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ORIGINAL ARTICLE
Year : 2013  |  Volume : 1  |  Issue : 2  |  Page : 49-54

Effect of curvature angle and rotational speed on fracture of various Ni-Ti rotary files used in extracted molars


1 Department of Conservative Dentistry and Endodontics, Swami Devi Dyal Dental College and Hospital, Panchkula, Haryana, India
2 Department of Prosthodontics, Swami Devi Dyal Dental College and Hospital, Panchkula, Haryana, India

Date of Web Publication3-Aug-2013

Correspondence Address:
Pardeep Khurana
1011/13, Civil Hospital Road, Shahabad (M), Kurukshetra - 136 135, Haryana
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/2321-4619.116033

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  Abstract 

Objectives: The objective of this study was to evaluate the effect of rotational speed and angle of curvature in curved root canals on the fracture of different nickel-titanium rotary instruments that is Profiles, ProTaper, and K 3 files. Materials and Methods: Freshly extracted 180 human maxillary and mandibular molars were selected and divided into two groups of 90. Each group was then divided into three sub groups A, B, and C according to angle of curvature, Group I < 30° and Group II > 30°, which was measured by Schneider's method. Instrumentation was performed using the profile, ProTaper, and K3 (Company and in Literature name of Rotary file is K3) rotary instrument at 3 rotational speeds of 150, 250, and 350 rpm (10 molars at each rotational speed). Results: There were total of 32 instrument fractured out of 120 files. In a multivariable analysis, it was demonstrated that the rotational speed of 350 rpm fractured than those used at 150 rpm (odd's ratio [OR]: 3.6, 95% confidence interval [CI]: 1.3-10.5). An increase in the angle of curvature of the canal did not significantly increase the likelihood of fracture (OR: 1.9, 95% CI: 0.8-4.2). No significant differences were found between various designs of files used. Conclusion: Instrument fracture was associated with rotational speed and the angle of curvature.

Keywords: Angle of curvature, direct digital radiography, fracture, nickel-titanium, rotary instruments, rotational speed


How to cite this article:
Khurana P, Khurana KK. Effect of curvature angle and rotational speed on fracture of various Ni-Ti rotary files used in extracted molars. J Res Dent 2013;1:49-54

How to cite this URL:
Khurana P, Khurana KK. Effect of curvature angle and rotational speed on fracture of various Ni-Ti rotary files used in extracted molars. J Res Dent [serial online] 2013 [cited 2020 May 28];1:49-54. Available from: http://www.jresdent.org/text.asp?2013/1/2/49/116033


  Introduction Top


The chemical-mechanical preparation of a curved root canals to enlargement and maintain its original form is a challenge to endodontics. Walia et al. were first to investigate Nickel-Titanium endodontic instruments, the superior fracture resistance of the Nitinol instruments was attributed to the ductility of this Nickel-Titanium alloy. [1] Root canal treatment causes stress to Nickel-Titanium files and a stress-induced Martensitic transformation takes place from the Austenitic to the Martensitic phase. [2] This ability of Nickel-Titanium alloys to undergo extensive deformation resulting from a stress-assisted phase transformation, with the reverse transformation occurring on unloading is called super-elasticity. Serene et al. reported fracture of rotary instruments takes place in two different ways: due to torsion and due to fatigue through flexure. [3] Fracture due to torsion occurs when the tip or any part of the instrument binds in the canal whilst the hand piece keeps turning. This type of fracture has been associated with the application of excessive force during instrumentation, instrument fractured because of torsional load often carry specific signs such as plastic deformation. [4]

Fracture owing to fatigue through flexure occurs because of metal fatigue. The instrument does not bind in the canal but rotates freely until the fracture occurs at the point of maximum flexure. [4] As the instrument is rotated within curved canals one half of the instrument shaft on the outside of the curve is in tension, whereas the half of the shaft on the inside of the curve is in compression generating tension-compressions cycle, in instrument. [5]

