Trends in Rotary File Design

By Dr. Matthew Malek

It has been over a century since the first rotary file was introduced, when Oltamare used fine needles with a rectangular cross-section mounted on a handpiece ¹; however, early efforts to implement engine-driven files faced significant challenges, including poor resistance to torsional and cyclic fatigue, alteration of the canal’s internal morphology, and a high risk of iatrogenic errors.  Many engineers of the time concentrated on kinematics, exploring various motion patterns to enhance the technology’s functionality. However, the concept failed to capture the enduring attention of a broad base of clinicians.

A renewed focus on kinematics in the following decades led to notable enhancements. In 1963, MicroMega engineers located in France introduced the Giromatic system, one of the earliest commercially available devices to employ reciprocating motion ². This system utilizes stainless steel reamers powered by an engine to generate controlled reciprocation, aiming to enhance safety and reduce instrument fatigue. Micromega credited the Giromatic as one of the first systems to implement this form of kinematic motion in endodontic instrumentation ³. While researchers were assessing the impact of emerging kinematic systems, they remained largely unaware that just a few years earlier, in 1960, a transformative alloy—nickel-titanium (NiTi), also known as Nitinol—had been developed in Silver Springs, Maryland.  Decades later, this alloy would revolutionize root canal instrumentation and fundamentally alter endodontic practice. The endodontic community would wait nearly three decades for the transformative potential of NiTi to be realized. In 1988, Dr. Harmeet Walia first reported the use of  NiTi (56% nickel: 44% titanium) in endodontic files, highlighting its superior flexibility and shape memory ⁴. This innovation paved the way for further advancements. In 1990, Dr. John McSpadden developed the first engine-driven NiTi rotary instrument. By 1992, Dr. Ben Johnson had introduced and commercialized the first 0.04 and 0.06 taper rotary files, marking a significant milestone in the evolution of rotary endodontics⁵.

Researchers quickly recognized the unique properties of the NiTi alloy, particularly its superelasticity and shape memory. These characteristics allow the material to undergo reversible deformation of up to  8% strain—significantly higher than the ~0.2% strain limit typical of most conventional metals—without permanent or plastic deformation.⁶ This enhanced flexibility and resilience translated into notable clinical advantages, including a reduced incidence of instrument separation due to cyclic fatigue, improved preservation of the original canal morphology, and minimized canal transportation ⁷.

However, it was soon recognized that while the shape memory property of NiTi allows instruments to return to their original form after deformation, it also makes them susceptible to cyclic fatigue. Additionally, despite significantly outperforming stainless steel files in maintaining canal curvature, NiTi instruments can still exert a tendency to straighten the canal, albeit to a much lesser extent ⁸. To address these limitations, engineers shifted their focus to modifying the phase transformation behavior of NiTi alloys through various heat treatment procedures. By altering the transformation temperatures, they aimed to enhance flexibility, fatigue resistance, and clinical performance. Over the next three decades following the introduction of the first NiTi rotary files, the focus of innovation gradually shifted from kinematics to metallurgy, marking a new era in the evolution of endodontic instrumentation ⁹. The rapid pace and breadth of advancements in NiTi file technology have given rise to the development of numerous novel technologies and heat treatment processes. As a result, there are now approximately 150 to 200 different file types available on the market, combining metallurgical advancements with innovative geometry and kinematics to revolutionize the cleaning and shaping of canals.

The wide variety of files available on the market can often confuse dentists, as each company promotes its file system as uniquely superior. However,  a careful evaluation reveals that many of these systems share fundamental physical similarities. Understanding these shared characteristics can help clinicians choose the most effective option within each category, maximizing the benefits of current file technologies. Next, we will review contemporary trends in NiTi rotary file design, focusing on metallurgy, geometry, and kinematics.

Metallurgy:

Shortly after the introduction of Nitinol, scientists discovered that its temperature-dependent, two-phase crystalline structure—characterized by transformations between austenite and martensite—could be altered ¹⁰. Austenite, the high-temperature phase of the alloy, is known for its superelasticity and rigidity, whereas martensite, the low-temperature phase, is distinguished by its flexibility and softness. The rhombohedral phase (R-phase) is an intermediate state that appears during cooling, characterized by a coexistence  of both austenite and martensite structures, and occurs just before the complete transformation to the martensite phase ¹¹. An austenitic file can also transform into martensitic under mechanical stress—a process known as stress-induced martensite transformation. While this change is reversible with heat, excessive stress beyond a certain threshold can lead to permanent plastic deformation of the file ¹².

