Past,Present,and Future Trends of Nickel Titanium Rotary Instrumentation

Introduction

The introduction of nickel-titanium (NiTi) rotary files in endodontics has created a significant and positive impact on how root canal preparation is performed. Until the 1990s, root canal therapy was conventionally completed using stainless steel hand files. ¹ NiTi is a metal alloy comprised of nearly equal amounts of both nickel and titanium and is commonly used in endodontics. ² It is far more flexible than stainless steel, which has an inherent stiffness and tendency to straighten a curved canal. The stiffness of stainless-steel files becomes more apparent as the file size increases. ³ NiTi alloy contains properties of superelasticity and shape memory along with an increased strength compared to stainless steel alloys.^4,5 The unique properties of NiTi arise from the alloy composition and reversible crystallographic changes that take place when heat or stress are applied. ⁴

A wealth of sources exists for NiTi research and usage in human endodontics, but there are relatively few dedicated to veterinary endodontics. Also, relatively few rotary systems are available to the veterinary endodontist compared to their human counterparts. The standard rotary file lengths available in human endodontics include 21, 25, and 31mm files. ⁶ These lengths may suffice for many teeth in veterinary patients; however, they are insufficient for the canine tooth in most medium to large-sized dogs. Despite having far fewer available rotary systems and files available to the veterinary endodontist, NiTi instrumentation has grown in popularity. This article will review the past, present, and potential future trends of nickel-titanium file instrument use for the veterinary endodontist.

Endodontic History

The study and application of endodontics has been documented since the 17th century.^7,8 Endodontics in veterinary dentistry has gained popularity as pet owners have gained a greater appreciation for the value of saving the structure and function of teeth as opposed to exodontia. Some of the earliest attempts at root canal treatment in human dentistry were reported in 1820 and involved cauterizing exposed pulp with a heated instrument and protecting it with lead foil. Sixteen years later, the application of arsenic trioxide was recommended for pulp devitalization. In 1838, Edwin Maynard introduced the first root canal instrument when he used a watch spring to file canals. ⁸ Significant progress continues to be made in endodontic instrumentation development. There are currently more than 160 available automated instrumentation systems in human endodontics utilizing NiTi alloy. ⁹

History of Nickel Titanium

NiTi alloy was initially developed for military purposes in the 1960s at the Naval Ordnance Laboratory in Maryland. Buchelier and Wang were investigating nonmagnetic, salt-resistant alloys for naval use and discovered the thermodynamic properties of NiTi by chance.^5,10 In 1963, the newly discovered alloy was given the name Nitinol. Nitinol is an acronym of the elements of which it consists, nickel "ni," titanium "ti," and "nol" for the Naval Ordnance Laboratory where it was discovered. NiTi alloy is comprised of 55 wt % Ni and 45 wt% Ti and is commonly referred to as 55-Nitinol.^2,5

NiTi alloy was first used in dentistry in 1971 by Andreasen and Hilleman in the manufacture of orthodontic wires due its low modulus of elasticity and super flexibility. A shape memory effect was additionally noted with NiTi alloy. ⁹ The modulus of elasticity, also known as elastic modulus and Young's modulus, refers to the inherent stiffness of a material. ¹¹ A high modulus of elasticity denotes that the material has more inherent stiffness and will reach plastic or permanent deformation with applied stress more quickly in comparison to a material with a low modulus of elasticity. Most metallic materials will have a deformation that is directly proportional to the applied force which is known as Hooke’s Law.^9,11 The elastic modulus of Nitinol is one-fourth to one-fifth that of stainless steel, contributing to increased flexibility in comparison to stainless steel.^2,12 Super flexibility, also described as superelasticity, is the ability of a material to resist stress without permanent deformation. NiTi's ability to return to its original shape without showing signs of permanent deformation is described as shape memory effect. ²

In 1988, one study discovered that Nitinol endodontic files had resistance two to three times the elastic flexibility in bending and torsion compared to stainless steel files. ¹² In the late 1980s, NiTi was developed for use in endodontics since its flexibility allowed better navigation of a curved root canal while minimizing canal transportation. ¹⁰ The newly discovered super flexibility was considered desirable due to the challenges of instrumenting more complicated root canal systems. This resulted in fewer procedural errors such as ledging, zipping, elbows and perforations commonly seen with stainless steel files.^4,6 By the 1990s, NiTi files were commercially available. NiTi rotary instruments began to gain popularity since the files offered a sturdy, corrosion-resistant instrument with unique properties that were seen as a way to increase the efficiency of root canal therapy. ¹⁰

