Background

Endodontic and periapical diseases are the more common pathologies in dentistry [1, 2]. The cause of the disease is an invasion of bacteria into the pulp through periodontal or defective areas of teeth, causing infection, or through physical channels, causing pain, bleeding, or even necrosis of the pulp [3]. Currently, the most effective treatment for endodontic and periapical diseases is root canal therapy [4]. Root canal therapy, also known as endodontic treatment, is a procedure in dentistry to treat necrotic pulp and root infections. The process of root canal therapy, i.e., mechanical preparation and chemical flushing to remove most of the infected material from the root canal [5, 6], followed by root canal filling and crown sealing [7]. The aim is to prevent the occurrence of periapical disease or to promote the healing of periapical disease that has already occurred [8]. Theoretically, the success rate of root canal therapy is between 83% and 97.1% [9]. However, the success rate of endodontic treatment is lower than the theoretical success rate, as shown in the surveys of the past decades [10]. The structural complexity of root canals, unskillful preparation technique, and insufficient performance of root canal files are all factors that contribute to the low success rate of root canal therapy in practice [11]. The solution to the first two problems will face great difficulties: for one, the root canal of a tooth is complex and variable, with a multidimensional curvature [12, 13], as well as many finer branches [14], which are inherent factors and cannot be changed artificially. Secondly, the current imbalance in the doctor–patient ratio in dentistry requires reliance on the clinician's manual operation combined with extensive clinical experience, so it is difficult to improve preparatory technology in a short period. Therefore, to solve the current problem of low success rate, improvement of root canal file performance is a key aspect.

As an important tool in the mechanical preparation step, the quality of mechanical preparation is directly influenced by the performance of root canal files, which should create a regular and smooth tapered structure to avoid significant deviation from the original shape and orientation of the canal. The quality of mechanical preparation directly affects the success rate of root canal therapy. However, from the clinical use, the performance of root canal files does not meet the requirements of required mechanical preparation. As shown in Fig. 1(a), the poor performance of root canal files can cause several problems, such as apical inflammation, incorrect preparation, and instrument separation.

Fig.1
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(Draw by Figdraw). Introduction to root canal therapy. a Problems of root canal therapy. b Problems with files

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This paper introduced the main solution to these problems is to improve the performance of root canal files. The properties of files can be divided into mechanical properties and use properties. The deficiency of mechanical properties is reflected in material limitations and poor geometric design, and the deficiency of use properties is reflected in a single function. As shown in Fig. 1(b), the factors affecting the performance of root canal files and their current limitations were described. For dental medical devices [15,16,17,18] in dentistry, an extensive review of literature exists but a detailed overview of root canal files is, to the author’s best knowledge, missing.

The main motivation of this paper is to provide a comprehensive survey of improved methods of root canal files, focusing on the improved principles as well as improved characteristics of various methods, which are expected to improve therapeutic efficiency, reduce accident occurrence, lower costs, and eventually achieve the high-precision treatment of pulp disease and periapical disease, and occlusal relationship of patients can be reproduced.

The rest of the paper is organized as follows. In "Material" Section, this review proposed the methods of literature analysis and structure of this paper. Secondly, the root canal files were divided into materials, geometry, and functions, which are in "Additional function" Section. In "Discussion" Section and "Future trends" Section, the current status of root canal files and their future directions were discussed. Finally, the full paper is summarized in "Conclusion" Section.

Root canal files patent data acquisition

Mathematical and statistical methods were used in bibliometrics to quantitatively describe and evaluate various external characteristics of scientific literature to understand the state of research and predict trends in scientific and technological development. In this paper, Patentics [19, 20] was selected as the literature analysis tool. With the world's original Patentics intelligent semantic mathematical model, only by inputting a key technology point, the technology lineage and technology route related to the technology can be automatically analyzed by correlation clustering, and the high-precision correlation relationship from hyperspace can be projected to a 2D map space through the map** of correlation retention. It can retrieve and download full text of patents of the US, Japan, China, Europe, and WIPO(PCT), and make patent analyses on retrieval results [21]. Patentics is currently used in the inspection of complex devices, such as chemical devices [30]. However, when a nickel-titanium root canal file is used to prepare a bent root canal, it may break due to torsional fatigue or bending cycle fatigue, which can seriously affect the completion of the root canal therapy. Therefore, how to reduce fracture is the focus of clinical research [31]. Scholars have conducted numerous studies on the matrix materials intending to improve the fatigue resistance and flexibility of root canal files. These methods can be better adapted to the root canal pattern and reduce the risk of instrument separation during root canal treatment.

