Modern Endodontic Rotary File Systems - A Review

Introduction

An accurate diagnosis and sufficient mechanical preparation of the pulp space for restoration are essential for a successful endodontic procedure. Over the past 40 years, the endodontic toolbox has become more complicated. Various methods of preparing roots canals have been published in the relevant literature.¹ Stainless steel (SS) instruments have been used for canal preparations for a long time. The advent of rotary instruments has successfully revolutionised both the practise and technique of endodontics over the last ten years. The metallurgy, design elements, and technique of propulsion (rotary/reciprocation) of rotary files have all undergone continual alteration, resulting in revolutions both inside the canal's walls and in the field of current endodontics.^1-2

Nickel-titanium (NiTi) alloy combinations have been transforming the practise of endodontics ever since they were first introduced. In terms of mechanical qualities, they outperform regular stainless steel in a striking way.²

Despite its benefits, the biggest disadvantage of the NiTi rotary system, according to researchers, is the instrument's unexpected breaking, which has a negative impact on the root canal therapy outcomes. Some of these mishaps can be prevented, though.

For these various reasons, the objective of this article is to examine the concepts and information essential for all professionals gearing up for endodontic procedures. This encompasses insights into rotary instrument features, developments in nickel-titanium alloys, instrument design, modes of rotation (continuous and reciprocating), and the occurrence of instrument fractures.²

History

Dentists utilised carbon steel and stainless steel endodontic instruments to clean and shape canals for a very long time before NiTi files were developed. These materials were less flexible and more prone to procedural errors. In the 1960s, metallurgist W.F. Bueler created the first NiTi alloys, which differ from other alloys in that they have unique super elastic and shape-memory qualities. Endodontic files made of NiTi arch wire were for the very first time invented by Harmeet Walia in 1988. NiTi alloys have now grown to be a crucial component of endodontic treatment.³

Characteristics

Key features of nickel-titanium alloy

The majority of alloys of nickel- titanium utilised in root canal therapy have a chemical composition that is similar to 55-Nitinol and comprises Nickel (Ni) around 56% (wt.) and titanium (Ti) which is approximately 44% (wt.). Ni-Ti has the innate capacity to modify the atomic bonds it forms, which results in distinctive and notable alterations to the alloy's mechanical characteristics and crystallographic arrangement.⁴

There are three microstructural phases in NiTi alloys:

1. Austenitic phase: Also referred to as the parent phase or the high-temperature phase. This phase of the Ni-Ti alloy exists at room temperature. One of the hallmarks of the this phase is the material's capacity to revert to its original state after yielding to the force that produces deformation.⁴

2. Martensitic phase: The nickel titanium alloy is also referred as the low-temperature phase because it is in the martensitic phase at low temperatures. When the force that generates the deformation is removed, the alloy maintains the deformation, which is a characteristic of this phase.

3. R phase: During the forward transformation from martensite to austenite on heating and the cooling-induced transition of austenite into martensite, an intermediate phase known as R-phase (rhombohedral structure) can occur.

Characteristics

1. Taper: The extent to which the file diameter increases for each millimeter along its operative surface, extending from the tip to the handles of the file.⁵

2. Diameter: A linear connection between two points on a circular cross-section, such as that of a file, passing through its center. This can fluctuate along the entire length of the file due to its taper. Understanding the file's diameter enables the practitioner to compute the file's size at the curved point and determine the corresponding tension applied to the file.⁵

3. Cross-section: The mass of a file is defined by its geometric shape when sectioned perpendicular to its longitudinal axis.

4. Edge: The section of the file that comes into contact with the dentin and connects two grooves.

5. Radial land: When a plane is made in place of a cutting edge, a radial land is created. Radial lands were constructed to reduce canal traffic and cutting depth.

6. Grooves or flutes: The section of the instrument used to gather dentin and soft tissue remnants retrieved from the canal wall. Large grooves allow for more cutting than narrow grooves since they take longer time.

7. Helical angle: The cutting edge angle and the file's longitudinal axis. It is in charge of cutting by rotating or traction, as well as eliminating clogged debris in the grooves. It can be fixed or variable, defining the file's degree of screw-in and thus being crucial in terms of the instrument's torsion.

8. Rake angle: The angle created in the cross-section between the forefront edge and the radius of the file. The rake angle is considered positive or indicative of cutting when the angle between the leading edge and the cutting surface is obtuse, and negative or indicative of scraping when the angle is acute.

