Rotary science and its impact on instrument separation: A focused review.

Efficient endodontic treatment demands thorough debridement of the root canal system with minimal procedural errors. The inherent weakness of nickel-titanium alloys is their unexpected breakage. Modifications in the design, manufacturing, thermomechanical and surface treatment of alloys and advancements in movement kinetics have shown to improve the fatigue properties of the alloys, reducing the incidence of separation. This review enlightens the impact of these factors on fatigue properties of the alloy.

INTRODUCTION

Separation of rotary nickel–titanium (RNT) has been an enigma to endodontists and practitioners since their introduction by Walia et al. in 1988.[1] The reported incidence of separation of RNT is 1.3%–10% in the literature,[23] 44.3% of which are attributed to cyclic fatigue and 55.7% to torsional failure. Cyclic or flexural fatigue occurs when the instrument rotates freely in a curved canal, with repeated compression/tension cycles to the point of maximum flexure to cause fracture of the instrument. Torsional fatigue failure occurs in a narrow canal, when the tip of a rotary file binds while the shank of the handpiece continues to rotate exceeding the elastic limit of the file.[4] Separated instrument compromises disinfection and obturation protocols, leading to failure of endodontic treatment.[5] Removal of RNT is not only challenging, but none of the retrieval systems ensure 100% success rate.[6]

Hence, “To prevent rather to treat the breakage” is the dictum among the clinicians. Improvements in instrument design, manufacturing process, and movement kinetics have been attempted to reduce the risk of separation. This review provides an insight into scientific causes of instrument separation, prevention strategies, and clinical recommendations for safer and efficient rotary endodontics.

MATERIALS AND METHODS

An electronic search was carried out using the following keywords: “cyclic fatigue/torsional resistance of NiTi rotary” to collect literature, from Journal of Endodontics, International and Australian Endodontic Journal, Journal of Conservative Dentistry, Endodontic Topics, from 2000-2017 July. The abstracts and full text of relevant articles were obtained to collect comprehensive update on instrument separation.

DISCUSSION

The review is discussed under the following headings:

Abbreviations

Ruddle's classification

Nickel–titanium (NiTi) metallurgy, phase transformation, and properties

Factors influencing separation

Canal curvature

Manufacturing process, heat and surface treatments

Rotary design which includes cross section, tip, taper, pitch, radial land, rake angle, and helical angle

Movement kinetics which includes torque, rotational speed, rotation, reciprocation, and adaptive motion.

Abbreviations

Cyclic fatigue resistance (CFR), torsional resistance (TR), superelasticity (SE), shape memory (SM).

Clifford Ruddle's classification of evolution of rotary instruments

First generation (the mid to late 90s)

Rotary files with passive cutting radial lands and fixed taper such as 4% and 6% over the length of their cutting blades are included. For example, profiles 0.04 and 0.06.[7]

Second generation (2001)

Files with active cutting edges, alternating cutting points, multiple tapers on a single file and electropolished files are included; for example: RaCe, EndoSequence and ProTaper.

Third generation (2007)

Thermally treated alloys mark this generation. These alloys exhibited superior flexibility and remarkable fatigue resistance; for example: Twisted File, HyFlex® CM, and ProfileVortex.

Fourth generation (2008)

Introduction of reciprocation concept: Endo EZE, M4, and reciprocating handpieces use equal clockwise/counterclockwise angles of 300. Yared et al. advocated unequal angles of reciprocation; For example: Wave One and Reciproc. Self-adjusting files by ReDent Nova, with new design and mode of operation, serve as a conservative approach for root canal preparation.

Fifth generation

Files with offset designs to reduce the screwing in and breakage mark the fifth generation; for example; ProTaper Next and Revo-S.

Nickel–titanium metallurgy

Equiatomic NiTi has 56% Ni and 44% Ti. At this ratio, NiTi alloys exhibit SE and SM. While SE endows the file with superior flexibility to negotiate canal curvatures, SM allows the file regain its original shape. The surface of RNT has metallic nickel, oxygen, carbon, and oxides of NiTi.[8] Decrease in the nickel content improves the fatigue resistance of NiTi alloys. HyFlex® has 52 wt% Ni (conventional RNT has 54%wt–57%wt), which lowers the Af to 47°C.[9]

Phase transformation and properties

Clinically, NiTi exists as one of the three phases: austenite, martensite, and an intermediate R-Phase. Their interconversion is a reversible phenomenon and is a function of temperature and stress [Figure 1]. Transformation temperature is influenced by the composition, impurities, and heat treatment of the alloy, which in turn influences its mechanical properties[10] [Figure 2].