The cyclic repletion of these stresses promotes cumulative micro-structural changes that induce the nucleation of cracks that increase, coalesce, and spread until the fatigue of the endodontic instrument. [6]

Profile (Tulsa Dental) rotary files have cross-sectional geometry of three equally spaced "U-Shaped" grooves. [7] ProTaper system (Dentsply, Switzerland) has "convex triangular" cross-section. [8] K3 rotary Nickel-Titanium file system (Sybron Endo, Orange, California) has "asymmetrical" cross-section. [9] There are a number of factors that are associated with fracture of rotary Nickel-Titanium instruments including the speed of rotation and the angle of curvature. The aim of this study is in-vitro evaluation of the fracture in curved root canals by using the different Nickel-Titanium rotary endodontic instruments.


  Materials and Methods Top


Freshly extracted 180 maxillary and mandibular molar were selected based on the following criteria. Free of caries, closed apices, without dilacerations, and restoration. The teeth were cleaned of debris and calculus and then stored in normal saline.

Measuring canal curvature

After placing the teeth in a vertical position into the acrylic block of radiographic platform it was stabilized with modeling wax. [10] Conventional radiographs were taken in bucco-lingual direction using the E speed film (Kodak), which was kept parallel to the teeth. These radiographs were used to determine canal curvature. Tracing of the radiograph was carried out on cephalometric tracing paper and Schneider's method was employed to determine canal curvature. The Schneider's method employs first drawing a line parallel to the long axis of the canal in the coronal third and second line is then drawn from the apical foramen to intersect the point where the first line left the long axis of the canal [Figure 1]. The Schneider angle is the intersection of these lines. [11] Mesial roots of mandibular molars and buccal roots of maxillary molars were measured. Most severe angle of the particular tooth was used to categorize the teeth. Grouping is performed as follows teeth were divided in two groups I and II having 90 teeth each. Group I has root curvature less than 30° and Group II have root canal curvature more than 30°. They were further divided in sub groups A, B, and C based on which rotary Ni-Ti file used having 30 teeth each, for subgroup IA and IIA Profile is used for instrumentation, for sub group IB and IIB ProTaper is used, for subgroup IC and IIC K3 files are used. Minor subgroups 1, 2, and 3 were based on speed at which files were used having 10 samples each. In Minor subgroup 1 (150 rpm), 2 (250 rpm), and 3 (350 rpm) is used for instrumentation.
Figure 1: Diagram showing Scheinder's angle

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Conventional access cavity was prepared in each tooth, with No. 2, 4, and 6 round burs and taper fissure bur was used to refine the access cavity. Each root canal located with an endodontic explorer. Pulp extirpation was carried out with barbed broach. The canals were made patent with 08 and 10 K-file. Mesial root canals of mandibular molars and buccal root canals of maxillary molars were measured. All the canals were prepared using the crown-down technique in sequence recommended by manufacturers. Files were mounted on a low-speed, high torque electric motor (TC motor 3000) with a contra-angle 20:1 reduction hand piece (WH 975, dental work Burwoos, Austria). Light pressure during an instrumentation procedure was used together with back and forth movements of amplitude of between 2 mm and 3 mm. [12] Usage time for each instrument was maintained between 5 sec and 10 sec. Each file was used for a maximum of 10 times. All files were inspected and cleaned after each use. Fracture was checked with visual examination and tactile sensation and by direct digital radiograph the level at which file had fractured in the tooth was evaluated. If fracture occurred instrument was replaced with new one.

For profile orifice shapes No. 4 (0.07/50), No. 3 (0.06/40), 0.06/30 and 25, 0.04/30 and 25 were used in sequence recommended by the manufacturer. [12] ProTaper rotary Ni-Ti endodontic instruments were used crown down technique. [8] K3 (Sybron Endo, Orange, California) rotary Ni-Ti endodontic instruments were used as recommend by the manufacturer in the procedural kit. [9] Orifice shapes (0.10/25), (0.08/25) was used to enlarge coronal third. 0.06/40, 35, 30 and 25 were used to enlarge middle third of the canal. For apical finishing, 0.04/25 and 30 were used.


