The shaping advantage of M-wire compared with conventional nickel-titanium rotary instruments in heavy curvature canal

Background The purpose of this study was to evaluate the shaping advantage of M-wire NiTi ProTaper NEXT (PTN) compared with a conventional NiTi ProTaper Universal (PTU) le in heavy curvature canal. The shaping ability was measured by the amount of canal cutting and transportation between the PTN and conventional PTU. Methods Root canal shaping by the PTN and conventional PTU was classied into two experimental groups according to the nal tip size, ISO #25 or ISO #40. Eighty-four J-shaped root canals (10°, 20°, 30° apical curvature) in resin block were used. Results After adjusting for the level and canal wall side, the mean transportation did not signicantly decrease between the PTN and PTU with ISO #25. Signicantly less deviation occurred with the PTN and PTU between 10° and 30° using ISO #40. Conclusions The M-Wire NiTi PTN improves le exibility and enables accurate canal shaping for heavy curvature canals.


Abstract
Background The purpose of this study was to evaluate the shaping advantage of M-wire NiTi ProTaper NEXT (PTN) compared with a conventional NiTi ProTaper Universal (PTU) le in heavy curvature canal. The shaping ability was measured by the amount of canal cutting and transportation between the PTN and conventional PTU.
Methods Root canal shaping by the PTN and conventional PTU was classi ed into two experimental groups according to the nal tip size, ISO #25 or ISO #40. Eighty-four J-shaped root canals (10°, 20°, 30°a pical curvature) in resin block were used.

Results
After adjusting for the level and canal wall side, the mean transportation did not signi cantly decrease between the PTN and PTU with ISO #25. Signi cantly less deviation occurred with the PTN and PTU between 10° and 30° using ISO #40.
Conclusions The M-Wire NiTi PTN improves le exibility and enables accurate canal shaping for heavy curvature canals.

Background
Rotary dental les with a NiTi alloy containing 56% (wt) Ni and 44% (wt) Ti have signi cantly changed root canal preparation. The NiTi alloy has a cubic crystal structure with an austenite phase above the shape recovery temperature (1). The austenite phase changes to martensite under force and temperature, but when the stress is released, the phase returns to austenite (2,3). Consequently, the NiTi alloy has a shape memory effect and superelasticity without plastic deformation, which is advantageous for instruments used for root canal preparations.
Given the development of NiTi endodontic instruments targeted at root canals with complex anatomical con gurations, the shape memory and superelastic properties of such NiTi instruments has helped prevent endodontic accidents (such as ledges, root canal perforations, apical transportation, and le fractures) (4,5). Currently, unique heat and mechanical treatments can be used to complement the mechanical properties of NiTi alloys to improve exibility and fatigue resistance.
In 2007, Sportwire LLC (Langley, OK, USA) developed a unique heat treatment process with the aim of producing a more exible NiTi alloy with better fatigue resistance than conventional NiTi alloys (6).
Additionally, Berendt developed a NiTi composition in which trace elements of less than 1% were added to produce a NiTi alloy (7). Under a heat treatment process, the M-Wire alloy converts to a rhombohedral (R) phase (intermediate temperature phase between austenite and martensite) and the elasticity changes with the change from austenite to martensite. Therefore, the exibility of a NiTi endodontic le can be enhanced by modi cation (8,9). Additionally, NiTi alloy converted to the R phase is conventionally produced by cutting; however, NiTi les are produced by a non-cutting rolling step, thereby increasing the le fatigue resistance and fracture resistance.
The ProTaper NEXT (PTN) le used in this study is a next generation model of ProTaper Universal (PTU), which is a modi cation of the conventional NiTi alloy PTU and uses M-Wire. PTN was developed as a le suitable for severely curved root canals given its increased exibility and fracture resistance. Compared with conventional NiTi PTU, the M-Wire NiTi PTN suppresses the occurrence of micro-cracks during root canal formation (10,11). Root canal wall displacement after cutting (12)(13)(14)(15), fatigue resistance (16)(17)(18), and le breakage resistance (19)