9. Pitch: Distance between the two corresponding sites where the pattern does not repeat itself. The number of spirals and the pitch of the file increase with the helix angle.⁵

Properties

Superelasticity: The super-elasticity of the nickel titanium alloy is due to the stress-induced unstable martensite transition, which tends to return to its former shape upon unloading, to the stable austenitic (stress induced property).⁶

Shape memory: The NiTi alloy's shape memory property is a specific heat-controlled property that is induced by a transition from a stable austenite to a stable martensite phase (martensitic re-orientation), and it does not resume its original shape upon unloading. As a result, when heated above its transition temperature, the alloy can keep its original shape.⁶

Transformation temperature: Transformation temperature is the one that causes the transition from twinned martensite to austenitic structure. Each phase of the NiTi alloy has a specified transformation temperature for the beginning and end. NiTi typically transforms at temperatures that are much below or very near to body temperature. Additionally, the temperature of transformation affects various properties of NiTi alloy.

Electropolishing

In order to minimize surface defects in metal instruments, the electropolishing procedure, often referred to as electrochemical surface treatment, involves the deposition of metal ions on an object's surface through electroplating. FKG used this technique to create NiTi instruments for the first time in 1999. To reduce manufacturing flaws and produce a smoother surface, mechanically turned NiTi instruments are submerged in an electrolyte bath. This causes an oxidationreduction reaction to form a uniform oxide layer on the instruments' surface, increasing their resistance to cyclic fatigue, cutting effectiveness, and corrosion resistance without compromising their superelasticity (SE). This process is used to produce the Race, EndoSequence, and One Shape instruments.⁸

What's New?^1,9,10-15

1. BT-RaCe (FKG Dentaire)

2. Hyflex CM (Coltène/Whaledent)

3. The Hyflex EDM

4. Vortex Blue and Protaper Gold (Dentsply)

5. Reciproc Blue, VDW; WaveOne Gold)

6. ProFile Vortex Blue; ProTaper Gold

7. Max-Wire (Martensite-Austenite-electropolish-fileX)

8. XP-endo Shaper

9. XP-endo Finisher 

10. 2Shape File System (MicroMega)

11. One Curve (MicroMega)

12. TruNatomy (Dentsply Sirona)

13. TRUShape (Dentsply Tulsa Dental Specialties, Tulsa, Oklahoma (OK), USA)

14. ZenFlex

The new rotating file system should be adopted for variety of factors:⁹

- Increased outcome consistency and safety.

- Increased effectiveness in achieving or exceeding safety and result standards.

- Enhancing patient endodontic service cost and/or treatment ease.

1. BT-RaCe (FKG Dentaire) (Figure 4a): A brand-new file system called BT-RaCe, which was developed from standard austenite NiTi, was released in 2013. These files have a distinctive booster tip design with a blunt tip that has a consistent triangular cross section. Their resistance to cyclic fatigue is increased by a surface electrochemical process. It is asserted that the booster tip will lessen transportation and deviation. This system's distinctive feature is a customised tip, allowing it to trace canal curvatures with lower strain and greater safety.¹⁰

2. Hyflex CM (Coltène/Whaledent) (Figure 4b): It is the first nickel titanium endodontic alloy and the rotary system introduced in 2011 that has undergone thermo mechanical treatment. In contrast to ordinary NiTi files, NiTi files fabricated with CM wire lack super elastic properties that exist at both room and body temperatures. Hyflex CM has an austenite finish temperature of 47-55°C, which is higher than the intracanal temperature. According to reports, these files are 300% more resistant to cyclic fatigue than traditional NiTi systems. This technique tends to diminish the straightening effect on the canal because of its controlled memory impact.

3. Hyflex EDM files (Coltene/Whaledent, Altstätten, Switzerland) (Figure 4c): First launched in 2016 and made from CM alloy employing EDM technology. It is seen as an extension of the Hyflex CM file system. EDM is a type of thermal erosion technique that produces a crater-like finishing surface on the instrument when used with electrically conducting materials. While maintaining flexibility comparable to CM Wire instruments, this file system has a substantially higher wear resistance than HyFlex CM, M-Wire, and traditional NiTi.¹¹

4. Vortex Blue and Protaper Gold (Dentsply) (Figure 4d,4e): In 2011, ProFile Vortex Blue debuted Vortex Blue and Protaper Gold. These files are produced using a sophisticated heating-cooling proprietary process. The titanium oxide is apparent coating on the surface and gives the alloy a blue hue. They are currently offered as two heat-treated systems in gold and two in blue. Reciproc Blue, VDW, and WaveOne Gold are used in reciprocating motion, while ProFile Vortex Blue and ProTaper Gold are utilised in rotary motion. These systems are susceptible to deformation, which results in the controlled memory effect.

