The Hidden Variable in Root Canal Shaping: How Endodontic Motor Kinematics Influence NiTi Instrument Performance.

How does an endodontic motor influence NiTi file performance? Explore how kinematics interact with file design, canal anatomy and axial load, and examine the biomechanical rationale for adaptive motion alongside current evidence and its clinical limits.

This webinar relates to the following article published in roots magazine:

The Hidden Variable in Root Canal Shaping: How Endodontic Motor Kinematics Influence NiTi Instrument Performance

Author Information :

Dr. Renaud Giess

• Maître de Conférences des Universités – Praticien Hospitalier (MCU-PH),
• Faculté d’odontologie de lorraine, Université de Lorraine and CHRU de Nancy, France.
• His clinical and academic work focuses on restorative dentistry, endodontics, micro-endodontics, operating microscopy, heat-treated nickel-titanium instruments, endodontic kinematics, and dental ergonomics.

Co-Authors:

Dr. Remy Blathazard

Pr. Marin Vincent

Corresponding author:

Dr. Renaud Giess Faculté d’odontologie de Lorraine, Université de Lorraine

7 Avenue de la Forêt de Haye, 54500 Vandoeuvre-lès-Nancy

renaud.giess@univ-lorraine.fr

The motor is part of the instrumentation system

When a nickel-titanium (NiTi) instrument shapes a root canal, its behavior is determined by more than the alloy, taper or cross-sectional design. The instrument interacts continuously with the canal wall, and the mechanical load changes as the file advances. Axial loading, curvature, dentin contact, cutting efficiency and torsional deformation all contribute to the forces recorded at the instrument–canal interface.

The motor kinematics is part of that system.

This point is rarely considered in the literature, because endodontic files are usually classified according to their geometry and metallurgy, whereas the motor is often treated as a separate medical device. Experimental data show that changing the motor can alter the mechanical behavior of the same instrumentation system. Reynette et al. (2023) compared several endodontic motors using standardized instrumentation conditions and found differences in cutting efficiency and screwing effects, particularly when reciprocating motion was used. Furthermore, a recent study by Delanoë et al. (2025) specifically demonstrated that altering clockwise and counterclockwise reciprocation angles directly influences the mechanical behavior of the instrument [1].

The practical implication is straightforward: the manufacturer’s stated speed and maximum torque alone do not fully explain the mechanical response of the instrument during root canal shaping. Rather, this behavior depends on how these parameters are actively delivered and modified by the motor.

Instrument design and motor kinematics cannot be considered separately

Alongside canal anatomy, the instrumental geometry of NiTi files is one of the most influential parameters determining how stress is distributed along the instrument. Taper, pitch, cross-sectional configuration and core dimensions influence bending, stiffness, torsional resistance and the amount of contact between the instrument and canal wall. Finite element studies and in vitro analyses from our group and others have repeatedly demonstrated that this instrumental geometry fundamentally alters the mechanical response of rotary instruments [3-5].

Metallurgy adds another layer. Heat treatment can modify, at a given temperature, the phase composition and mechanical properties of NiTi, with important consequences for flexibility and cyclic fatigue behavior [4,6]. These properties are clinically valuable in curved canals, but greater flexibility does not automatically translate into safer instrumentation under every loading condition.

Axial loading is a good example. Maki et al. (2022) demonstrated that increasing downward load changes the vertical forces and torque generated during rotary instrumentation [7]. More recently, Yamamoto et al. (2026) showed that both downward load and rotational kinematics influence the mechanical response of a heat-treated NiTi instrument during root canal shaping of extracted teeth [8]. Therefore, the file does not have a fixed mechanical behavior that can be predicted from its alloy and geometry alone. Its response changes with the way it is driven.

This distinction is particularly relevant when discussing screw-in behavior. A file with an aggressive cutting geometry can generate a substantial apically directed force when its flutes engage dentin. That tendency is influenced by the direction of rotation (i.e., clockwise versus counterclockwise) and its characteristics, the amount of axial pressure and the geometry of the instrument. If the file becomes bound while the motor continues to drive it, torsional stress rises rapidly.

