Dynamism in Rotary Endodontics

Rotary endodontics is an asset to modern dentistry. Nickel–titanium (NiTi) instruments make root canal preparation more predictable with reduced procedural time, along with more ergonomic comfort and benefits to the clinician and patient, eventually directing toward successful outcome of treatment. However, the risks involved include loss of tactile sensitivity, chances of instrument separation, high armamentarium cost, and a learning curve.[1]

During endodontic instrumentation, endodontic files undergo flexion, torsion, tension, etc.

Dynamism in rotary endodontics refers to the mechanics and stress analysis of NiTi files during the shaping of the root canal. Performance of instrument is directly related to cyclic fatigue (fracture from repeated bending) and torsional stress (twisting), which are influenced by metallurgy, file design (pitch, cross-section), and root canal anatomy (curvature).[2]

Endodontic instrument manufacturing companies are consistently innovating instruments with newer instrument geometry, kinematics, and thermal treatments. Design improvements using advanced alloys increase flexibility, improve cutting efficiency, reduce instrument separation, enhance safety, and thus impact positive clinical outcomes with more predictibility.[2]

Based on the analysis of a systematic review, it was inferred that heat treatment and the type of instrument motion used in cleaning and shaping of the root canal have an effect on the cyclic fatigue strength. Improvements in fatigue strength were noted with reciprocating instrument motion as opposed to continuous rotation. The sterilization process has been observed to reduce the cutting ability of the files. On the other hand, upon sterilization, there is a shape recovery effect along with enhanced property of resistance to torsional and cyclic fatigue.[3]

By combining metallurgy, geometry, and kinematics, a diverse range of file systems has emerged, each with unique properties. This variety gives clinicians the freedom to choose the most suitable system for different clinical situations. For example:

• A progressively tapered martensite-austenite file is ideal for shaping large canals

• An austenitic file works well for straight canals or retreatment cases

• A thin martensitic file is perfect for navigating curved canals

• An adaptive three dimensional-file is designed for C-shaped or anatomically wide canals.[3]

Finite element analysis showed that endodontic files with cross-section design of convex triangle and triple helix designs experienced less stress as compared to S-shaped and concave triangle designs. As canal curvature increased, stress levels rose, with the S-shaped instrument showing the largest increase (up to 12%). Instruments with shorter pitch lengths distributed stress more evenly, which improved fatigue life. Maximum stress was found to be at 5–8 mm from the tip and varied across cutting edges. S-shaped sections had lower forces but higher stress because they have lower moments of inertia.[4]

A systematic review concluded that higher torque/force in endodontic files is linked to convex triangle cross-section, regressive taper, short pitch, larger instrument size, smaller canal size, single-length technique, longer prep time, deeper insertion, slower insertion rate, continuous rotation (torque), reciprocating motion (force), lower speed, and conventional alloy.[5]

Endodontic files with square cross-section exhibited the highest screw-in force and torsional stress, followed by rectangular, triangular, and slender-rectangular designs. Files with a closer pitch produced less stress compared to those with a longer pitch. Furthermore, more the root canal curvature greater is the screw-in force and reaction torque.[6]

Narrow, curved canals are the hardest to shape into a smooth taper while preserving the original anatomy. TruNatomy is one of the latest with an advanced design offering greater flexibility. A comparative study found that TruNatomy rotary files preserved pericervical dentin better than WaveOne Gold reciprocating files, leading to higher fracture resistance.[7] Analysis of photoelastic stress suggested that early instrumentation concentrated stress at the coronal region; later, stress shifted to mid and apical areas.[8]

MaxWire is a proprietary NiTi alloy that blends shape memory and superelasticity in clinical use (XP-endo Shaper and XP-endo Finisher). At room temperature, they are mostly straight in their martensitic (M-phase) state, but at body temperature inside the root canal transform into austenitic (A-phase) and take up a curved shape, adapting to complex canal anatomy and irregularities.

XP-endo Shaper shows higher cyclic fatigue resistance compared to Hyflex CM, Vortex Blue, and iRaCe, but lower torsional resistance when compared to Vortex and FlexMaster. Low taper (0.01) of the instrument increases fatigue life but reduces torsional strength, typical for smaller diameter NiTi files.[9]

The root canal system is a very complex entity. Selection of appropriate endodontic instruments for a particular clinical case is often a dilemma for the clinician, particularly in an anatomically challenging tooth. The array of armamentarium available to us today allows customised treatment approaches for various root canal anatomies. During root canal preparation, endodontic instruments do cause stress accumulation within the tooth with varying distribution at different levels. The know-how and expertise to combat the same lies in the hands of an endodontist.