic fatigue are the main reasons for the occurrence of a fracture, and the frequency of NiTi file fractures is exactly proportional to the degree of bending of the canal, knowledge of the features of the file, ie. the skill and expertise of the therapist. The most dominant factor for the occurrence of fractures is the dentist who, in addition to manual dexterity, must have a good knowledge of the anatomy of various canal systems and file design in order to make the best choice for each individual case. Properly formed access cavity and knowledge of the working part of NiTi files enable maximum realization of endodontic skills, increased efficiency of intervention and reduced possibility of error during canal instrumentation. The special treatment of NiTi alloy and new solutions related to the design of the working part have significantly increased file flexibility and resistance to cyclic fatigue. Specific design (reduced number of threads, change of conicity, interruption of blade continuity), reduced engagement of file, ie. change of dynamics of movement in the canal (full rotation, reciprocal, eccentric, transaxial), as well as reduced number of processing files, have also significantly influenced the reduction of fracture risks. The aim of this paper was to analyze the most common reasons for the fracture of NiTi files and emphasize measures and factors that can increase their safe use and reduce complications during canal instrumentation.


INTRODUCTION
Endodontic treatment has become increasingly safe and effective due to new concepts and methods of "cleaning" and shaping the canal, ie. introduction of new files [1]. More certain therapeutic and significantly facilitated solutions of everyday endodontic problems are provided by the introduction of NiTi rotary files and their proper application [1,2]. The use of these files, with limited speed and marked flexibility, significantly accelerates canal instrumentation, but unexpected and sudden fractures are still an important frustrating factor that diminishes their numerous benefits [3,4].
NiTi files have become the standard in endodontic dental treatment over the last two decades and their widespread use has caused more frequent fractures [3,5,6]. It has been confirmed that due to rotating, almost all sets of NiTi files are subject to fractures, and that is more common during instrumentation of curved canals [3,6]. The main reasons for the occurrence of fracture are torsional stress and cyclic fatigue during canal instrumentation [7][8][9][10]. Torsional stress occurs due to the friction of the files against the walls of the canal when one part of the file is screwed in the wall, and the other part continues to move, so the fracture occurs due to exceeding the elastic limit [7,8]. Cyclic fatigue is a consequence of the cumulative effect of bending forces that are repeated in the same place (usually in the area of the curve), so the file is alternately exposed to tensile forces on the outside and compression forces on the inside [9,10]. It has been confirmed that NiTi files that were previously exposed to torsional stress showed higher resistance to cyclic fatigue, ie that files of smaller diameter are generally more resistant to cyclic fatigue [11,12].
The frequency of NiTi file fracture is exactly proportional to the curvature of the canal (7% in straight, 35% in slightly curved and 58% in very curved canals), and most often occurs in the apical segment .7%), then in the middle third (14.8-32%) and least often in the coronal third (2.5-20%) [13,17].
In a study that examined the frequency of NiTi file fractures in general dental and specialized endodontic surgeries, 88.8% of dentists reported fractures, which were much more common among endodontists who performed multiple interventions and generally treated more severe cases [18].
The aim of this paper is to assess the most common reasons for the occurrence of NiTi file fractures and emphasize measures and factors that can increase the safety and reduce possible complications during canal instrumentation.

Factors that influence the occurrence of NiTi file fractures
File fractures seriously complicate and compromise endodontic treatment and its prognosis, and therefore require various precautions [13]. Numerous studies have examined factors that can affect the fracture of endodontic files with the main goal of increasing the safety and reducing possible complications during canal instrumentation [3,6,9,10,11,14,17].
Different factors that can be classified into four categories, affect the occurrence of NiTi file fractures: a) factors related to the therapist (skill and expertise); b) anatomical factors (access cavity and canal anatomy); c) factors related to the file (material, design, production process) and d) technical factors of instrumentation (dynamics of file movement, irrigation, reuse, sterilization) [19][20][21][22].
In order to eliminate the risk of possible fractures and deformations of NiTi rotary files during canal preparation, dental technology has developed several new strategies in creating and designing their working part (conicity, cross section, working part design), special thermomechanical procedures in alloy preparation, ie different kinetics of file movement in the canal [1,9,20,21,22].
A dentist is certainly the most important and most dominant factor influencing the appearance of defects and fractures, ie. his knowledge, expertise and clinical training. In addition to manual skills and knowledge of instrumentation techniques, the practitioner must have a good knowledge of the anatomy of different canal systems (number, arrangement, angle and diameter of canal curvature, additional canals), or file design, to make the best choice for each case [3,16,18]. Good planning and detailed analysis of the course of endodontic treatment are necessary for success, but also for prevention of possible fractures [16,17].
The first and certainly the most important step in preventing fractures is the proper formation of the access cavity and adequate opening of the canals. This provides good visualization of endodontic space, facilitates the control of bending and preparation of the complex apical segment of the canal [3,15,16,23]. Properly formed access cavity is the first step in preventing fracture, because it enables easier movement of the files in the canal, significantly minimizes the stress of bending and twisting, and prevents possible fractures when it comes to patients with limited mouth openings. Canal entrance processing (Gates-Gliden) and checking the patency with hand files significantly facilitate rectilinear access to the canals and reduce the cyclic fatigue of the file during movement in the canal [3,15,16].
Apical preparation is a special problem due to inaccessibility and inadequate diameter, reduced effect of irrigation solution and frequent existence of apical curvature that makes it difficult to reach the file to the apical narrowing. Therefore, the blade efficiency of the file and the solvent effect of irrigation are reduced in this part of the canal, and the possibility of retention of debris is increased, as well as the frequent formation of "blockage" that can cause fracture of the file [24,25].