  Results Top


Total 120 files were used for instrumentation of 180 molars out of which 32 files fractured when Group I and Group II were compared more number of fractures occurred in-Group II, i.e. 20 and 12 fractures occurred in-group I. Fisher's exact test P0 = 0.12, NS (Not Significant), odd's ratio (OR) - 1.9 with 95% confidence interval (CI) variation of (0.8-4.2) [Graph 1] [Additional file 1]. When different design of files were considered profile showed 9 fractures in group II (30%) and 5 fractures 16.7% in group I; P = 0.23, NS ProTaper files showed 6 fractures in Group II and 5 fractures (16.7%) in Group I. Fisher's exact test P = 0.51, NS. K3 files showed 5 fractures (16.7%) in group II and 3 fractures (10%) in group I, Fisher's exact test P = 0.47, NS [Table 1] [Graph 2] [Additional file 2]. When fractures in relation to the rotational speed were compared 150 rpm showed 6 fractures (10%), whereas 250 rpm showed 9 fractures (15%) and 350 rpm showed 17 fractures (28.3%).
Table 1: Number of fractures in relation to instruments used (angle and rotation combined)

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Comparison between rotations showed a significant difference in the likelihood of fracture when the rotational speed was increased from 150 rpm to 350 rpm with P < 0.05. OR, which represents the probability of occurrence = 3.6, and 95% CI varied from 1.3-10.5. Whereas, comparison between 150 rpm versus 250 rpm showed P = 0.42, which is not significant and OR is 1.6 with 95% CI varied from 0.5-5.1. In comparison between 250 rpm versus 350 rpm P = 0.08, which is not significant; OR = 2.2 and 95% CI from 1.0-5.7 [Table 2].
Table 2: Number of fractures in relation to rotation (instruments and angle combined)

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14 Profiles (23.3%), 10 ProTaper files (16.7%) and 8 K3 files (13.3%) fractured [Table 3] [Graph 3] [Additional file 3]. When overall fractures were compared after combining angle and rotation depending on the type of file used. Profile versus ProTaper files showed P = 0.37, which is not significant. Profile versus K3 files showed P = 0.13, which is significant and ProTaper and K3 files P = 0.62, which is not significant [Table 3] [Graph 4] [Additional file 4].
Table 3: Number of fractures in relation to rotation and angulation

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Most fractures occurred at apical third and middle third very few fractures occurred at coronal part of the canal. Smaller taper 0.04 fractured more frequently than 0.06 taper instruments.


  Discussion Top


Clinically nickel-titanium instruments, have a higher risk of separation. Specifically, retrospective analysis of routinely discarded NiTi instruments indicated two distinct fracture mechanisms, namely torsional fractures, and flexural fractures. [5] For Sattapan et al. [4] torsion fracture occurred in 55.7% of all fractured NiTi instruments during routine clinical use [Figure 3]. In a recent study of 100 fractured NiTi instruments, in 91% of the cases the fracture occurred as a result of cyclic fatigue, in 3% by torsion and in 6% by a combination of these. [13] whereas, other studies Zelada et al. [14] and Martín [15] show result in accordance to this study.

When an instrument is rotating around the curve, it is compressed on the inner side of the curve and stretched on the outer side of the curve resulting in cyclic fatigue and even fracture [Figure 2]. Evidence of these stages was visible upon higher magnification under Scanning Electron Microscopy examination. Crack initiation and growth, was characterized by the smooth almost featureless area at the periphery of fracture face. Crack propagation, and was typified by striations. Each striation represents the progression of the crack caused by tension during one rotation of the instrument. Fractures propagated from the periphery of the instrument toward the center. [6]
Figure 2: Scanning electron microscopy image of fractured pro taper Ni-Ti file showing cyclic fatigue.

