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Aust Endod J 2013; 39: 151–154
ORIGINAL RESEARCH
Cyclic fatigue resistance of newly manufactured rotary nickel titanium instruments used in different rotational directions aej_353
151..154
Gianlucca Gambarini, DDS, MSC, PhD1; Richard Gergi, DDS, MSC2; Nicola Maria Grande, DDS, MSC1; Nada Osta, DDS, MSC2; Gianluca Plotino, DDS, MSC1; and Luca Testarelli, DDS, MSC1 1 Department of Endodontics, La Sapienze University, Rome, Italy 2 Department of Endodontics, Saint Joseph University, Beirut, Lebanon
Keywords clockwise rotation, counterclockwise rotation, cyclic fatigue, heat treatment. Correspondence Dr Richard Gergi, Department of Endodontics, Saint Joseph University, PO Box 166255, Beirut, Lebanon. Email: [email protected] doi:10.1111/j.1747-4477.2012.00353.x
Abstract The aim of this study was to investigate whether cyclic fatigue resistance is increased for nickel titanium instruments manufactured with improved heating processes in clockwise or counterclockwise continuous rotation. The instruments compared were produced either using the R-phase heat treatment (K3XF; SybronEndo, Orange, CA, USA) or the M-wire alloy (ProFile Vortex; DENTSPLY Tulsa Dental Specialties, Tulsa, OK, USA). Tests were performed with a specific cyclic fatigue device that evaluated cycles to failure of rotary instruments in curved artificial canals. Results indicated no significant difference in resistance to cyclic fatigue when rotary nickel titanium instruments are used in clockwise or counterclockwise continuous rotation. In both directions of rotation, size 04-25 K3XF showed a significant increase (P < 0.05) in the mean number of cycles to failure when compared with size 04-25 ProFile Vortex.
Introduction Nickel titanium (NiTi) root canal files were first introduced in 1988 by Walia et al. (1) to overcome the rigidity of stainless steel instruments and thereby improve the instrumentation of curved canals (1). NiTi is far more flexible then stainless steel and its super-elasticity reduces the restoring forces (2), thereby allowing improved canal shaping and reduced transportation (3). Despite the many advantages of NiTi instrumentation, instrument fracture is still a major concern when using NiTi rotary files (4,5). Cyclic fatigue is reported to occur unexpectedly in the absence of any sign of previous permanent deformation (6,7). Cyclic fatigue is most likely to occur in a canal with an acute curve and a short radius of curvature as defined by Pruett et al. (8) and is the leading cause of NiTi instrument separation. Increasing the resistance to file separation has been a focus in new NiTi rotary instrument manufacture and design (9). Many variables that might influence the resistance to cyclic fatigue fracture of NiTi rotary files have been investigated, such as operational speed, design of the
© 2012 The Authors Australian Endodontic Journal © 2012 Australian Society of Endodontology
instruments, metal surface treatment and the effect of the irrigating solution. Possible strategies to increase the resistance to cyclic fatigue of NiTi rotary instruments include an improvement in the manufacturing process or the use of new alloys with superior mechanical properties. Recently, several manufacturers have developed special thermomechanical processing, with the aim of producing a NiTi super-elastic alloy that contains primarily a martensitic phase stable under clinical conditions. They are reported to have shown better physical and mechanical properties than instruments manufactured with traditional NiTi alloy (3,10). Recently, two new file systems, manufactured from this super-elastic alloy, have been introduced commercially. The K3XF (Sybron Dental Specialties, Orange, CA, USA) is a NiTi engine file identical to the K3 file (Sybron Dental Specialties), but with improved R-phase heat treatment (similar to the Twisted File). DENTSPLY Tulsa Dental Specialties introduced ProFile Vortex Rotary Files using M-Wire NiTi raw materials, which have been developed at SportsWire of DENTSPLY Tulsa Dental Specialties 151
Cyclic Fatigue Resistance
through a proprietary heat treatment process. Parallel to the introduction of newly heat-treated rotary NiTi instruments, a change in the movement kinematics is being introduced (11). The technique that was originally proposed by Yared (11) has recently led to the commercialisation of new specific reciprocating NiTi instruments and proprietary, undisclosed movements and angles of reciprocation (Reciproc, VDW, Munich, Germany; and Wave One, Dentsply Maillefer, Ballaigues, Switzerland). There have been no previous studies published investigating the mechanical properties of these new NiTi rotary instruments. The purpose of this study was to evaluate the cyclic fatigue life of 0.25 taper 0.04 K3XF and ProFile Vortex engine-driven files used with two different movement kinematics.