In other words, the clinically relevant question is not simply how much torque a motor can deliver, it is how the drive responds when resistance develops.

Torque is a dynamic variable

Torque is commonly used to describe the mechanical limit at which an endodontic motor reverses or stops the instrument. In practice, the situation is more complex. Resistance may develop gradually as the file contacts dentin, or abruptly when the apical portion binds. The location of that resistance also matters.

Thus, a file can experience different mechanical conditions at different points along its active portion. A preset torque value measured at the handpiece does not reflect the actual distribution or intensity of torsional stress along the entire length of the file.

The influence of axial load is particularly important. Experimental work has shown that changes in downward load affect both torque and canal-centering behavior during NiTi instrumentation [7]. These findings help to explain why operator pressure remains relevant even when a motor is equipped with torque control. Because a torque limit is fundamentally a reactive response to a mechanical overload that is already taking place, it cannot compensate for every mechanical consequence of excessive axial loading.

Canal anatomy adds another variable. Curvature changes the bending environment of the instrument and can concentrate stress in specific regions of both the file and the surrounding dentin. Furthermore, canal width plays a critical role: narrower canals significantly increase the engagement between the file and the dentinal walls, directly amplifying torsional stress and altering the required apical pressure. Finite element and experimental studies, including recent work by our group, have demonstrated changes in stress distribution as canal curvature, diameter, and preparation conditions vary [9,10].

The result is a moving mechanical target. The file is exposed to bending, torsion and axial loading simultaneously, and the relative contribution of each component changes as the instrument progresses through the canal. The experimental setup used to evaluate these dynamics, along with the real-time adaptive logic, is illustrated in Figure 1.

Fig. 1. Schematic representation of the experimental traction-compression setup used to evaluate penetration and removal forces during root canal shaping. The endodontic motor is integrated into the upper grip to drive the NiTi instrument. The inset illustrates the unified real-time response logic of adaptive kinematics, where the motor dynamically adjusts its motion based on the specific location (e.g., tip stress vs. widespread load) and magnitude of torsional stress detected along the file.

From fixed kinematics to adaptive control

Conventional continuous rotation and fixed reciprocating motion treat the movement pattern as a predetermined parameter. Continuous rotation maintains the cutting direction until a programmed torque threshold is reached. Fixed reciprocation alternates the direction of rotation according to a defined angular pattern.

Dynamic adaptive systems introduce another principle: the motor modifies its motion according to the mechanical stresses experienced by the instrument.

Several systems use some form of adaptive control, but the underlying algorithms are not interchangeable. This distinction is important when discussing adaptative motion.

Certain contemporary adaptive movements (designed primarily for left-cutting instruments) begin with a counterclockwise (CCW) rotary motion. According to the technical description of such systems, brief measurement pauses are used to assess torsional loading of the instrument. The system is designed to distinguish whether the principal torsional deformation is located toward the coronal, middle or apical portion of the file. When the measured loading reaches a critical condition, the motor modifies its motion and can introduce a relieving reverse (clockwise, CW) motion. [12]

The important feature is not simply the change from rotation to reciprocation. It is the attempt to use the instrument’s torsional response as information for the control algorithm.

A simplified mechanical sequence can be represented as follows:

canal resistance → file torsional deformation → measurement → assessment of load and location → modified motor motion.

This is fundamentally different from a motor that simply waits until a preset motor torque threshold is exceeded and then applies the same reversal response regardless of where the resistance has developed. The concept of adaptive control is illustrated schematically in Fig. 1.

Fig. 2. Schematic comparison of continuous rotation, fixed reciprocation and adaptive kinematics under increasing torsional resistance. Continuous rotation maintains the cutting direction until a predefined torque response is reached; fixed reciprocation follows a predetermined angular cycle; adaptive motion incorporates intermittent measurement phases and modifies the subsequent movement according to the detected torsional state of the instrument.

The distinction between the technology and its evidence base should be maintained. Manufacturer documentation describes these adaptive dynamics as patented movements designed to detect torsional deformation and adapt the motor response according to the location and magnitude of loading. [12] Independent studies have also examined adaptive movements, including their effects on cyclic fatigue and mechanical resistance. [11,13,14] These studies provide mechanistic evidence, but they do not establish that the system improves long-term clinical outcomes.