Properties of NiTi Files

Nickel titanium’s thermodynamic properties are due to its ability to phase change. NiTi alloy is quite complex and has numerous intricacies within its phase change characteristics and transformation that are beyond the scope of this article. A description of the fundamental principles of metallurgical transformation has been described to provide a working understanding of the alloy for the veterinary endodontist.

NiTi alloy exists in two main predominant conformations, austenite and martensite. ¹³ Phase (crystallographic) changes occur either due to stress during procedures or due to changes in temperature during manufacturing. ⁵ The austenite phase (A-phase), known as the parent phase, is a hexagonal lattice that is quite strong and hard. As stress occurs, such as in root canal instrumentation, there is a transformation from the austenite phase to the martensite phase (M-phase). Many conventional rotary instruments, including ProFile^a and ProFile GT^b, LightSpeed^c, and K3^d are used clinically in the austenite phase. ¹⁴ The flexibility of the austenite phase allows the file to be inserted readily within a curved canal. Once stress is encountered, as the file is in contact with the canal, a shearing transformation occurs, transforming the hexagonal lattice to a body-centered cubic lattice. This transformation gives rise to the martensite phase which is more elastic and has greater tensile strength than the austenite phase. ¹⁴ The martensitic transformation causes a physical change in the austenite lattice which gives rise to the shape memory trait. Once the stress is released, the M-phase transforms back to austenite.^5,15

The superelastic nature of NiTi is attributed to the reversible austenite to martensite phase changes (Figure 1A and B).^2,16 Loading and unloading occur as the atomic arrangement shifts. If the austenite phase is not present, the superelastic behavior does not exist. ¹⁶ Without the martensite phase, a plastic strain recovery or shape memory would not transpire. A third, special type of martensite transformation may occur called R-phase. R-phase is a rhombohedral distortion of the cubic austenite phase which can be present when precipitates or dislocations exist such as in aging NiTi files. This type of martensite transformation occurs prior to the return transformation to austenite and has a 0.5% reduced strain in comparison to the martensite/austenite transformation. Additionally, there is good stability of the transition from R-phase to A-phase with a Young’s modulus that is lower than austenite.^4,14,15 It is important to note that R-phase is not consistently present. R-phase technology has been a focus of NiTi research.^15,17,18

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Figure 1.

Crystallographic phase change of NiTi alloy with temperature (A) and applied strain (B). As and Ms are the start temperatures at which phase transformation occurs. Image obtained from Cohen’s Pathways of the Pulp, 11th ed, reprinted with permission from Elsevier.

The metallurgy of NiTi gives it a significant advantage over traditional stainless-steel files (Figure 2).^6,19 The ductile transformation from austenite to martensite allows the NiTi file to navigate curved canals while returning to its previous shape. ¹⁶ Super flexibility allows for higher cutting efficiency compared to stainless steel instruments. This allows for the NiTi instruments to maintain the original canal shape with fewer irregularities. NiTi’s low modulus of elasticity in comparison to stainless steel allows for the alloy to return to its normal shape on unloading before deformation or shape memory. Steel allows a 3% strain before plastic deformation occurs. ¹³ Plastic deformation is an irreversible change that remains once the applied external force has reduced or ceased. ¹¹ In contrast, a strain of up to 8% in NiTi can be overcome, but as wear occurs with repeated use, the files become less effective. After 100 deformations, the tolerance for elasticity decreased to 6% of the original strain ability. ² Multiple deformations can occur as the file moves through the canal in rotary motion, with the instrument being bent once per rotation. Constant rotary motion contributes to cyclic fatigue. ¹³ Steel can withstand up to 20 complete bending cycles, while NiTi can be bent up to 1000 times. NiTi's resilience is attributed to the anatomical structure of steel compared to NiTi.^12,13 It remains essential to consider fatigue and increased strain with repeated use of files. NiTi rotary files should be operated with constant speed and pressure as phase changes occur to minimize iatrogenic errors.^10,13

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Figure 2.