DESIGN OF NITI FILES
Knowledge and understanding of the design of NiTi files enables the practitioner to maximize endodontic skills, increased efficiency and reduced possibility of error during canal instrumentation [3,26]. Cyclic fatigue and torsional stress are the most common causes of the damage and fractures during canal instrumentation and are a function of the material and design of the working part, i.e the dynamics of movement [7,8,9,26]. The resistance of files to cyclic fatigue depends on numerous factors, primarily on the metallurgical features of the alloy and production process, file design (cross-sectional design, length, conicity, blade thread depth), preparation technique, irrigation, rotation speed, number of used files and of course canal curvature and dentist training [4,8,26,27].
The first NiTi rotary file was introduced in 1992 (standard conicity of 2%) and was made of conventional NiTi wire (56% Ni, 44% Ti) with a "shape memory" effect (after unloading, it returns to its original shape), and with extremely biocompatible properties and corrosion resistance [1,4,8,16].
A large number of different NiTi sets have been presented during the last decades (more than 160), with the basic goal of increasing their clinical efficiency and safety with innovative file design, new production processes and special treatments of NiTi alloy, ie affecting resistance to torsional loads, cyclic fatigue and flexibility [1,8,20]. High flexibility and shape memory are based on the fact that atoms in the alloy exist in two conformational forms (martensite and austenite), which significantly depend on the ambient temperature and stress (tension) during the movement of the files. The elasticity of the alloy increases with the change of phase, ie martensitic transformation into austenitic (in the martensitic phase the file bends with a slight force), and returns to its original shape after the cessation of stress [4,9,20,26].
Files with an austenitic phase are used for instrumenting straight canals, and NiTi files with a higher share of the martensitic phase show greater flexibility and resistance to cyclic fatigue and are used for instrumenting highly bent canals [28]. An important step in improving physical and mechanical properties of NiTi files is the introduction of new design solutions and production technologies of alloys related to thermal and electrochemical treatment of the working part, as well as the procedure of ion implantation and protection of alloy with various nano coatings [1,3,20,28,29].
Electrochemical surface treatment of NiTi alloys was a key factor for increasing blade efficiency and increased resistance to cyclic fatigue [29], and the introduction of new heat-treated alloys (M-wire, CM-wire, Max-wire) significantly increased flexibility and resistance to torsional stresses [4,9,30,31]. The alloy is alternately and under controlled conditions heated and cooled by heat treatment, in order to provide its specific properties related to increased flexibility and greater resistance to cyclic fatigue [9,28].
Knowledge of the design features of the working part of the NiTI file is one of the most important prerequisites for efficient preparation, but also a significantly lower possibility of fracture [1,3,26]. Resistance to cyclic fatigue mostly depends on the size of cross section, conicity, length of the working part, depth of the cutting edges, surface imperfection of the file, rotation speed, etc. [15,17,26,31]. It has been observed that thinner and more flexible files are more resistant to cyclic fatigue and more sensitive to torsional loads, while thicker ones are more sensitive to cyclic fatigue, but can withstand higher torque [17,26]. Since increased conicity is often the reason for fracture, dental technology has offered shorter files with progressive multiconicity, which significantly reduces the engagement (contact) of the file with the walls, and thus stress and the possibility of screwing, with more efficient cutting and removal of detritus from the canal [7,8].
The specific cross-section of the file (U, S shape), ie. number and depth of blades are also factors that lead to screwing and possible fractures, so files with shallow blade edges and constant cross section shape are more resistant to fractures [3,26]. Surface imperfections of the new NiTi files and higher speed also affect the appearance of fractures. Deformations and fractures of the file are four times more frequent at higher speeds (over 350 rpm) compared to lower speeds (160 rpm), as well as at higher torsional load (3N/cm 2 ) compared to 1N/cm 2 [3,11,12].
Reduced engagement of the NiTi file in the canal can be solved by shortening the working part, reducing the number of threads, changing the conicity, interrupting the continuity of the blade, using files in correct order and changing the dynamics of movement [3,14,26]. Since the torque is directly proportional to the surface of engaged file in the canal, smaller torque requires higher number of revolutions and sharper blade needs less threads [3,12,26,27).