By applying various heat-treatment processes, engineers have successfully modified the alloy’s phase transformation between austenite and martensite, allowing them to tailor the file’s mechanical behavior to perform optimally at specific temperatures. This advancement paved the way for the development of a wide range of files with distinct mechanical behaviors, ranging from the original austenitic files, which are stable at room temperature, to those exhibiting martensitic properties. Over the past 15 years, the evolution of this technology has prompted numerous companies to introduce their own proprietary heat-treatment methods. Some of these notable technologies are listed below:

Technology	Company	Date	Significance	Examples
M-Wire	Dentsply Tulsa Dental Specialties	2007	Higher R-phase and martensitic structures compared to original austenitic files	ProFile®, GT®, ProTaper® Next, WaveOne®
R-phase	Sybron Endo	2008	Austenite; twisted in R-phase	Twisted files (TF™)
CM-wire	DS Dental	2010	Higher total stable martensitic phase	Hyflex® CM, V-Taper® 2H
Blue-wire	Dentsply	2011	Higher total stable martensitic phase	Vortex Blue®
Gold-wire	Dentsply	2011	Higher total stable martensitic phase	ProTaper® Gold, WaveOne® Gold,
EDM	Coltene	2016	Higher total stable martensitic phase; includes the electric discharge machining technology	Hyflex® EDM
Maxwire®	FKG	2016	Martensitic in room temperature changing to austenitic in the canal	XP- 3D Shaper™, XP-3D Finisher™, XP – 4D ™
DualWire®	Zarc	2021	Includes two heat treatments (Gold and Blue) in a single instrument.	BlueShaper PRO®
Compared to earlier heat treatment technologies such as M-wire and R-phase,, each subsequent advancement has aimed, among other goals, to increase the proportion of stable martensitic crystals in the alloy at body temperature. This shift results in files that are  more martensitic, offering greater elasticity and improved resistance to cyclic fatigue. Two notable exceptions to  this trend are MaxWire and DualWire heat treatment technologies. MaxWire produces  files that are martensitic at room temperature but transform to the austenitic phase inside the canal, allowing the file to adapt to the canal’s shape. DualWire technology takes a unique approach by combining two different heat treatments within a single file (Blue and Gold), with the apical third treated to be more austenitic (Gold) than the shaft (Blue), optimizing flexibility  where it is most needed13.

 

Files with more stable martensitic crystals are softer and more elastic, enabling them to deform reversibly without exhibiting shape memory or a tendency to return to their original shape, unless heated above a specific threshold known as the austenite finish temperature. If the austenite finish temperature is higher than body temperature, the file will retain a proportion of  martensitic crystals during use in the root canal ¹⁴. This increased martensitic content enhances the file’s flexibility, allowing it to bend or deform within a certain range without permanent plastic deformation, thereby improving its resistance to cyclic and, to some extent, torsional fracture.

Geometry

Geometry refers to the physical characteristics of a file, including its taper, thickness, cross-sectional design, helical angle, and pitch.

Taper and thickness: In general, a file’s thickness and taper are directly related to its overall mass. In other words, for any given cross-section, a file with greater thickness and taper will possess a higher mass,  resulting in increased stiffness ¹⁵. These characteristics make thicker and higher-taper files useful in wider canals or when increased stiffness is needed to resist torsional fatigue. However, this comes at the cost of reduced resistance to cyclic fatigue. NiTi files inherently possess superelasticity and flexibility, allowing manufacturers to increase taper and thickness with less concern for cyclic fatigue and canal straightening. Still, under normal clinical conditions, thicker, higher-taper files are more likely to alter the internal canal and are at a greater risk of separation in curved canals compared to thinner files. Conversely, lower-taper files tend to preserve canal curviture, which is advantageous in curved canals. To offset the increased stiffness caused by greater taper or thickness, modifications to the cross-sectional design have been employed.

Cross-section: Among other properties, the cross-sectional design influences cutting efficiency, debris removal, and flexibility. A larger cross-sectional mass increases stiffness, which is beneficial for enhancing torsional fatigue resistance, whereas a smaller cross-section is more favorable for resisting cyclic fatigue ¹⁶. A square cross-section generally exhibits greater stiffness than a rectangular one,¹⁷ while a triple helix design tends to be stiffer than a triple U cross-section ¹⁸. Cross-sectional modifications have been strategically used to reduce stiffness in larger files. For instance, ProTaper^® Gold files feature a convex triangular cross-section in sizes  S1 to F2, and a concave cross-section in sizes F3 to F5—an intentional design that decreases core mass in larger files, improving flexibility without compromising cutting performance.