Comparison of stainless steel and nickel-titanium crystal lattices under a stress load. Stainless steel has limited elastic behavior (E) while NiTi exhibits super elasticity (SE). Image obtained from Cohen’s Pathways of the Pulp, 11th ed, reprinted with permission from Elsevier.

Manufacturing of NiTi files is different from traditional stainless-steel K files, which are created through a twisting process. Machining and grinding are necessary for manufacturing of NiTi files as superelasticity renders twisting ineffective.^2,5 The exception is the manufacturing of Twisted Files^e in which thermal processing is used to transform the file from an austenite phase to R-phase in order to achieve a twisted shape. Once the twisted shape is achieved, additional heating and cooling methods return the file to the austenite structure. ²⁰ One study found the bending load values for elasticity were lower with Twisted Files than with a conventional NiTI rotary file. ²¹ Milling marks, such as surface irregularities and grooves, are produced during the traditional manufacturing of NiTi files. These inconsistencies compromise the instrument's cutting ability and they lower fatigue resistance. ² They can also act as stress raisers that may make the file more susceptible to fracture. ²²

Fracture is a significant concern with NiTi rotary files and can occur without warning. File fractures may occur with or without visible detection, making visual examination before use unreliable.^22,23 Fracture of NiTi instruments occurs due to either cyclic fatigue or torsional fracture. Cyclic fatigue occurs when an instrument rotates in a curved canal, leading to motion-induced repeated compressive and tensile stress.^24,25 It is a product of hardening and metal fatigue, resulting in failure. Torsional fracture occurs when the instrument tip is locked within the canal while the shaft continues to rotate. Cyclic fatigue and torsional fracture both occur in straight and curved canals. However, cyclic fatigue tends to occur more frequently within curved canals. ²⁴ A previous study evaluated defects in rotary NiTi files after clinical use and found that fractures occurred due to torsional failure with smaller files (55.7%), while larger files were prone to fracture from cyclic fatigue (44.3%). ²³

Generations of NiTi Files

The use of NiTi has been evolving from its first use as an orthodontic wire to its present use in endodontic rotary instrumentation. ¹² NiTi alloy is quite intricate and the complexities in its anatomic arrangements have taken many years to understand. ¹⁵ Numerous generations of NiTi files highlight the progress that continues to be made with this alloy.

John McSpadden developed the first-generation files that came to market in 1992. A U-shaped tip, radial lands, and fixed tapers (0.02 mm, 0.04 mm, 0.06mm) characterized first-generation files. ³ This generation initially included the ProFile system^a. Machining of three equally spaced U-shaped grooves makes up the cross-sectional shape on the shaft of these tapered files. The space next to each groove is called a radial land. Radial lands are the peripheral portion of a rotary instrument that is flat and smooth. Described as a flawed area that prevents the file from locking within the dentin, radial lands allow cutting to occur through a passive planing action, and its design is to center the instrument in the central space (Figure 3A, 3B, 3C). Neutral or slightly positive rake angles were also present in this generation. Positive rake angles result in cutting action. Negative rake angles result in scraping action. ⁶

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Figure 3.

Profile Instrument. (A) Scanning electron micrograph lateral view. (B) Scanning electron micrograph cross-section illustrating radial lands noted by the dashed line. (C) Profile instrument. Image obtained from Cohen’s Pathways of the Pulp, 11th ed, reprinted and modified with permission from Elsevier.

LightSpeed by Senia and Wildey followed suit soon after Profile’s appearance. The LightSpeed files were considered unique due to their long thin shaft, and short anterior cutting part (Figures 4 and 5).^3,6 The design of the instrument tip and the radial land rendered them fairly safe in avoiding preparation errors such as ledging and transportation. ⁶ File separation with the LightSpeed rotary system was a concern. Additionally, a large number of files were needed to prepare a canal for the LS1 system. ³

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Figure 4.

LightSpeed files size 35-160.

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Figure 5.

LightSpeed file. Note the long, thin shaft and short anterior cutting part.