A gradient flexible nickel-titanium root canal files [32] was proposed by Wang Z. The files have excellent cutting properties, but the file material is not set for the preparation requirements of the root canal. Later, he improved the files according to the demand for material [33] in the different part of canal. The internal organization of the tip part of the root canal files was improved to the martensitic M phase, the middle part to the R phase, and the root part to the austenitic A phase. After this method of manufacturing, it can effectively prevent side penetration. This treatment solves the problem of hardness and wear resistance of existing nickel-titanium root canal files. Zheng YF proposed ultrafine-grain nickel-titanium alloy root canal files [34], and the preparation process is shown in Fig. 4(a). The material composition of the files is martensitic and austenitic when not in use, and austenitic in clinical use. From Fig. 4(d, e), it can be seen that the hardness and wear resistance of the superfine grain-treated nickel-titanium alloy and the superfine grain nickel-titanium alloy after heat treatment by holding at 400 °C for 60 min are improved [34]. As shown in Fig. 4(g–i), nickel-titanium and superfine grain nickel-titanium alloys are dominated by abrasive wear, while superfine grain nickel-titanium alloys after heat treatment are dominated by adhesive wear.

Fig. 4
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Preparation of ultrafine-grained nickel-titanium alloy root canal files. Reproduced with permission. Source: CNIPA, www.cnipa.gov.cn. a The preparation process of ultrafine-grained nickel-titanium alloy root canal files. b XRD graph. c DSC curves. d Microhardness plots of nickel-titanium, superfine-grained nickel-titanium, and superfine-grained nickel-titanium alloys after heat treatment at 400 °C. Figures 1, 2, and 3 indicate the microhardness plots of nickel-titanium alloy and ultrafine-grain nickel-titanium alloy, and 3 indicates the microhardness plots of ultrafine-grain nickel-titanium alloy after heat treatment at 400 °C. e Wear rate diagrams of nickel-titanium alloy, ultrafine-grain nickel-titanium alloy, and ultrafine product nickel-titanium alloy after heat treatment at 400 °C. Figures a and b indicate the wear rate graphs of nickel-titanium alloy and ultrafine-grain nickel-titanium alloy, and c indicates the wear rate graph of ultrafine-grain nickel-titanium alloy after heat treatment at 400 °C. f Electron micrographs of the prepared ultrafine-grain nickel-titanium alloy root canal files. g Wear surface morphology of nickel-titanium alloy. h Wear surface morphology of ultrafine-grain nickel-titanium alloy. i Wear surface morphology of ultrafine-grain nickel-titanium alloy after heat treatment at 400 °C

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In order to solve the problem of large wobble at the end of the root canal file, many methods have been proposed by many scholars. Liu S proposed austenitic nickel-titanium alloy as the material for the connecting rod [35], and the threads at the edge are machined from a martensitic nickel-titanium alloy. This type of root canal file can effectively reduce the oscillation of the blade. With the development of minimally invasive dental techniques, the use of minimal-size root canal files is required. However, the conventional material of root canal files is not designed for the micro-preparation of root canals and is prone to fracture. In response to this problem, Steven S proposed polymeric materials [36]. It includes stainless steel, nickel-titanium, titanium, carbon steel, plastic, carbon fiber, or composite materials. In response to the problem that root canal files are not easily removed after accidental breakage, Duan JH proposed biodegradable magnesium alloy root canal files [37]. When a root canal file is broken, a corrosive degradation reaction occurs when the magnesium alloy at the section encounters the root canal flushing fluid. Because magnesium is a macronutrient in the body, it has good biocompatibility and is harmless to humans.