Helical angle and pitch: The helical angle refers to the angle of the cutting edge relative to the instrument’s long axis, while the pitch is the distance between two consecutive cutting edges when viewed laterally. Generally, the helical angle and pitch share an inverse relationship—an increase in helical angle corresponds to a decrease in pitch, and vice versa. Studies have shown that increasing the pitch or decreasing the helical angle of a file enhances its resistance to cyclic fatigue but reduces its resistance to torsional fatigue¹⁹. Additionally, these changes are associated with lower cutting efficiency ²⁰. It is important to note that the unwinding of rotary files after use results in an increased pitch and a decreased helical angle. These changes not only reduce cutting efficiency but also increase the file’s susceptibility to torsional fracture, particularly in tight or constricted canals.

Kinematics

While full reciprocation—equal clockwise and counterclockwise rotation—had been incorporated into rotary devices as early as the 1960s, the concept of partial reciprocation, involving unequal clockwise and counterclockwise rotation, was introduced to endodontics in 1985 by Dr. James Roane through the balanced force technique. This innovative technique, initially applied to hand instrumentation, was effective in maintaining the centering of stainless steel files within the canal ²¹. In 2008, Dr. Ghassan Yared adapted the concept to engine-driven instruments by investigating the cyclic fatigue resistance of a standard ProTaper^® F2 file in both continuous rotary and reciprocating motions ²². Building on this foundation, Dentsply Tulsa Dental Specialties introduced the WaveOne^® file system in 2011—one of the first to utilize reciprocating kinematics in a commercially available engine-driven file. Following the introduction of WaveOne^®, several other reciprocating file systems have been developed and are now widely available on the market. Research has demonstrated that reciprocating motion—particularly in the horizontal plane—offers significant advantages, including improved resistance to both cyclic and torsional fatigue ²³. Additionally, alternative kinematic approaches, such as the vertical oscillations used in the Self-Adjusting File (SAF) system, have been introduced, offering similar mechanical and clinical benefits²⁴.

Summary

The integration of metallurgy,  geometry, and kinematics has resulted in the development of a wide variety of file systems, each with distinct properties. For instance, the WaveOne^® Gold file combines a heat-treated gold alloy rich in martensitic phase for enhanced flexibility, with reciprocating motion to maximize both cyclic fatigue resistance and cutting efficiency, leveraging the benefits of both advanced metallurgy and kinematics.

The era of just relying on one or two file systems to manage all cases is long gone. Today, with a wide range of files available – each offering distinct characteristics – clinicians have the advantage of selecting the most appropriate system tailored to the specific demands of each case. Nonetheless, it is essential to recognize that many of the files currently available on the market share similar core characteristics, with only subtle differences between them. A typical file selection  in an endodontic practice will often include at least one file representing  each of the following categories (glide path rotary files are not the subject of this article):

• A thick or progressively tapered martensite-austenite file for shaping large canals
• A thin or thick austenitic file for straight canals or retreatment cases
• A thin martensitic file for negotiating curved canals
• A 3D-adaptive file designed for C-shaped or anatomically wide canals

A wide canal may require a file with a greater taper for effective shaping, while a narrower canal is better suited for a file with a lower taper. In curved canals, a more martensitic file is preferred to minimize the risk of cyclic fatigue and canal transportation. Conversely, in straight but constricted canals,  a slim yet more austenitic file may be more effective, offering increased resistance to unwinding and torsional fatigue fracture. Another file system that can be particularly useful in certain cases—such as c-shaped canals—is the so-called “3D” file. A notable example is a file utilizing Maxwire technology, which adapts to the canal’s shape and helps preserve its internal anatomy ²⁵.

When selecting a file within each category, it is important to consider design features such as cross-section, helical angle, and pitch, as each significantly influences the file’s mechanical behavior. For example, a file with a higher pitch generally offers greater resistance to cyclic fatigue but may be more vulnerable to torsional stress. Similarly, a triangular cross-section typically provides greater flexibility compared to a rectangular one.

The following table summarizes key file specifications and their relative effects on file performance:

	Higher cyclic fatigue resistance	Higher torsional fatigue resistance
Higher Martensite crystals	+
Higher thickness		+
Increased taper		+
Cross-section	Triangle, S-shaped, U-shaped	Rectangular, triple helix, convex triangle
Increased helical angle / reduced pitch	+
Reciprocation movement	+	+

 

Ultimately, it is essential to acknowledge the significant impact of the operator’s experience on the successful cleaning and shaping of the canal²⁶. It is well understood that it is the clinician—not the files themselves—who ultimately performs the treatment. It has also been shown that the outcome of root canal treatment depends on other significant factors, rather than the file choice.²⁷ Nevertheless, when properly selected, files used by a skilled operator become invaluable tools for delivering high-quality, efficient care that meets patients’ expectations.

References

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