Second generation files were introduced in 2001. These files had active cutting edges without radial lands in comparison to the cutting U-shaped tips of the first generation. ³ The angle between the cutting blade and the longitudinal axis of the instrument became smaller compared to first-generation files and fewer instruments were used to prepare a canal. A smaller angle minimized a screwing effect during use.^3,19 Some of the files such as K3^d had positive rake angles, which led to a higher cutting efficiency. ProTaper^f was introduced, with multiple tapers of increasing and decreasing size on a single file. Unfortunately, the increase in taper led to considerable instrument breakage. ¹⁹ Additionally, manufacturers began to focus on enhancing the work surface of the files through electropolishing. Electropolishing is an electrochemical process that reduces the microscopic surface roughness by immersing the material in a highly ionic solution with an electric current. ²⁶ RaCE^g, one such system, was found to have an increased resistance to fatigue in a corrosive environment with hypochlorite compared to a non-electropolished file system. ²⁷ However, electropolishing has been reported to dull the cutting edge leading to an increased forward pressure when advancing the file which can invite taper lock. ²⁸

Improvements in metallurgy were the focus in the development of third-generation files. ³ In late 2007, manufacturing focused on the utilization of heating and cooling methods to reduce cyclic fatigue and improve safety when rotary NiTi instruments operate in curved canals. ¹⁹ A reduction in cyclic fatigue would theoretically decrease the incidence of broken files. ¹⁷ A few of the brand lines that offer heat treatment technology are Twisted Files^e, Hyflex^h,Vortexⁱ, and WaveOne^j.^29,30

The third generation also produced M-Wire^k technology. M-wire is produced by applying heat treatments at various temperatures to wire blanks which results in a material that includes both martensite and R-phase providing for an improved elastic state. ³ Following M-wire technology, was the development of Vortex Blue instruments^l. The color of the Vortex Blue files was attributed to a relatively hard titanium oxide surface layer, which was designed to improve cutting efficiency and wear resistance (Figure 6).^3,4,18,20

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Figure 6.

Vortex Blue file. Note the blue color on the file surface from titanium oxide enhancement. Image courtesy of Dentsply Sirona.

Fourth-generation files focused on reciprocation, which is defined as a repetitive back-and-forth or up and down motion.^3,19 Reciprocating systems began to use smaller and equal angles less than the previous 90 degrees clockwise and counterclockwise rotation. ³ The self-adjusting file system^m is a popular representative system of this group with file length available in 21 mm, 25 mm and 31 mm. The SAF file uses a hollow, open tube design that exerts uniform pressure on the surrounding dentinal walls reciprocating irrigation throughout the mechanical preparation. Additionally, the SAF is mechanically driven by a handpiece that produces both vibrating movements and constant irrigation. This file has a compressible open tube design that is purported to exert uniform pressure on the dentinal walls, regardless of the cross-sectional configuration of the canal. ³¹ Adaptation to the canal can minimize file separation and breakage. Additional popular systems include WaveOne and Reciprocⁿ which have the added benefit of being produced with M-wire technology (Figures 7 and 8). ³² The WaveOne System is classified as third generation but represents a convergence of the best design features from the second and third generations of files with a reciprocating motor that places it within the fourth generation. ³⁰ WaveOne is presently superseded by the WaveOne Gold system^o (Figure 9). Endopulse^p is a veterinary endodontic system that utilizes reciprocation. However, the master files used to create a glide path, are comprised of stainless steel material and not NiTi.

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Figure 7.

WaveOne 4th generation files. Image courtesy of Dentsply Sirona.

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Figure 8.

Reciproc 4th generation files. Image courtesy of VDW.

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Figure 9.

WaveOne Gold files. Image courtesy of Dentsply Sirona.