Surface material

There are many advantages in nickel-titanium alloys, but their disadvantages cannot be ignored, such as low surface hardness and poor corrosion resistance in root canal irrigation fluids. And in mechanical preparation, due to factors, such as wear, corrosion, and fatigue, resulting in micro-cracks on the metal surface of nickel-titanium files at the same time, disintegrating metal debris will be produced. The metal debris reacts with the tissue fluid and residual flushing fluid that exudes from the root canal, which in turn leaches out metal particles that are harmful to humans. Some reports show that nickel-chromium alloys will produce varying amounts of nickel ions after 7 days of immersion in artificial saliva. Due to the strong toxic side effects of heavy metal nickel ions on the human body, the release of nickel ions has become one of the indicators of biosafety evaluation of medical devices containing nickel metal [38]. In recent years, scholars have been working on surface modification techniques to improve the defects and deficiencies of root canal files in terms of biosafety, corrosion resistance, and fatigue fracture resistance [38,39,40,41]. For the surface modification of root canal files, the main modalities include polishing [42,43,44,45], coating metallic materials [10(a) shows the schematic diagram of the file's structure with the cross-section of the body at different positions. A convex triangular design was proposed by Long XP to improve the fatigue strength of root canal files in the circumferential and axial directions [94]. Figure 10(b) shows a schematic diagram of the structure of the files. Wang Z proposed root canal files with a non-isometric cross-section [95]. As shown in Fig. 10(c). Due to the difference in axial dimensions, the bending deformation capacity is poor in the direction of the long axis of the cross-section. However, it has a good deformation ability in the short-axis direction. Thus, flexibility is improved while strength is maintained. Zhong S used integral machining and molding, and the cross-sectional shape [96] and the center of rotation of the file bodies were designed eccentrically. Figure 10(d) shows a cross-section of the files with one of the implementation forms. A rectangle-shaped file cross-section only has two adjacent corners on the cutting boundary when it is designed as a rectangle. The force on the root canal files is greatly reduced. Yue B proposed that the cross-section [97] is a convex quadrilateral, where three angles are obtuse or right angles and the other angle is acute. As in Fig. 10(e), the files allow overcutting of the dentin. Liu S proposed root canal files with a double-edged section [98], where the first edge was machined from a cone with a rectangular cross-section and the second edge was machined from a cone with a square cross-section. Figure 10(f) shows a schematic diagram of the shape of the files with enhanced chip evacuation. Liu S then proposed that the files have a quadrilateral cross-section [99,100,101], the convex ribs project outward and the edge part is twisted by the cone. Using this method, the root canal files automatically conform to the root canal shape, which facilitates its entry and prevents the occurrence of lateral penetration. Figure 10(g) depicts the structure of the files and the cross-section of the edge.

Fig. 10
figure 10

Special-section root canal files. Reproduced with permission. Source: USPTO, www.uspto.gov: CNIPA, www.cnipa.gov.cn. a A parallelogram cross-section [93]. b A convex triangular design [94]. c A non-isometric cross-section [95]. d A rectangle-shaped [96]. e A convex quadrilateral [97]. f A rectangular cross-section and a square cross-section [98]. g A quadrilateral cross-section [99]

