ni-ti file fatigue fracture and cuttability …

THE INTERNATIONAL JOURNAL OF MICRODENTISTRY40

1 Department of Endodontics, Nihon University School of Dentistry at Matsudo, Japan

2 Research institute of Oral Science, Nihon University School of Dentistry at Matsudo, Japan

3 Department of Advanced Dental Treatment & Microscopic Dentistry, Nihon University School of Dentistry at Matsudo, Japan

Correspondence to:

Masato IzawaDepartment of Endodontics, Nihon University School of Dentistry at Matsudo 870-1, Sakaecho, Nishi-2, Matsudo, Chiba, JapanE-mail: [email protected]

Ni-Ti FILE FATIGUE FRACTURE AND CUTTABILITY INVESTIGATION, AND DESIGN WITH DIFFERING CORE RATIOS

Masato Izawa, DDS, PhD1, 2/ Yasuhisa Tsujimoto, DDS, PhD2, 3

In this study, we investigated the effects of changing the core ratio of a passive instrument with a radial land using a rotary motion on changes in fatigue frac-ture and on centering ability in root canal shaping. Passive instruments consisting of R-phase Ni-Ti files with one radial land, core ratios of 45%, 50%, 55%, 60%, 65%, and 70%, D1#25, and 0.06 taper were de-signed and subjected to torsional, bending, fatigue-fracture, screwing, and centering ability tests, with investigational comparison. The correlation with in-creasing core ratio was positive for torsional torque (r=0.9480) but negative for torsional angle (r=-0.8139), positive for bending torque (r=0.9689) but negative for both fatigue fracture (r=-0.8177) and screwing (r=-0.7746), and positive for centering ability displacement (r=0.6874).With a core ratio of 45%, file strength could not be maintained. Correlation with increasing core ratio was high nega-tive for screwing. At core ratios of 50% to 65%, the time to fatigue fracture was long and free from root-canal screwing, indicating that root canal shaping can be performed safely at those core ratios. Int J Microdent 2019;10:40–46

INTRODUCTION

Waila et al.1 produced the Ni-Ti file in 1988 and reported its superior-ity to existing stainless steel files in flexibility, which led to growing research and development of Ni-Ti files. The main focus was initially on the use of austenite-phase files, but today, files characterized by an R-phase intermediate be-tween austenite and martensite phases are considered superior with regard to fatigue fracture2, root canal centering ability3, 4, and other aspects, while R-phase files and method of use have become important topics of research. Other than rotary motion, recip-rocation is thought to prevent file fracture5. As a means of shorten-

ing root canal enlargement time or emphasizing safe performance despite a longer time require-ment, active instruments (AIs) with no radial land and with a large rake angle, or passive instruments (PIs) with a radial land and a small rake angle are also observed, as are products with different char-acteristics. PIs with a radial land are advantageous for resistance to rotary fatigue and for good root canal following, but AIs hold the advantage for cutting efficiency and resistance to torque fracture6.

Our goal is to design an R-phase file with better than existing file performance, as a PI superior in root canal following and in resist-ance to rotary fatigue fracture. There are no earlier studies on the effect of changing core ratios

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and the file properties, and in this study, we therefore investigated and herein report on changing the core ratio of a PI file of the form shown in Fig 1, and the effects of these changes on fatigue fracture and on centering ability in root canal shaping when used with ro-tary motion.

MATERIALS AND METHODS

Ni-Ti files

The R-phase Ni-Ti file, as shown in Fig 1, is a PI designed with a single radial land and a core ra-tio of 45, 50, 55, 60, 65, or 70% and produced as a prototype with D1#25 and 0.06 taper. The file with 45% to 70% core ratios is shown in Fig 1.

Torsion test

The universal testing machine (Mani, Tochigi, Japan) shown in Fig 2a was used.

Test conditions were in accord-ance with ISO3630-1:2008, 7.4.

Procedure: 1) hold the test sample shank in the torsion jig drill chuck; 2) secure the blade tip in the air chuck, in the region 3 mm from the blade tip; 3) twist the sample, rotating clockwise viewed from the shank; 4) twist until the sam-ple fractures and record the maxi-mum torque and rotation angle at the time of fracture. N=10.

Bending test

The universal testing machine (Mani, Tochigi, Japan) shown in Fig 2b was used. The test conditions were in accordance with ISO3630-1:2008, 7.5. Procedure: 1) secure the blade tip in the bending jig, in a region 3 mm from the blade tip; 2) bend the sample to 45°; and 3) record the maximum torque to at-tainment of 45°. N=10.