The potential advantage of such adaptive kinematics lies precisely in the timing of the motor response. Unlike a fixed reciprocating sequence which reverses according to a programmed cycle regardless of the load or continuous rotation which cuts until a preset threshold is reached an adaptive system inserts a critical decision point. It maintains efficient rotation when resistance is low, but modifies the movement as soon as the mechanical state of the file changes.

This distinction is particularly relevant in curved and narrow canals, where the instrument is exposed to simultaneous bending and torsional loading. It also becomes important when any rotary instrument is used under axial loads that increase its engagement with dentin.

The available evidence should nevertheless be interpreted carefully. Reynette et al. demonstrated that the motor itself influences penetration and removal forces during instrumentation. [1] Mena-Álvarez et al. examined several contemporary movement strategies and described differences between conventional and adaptive motions. [13] Laboratory studies have also reported effects of kinematics on cyclic fatigue. [11,14]

These observations support a biomechanical rationale for adaptive control, but they do not justify assuming that every adaptive motor, file system, or clinical protocol will produce the same result. Therefore, the clinical efficacy of an instrument cannot be evaluated independently from the motor kinematics used to drive it.

Why the location of torsional stress matters

Torque measured is not equivalent to the torsional stress experienced at every point along the instrument.

When the tip of a file binds while the rest of the instrument and the handpiece continue to rotate, the active portion of the instrument undergoes elastic torsional deformation. The greater the deformation and the longer it persists, the greater the mechanical demand placed on the instrument. If the file is released before permanent deformation occurs, the stored elastic energy can dissipate. If the file remains locked while rotation continues, torsional stress can approach the instrument’s failure threshold.

This is where the concept of load-adaptive motion becomes mechanically interesting. Rather than treating the file as a passive component attached to the motor, the system attempts to extract information from the file’s own deformation. The manufacturer describes this as a means of distinguishing the location of torsional loading and adjusting the motor response accordingly (for instance, by triggering a specific reverse angle to unbind the tip without over-stressing the rest of the file) [12].

That principle should not be confused with conventional torque control. Both systems respond to mechanical resistance, but they do not necessarily obtain or process the same information.

The distinction also explains why the term “adaptive” should not be used as a synonym for “reciprocating.” Reciprocating motion describes a movement pattern. Adaptation describes a control strategy in which the motion can change in response to a measured or inferred mechanical condition.

Clinical relevance

For the clinician, the significance of this distinction may vary according to the anatomical complexity of the root canal and the mechanical conditions encountered during instrumentation.

A straight and wide canal with limited curvature imposes different demands on an instrument from those encountered in a severely curved or double-curved and narrow canal. In the latter situation, bending and torsional loads may fluctuate rapidly as the file advances. Operator-applied axial force adds another source of variability.

Therefore, adaptive control may have its greatest theoretical value in situations in which the instrument is repeatedly transitioning between low and high mechanical loads.

That does not mean that continuous rotation is inappropriate, nor that adaptive motion should be regarded as a universal replacement for conventional kinematics. Continuous rotation remains a well-established approach, and instrument selection, glide-path preparation, irrigation, apical shaping, working length and operator technique all contribute to the mechanical outcome.

The selection of motor kinematics should be evaluated alongside these variables rather than as an independent consideration.

The same principle applies to torque settings. A lower programmed torque does not automatically create a safer preparation if the operator compensates with excessive axial force or repeated engagement of the file. Conversely, a motor capable of adaptive control cannot eliminate the consequences of inappropriate file selection or poor canal negotiation.

Kinematics can modify the mechanical environment. They cannot remove the mechanical problem.

Limits of the current evidence

The principal limitation of the current evidence is its distance from the clinical endpoint.

Much of the literature consists of finite element analyses, cyclic fatigue experiments, resin-block models, traction/compression measurements and extracted-tooth studies. These approaches are valuable because they isolate mechanical variables that cannot be controlled as easily in vivo. They also remove important clinical variables, including differences in dentin structure, hydration, canal morphology, irrigation, operator technique and tooth-specific anatomy.