Root canal preparation
Before starting root canal preparation with the M-wire PTN and conventional PTU les, are formation (straight line) in the upper third of all root canal ori ce openings and smooth le guide path creation (glide path) up to the root apex were performed. Flare formation was performed using PTN SX and PT SX les (Dentsply Sirona), which are used for straight line formation only. Furthermore, the glide path was formed up to #15 using a ISO#15K stainless-steel le. All NiTi les were washed with puri ed water using a dedicated engine (X Smart plus, Dentsply Sirona), and 1 mL of EDTA-gel Glide (Dentsply Sirona) was applied to the NiTi les.
Root canal preparation using PTN and PTU les was performed by a single dentist with more than 7 years of experience using NiTi rotary les. The following procedure was performed and les were exchanged after every ve root canal formations.
1. ISO #25 PTN: Using two X1 and X2 les sequentially, select the X Smart plus PTN mode (300 rpm, 2.0 Ncm), perform root canal cleaning when exchanging les, and reach the working length of the X2 le to complete the root canal formation.

Evaluation of the canal shaping
The amount of resin removed in the outer and inner side of the curved root canal was compared. A stereoscopic microscope Olympus SZX16 and a digital camera DP71 (Olympus, Tokyo) were used for the measurement. The transparent root canal models before and after root canal preparation was superimposed with digital images, and the resulting image was transferred to a PC and measured. The measurement points were 1, 2, 3, 4, and 5 mm from the apex, and the increase in the root canal width on the outer and inner side was measured and statistical processing was performed (Fig. 2). Root canal transportation was measured as the amount of root canal wall cutting in the outer and inner sides of the curved root canal, and the median displacement of the root canal before and after preparation was measured.

Statistical analysis
The amount of root canal wall cutting and root canal transportation was measured using statistical analysis using one-way analysis of variance and a multiple comparison test using Bonferroni-Dunn test. The level of signi cance was test at p<0.05.

Results
The root canal shaping ability was evaluated according to the amount of resin removed and the root canal transportation.  Fig. 3a,4a,5a . Alternatively, the amount of resin removed by PTU on the inner and outer sides was small at 10° to 20° curvatures. The amount of resin removed by PTU a little increased at the 30° curvature: the apex side was 1 mm on the outer side, and the apical side is 4 mm and 5 mm on the inner side (Fig.5b).
The change in the root canal transportation for both PTN and PTU was within 0.05 mm for all root canals with a curvature of 10° to 30° (Table 1).
Shaping ability of the nal apical size ISO #40 Even with ISO #40 and PTN, the amount of resin removed from the inner and outer sides was less than those in the PT group for all root canals with a curvature of 10° to 30° Fig. 3b, 4b, 5b . For ISO #40 and PTN, the amount of resin removed was signi cantly less than the PTU group at 1 -5 mm on the inner side with a curvature of 10° (Fig.3b) and 1 mm on the outer side at 30° (Fig.5b). The amount of resin removed by PTN only increased from 4 to 5 mm on the inner side compare with outer side at 30° (Fig.5b). This tendency was similar to that of the group. The amount of resin removed by PTU with ISO #40 signi cantly increased by 1 mm on the apex side for all a curvatures, and the cutting amount on the inner side increased from 4 to 5 mm on the apex side p<0.05 (Fig.5b).
The change in the root canal transportation was less than 0.05 mm in the 10° to 20° curvatures using PTN, but 0.06 mm at the apex 4 to 5 mm into the root with a 30° curvature. The transportation further signi cantly increased to 0.08 mm p<0.05 (Table2).
Alternatively, the root canal transportation of PTU showed a displacement of 0.1 mm or more on the outer side of the apex side at 1 mm in all the root canals with a curvature of 10° to 30°. In the PTU, the canal transportation on the inner side increased to 0.07 mm and 0.08 mm at 3 and 4 mm, respectively, even with a curvature of 10°, and the inner side displacement of the root canal of 30° also increased to 0.07 mm and 0.08 mm at 4 and 5 mm, respectively (Table 2).