Fifth-generation files involve an offset center of rotation. In rotation the files produce a mechanical wave of motion that travels along the length of the file. ³ This offset design minimizes contact and engagement between the file and dentinal walls. Reduced engagement with the walls limits torque and locking of the file within the dentin. One example of this generation is ProTaper Next^q.^3,19

Present Trends and Use of NiTi Files

Advancements in the field of NiTi rotary endodontics have built on the progress from the previous generations. WaveOne Gold, Profile Vortex (Figure 10) and Vortex Blue are current popular NiTi rotary file systems used in the human endodontic field. WaveOne Gold’s properties include the reciprocating motion of the fourth and fifth generation files, with a parallelogram geometry which allows for improved centering within the canal. WaveOne Gold has two cutting edges in comparison to the WaveOne system. Additionally, a gold heat treatment is applied during the manufacturing process, which improves the elasticity of this single use file. ³³ ProFile Vortex files represent the newer generation of ProFile instruments and utilize M-Wire technology in their manufacturing. The files are triangular on cross section without radial lands as with other second generation files. Vortex Blue files are similar to the Profile Vortex with the addition of bluish color on the file surface due to enhancement of titanium oxide to increase flexibility and fatigue resistance. ¹⁸

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Figure 10.

Profile Vortex Series. Image courtesy of Dentsply Sirona.

Presently, LightSpeed LSX^r is a commonly used NiTi rotary system in veterinary endodontics notably due to the availability of 50 mm files. LightSpeed LX1 and LSX has remained a static and popular system since its first development over 25 years ago. File separation and breakage have been predominant sources of frustration since its first appearance in 1992. ¹ The incidences of file separation are multifactorial and complex. Reasons for file separation include skill and experience of the operator, instrumentation technique, instrument design, fatigue, wear due to excessive repeated use, or root canal anatomy. ³⁰

Use of NiTi Files

The American Association of Endodontists has outlined the following steps which are summarized to optimize the benefits of the unique metallurgic properties of NiTi files. ¹³

Access Preparation—Poor access will promote procedural errors. Adequate access is crucial for the use of NiTi rotaries. Always attempt to create straight-line access into the coronal or middle root canal third prior to rotary use.

Rotary NiTi instruments have proven to be efficient in shaping the root canal system; however, there has not been an appreciable difference noted in disinfection efficiency when rotary systems are compared to hand files. In another study, hand files and rotary files were both effective in the removal of intra-canal bacteria; however, rotary files were capable of achieving better removal of biofilm bacteria within the apical groove when compared to hand files. ³⁶ Frequent irrigation during shaping increases disinfection during root canal procedures and improve outcomes.

Future Trends of NiTi

Continued research into metallurgy and modification of the properties of NiTi with sufficient length for the majority of veterinary patients is needed. Desirable files will adapt to the shape of the canal without removing excessive dentin to preserve tooth structure. A file ideally would be able to engage all 360° of the dentinal wall within the canal, notably at the apex, to be fully effective. ³⁷ Further evolution and development of file systems should not result in excessive modification of the canal for file adaptation. Files with a high modulus of elasticity would potentially lead to ledging, zipping, and perforation complications during endodontic therapy. File adaptation to the shape of canals improves disinfection, decreases procedure time and contributes to root canal therapy success.

A progressing trend in human endodontics is the use of NiTi in a single file system with reciprocation rather than a rotary function. A reciprocal motion may reduce torsional stress by intermittently reversing the file’s rotation allowing it to remain under its plastic limitation. A single file NiTi system for instrumentation should shorten procedure time, decrease number of required instruments and result in a shorter learning curve time. ³⁸

Reciproc and WaveOne are examples of single-file reciprocating systems. Both systems utilize M-wire technology. ReciprocBlue^s is the new generation of the Reciproc system and is created using a modified temperature protocol (Figure 11). One study found that both Reciproc and ReciprocBlue had excellent resistance to cyclic fatigue in curved or S-shaped canals. This is due to the benefits of improved cyclic fatigue and flexibility afforded by M-wire technology. ³⁹ Both Reciproc and WaveOne were found to have greater than three times the number of cycles to fracture than ProTaper in a separate study. ⁴⁰ The WaveOne file has modified geometry compared to the LightSpeed and previous rotary file systems that focused on taper. The WaveOne file has a modified triangular cross-section that continues through the apex. The WaveOne Gold has a differing parallelogram shape on cross-section. Only one of the two cutting angles of the WaveOne Gold file is in contact with the canal wall during instrumentation to minimize potential taper lock. ⁴¹ WaveOne and WaveOne Gold systems were compared on extracted human molars to determine use associated with torsional resistance andboth systems yielded excellent torsional resistance with WaveOne having a slight advantage in constricted canals. ⁴² In comparing both Reciproc and WaveOne systems, the cyclic fatigue properties of Reciproc are superior to WaveOne; however, the resistance to torsional failure of WaveOne is superior to Reciproc. The improvements in cyclic fatigue and torsional resistance have led to increased fracture resistance of NiTi reciprocating files in comparison to NiTi rotary files. ⁴³

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Figure 11.

