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[ original article ] A NiTi rotary instrument manufactured by twisting: morphology and mechanical propertiesIntroductionIn 1988, Walia et al 1 used a new metal alloy to manufactureendodontic instruments, the NiTi alloy. The instrumentsproduced with this alloy had a lower Youngmodulus than the instruments made with stainless steel,thus allowing the endodontic treatment of cases withlarge root curvatures. The use of instruments made withstainless steel could make the treatment more difficult.The first endodontic instruments of NiTi where manufacturedby a machining process using burs. With thedevelopment of new NiTi alloys, the study of the mechanismsinvolved in the phase transformation and bettercontrol of the microstructure, it was possible to developa new manufacturing method based on twisting. TheTF ® instruments (Twisted Files, California - USA) aremanufactured by twisting. This new generation of instrumentshas better clinical properties.In the present work the surface morphology ofendodontic instruments manufactured by twistingwas investigated and microhardness and flexibilitymeasurements were performed. These properties areimportant to understand the clinical behavior and todevelop new instruments.Materials and methodsMorphologyThe TF ® endodontic instruments (Twisted Files,California) used in this study has a length of 27 mm, atip diameter (D o) of 25 mm. Three different tapers wereused (0.04, 0.06 and 0.08 mm/mm).The tip angle, the tip length and the taper were determinedwith an optic microscope Zeiss with a pixeLINKcamera model PL- a662 and a light source Zeiss 1500LCD. The taper was determined with an amplificationof 1.6X. The other dimensions were quantified with anamplification of 5X. All dimensions of the instrumentswere determined with the program AxioVison 4.4 ® .Five instruments with each taper were investigated.Bending tests (at 45º)The bending tests were performed with an apparatusconnected to a universal material testing system EMICDL1000 (EMIC Equipment, Brazil). A 20 N load cellwas used to measure the force necessary to bend thetip of the instruments by 10 o , 20 o , 30 o and 45 o . The testswere performed according to ADA standard 28, withthe force applied 3 mm from the tip of the instrument.Vickers MicrohardnessFor microhardness testing, the instruments wereembedded in epoxy resin. The fixation cable was parallelto the recipient base with the purpose of keepingthe central longitudinal surface outside of the resin afterpolishing. The instruments were prepared with sandpaper200, 300, 400, 600 and 1200 and polished with aluminaparticles of 0.5 µm.The Vickers indentations were made with 100 gf during15 s using a microdurometer Bhueler model 1600-5300. Five indentations were made in the working partand five in the neck of each specimen.Scanning electron microscopy (SEM)Two instruments of each taper were submitted toSEM (JEOL, LSM 5800LV) to evaluate the morphologiesof the cutting edge, the tip and interface of the neckregion with the fixation cable.Statistical analysisThe data of the bending tests and the Vickers microhardnesswere analyzed statistically by the Kruskal-Wallis method and complemented with the Student-Newman-Keuls multiple comparison test to comparethe tapers. The microhardness at the neck region wascompared applying the Mann-Whitney test. The levelof significance of all analyses was 5%.ResultsThe results of the statistical analysis are shown inTables 1 and 2. The bending testing results are shown inTable 3. Figure 1 shows a mean curve obtained from 10bending tests performed in instruments with taper 0.06.The tests for other tapers showed similar curves.The curves show a slope change that is attributed toa phase transformation. The values of the forces necessaryto bend the instruments by 10 o , 20 o , 30 o and 45 o areshown in Table 2 and the forces necessary to inducephase transformation by stress are shown in Table 3.Statistical analysis (Kruskal-Wallis test) demonstratedthat there was a significant difference between instrumentswith different tapers (P < 0.00001). Then, the Student-Newman-Keulsmultiple comparison test revealedthat the instrument of taper 0.04mm/mm is more flexiblethan instruments of tapers 0.06 and 0.08 mm/mm. Moreover,the instrument of taper 0.06 mm/mm proved to bemore flexible than the instrument of taper 0.08 mm/mm.© 2011 Dental Press Endodontics 22Dental Press Endod. 2011 apr-june;1(1):21-7
Vieira VTL, Elias CN, Lopes HP, Moreira EJL, Souza LCThe Vickers microhardness average values at theneck region and at the working region of the instrumentsare shown in Table 4.The Vickers microhardness results for each taperwere submitted to the Mann-Whitney test and therewas no significant difference between the values in theneck region and in the working region for all instruments(p> 0.05).Table 1. Tip angle, Tip length (L) and taper of the instruments.Instrument Taper 0.04 Taper 0.06 Taper 0.08 Taper 0.10Tip angle 26.56 + 4.39 32.41 + 7.59 32.39 + 13.89 25.48 + 4.92L (mm) 0.24 + 0.011 0.25 + 0.013 0.24 + 0.007 0.26 + 0.012Taper 0.039 + 0.0029 0.061+ 0.0016 0.077 + 0.001 0.099 + 0.0022Table 2. Average values of the maximum forces to bend at 45º (gf) and respective standard deviations.Instrument Taper 0.04 Taper 0.06 Taper 0.0810 o 67.82 + 7.02 130.7 + 17.21 179.7 + 20.6220 o 92.26 + 4.36 183.9 + 16.17 295.2 + 26.2730 o 120.3 + 7.27 247.5 + 20.61 390.3 + 23.1545 o 131.7 + 9.43 263.6 + 23.18 400.7 + 23.88Table 3. Average forces for phase transformation by stress.Instrument TF 0.04 mm/mm TF 0.06 mm/mm TF 0.08 mm/mmAverage force 100 gf 150 gf 250 gfTable 4. Vickers microhardness of the instruments.Instrument HV neck region HV working region0.06 272.4 + 31.6 291.2 + 240.08 292.8 + 33.8 293 + 170.10 315.5 + 33.7 279 + 10.72.52Figure 1. Mean curve for taper 0.06 mm/mm TF ®files. The red line represents the elastic region, thegreen line phase represents the transformationregion and the dashed line the superelastic region.Force/100 (gf)1.510.50Elastic zonePhasetransformationzoneSuperelasticzone0 5 10 15 20Strain (mm)© 2011 Dental Press Endodontics 23Dental Press Endod. 2011 apr-june;1(1):21-7
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