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Besides the cases described above, there are also ways to set different cross-sectional shapes according to different needs. Figure 11(a) illustrates the cross-sectional shape [102] of the root canal files in the reference, which ensures flexibility and strength. A concave helical groove [103] with continuous intervals was proposed by Jaunberzins A for the handle section. Additionally, it increases flexibility, reduces torsional resistance, and extends the prepared length. Figure 11(b) shows a schematic diagram of the structure of the files. Zhou L proposed that the cross-section [104] of the files gets smaller as it gets closer to the distal end. There is a difference in cross-sectional shape between the distal and proximal ends of the files. And in any two sections, the ratio of the area of the section near the proximal end to the area of its outer circle is not greater than the ratio of the area of the section near the far end to the area of its outer circle. As shown in Fig. 11(c), the strength of the middle section of the root canal files and the toughness of the distal end is ensured. William B proposed a polygon cross-section [105] at the proximal end of the files, which gradually becomes a square at the distal end. The cut angle of the files is achieved by rotating it in a positive direction, whereas the scra** angle is achieved by turning it in a negative direction. Figure 11(d) shows a schematic cross-sectional view of the files and their various embodiments. McSpadden JT proposed multi-tapered root canal files [106]. There are at least two grooves on the body of the file, which are, respectively, thinned along with the root canal files according to a predetermined taper function to form different sections. Torque loading should be reduced and the tendency to screw into the canal should be reduced. Figure 11(e) shows a schematic diagram of the profiles of the files and partial transverse cross-section views of additional alternative embodiments of a multi-tapered endodontic instrument. This kind of file meets the different performance priorities of the proximal and distal ends by setting polygons of different shapes at the proximal and distal ends of the body of the file. Under the premise of ensuring cutting efficiency, the body of the file can take into account the chip removal ability and strength requirements. Table 4 summarizes the special-section shape and their illustrations.

Fig. 11
figure 11

Special-section root canal files. Reproduced with permission. Source: USPTO, www.uspto.gov: CNIPA, www.cnipa.gov.cn. a The cross-sectional shape of the root canal files in the reference [102]. b Schematic diagram of the cross-section of the root canal files [103] in different positions. c Proximal cross-sections and distal cross-sections in the reference [104]. d Cross-sectional view of the files in the reference [105]. e Cross-sectional view of the files in the reference [106]

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Table 4 Overview of special-section methods of root canal files
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Additional function

Clinicians and patients look for safety and cleaning ability in root canal files when they use them in clinical settings. However, even with root canal files, problems, such as instrument separation, accidental falling off, secondary infection of the dental pulp, inadequate preparation, or over-preparation, may still occur. Root canal files perform insufficiently, which is obvious from their single function. Insufficiently performing root canal files cause these problems. The methods of adding additional functions have been proposed by scholars as a means of solving these problems. The paper categorizes them into three types: anti-fall methods, anti-infection methods, and anti-error methods.

Anti-fall

Clinically, root canal files are used in a humid oral environment, which is susceptible to accidental falls. Two types of accidental falls of root canal files are caused by saliva lubrication and loose connections between the file’s body and handle. When used in clinical settings, the root canal files are very close to the respiratory tract, throat, and other tissues. Accidental falling off will easily cause medical accidents. Recently, several methods have been developed to prevent falling off.

Hao ZY and Chi HY proposed files with a rope threading hole [107, 116,117] to the root canal files to remove debris. Sterilization and disinfection follow the cleaning of the dentin. To better restore the prepared root canal, light-based root canal files were proposed in the reference [118]. It can emit light of one or more wavelengths, which can realize a variety of therapeutic benefits, for example, disinfection, tissue regeneration, reconstruction of vascular tissue, and reduction of inflammation or pain. Figure 13(f) shows a schematic diagram of two embodiments of the files. A plurality of files [119] is provided and each file is insertable into a root canal. The fiber optic cable is insertable into the root canal to communicate the laser light onto an abscess for eliminating the abscess without conventional surgical intervention, as shown in Fig. 13(g).

Fig. 13
figure 13

Anti-infection root canal files. Reproduced with permission. Source: USPTO, www.uspto.gov: CNIPA, www.cnipa.gov.cn. a Root canal files with ultrasound function [113]. b–e Root canal files with flushing function [114–117]. f Root canal files with light anti-inflammatory function [118]. g Root canal files with laser anti-inflammatory function [119]. h Can bypass fractured root canal files [120]. i Can clamp fractured root canal files [121]

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For accidental fracture of stuck files fragments. Radwan S proposed files [120] that can bypass the fractured part. As shown in Fig. 13(h). Later, he proposed a cham**-type [121] root canal file that can remove the fractured files. These root canal files can help complete a complete root canal therapy without the influence of fractured files, as shown in Fig. 13(i)

Anti-error

The most common cause of pulp failure in root canal therapy is insufficient or excessive preparation of the root canals. Often, these complications occur as a result of a poor understanding of root canal length before the operation, so it is especially important to measure root canal length accurately. An apical locator is a traditional tool for measuring the working length of the root canal. During operation, large errors are often caused by cursor movement or personnel errors. Measurement steps are not only cumbersome but also inaccurate and time consuming. In addition, estimating the length of a root canal may lead to over-preparation or under-preparation due to mistake operation. Several methods to prevent error preparation have been proposed in recent years in light of these problems.