ReciprocBlue file system. Image courtesy of VDW.

Control Cut Rotary^t is a unique rotary NiTi file system (Figure 12). The endodontic files have an inactive tip and a working cutting edge on the bottom third of the file. The lack of a cutting surface along the entire file allows for shaping of the distal third of the canal. Available file lengths are 31 and 60 mm. The rotary endodontic motor for this system is Endo-A-class Vet LED which may be used with LightSpeed files in addition to Control Cut Rotary files. There is no independent published data available at this time with Control Cut Rotary. A new version of the existing system is presently on the horizon.

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Figure 12.

Control Cut Rotary file system. Image courtesy of IM3.

Dentsply Sirona has recently developed Vet Flex&Preserve^u, a veterinary specific reciprocating NiTi file system in 45 and 70 and 120 mm lengths (Figure 13). Though limited information is available, the file system allows the veterinary endodontist to instrument the canal through only one access site. Elimination of a second access site allows the preservation of tooth structure and decreases procedure time. Their present focus on a veterinary specific endodontic system is promising for future advancements in NiTi instrumentation. There is no independent published data available at this time with VetFlex&Preserve.

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Figure 13.

Vetflex & Preserve Reciprocating File. (A) Top image: 70 mm file shown in handpiece using reciprocating motion. Bottom image: Diagrammatic view of forward and reverse action. (B) 70 mm file side view. Image courtesy of Dentsply Sirona.

Dentsply Sirona has developed an X-Smart iQ motor, which is operated by the DENTSPLY iOS application on an iPad (Figure 14). Bluetooth technology allows the procedure to be cordless. The X-Smart iQ motor has the advantage of observing real-time torque and subsequent potential binding of a file within a canal before fracture. This motor is used with the WaveOne Gold and Vet Flex&Preserve systems, which are both reciprocating systems.

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Figure 14.

Vetflex & Preserve X-Smart Q motor. Image courtesy of Dentsply Sirona.

The future of NiTi instrumentation in human endodontics is progressing from rotary instrumentation to single use reciprocating file systems. Advancing technology with torque control provides the endodontist with real-time visualization and control during instrumentation. This will hopefully contribute to endodontic success. Although LightSpeed is the only available NiTi rotary system that is available in veterinary endodontics for the majority of teeth needing root canal therapy, a reciprocating NiTi system is available with the VetFlex&Preserve to provide endodontists with the file length required for the majority of patients. The use of the X-Smart Q motor provides valuable feedback during instrumentation. The Control Cut Rotary file system has both a rotary and reciprocating function through use of the Endo-A-class Vet LED motor. This unique system provides the veterinary endodontist with the ability to modify the shaping technique for a specific tooth.

Conclusion

The successful use of NiTi relies on a basic understanding of the properties and metallurgy of the alloy. NiTi's characteristics of superelasticity and shape memory have improved speed in root canal therapy and have decreased operator fatigue, leading to reduced procedural errors. Unfortunately, instrument fracture and separation due to torsional failure and cyclic fatigue remain a concern. Recent advancements in NiTi technology include single file reciprocating systems, M-wire technology that decreases cyclic fatigue, and modified file geometry that improves torsional resistance.

There are numerous NiTi instrument systems available in human endodontics that are not adequate for veterinary use due to file length. LightSpeed’s (LS1, LSX) 50 mm files have historically met the need for longer files. Although data is sparse, and it is not a rotary system, Dentsply Sirona's recent development of a NiTi reciprocating file system that requires only one access site is promising for the veterinary endodontist. The future development and continued use of NiTi instrumentation rely on the basic tenets of shaping canals: safety, effectiveness, and simplicity. ³ Regardless of the materials used, these tenets will determine the future of systems in human and veterinary endodontics.