The 2023 study by Reynette et al. was performed on a standardized preclinical platform and measured penetration and removal forces rather than clinical treatment outcomes. [1] Maki et al. used controlled downward loads to characterize torque and force generation. [7] Yamamoto et al. subsequently examined the interaction between downward load and rotational kinematics in extracted teeth. [8] These studies strengthen the mechanical argument, but they remain laboratory investigations.

The same caution applies to claims about postoperative pain, flare-ups or long-term tooth survival. A reduction in apical extrusion or an increase in cyclic fatigue resistance does not systematically translate into a clinically meaningful reduction in postoperative complications.

At present, the evidence supports a more restrained conclusion: motor kinematics are an independent mechanical variable that can alter the behavior of NiTi instruments. Whether a particular adaptive strategy produces a clinically relevant advantage (figure 2) over conventional rotation or reciprocation remains a question for comparative clinical research.

Figure 3: Clinical cases illustrating the use of adaptive kinematics to facilitate root canal shaping.

Conclusion

Rotary NiTi instruments do not operate efficiently independently of the motors that drive them. Their mechanical behavior emerges from the interaction between alloy, geometry, canal anatomy, axial loading and kinematics.

The available evidence shows that changing the motor or movement pattern can alter penetration forces, removal forces, torque generation and fatigue behavior. [1,7,8,13,14] Adaptive systems extend this concept by modifying the movement in response to the mechanical state of the instrument rather than relying exclusively on a fixed rotational pattern.

Adaptive motion is a particularly interesting example because its control strategy is based not only on motor torque but also on the torsional deformation of the file. That distinction provides a plausible biomechanical explanation for its behavior in situations in which resistance changes rapidly along the canal.

The clinical significance remains less certain.

For now, adaptive kinematics should be viewed as an additional mechanical control variable rather than as a substitute for sound instrumentation principles. The most useful question is not whether one movement is universally safer than another, but how the selected kinematics interact with the specific file, the applied axial load and the anatomy being prepared.

Although further comparative clinical research is required, adaptive kinematics represent a highly promising evolution in endodontics. Looking ahead, Komet Dental is currently working on the testing and development of new motion concepts, including the forthcoming EnDrive motor, to push the boundaries of real-time kinematic adaptation. By actively responding to unexpected stress, these dynamic systems offer a logical strategy to minimize the risk of file separation, ultimately enhancing both preparation safety and clinical confidence. The motor is not simply the device that turns the file. It is part of the mechanical system that determines how that file behaves.

References

1. Reynette C, Giess R, Davril J, et al. Influence of endodontic motors on the behaviour of root canal shaping instruments: an in vitro comparative study. BDJ Open. 2023;9:51. doi:10.1038/s41405-023-00179-z.
2. Giess R, Mortier É, Delanoë M, et al. Influence of the rotation angles on the mechanical behavior of the one reci instrument. 2025.
3. Akkoç HI, Keskin C, Aslantaş K. Dynamic analysis of a NiTi rotary file by using finite element analysis: effect of cross-section and pitch length. Australian Endodontic Journal. 2024;50(3):649-657. doi:10.1111/aej.12892.
4. Martins SCS, Silva JD, Viana ACD, Buono VTL, Santos LA. Effects of heat treatment and design on mechanical responses of NiTi endodontic instruments: a finite element analysis. Mater Res. 2020;23(3):e20200087. doi:10.1590/1980-5373-MR-2020-0087.
5. Di Russo, F.M., Gisario, A., Zanza, A. et al. Finite element design optimisation by DOE technique of the torsional stress behaviour for NiTi rotary endodontic instruments. Int J Adv Manuf Technol 136, 2377–2395 (2025). https://doi.org/10.1007/s00170-024-14956-z.
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12. Technical documentation & user manual specifications regarding adaptive motor technologies.
13. Mena-Álvarez J, Almanzor-López M, Quispe-López N, De Pedro-Muñoz A, Rico-Romano C. Analysis of the importance of the motion used in the resistance of different mechanical instrumentation systems in endodontics: a comparative study. 2022;15(13):4443. doi:10.3390/ma15134443.
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