The references [122,123,124] provided root canal files for measuring root canal length, and Fig. 14(b–d) illustrates its structure. By setting an exposure window at the handle of the root canal files, Bagheri MJ attempted to overcome the problem of the traditional electronic apical locator sliding [125] when attached to the body of the files. The window structure is shown in Fig. 14(e). The snap of the electronic apical locator can be connected to the metal handle in the window exposure. These steps in Fig. 14(a) are performed in the root canal therapy of a patient. A solution [126] proposed by Du Y is to integrate the aluminum wire of the electronic apical locator with the metal of the handle so that the process of measuring root canal length does not involve repeated clam** and removal. Additionally, the measuring instrument collet can be avoided from affecting the field of vision and operation. Cai proposed a metal ring [127] fixed to the handle to solve the interference problem between the clip and the stopper. Figure 14(f) shows the structural diagram of the files. Curry AD proposed a method to limit the length of root canal files into root canal [128]: Root canal files have an adjustment scale and a nonius on the body. Once inserted into the root canal, the washer should be fixed to the tooth surface, which is shown in Fig. 14(g). Zhang XR proposed setting a micro camera at the end of the files away from the handle and embedding a video transmission line in the files [129], as shown in Fig. 14(h). Through the transmission line, images captured in real time in the root canal can be transmitted to the external display device, so that you can observe the root canal in real time. The relevant files and their improved methods are summarized in Table 5.

Fig. 14
figure 14

Effective preparation of root canal files. Reproduced with permission. Source: USPTO, www.uspto.gov: CNIPA, www.cnipa.gov.cn. a Process of using the electronic apical locator. B–d Root canal files for measuring root canal length [122–124]. e Root canal files with exposed window [125, 126]. f Structure to prevent disturbance [127]. g Structures limiting the working length of root canal files [128]. h Microscope working schematic [129]

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Table 5 Overview of additional function methods of root canal files
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Discussion

As shown in Fig. 15, we can see that the number of patents for root canal files has increased each year since the first patent for the root canal file, US04028810, was filed in 1975. This proves that in recent years there has still been a lot of researches into how to improve the performance of root canal files. By reviewing the paper, the development of root canal files in the last decade has been dominated by geometric designs. The geometric design [130, 131] influences the movement of the root canal files and determines the forces applied to files in the root canal. Further, due to the advantages of composite structures, such as hollow structures, compressibility, the ability to mount specific flushing devices, and continuous chemical preparation alongside mechanical preparation, research hotspots have shifted from the cross-section of root canal files to open composite structures. However, there are some limitations to the preparation of special root canals. Therefore, the application of the correct instrumentation in combination with the shape of the root canal can improve the sha** and cleaning ability [132] and reduce complications. Figure 15 shows when anti-infection methods began to appear in large numbers, indicating that patients are taking the safety of root canal treatment more seriously. In the future, anti-infection methods will be a hot new research topic.

Fig. 15
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The growing trend of root canal files

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However, as the number of ways to improve the performance of root canal files has increased, the performance of root canal files has become more and more sophisticated. Several studies have therefore investigated the differences from several perspectives [133], finding differences in design, phase transformation temperatures, and mechanical behavior of instruments. The low-cyclic fatigue resistance of counterfeit instruments makes them unsafe systems. To compare the performance of different root canal files in vivo and in vitro experimental studies, multimethod assessments [134] can be considered one of the main advantages of current research. This methodological approach allows for a more comprehensive assessment regarding the properties of the tested instruments, as it avoids “knowledge compartmentalization” a phenomenon in which knowledge structures about a specific domain are composed of several separate parts [131]. Understanding these characteristics may help clinicians make decisions regarding which instrument to choose for a particular clinical situation.