Figure 1

(a) Phase transformation of nickel–titanium alloy; As: Austenitic start, Af: Austenitic finish, Ms: Martensitic start, Mf: Martensitic finish. (b) Properties of conventional rotary nickel–titanium (i) Conventional rotary file exhibiting (ii) superelasticity when stressed in the canal, (iii) shape memory when the stress is released. (c) Properties of thermally treated rotary nickel–titanium alloy (i) Controlled memory file, exhibiting (ii) superelasticity (iii) but no shape memory when stress is released, also (iv) deformation under stress and (v) regains shape on heating

Figure 2

Impact of phase transformation on the properties of nickel–titanium alloys

Factors influencing the separation of rotary files

Various factors have an impact on the fatigue properties of rotary NiTi instruments [Table 2].

Table 2

Impact of various factors on cyclic fatigue and torsional resistance: A report of studies from 2010 to 2017

Fracture of instruments occurs commonly in the apical one-third where torsional and flexural stresses are highest. Hence, torsional and bending tests are performed at D3.[11] A flexible file has more CFR but less TR and enables a smoother preparation in curved canals without being deformed. Whereas, a rigid file would be good in torsion than in flexion and performs better in narrow, constricted canals. Thus, cyclic and torsional properties are in inverse relationship with each other.[12]

Canal curvature

As the radius of curvature of canal increases, the frequency for instrument separation increases.[13] A canal with 50° curvature gives rise to stress of about 700–800 MPa on the outermost part of the instrument, whereas ultimate tensile strength of NiTi is 1400 MPa.[14]

Manufacturing process

Grinding versus twisting: In grinding, NiTi is cut across grain of the crystalline structure which causes grooves and microcracks, leading to stress concentration.[15] To release these stresses or crystalline defects, annealing heat treatment is given. It increases the flexibility and modifies the phase transformation behavior.[16]

Thermal treatment of alloys

R-Phase alloys

R-phase alloys have more flexibility, TR[11] and CFR[17] than conventional alloys with the same geometry, in both dry and aqueous environments,[18] and also when used in reciprocating motion.[19] However, they have low TR than M-wire alloys.[20]

M-wire alloys

They are produced by a series of heat treatment and annealing cycles which endow them superior strength due to stable nanocrystalline martensitic structure.[21]

Controlled memory alloys

Unlike conventional rotary files, controlled memory files do not exhibit SM, and hence do not straighten the canal or cause ledging [Figures 1b, c and 2]. Instruments with low transformation temperature exhibit (Hero, K3) higher maximum torque and resist fracture better than instruments with high transformation temperature (Endowave, Profile, and ProTaper).[21]

Alloys with gold thermal treatment

Files are subjected to a temperature of 370°C–510°C for 10–60 min depending on the size and taper of files. Files exhibit two-stage transformation behavior and high Af temperature of 50°C; for example ProTaper Gold.

In WaveOne Gold, a constant strain of 3–15 kg is applied at a temperature of 410°C–440°C. After machining, the working portion is again heat treated at 120°C–260°C.[22]

MaxWire alloys (Martensite-Austenite Electropolish fleX) alloys

This new technology allows the file XPendoShaper to attain martensitic phase when cooled (20°C) and austenitic phase at body temperature (37°C). At austenitic phase, it has a snake-like shape adapting to the canal irregularities, reducing stress on the file.[23]

T-wire alloys

The proprietary heat treatment claims to increase the flexibility and fracture resistance by 40%; for example: 2 shape files, it has two rotary files TS1 and TS2. The instruments regain their original shape after each use.[24]

Thermally treated alloys have low modulus of elasticity (20–40 GPa) than conventional (40–90 GPa) alloys and resist fracture better under stress.[21]

Surface treatment of nickel–titanium alloys

Rotary files are subjected to various surface treatments to improve their properties [Table 1].[8] Implantation of Argon ion increases the fatigue resistance. Nitrogen ion implantation has negative effect on fatigue resistance. Thermal nitridation at 250°C has shown more fatigue resistance because at 300°C the superelastic behavior is lost. Deep dry cryogenic treatment of 24 h, at −185°C, provides adequate time for transformation of retained austenite to martensite, improving the fatigue life.[2526] Electropolished instruments have more number of cycles to fracture than that of nonpolished instruments.[8] However, it does not prevent the development of microcracks on the instrument surface.[827]