Under clinical circumstances, ProTaper Gold transforms at a temperature that is significantly higher than the body temperature, resulting primarily in martensitic or R-phase. In comparison to the standard NiTi and M-wire instruments, it was discovered that all gold and blue heat-treated instruments had higher levels of elasticity and fatigue resistance. In particularly sharp curved canals, all these heat-treated systems yield well-centred canal preparations.¹⁵

5. Maxwire (Martensite–Austenite–electro polish-file X) (2016) (FKG) (Figure 4f): It is the first Nickel-titanium alloy used in endodontics to combine the effects of shape memory and super elasticity. These devices are martensitic at room temperature, but they transition to an austenite phase at intraduct temperature, giving them their curved shape. Their curved design makes it possible to create complex canals with the possibility to handle canal imperfections. Their mechanical impact, combined with the irrigant's agitation, encourages better bacterial reduction and biofilm removal.¹²

6. 2Shape File System (MicroMega) (Figure 4g): It is a recent generation file system created using a patented heat treated T-wire, which seeks to increase both cycle fatigue resistance and flexibility by 40% over One Shape. With its most recent version of triple-helix cross section, this technology achieves the ideal ratio of two primary and one secondary cutting edge for cutting effectiveness and debris removal. Two files make up the 2Shape rotary file system: TS1 (25/.04) and TS2 (25/.06).¹⁴

7. One Curve (MicroMega) (Figure 4h ): A single file NiTi rotary method for root canals shaping called One Curve was released in 2017. C-wire heat treatment technique was used to create the One Curve file system. According to the manufacturer, this tool has a regulated memory and can prebend, which improves root canal formation. For improvized cutting efficiency and centering capabilities, this file contains a changeable cross section. According to reports, compared to OneShape system, One Curve files are 2.4 times more resistant to cyclic fatigue.^9,13

8. TRUShape (Dentsply Tulsa Dental Specialties, Tulsa, Oklahoma (OK), USA) (Figure 4i): This system was made by performing heat treatment after cutting flutes with characteristic S-curve bends from NiTi file blanks. TRUShape's R-phase and martensitic transformation temperatures overlap throughout cooling and heating cycles, making it impossible to discern between the two phases. The literature typically indicates that bigger inner core diameter instruments have greater torsional resistance but less cycle fatigue.

9. TruNatomy (Dentsply Sirona) (Figure 4j): Utilizing a specialised post-machining heat treatment procedure, these instruments are made. As a result, this system is super-elastic and has less shape memory than traditional NiTi or M-wire. TN showed longer cycles to failure than VortexBlue in single and double bent canals because VortexBlue has a larger austenitic concentration at body temperature. However, Vortex Blue and HCM showed a higher number of cycles to failure during the dynamic cyclic fatigue test with axial movements. WS Chan et al. found that TN had the lowest canal transit and the highest capacity to preserve tooth structure.

10. ZenFlex (Kerr Corporation, Pomona, CA, USA) (Figure 4k): It outperforms VortexBlue in terms of cycle fatigue resistance while also having higher torsional stress resistance. However, Zanza et al. discovered that this file system had fewer cycles to failure than VortexBlue. This may be because it contains a higher proportion of austenitic phase at ambient temperature than VortexBlue.¹²

The GenENDO files system (Figure 4l): This system was designed with user-friendliness in mind, and it comes with special files for coronal flaring, choosing glide pathways, and for efficiently preparing & shaping the canals and for finishing of the canals. These files were all created using various technologies, which gives each one of them a unique identity.

The four GenENDO file sets that are available are Coronal flare (CF), Glide Path File (GPF), Preparation file (PF), and Universal Finishing file (UF1), as well as a set of six files (FF2 and FF3 in addition to the four file set). The suggested speed for CF and GPF is 400 rpm, while the suggested torque for PF, UF1, FF1, and FF2 is 2.5 Ncm.

Advances in Rotation (Figure 4m)

Two reciprocal rotation systems— Reciproc (VDW, Munich, Germany) and WaveOne (Dentsply, Mallifer) have been released in the market. These instruments' primary characteristic is that they cut anticlockwise; hence, they cut actively if the rotation is more anticlockwise than clockwise. The file's cross-sectional design is flawed; as it is rotated anticlockwise, cutting angle with the canal walls shifts from negative to positive. With a few notable exceptions, all instruments are made to cut anticlockwise, which suggests a positive cutting angle. Any instrument built for continuous clockwise rotation can also operate with reciprocating movement, provided the clockwise revolution in this movement is wider than the anticlockwise revolution.^16,17

In two distinct categories of motors, a reciprocating rotation option is available. Open motors, such as those used in the ATR Vision (ATR, Pistoia, Italy), iEndo Dual (Acteon, Merignac, France), and SAF system pro (ReDent-Nova, Ra'anana, Israel), enable the adjustment of the angles of clockwise and anticlockwise rotation as well as their speed.