Although we have now improved the performance of root canal files, there are more factors affecting root canal therapy than just the performance of the files. As mentioned above, the experience of the clinician and the complexity of the root canal affect the success rate. The same root canal file used by different clinicians may give different results for root canal therapy. It may be that experienced clinicians have a better understanding of what kind of root canal file and what form of motion (reciprocation or OTR) is appropriate for the root canal. Root canal therapy capacity, working length variation, mid-axis offset, bending variation, root canal therapy time, and success rate all vary with different file motions. It is important to study the ability of root canal therapy and sha** with different file motions (reciprocation or OTR). Reciprocating motions can reduce the formation of dentin cracks to some extent in terms of safety compared to conventional rotary motions [135,136,137], but this advantage needs to be based on the selection of a suitable model according to the size of the root canal. Whereas the introduction of apical debris is closely related to both the motion pattern and the cross-sectional design of the instrument [138,139,140], debris removal is facilitated if a large amount of flushing fluid is used during root canal therapy [141]. Reciprocating Nitinol files are made from especially tensile machined and heat-treated M-wires, which are significantly more resistant to cyclic fatigue behavior and wear than other Nitinol instruments [142, 143]. In terms of microbial clearance, when applying reciprocating motion for root canal therapy, although the mechanical preparation time is significantly reduced, the flushing time should be longer than when using a continuous rotary motion system for root canal therapy due to the need to flush with an adequate amount of flushing fluid [144]. The reciprocating motion has the least change in working length and root canal curvature in terms of natural root canal morphology maintenance [145]. The two main directions affecting working length variation are the curvature of the root canal and the adjustment of the coronal access [146], and the more curved the root canal, the more pronounced the straightening effect of the adjustment of the coronal access. In terms of efficiency, the application of a single file reciprocating motion root canal therapy system avoids frequent instrument changes and, together with its greater dentin-cutting capacity, improves the efficiency of root canal therapy [147, 148]. And the OTR motion, Optimum Torque Reversal, is a torque sensitive, round-trip motion mode. OTR motion contributes to the release of spin-in forces compared to conventional rotational motion, and continuous rotation generated notably higher peak torque values than OTR motion [149]. The OTR movement reduces the torque and wedge forces during the crown-down preparation phase of the crown-down method and the apical preparation phase of the single-length method. The number of instrument separations can therefore be reduced.

This paper reviewed improved methods of root canal files. Figure 16(a, b) shows the percentage of root canal patents that is classified by the improved methods. According to the patents selected for this paper, the additional function methods account for the largest proportion of the three major classifications of root canal archives, at 40%. From Fig. 16(b), it can be found that among the eight subcategories of improvement methods, the special-shape methods accounted for the highest percentage at 24.4%, while the anti-fall methods and the matrix material methods accounted for the lowest percentage at 7.0%. The patents appearing in the paper are ordered in Fig. 16(c) by improved methods and time of appearance. The specific improvements to each root canal file are also shown in Fig. 16(c). The key techniques and features are represented in Table 6.

Fig. 16
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The patents of this paper. a–b Improvement method classification percentage. c Patent numbers of root canal files appearing in the paper

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Table 6 Overview of three improved methods of root canal files
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Future trends

Material

The methods for improving the performance of root canal files proposed in this paper pose several problems. Materials for root canal files should be developed to solve the problems encountered in the clinical use of root canal files. Considering the limitations of the material, it is possible to study experimentally how to instrument fatigue fracture that occurs during root canal therapy. According to the research results, find alternative materials, such as graphene [150]. Because graphene shows good toughness and high flexural strength, it can adsorb and desorption various atoms and molecules, and has biocompatibility and stability. As part of the root canal therapy process, it can serve as a cleaning and sterilization agent. By using graphene as a base material or surface treatment material, i.e., a highly thermally conductive graphite film, the mechanical properties of the root canal file will be greatly improved. Alternatively, inorganic non-metallic materials are used as the matrix materials for root canal files, such as ceramic materials [151], or medical materials, such as medical-grade Nitinol powder [152]. Vigorous development of medical additive manufacturing technology, breakthrough medical grade titanium powder and nickel-titanium alloy powder and other key raw material constraints, can be a new idea in the manufacture of root canal files. Three-dimensional (3D) printing technology [153] is driving changes in the medical and health care industry, and titanium and titanium alloys [154], as biomedical materials with excellent performance are develo** at an alarming rate. The combination of the two will help push personalized medicine to shine. Or making composite materials [155], such as preparing antibacterial materials by polymer polymerization, surface functionalization, and derivatization, and coating biomimetic micro nanostructures with bactericidal function. To prevent instrument separation, change the surface morphology and structure through physical methods. These future directions of materials are shown in Fig. 17.