Dynamics of movement of NiTi files in the canal
Frequent fractures of NiTi files in the canal influenced the introduction of new concepts of instrumentation primarily based on the change of movement dynamics, ie. reduction of the number of files for canal treatment [1,10,16]. Initially, only full rotation was used to start the NiTi file, and the instrumentation was most often realized with several file sets (initially with 5-6, and later with 3), while in recent years endodontics with single file has become more common, which significantly reduces the risk of fracture [2,6,8]. The fear of screwing (and thus fracture) in systems with full rotation influenced the introduction of systems with reciprocal movements that significantly increases the resistance to cyclic fatigue and prolongs life of files. Files with reciprocal movements are mainly represented by a single file that significantly reduces the instrumentation time, but also the stress during canal instrumentation [1,25,27].
Research confirms that the system with reciprocal movements is currently the most popular, because it allows greater flexibility and increased file resistance to cyclic fatigue, ie. efficient cleaning and shaping with reduced postoperative sensitivity [1,13,32,33]. Reciprocal movements are based on movements of balanced forces, where rotation counterclockwise (cutting direction) and a much shorter movement in a clockwise direction significantly reduce both torsional stress and cyclic fatigue, and thus the possible screwing of the file [1,26,34,35,36]. The benefits of these files are shorter processing time, reduced possibility of cross-contamination and reduction of fear of fracture, because only one instrument is used [16,32,33,34].
A system that uses a combination of full rotation and reciprocal movements (Genius system, Ultradent, USA) has been introduced in recent years, with instrumentation performed reciprocally and final instrumentation with full rotation files, which significantly increases resistance to torsional fractures [1,34].
Some NiTi systems that use individual files rotate eccentrically (asymmetrically) in the canal and thus provide efficient canal cleaning with irregular morphology [1,35]. Representative of these files is XP ENDO Shaper, a new generation of NiTi files, made of a special alloy (Max-wire), with a unique design of the working part (snake shape), which provides exceptional flexibility and increased resistance to cyclic fatigue. The specific design of the file also enables higher rotation speed (800 rpm), more efficient irrigation and more efficient removal of detritus from inaccessible parts of the canal [1,35,36].
The Self Adjusting File (SAF) has a completely different design and movement kinetics than existing NiTi systems. This NiTi file is hollow, mesh and flexible, and during transaxial movement (vibration) it enables more efficient and always fresh irrigation, that is continuously delivered to the canal via a silicone tube. This file has high resistance to fracture, adapts three-dimensionally and cleans the canal system very efficiently [1,3,7,37].
In vitro studies have confirmed that prolonged clinical use of NiTi files reduces their resistance to cyclic fatigue, and therefore a single application is recommended [9,10]. Sterilization of new or used files also reduces the resistance to cyclic fatigue and affects the occurrence of corrosion due to changes in the surface layer of titanium oxide [16,20,38]. The use of different lubrication gels, ie irrigation solution (NaOCl) can also have corrosive effects of NiTi alloy. It has been confirmed that NaOCl in concentration of 1% affects torsional and cyclic resistance after a cumulative exposure of 2.5 h, while longer-term exposure (18 h) also shows clear signs of corrosion [16,38,39]. The problem caused by immersion of files in NaOCl solution is related to metallurgical features and the occurrence of galvanic currents (handle and working part are made of two different metals) can accelerate corrosion and reduce fracture resistance [40,41,42].

CONCLUSION
File fracture during canal instrumentation is a serious iatrogenic complication that compromises endodontic treatment and largely depends on the therapist. Exceptional skills and expertise are required, as fractures are more common in less experienced practitioner. Preclinical training on extracted molars is mandatory and necessary in order to provide routine and experience with NiTi files, before using them in clinical conditions. The therapist also must be well acquainted with the canal anatomy of the teeth, the number and shape of the canals, the position of the curve and must plan the whole endodontic intervention well. In addition, therapist must clearly and precisely form the access cavity and clearly shape the entrances to the root canals.
The clinician must know the design of NiTi files and material they are made of (type of alloy), and choose files with working parts that will ensure reduced engagement of the file during canal instrumentation. Also, lower speed and less torque should be used during canal instrumentation and proposed protocol must be followed. The patency of the canal should be checked with hand instruments and it is obligatory to use lubricants and abundant irrigation with solutions during canal instrumentation.
The therapist must know the dynamics of the file movement and choose a preparation technique that prevents a possible fracture. Reciprocal movements are currently the most efficient, because they significantly reduce stress and the possibility of screwing during canal instrumentation due to the specific dynamics of movement in the canal. Numerous studies have confirmed that the frequency of file fractures is extremely low in clinicians who are well aware of possible fractures.