Closed motors are fixed and immutable. Because the tool does not cut or penetrate the canal, files with clockwise cutting and reciprocating movement cannot be utilised in these motors. For the use of reciprocating option, files made specifically for cutting anticlockwise should be used in these motors. Two examples are VDW Silver and WaveOne.

Metallurgical Development

CM NiTi wire was developed in 2010, and it has low nickel content (52%) than normal NiTi alloy, which has a nickel percentage of 54.5% to 57%. This enhances the alloy's mechanical qualities. Additionally, the post-manufacturing heat treatments to alloy SE508, raises the CM-wire's austenite temperature at phase transition to 55°C and causes martensite to predominate in its crystal structure at room temperature. In conclusion, metal alloys with regulated memory, enhanced fracture resistance, high flexibility, and ability to withstand cyclic fatigue have been developed using CM wires. This file could be prebent to match the root canal's curvature.¹⁸

Blue and Gold Wires: Titanium oxide layer is formed on the CM-wire alloy through repeated heating and cooling processes, and the titanium oxide layer's thickness affects the alloy's surface colour. The CM-Blue wire has a surface colour of blue between 60 and 80 nm, and the CM-Gold wire has a surface colour of golden between 100 and 140 nm.¹⁹ The titanium oxide layer improves cutting effectiveness and wear resistance by compensating for the hardness that was lost during the manufacturing of the CM-wire alloy.²⁰ The Vortex Blue system, the Sequence Rotary system, the Reciproc Blue system, and the ProTaper Gold system are examples of representative equipment.

EDM: Electrical discharge machining, often known as EDM, is a popular noncontact thermal erosion method in engineering that generates a corroded rough metal surface by melting the metal surface and evaporating small amounts of metals in the presence of a dielectric fluid. This treatment gives NiTi tools a rougher, harder surface, which enhances cutting effectiveness.^21,22 The HyFlex EDM system was introduced in 2016 and was produced using this spark-erosion technology. According to X-ray diffraction studies, the HyFlex CM system is made up primarily of martensitic NiTi alloy, while the HyFlex EDM system is made up primarily of R-phase NiTi alloy and a significant amount of martensitic NiTi alloy.²³

Max Wire: The Max-wire alloy was developed through alloy heat treatment. The alloy exhibits a shape memory property, causing it to shift from martensite to austenite when inserted into the canal at a temperature equal to or exceeding 35°C. Consequently, the instrument adopts a semi-circular shape from its initial form. When coupled with the agitation of irrigation fluid, the eccentric rotation against the root canal walls enhances the contact area between the instrument and the canal wall. This action serves to both beat and scrape the wall, thereby facilitating effective cleaning of the root canal. The XPendo Shaper family system stands out as an exemplary tool in this regard.²⁴

Instruments with a patency file or mechanical glide path

The unobstructed, clear route from the root canal orifice to the apical stop is termed root canal patency, facilitating subsequent access with larger instruments. A smooth root canal patency reduces the likelihood of iatrogenic errors during root canal preparation. Establishing a gliding path for calcified and/or narrow canals is the most challenging aspect of root canal preparation, requiring high technical proficiency. The watch-winding method often involves the gradual clearing of the root canal using small stainless steel hand tools like #6, #8, and #10 root canal files.²⁵

The small tip diameter and taper are defining characteristics of mechanical patency files. For instance, M-wires' ProGlider and PathFile systems have excellent flexural strength and flexibility. ProGlider file with tip diameter #16 and a taper of 0.02 to 0.085 is used to achieve a wider canal compared to hand stainless steel file #15 (ISO standard) after unclogging tiny canals with stainless steel files #6, #8, and #10. When employed in a reciprocating motion, R-Pilot and WaveOne Gold Glider NiTi glide path files would offer exceptional cycle fatigue resistance.

Conclusion

As outdated systems are replaced, new endodontic file systems for root canal instrumentation are introduced. The most reliable design elements from the past are still used in a safest, most effective way, together with the most recent technical developments. The endodontic file industry has undergone a revolution, owing to NiTi's super-elastic and shape-memory capabilities. The present literature demonstrates widespread use of the nickel titanium alloy which has controlled memory and can be heat treated. These more recent designs have less shape memory and greater flexibility. These tools can be pre-bent to maintain the flexed shape, which facilitates the canal entry when clinically used. While we are riding the wave, let's not forget that the key to success is a skilled endodontist, with the type of material employed being merely a supporting element. When modern technology is used carefully and combined with a basic understanding of anatomy, root canal treatments will be consistently of higher quality, helping to retain teeth longer.

Conflict of Interest

None