Fig. 17
figure 17

Future directions of root canal files and future root canal therapy procedures

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Structure

The mechanical properties are influenced by the structure of the root canal file. Three future trends in the structure were discussed in this section. Bionic structures [156, 157] are one of them. As bionic technology keeps evolving, through the design morphology under the biological incentive mode and integration of multi-disciplinary knowledge to learn, simulate, and copy the function, behavior, or structure of organisms, to develop the root canal files with bionic structure. These methods can improve the debris removal ability and cutting effect of root canal files. For example, the tongue of the pangolin [158] can clean out anthills quickly and accurately without causing harm to the anthills, and its method of cleaning out anthills is similar to that of root canal therapy and is worth learning from. The second is the foldable structure [159]. It has a unique space-occupying volume. To introduce a foldable structure that can remove the necrotic pulp without over-preparing the root canal, thereby ensuring the maximum retention of the affected teeth. Finally, we can also start with the size of the structure and introduce a micro nanostructure [160]. Root canal files can be converted into micro nanorobots to realize minimally invasive treatment, remove the necrotic dental pulp, and eliminate bacteria. After you input the digital root canal therapy information, the robot will select the most suitable surgical scheme in the database for this kind of root canal. Automated preparation can replace manual operation completely. These structures are shown in Fig. 17.

Function

In the future, more functions will be added to root canal files to simplify the treatment. Given the current problems encountered with the clinical use of root canal files, this section proposed the following aspects of future functions. For example, the micro-display will be created inside the root canal, by using 3D display technology [161], and the clinicians can observe the progress of root canal therapy in real time to determine the next step needed. With sensing technology [162] and computer graphics technology [163], it is possible to perform under-preparation detection, over-preparation detection, collision detection, and detection of whether the root tip is reached, allowing for safer and more complete preparations. As computer and artificial intelligence technologies [164] continue to develop, a large number of root canal therapy cases have been compiled on the Internet, which can provide more comprehensive information on the problems of root canal therapy. So, before formal root canal therapy, the root canal model can be generated based on the patient's real dentition. Clinicians can then select the appropriate root canal files based on the patient's 3D root canal model with biomechanical properties and perform root canal therapy. When faced with this type of root canal, the large number of root canal models will help clinicians choose the right root canal files. A trial preparation with root canal files will be conducted based on the virtual root canal model. The sensor on the root canal files will examine the problems encountered. If the files are not suitable for preparing that type of root canal at this time, the clinician can replace them in advance without causing a break during clinical use. It is possible to anticipate the risk and make timely adjustments. In Fig. 17 you can see some general directions based on the additional functional methods.

Conclusion

In this paper, we analyze the problems encountered in the use of root canal files and the factors affecting their performance. Existing improved methods (theoretical research) of root canal files have been surveyed and classified into three categories, i.e., material-based methods, geometry-based methods, and those based on additional function methods. The basic information of each classification and the advantages and limitations of each are also described in detail. And this paper further explains the percentage of different methods and determines the development trend of root canal files at this stage and the popular advance direction. In addition, this paper proposes the future development direction for root canal files based on three principal methods. The future progress of root canal files will be guided. This paper understands the state of the art and identifies future research directions for the improved methods of root canal files. It contributes to the accuracy, effectiveness, and reliability of root canal therapy. In addition, the proposal and overview of the improved methods for root canal files are of great importance in promoting the precise diagnosis and treatment of dental pulp disease and periapical disease.