Fatigue fracture test

The fatigue fracture test machine (Mani, Tochigi, Japan) shown in Fig 2c was used. Test procedure: 1) secure the sample shank in the test machine drill chuck; 2) advance the blade until its tip ex-tends 1 mm from the root canal

forma; 3) rotate clockwise viewed from the shankb; and 4) record the number of cycles to sample frac-ture. N=10.

aroot canal form: curved to R5 (jig simulating root canal with 5 mm radius curvature)

brotational speed: 500 rpm

Screwing test

The screwing test machine (Mani, Tochigi, Japan) shown in Fig 2d was used. The VDW-Zipperer training block shown in Fig 2d was used for the artificial root canal simulation form. The root canal length was approximately 19 mm, the root apex diameter 0.20 mm, the taper 0.02, and the Hv (hard-ness) 12.1. Procedure: 1) Secure the sample shank in the test ma-chine drill chuck; 2) place the root canal form in the test jig; 3) rotate clockwise viewed from the shank, advance at slow speed and cut the root canal forma; and 4) measure and record the maximum drawing force until the root canal form tip is reached. N=10.

aRotational speed: 500 rpm; for-ward motion: 0.5 mm/s.

Fig 1 Ni-Ti files designed in this study, with 45% core ratio design to actual file on the left and 70% core ratio design and actual file on the right.



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Fig 2b Universal tester (Mani, Tochigi, Japan) used in bending test.

Fig 2a Universal tester (Mani, Tochigi, Japan) used in twisting test.

Fig 2d Screwing tester (Mani, Tochigi, Japan) used in screwing test (left) and artificial root canal model used (right).

Fig 2c Fatigue fracture tester (Mani, Tochigi, Japan) used in fatigue fracture test.

Fig 2e Determination of measurement conditions for ar-tificial tooth used in centering ability test and microscope (TM Generation B. Mitutoyo, Kanagawa, Japan) used in measurements (left), and artificial tooth used (right).



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Centering ability test

By the method shown in Fig 2e, we determined the artificial tooth measurement conditions and per-formed the measurements with a TM Generation B microscope (Mitutoyo; Kanagawa, Japan). Test procedure, for measurement of the Fig 2e upper side outer cur-vature and the bottom inner cur-vature: 1) in the pretreatment root canal form, perform measurement every 1mm; 2) using the test sam-ple, perform enlargement shaping to/as far as the root apexa; 3) after shaping, again measure the root canal form at every 1mm; 4) Re-cord the inner curve and the outer curve variationsb

aShaping performed at 300 rpm, 3.0 N, and appropriate penetration confirmed.

bFor the 025/.06 sample in this study, root canal form with 025/.04 enlargement.

Statistical analysis

One-way analysis of variance and multiple comparison test (Tukey’s test) were performed. The p-value was 0.05.

RESULTS

Torsion test

Test results are shown in Figs 3a to d. The torsional torque and torsional fracture angle results are shown. The torsional torque increase was nearly linear with the increase in core ratio. In the multiple comparison test, a signifi-cant difference was not observed between core ratios of 55% and 60% but was found between all others. The torsional angle showed a high value of 1000° at 45% but was approximately 600° at 50% and 55% and decreased almost linearly in the range 60%, 65%,

and 70%. In multiple comparison test, no significant difference was observed between core ratios of 50% and 55%, 50% and 60%, 55% and 60%, 60% and 65%, or 65% and 70%. The correlation with core ratio was strongly posi-tive (r=0.9480) for torsional torque but strongly negative (r=-0.8139) for torsional angle (Table 1).

Bending test

Figs 3e and f show the bending test results. The values for bend-ing torque increased linearly with increasing core ratio. The value was approximately 7 gf·cm at a core ratio of 45%, but approxi-mately 53 gf·cm at a core ratio of 70%. A fairly large difference thus occurred with changes in the core ratio. In multiple comparison test, the difference was significant for all cases. The correlation with in-creasing core ratio was positive high for bending torque (r=0.9689) (Table 1).

Fatigue fracture test

Figs 3g and h show the fatigue fracture test results. The fatigue fracture test yielded a high value of approximately 8,500 rotations at a core ratio of 45%, but the values numbered in the 3,000s at 50% and 55%, and in the 2,000s at 60% and 65%, and decreased to approximately 1,500 at 70%. Multiple comparison test showed no significant differences between core ratios of 50% and 55%, 55% and 60%, 55% and 65%, 60% and 65%, 60% and 70%, or 65% and 70%. The correlation with in-creasing core ratio was strongly negative for fatigue fracture (r=-0.8177) (Table 1).

Screwing test

Figs 3i and j show the screwing test results. The screwing load was approximately 1.5 N at core

ratios of 50% and 55%, approxi-mately 0.9 N at core ratios of 60% and 65%, and approximately 0.6 N at a core ratio of 70%. Torsion return occurred at a core ratio of 45% and it was therefore exclud-ed from the test results. Multiple comparison test showed no sig-nificant difference between core ratios of 50% and 55%, 60% and 65%, 60% and 70%, or 65% and 70%.The correlation with increas-ing core ratio was strongly nega-tive for screwing (r=-0.7746) (Table 1).