Table 1

Impact of surface treatment on properties of nickel-titanium alloys

Electric discharge machining technology

Electric discharge machining (EDM) technology is a noncontact thermal erosion process in which electric sparks are used to melt and vaporize the top layer of NiTi alloy, reducing the surface defects. This increases the fracture resistance of files.[28] After cutting, cleaning is done ultrasonically in an acid bath. Then, they are heat treated at 300°C–600°C for 10 min, 5 h before and after the cleaning;[22] for example: HyFlex® EDM

Blue phase treatment

In this, the surface of a NiTi alloy is treated with titanium oxide by proprietary manufacturing process, which increases the surface hardness, wear resistance, cutting efficiency and flexibility of the file; for example: Vortex Blue and Reciproc Blue.[10]

Design features related to instrument separation

Cross-sectional area

Rotary files are manufactured with different cross-sectional designs; for example: Convex triangle-ProTaper, Wave One, Triple U-Profile, Equilateral triangle-Race, S-type-M-two, Reciproc, and Rectangular–ProTaper Next

Inner core and CSA: More the core and CSA, more is the TR but less is the CFR.[122930] This is the reason for improved fracture resistance of ProTapers than profiles in narrow canals[31]

Cross-sectional design: Instrument separation occurs in the decreasing order with the following cross sections

Square > rectangular > triangular and slender rectangular triangle[3233]

S-shaped, H-file fracture more compared to triangular cross section[33]

Alloy type: With the same cross section, M-wire alloy resists fracture better than conventional alloy[34]

Different cross sections along the length of an instrument improve the fracture resistance; for example: One shape and WaveOne[35]

Asymmetric cross sections of ProTaper Next and Revo-S also reduce screwing in and breakage[36]

Protaper universal files (F2, F3, F4, F5) are made more flexible by incorporating an additional groove in the middle of side of convex triangular cross section.[3136]

Tip

In general, NiTi rotary instruments are designed with noncutting tips to prevent ledging, torsional failure, and fracture.

Booster tip is a lead tip, that is incorporated in XPendoShaper. The lead section enters canal ensuring fit into the pre-established glide path. There are no cutting flutes on this section (¼ mm) where as the next ¼ mm has 6 cutting flutes, which shapes canal to #25/.02 to #60/.02 instrument. The repeated use of the shaper prepares the canal to a taper size consistent with the intracanal dentinal anatomy and hardness.[37]

Taper

Taper is increase in the diameter of file per mm increase in length. Fixed taper files cause excessive screwing in and taper lock than graduating or variable taper files.[363839] An instrument with larger taper and tip diameter is more likely to fracture in a canal with more acute and coronally located curvature.[40]

Pitch

It refers to the number of flutes per unit length of the file. More the flutes on the file, lesser the pitch and more is the fracture resistance of the file. Hence, it is recommended to use a file with smaller pitch (more flutes) for both curved and straight canals.[41]

Radial land

Radial land is a surface that projects axially from the central axis between the flutes, it as far as the cutting edges. Increased width of land increases the peripheral strength and canal-centering ability, but induces stresses due to increased contact with the canal wall causing fracture.[42] To balance these properties, K3 is designed with two recessed and one full land: ProTaper and Race lack radial lands.[36]

Rake angle

It is the angle formed by the cutting edge and cross-section taken perpendicular to the long axis of the instrument. Slight positive rake angle is recommended to for both good cutting action and reduced screwing in; for example: K3.

Helical angle

Helical angle is the angle that the cutting edge makes with the long axis of the file. Varying the helical angle through the working part has been shown to reduce the screwing in tendency; for example; in K3, helical angle is increased from tip to the handle. In Race, alternate helical design reduces the torque.[42]

Movement kinetics

Torque

It is the force that is exerted on an object in rotation. It is measured in gram centimeters.

In endodontics, torque is related to the apically directed force and preoperative canal volume.

Conventional or slow-speed, high-torque motors (>3 Ncm) cause fracture of instruments in curved canals. Slow-speed, low-torque motors provide the right torque for the specific file.