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Figure 3: Photomicrography image of K3 files showing elongated flute of file along with fracture indicative of torsional fracture

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The intensity of the stresses is a specific parameter and it is related to the radius of the curvature of the root canal, length of the arch, and diameter of the instrument used. The smaller the radius of curvature of the root canal, greater will be the length of the arch, and the greater the diameter and rigidity of the instrument used, the lower will be the number of cycles until the fracture withstood by the evaluated endodontic instrument. [6],[16],[17]

Immobilization of an endodontic instrument inside a root canal can be controlled by reducing the advance of the instrument in apical direction and performing the root canal preparation in the crown-down direction. According to Yared et al. [18] for experienced professionals the use of motors with torques smaller than the limit of resistance to torsion fracture of the instrument used is not important to reduce the incidence of instrument fracture. For some authors, the rotation at failure decreases with the increase in diameters. [19]

When fractures in relation to the rotational speed were compared higher rpm will consume the useful life of the instrument much faster than a lower rpm. Thus, a lower rpm would be beneficial and provide a greater clinical life, more slowly using the finite number of cycles to failure available. [20],[21] According to Yared et al. [22] for an inexperienced operator, however, using the slower speed of 150-170 rpm would be more likely to prevent instrument from deformation and fracture The higher speed reduce the time required to reach the number of cycles until the fracture occurs. [6],[14],[23] Higher speed produce more heat than lower speeds and thus increase the temperature of the test specimen faster, which leads to faster increase in the tension on the surface making the fatigue fracture occur sooner. [24]

There were no statistically significant differences in incidence of fracture when Profile, ProTaper and K3 files were compared. Berutti et al. [25] studied that profile model is more elastic than ProTaper model and has a very long transformation phase and thus can operate even with high loads in the transformation phase without accumulating dangerous stresses. The results of this study reveals that profiles had more distortion than the ProTaper and K3 although it is not statistically significant similar to study by Ankrum et al. [26] Most fractures occurred at apical third and middle third very few fractures occurred at coronal part of the canal. Smaller taper 0.04 fractured more frequently than 0.06 taper instruments, explained by Blum et al. who recorded torque values and the location on the instruments of the areas of contact with dentin during its development of torque that is at or near the tip, indicate that greater caution should be used with the rotary technique, particularly with the taper 0.04 instruments. Increased torque at the tip and increased cyclic fatigue of the file at the tip, which explains more fracture at the tip of the file. [27]

 
  References Top

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15.Martín B, Zelada G, Varela P, Bahillo JG, Magán F, Ahn S, et al. Factors influencing the fracture of nickel-titanium rotary instruments. Int Endod J 2003;36:262-6.  Back to cited text no. 15
    
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24.Eggeler G, Hornhogen E, Yawny A, Heckmann A, Wgner M. Structural and functional fatigue of NiTi shape memory alloys. Mater Sci Eng A 2004:378;24-33.  Back to cited text no. 24
    
25.Berutti E, Chiandussi G, Gaviglio I, Ibba A. Comparative analysis of torsional and bending stresses in two mathematical models of nickel-titanium rotary instruments: ProTaper versus ProFile. J Endod 2003;29:15-9.  Back to cited text no. 25
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26.Ankrum MT, Hartwell GR, Truitt JE. K3 Endo, ProTaper, and ProFile systems: Breakage and distortion in severely curved roots of molars. J Endod 2004;30:234-7.  Back to cited text no. 26
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27.Blum JY, Machtou P, Micallef JP. Location of contact areas on rotary Profile instruments in relationship to the forces developed during mechanical preparation on extracted teeth. Int Endod J 1999;32:108-14.  Back to cited text no. 27
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    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]


This article has been cited by
1 Comparison of cyclic fatigue resistance of novel nickel-titanium rotary instruments
Ismail Davut Capar,Huseyin Ertas,Hakan Arslan
Australian Endodontic Journal. 2014; : n/a
[Pubmed] | [DOI]



 

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