Centering ability test

Figs 3k to m show the results of the centering ability test. Dis-placement at the inner curvature and outer curvature of the root canal was more at a core ratio of 70% than at the other core ratios. Displacement was smallest at a 45% core ratio, and was approxi-mately the same at core ratios of 50%, 55%, 60%, and 65%. Multi-ple comparison test showed sig-nificant differences between core ratios of 45% and 65%, 45% and 70%, 50% and 70%, 55% and 70%, 60% and 70%, and 65% and 70%. A significant difference was thus found between the 70% core ratio and all other core ratios. The correlation with increasing core ratio was somewhat positive for centering ability displacement (r=0.6874) (Table 1).

DISCUSSION

Ni-Ti files are considered superior to stainless steel files in small dis-placement in root canal following and shaping in therapy for curved root canals7, and it is known that fractures occur in Ni-Ti files dur-ing root canal enlargement, with 55.7% of fractures being torsional and 44.3% being flexural fatigue8. Ni-Ti files with superiority in both aspects are therefore desired. In



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Figs 3a and b Twisting fracture test.a b

Figs 3c and d Twisting fracture an-gle test.c d

Figs 3e and f Bending torque test.e f

Figs 3g and h Fatigue fracture test.g h

Figs 3i and j Screwing load test.i j

Figs 3k to m Centering ability test.

k l m



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an effort to prevent fracture, de-velopment has progressed from austenite-phase to R-phase Ni-Ti files. We performed actual com-parisons of R-phase and auste-nitic-phase files with the same shape3, 9. In the fatigue fracture test, the R-phase was found to be more resistant to fatigue fracture in both NRT (Mani, Tochigi, Japan) and Silk (Mani, Tochigi, Japan) files. In centering ability, R-phase files showed less transportation and better values. Good results have been obtained by changing from austenitic-phase to R-phase2-4. Manufacturers have considered innovative design section forms in R-phase files to enhance cutting ability, transportation prevention, resistance to fatigue fracture, and other aspects. There have, how-ever, been no reports of research on changing and investigating the core ratio.

In the present experiment, PI R-phase Ni-Ti files with safety considered were designed, al-though PI is deemed inferior to AI in cutting ability, but the core ratios were varied and the effec-tiveness of each was investigated. In the torsion test, the torsional torque value showed a positive correlation with increasing core ratio. This indicates that the force of resistance to twisting increases

with core rate. The correlation of torsional fracture angle with the core ratio, moreover, was strongly negative. Nakagawa et al.10 re-ported that bending and torsional resistance increases significantly with rising Ni-Ti file diameter and cross-sectional area. In the pre-sent study, too, the results indicat-ed that increasing the core ratio of Ni-Ti files decreases the flexibility and torsional fracture angle. The bending test results also indicated that raising the core ratio, and thus increasing the size of the core re-gion, increases the torque needed to bend the file. In the fatigue fracture test, moreover, a strong negative correlation with core ra-tio was noted, indicating that as core ratio increases, flexibility de-creases and fatigue fracture more readily occurs. In summary, these results for file characteristics in-vestigation show that files with low core ratios have more flexibil-ity than those with high core ra-tios, and thus, files with high core ratio require a larger torque to twist and exhibit a lower torsional fracture angle. In bending torque as well, higher core ratio naturally requires higher torque. The corre-lation between core ratio and fa-tigue fracture is strongly negative, and the number of rotations to fa-tigue fracture is more than twice

as high at a core ratio of 45% than at any other, and the resistance to fatigue fracture thus appears high. That core ratio was excluded from in the screwing test, however, because of torsion return. It was deemed best to exclude the core ratio of 45% from the trial manu-facture, as flutes tend to clog with chips, screwing load increases and screwing tends to occur, and high resistance to rotation would develop in files with a high core ra-tio. Finally, with respect to curved root canal shaping, we performed the centering ability test and found that the 45% core ratio file resisted transportation whereas the file with a 70% core ratio ex-hibited larger transportation than the files with other core ratios.

There have been no prior reports on producing Ni-Ti files starting from the core ratio. In the present study, PI morphology investigation showed an inability to maintain file strength with a core ratio of 45%, together with the strongly nega-tive correlation between screwing and increasing core ratio, thus in-dicating the desirability of forming with a core ratio of 50% to 65% for safe, certain root canal shap-ing free from root-canal screw-ing with a long period of freedom from fatigue fracture.

TABLE 1 Correlation with core ratio.

Torsional torque

Torsional bending racture angle Bending angle

Cyclic fatigue failure

ScrewingCentering

ability

R2 0.899 0.662 0.939 0.669 0.600 0.473

| R | 0.948 0.814 0.969 0.818 0.775 0.687



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REFERENCES


ni-ti file fatigue fracture and cuttability …

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