Discussion
The mechanical properties of the NiTi le depends on the processing method of the le and the surface nishing method, but it has been reported that the change in crystal structure by the heat treatment process affects the exibility of NiTi alloys (20)(21)(22)(23). The PTU used in this study has super-elasticity by the stress-induced transformation of the martensitic phase with a monoclinic crystal structure to the austenite phase, which is the cubic crystal structure of a conventional NiTi alloy (24). Alternatively, PTN, which is a NiTi alloy processed into M-wire by heating and cooling, coexists with austenite, martensite, and R phases, and has a monoclinic martensite phase and R phase (16)(17)(18). Because the elastic modulus is less than that of the austenite phase, the initial exibility is improved. Additionally, the M-wire NiTi alloy is more exible than the conventional NiTi alloy because the austenite phase undergoes a martensitic transformation due to the stress on the le when forming a curved root canal. The stress load on the le is reduced, which reduces le corruption (19).
In this study, the usefulness of the M-wire NiTi alloy was analyzed by comparing the cutting characteristics of PTN and PTU under a stress load (formation of a curved root canal). The root canal formation using root canal models with different apex curvatures of 10° to 30° were analyzed with nal apical sizes of ISO #25 and ISO #40. There was a large difference in the displacement of the root canal wall between the groups. In the ISO #25 group, there was no signi cant difference in the amount of resin removed or canal transportation associated with the change in the bending angle in PTN and PTU.
In the ISO #25 group, the amount of resin removed by PTN and PTU was 1 to 5 mm from the apex of the inner and outer bays for curved root canal angles of 10°, 20°, and 30°. The measured value was less than 0.05 mm. Furthermore, the root canal transportation was less than 0.05 mm and both les could form the root canal and maintain the original anatomical root canal morphology. The NiTi le with a ISO #25 tip showed that the shaping ability of the conventional and M-wire type NiTi alloy les were similar and useful for root canal preparation of curved root canals between 10° to 30°.
Alternatively, in the ISO #40 group, the amount of resin removed signi cantly changed depending on the root canal curvature in the PTN and PTU le. The PTU increased the lateral displacement by 1 mm from the apex, and 4 mm and 5 mm from the apex in the inside of the root canal between 10° to 30°. The risk of transportation and canal perforation in the curved root canal was observed. However, the PTN maintains the original root canal morphology when the amount of resin removed in the inner and outer sides was 1 to 3 mm and from the apex is 0.1 mm or less between 10° to 30° canals. Alternatively, for the 30° curvature canal, the inner side cutting of 4 mm and 5 mm from the apex tended to be as high as that of the PTU. The PTN obtained by converting the NiTi alloy into the R phase by heat processing improved the root canal followability and suppressed the transport on the outer bay side of the apex. It is necessary to be careful of accidents, such as strip perforation during root canal formation on the inner side, when using the 30° curvature.
Because the PTN and PTU used in this study are similar le systems for complete root canal formation with multiple lines, the exibility of the M-Wire NiTi alloy is useful for proper root canal formation. Additionally, the M-wire type NiTi alloy single le preparation system (Reciproc, WaveOne le) completes root canal formation with only one le, the root canal wall displacement is less than the conventional NiTi PTU, and the root canal transportation is reduced. Additionally, the M-wire PTN is reported to retain anatomical morphology (23,24). Finally, the M-wire PTN has excellent fatigue resistance and torsion resistance regardless of the difference in le systems used for root canal preparation.

Conclusion
The amount of canal wall removed and transported by M-wire ISO #40 PTN was less than that of conventional ISO #40 PTU in 10° to 30° curvature canals and the original root canal form was maintained, although no signi cant difference in the shaping ability for curvature canals by both PTN and PTU of ISO #25. The M-Wire PTN improves le exibility and enables accurate canal shaping for wide and heavy curved canals   Pre-and post-instrumentation images were superimposed and the difference between the canal con guration before and after instrumentation were measured in each of the ve traced levels.