The values are smaller (low torque) for smaller and less tapered instruments and higher (high torque) for bigger and more tapered instrument.[43]

Optimum torque reverse

It is based on the principle of torque-provoked reversal, which is activated when preset torque is exceeded during 1800 forward rotation, and then the file rotates backward 900 to release itself. It provides high cutting effectiveness at very low torque values and moderate speed. Files have shown better fatigue resistance in Optimum Torque Reverse (OTR) motors.[44]

Safety quotient = RFT/MWT.

Where RFT – “Rotation to failure torque” is measured by turning a file that has been grasped and immobilized at D3 at a constant revolutions per minute (rpm).

MWT – “Mean working torque”– the torque required for any given NiTi to cut the dentin.

Ideally, safety quotient should be greater than one to reduce the risk of file fracture.[45]

Higher is the torque, greater is the breakage. Torque is higher in the following:

Narrower canals,[46] large diameter file,[474849] acute canal curvature,[48] and rotary instrumentation without glidepath.[50]

Rotational speed (revolutions per minute)

The recommended rpm for most of the rotary files is between 250–500 rpm.

Higher speed causes increased fracture of files due to the following:

Increased stress[13] and strain rate and decreased time for stress relaxation

Increased chance for taper lock[51]

More heat is generated during conversion of austenite to martensite, leading to precocious fatigue of the metal.[52]

The inverse relationship between speed and torque

As speed increases, torque decreases, which compensates for negative effect of increased speed on cyclic fatigue. Hence, endodontic handpieces deliver low speeds for higher working torque for the individual file to reduce separation.[51]

Rotation, reciprocation, and adaptive motion

Rotation

It is continuous clockwise rotation of file. Rotary files undergo plastic deformation when their endurance limit is exceeded. Endurance limit is the level of stress or strain at which a file can be subjected to a virtual infinite cycles without failure.

Reciprocation

It is defined as repeated CW/CCW movements of the file. Reciprocation can be as follows:

Complete: Reciprocating angles are same in both CW and CCW directions

Partial: It has angles of 3700 in cutting verse CW and 500 noncutting verse CCW

Hybrid reciprocation: Here, the angles change depending on the intracanal torque.

In reciprocation, the CW rotation changes to CCW before the file is subjected to endurance limit, hence reducing the risk of separation.[53] The reasons include, in reciprocation, fatigue of file occurs at multiple sites, whereas in rotary, a single site undergoes repeated fatigue. During one reciprocation, i.e., CW/CCW motion, crack formed in the file opens and closes once. A higher number of reciprocating cycles are required to complete one full rotation, which extends the CFR.[54]

Heat treatment[55] and reciprocating motion have been shown to enhance the CFR of some rotary files, such as ProTaper F2 and K3XF.[56]

Adaptive motion

The feature of this motor is that it changes kinematics from an interrupted complete rotating movement (6000 CW and 00 CCW) to a partial reciprocation (3700 CW/500 CCW) according to the intracanal pressure.[53] Adaptive motion does not improve the CFR.[57]

Other factors affecting fracture resistance

Fracture resistance is not affected by autoclaving of new files in vitro[5859] unlike during clinical use.[58] CM and R-phase alloys exhibited increased CFR on autoclaving.[59]

A rotary file is often subjected to a combination of torsional and cyclic stresses in the canal. In vitro studies have shown that torsional preloads increase the fatigue,[60] but improve torsional strength,[61] where as cyclic preloads add to tosional failure.[62]

Clinical recommendations for a safer rotary

A generous access with flexible files for severe curvatures and stiffer ones for straight canals

Stick to the rpm and torque as recommended, no high rpm for curvatures

Secure a patent glidepath. It takes off that extra torque on the files

Lubrication and light touch, follow the sequence of files

Monitor the files before reuse, there is no SAFE NUMBER

Pecking motion for radial landed and brushing motion for nonlanded files[40]

Lateral brushing motion for oval canals[63]

Small taper and tip diameter file for more coronal and/or acute curves[40]

Be selective for single-visit cases

Practice hybrid instrumentation, i.e., more than one rotary system for individual case.

CONCLUSION

Selection of proper file by the clinician, for the individual case, would balance the conflict between fatigue properties of the alloy, enabling a more efficient preparation of the canal with reduced risk of separation.106

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.