Materials and methods Two groups of NiTi endodontic instruments consisting of identical instrument sizes (constant 0.04 taper and 0.25 tip diameter) were tested under continuous clockwise (CW) and counterclockwise (CCW) rotation until fracture. Ten instruments from group 1 compared cyclic fatigue rotation of K3XF under continuous CW rotation with K3XF used in continuous CCW rotation until fracture. Resistance to cyclic fatigue fracture was also compared using 10 NiTi ProFile Vortex instruments (group 2) in CW and CCW continuous rotation. Ten instruments from each system were tested for cyclic fatigue resistance, resulting in a total of 40 instruments. The cyclic fatigue testing device used in the present study has been used for studies on cyclic fatigue resistance previously performed by the authors (6,12). The device consists of a mainframe to which a mobile plastic support is connected for the electric hand-piece and a stainless steel block containing the artificial canals. The electric hand-piece was mounted on a mobile device to allow precise and reproducible placement of each instrument inside the artificial canal (Fig. 1). This ensured three-dimensional alignment and positioning of the instruments to the same depth. The artificial canal was created by reproducing an instrument’s size and taper, thus providing the instrument with a suitable trajectory that respects the parameters of the curvature chosen. A simulated root canal with a 60° angle of curvature and 5 mm radius of curvature was constructed for each instrument. The centre of the curvature was 5 mm from the tip of the instrument, and the curved segment of the canal was approximately 5 mm in length. The instruments were rotated at a constant speed of 300 rpm by using a 16:1 reduction hand-piece (W & H Dentalwerk, Burmoos, Austria) powered by a torque-controlled electric motor (VDW, Munich, Germany). To reduce the friction of the 152
G. Gambarini et al.
file as it contacted the artificial canal walls, special highflow synthetic oil designed for lubrication of mechanical parts (Super Oil; Singer Co Ltd, Elizabethport, NJ, USA) was applied. All instruments were rotated until fracture occurred. The time to fracture was recorded visually with a 1/100 s chronometer. The time to fracture was multiplied by the number of rotations per minute to obtain the number of cycles to failure (NCF) for each instrument. The length of the fractured tip was also recorded for each instrument. Means and standard deviations of NCF and fragment length were calculated for each system. Data were subjected to one-way analysis of variance to determine significant differences between groups. When the overall F-test indicated a significant difference, the multiple-comparison Holm t-test procedure was performed to identify the mean that differed from the others. Significance was set at the 95% confidence level.
Results Mean values standard deviations expressed as NCF are displayed in Table 1. A higher NCF is caused by a higher
Figure 1 Artificial canal with 60° curvature and 5 mm radius.
Table 1 Fatigue resistance of size 25, taper 0.04 instruments used in CW and CCW rotation NCF
FL
Instruments
Mean
SD
Mean
SD
K3XF CW K3XF CCW ProFile Vortex CW ProFile Vortex CCW
1079 1164 877 896
151.17 147.77 114.43 120.65
6.15 6.20 5.85 5.90
0.43 0.32 0.24 0.17
CW, clockwise; CCW, counterclockwise; FL, fragment length; NCF, number of cycles to failure; SD, standard deviation.
© 2012 The Authors Australian Endodontic Journal © 2012 Australian Society of Endodontology
Cyclic Fatigue Resistance
G. Gambarini et al.
resistance to cyclic fatigue of the tested instruments. K3XF instruments, size 25 and 0.04 taper, showed a significant increase in the mean NCF when compared with size 25 and 0.04 taper ProFile Vortex files (1079 vs. 877 NCF in CW rotation and 1164 vs. 896 NCF in CCW rotation). No statistically significant difference (P < 0.05) was noted between instruments from group 1 and between instruments from group 2 (Table 1) used either in CW or CCW continuous rotation. The mean length of the fractured segment was also recorded to evaluate the correct positioning of the tested instrument inside the canal curvature and whether similar stresses were being induced. No statistically significant difference (P > 0.05) in the mean length of the fractured fragments was evident for all of the instruments (Table 1).
Discussion The grinding of file blanks during the manufacture of NiTi rotary instruments causes many machining defects. Kuhn et al. (13) reported in their microstructural analysis that ground- and work-hardened NiTi rotary instruments had a high density of defects that might disturb the phase transformation. Defects left on the instrument surface can increase the level of stress during instrumentation (14). This means that the resistance to fatigue failure can be enhanced by a smooth defect-free surface. One of the many promising approaches to improve fatigue resistance of rotary instruments is using better NiTi raw materials through optimised thermomechanical processing or new manufacturing technology (3,10). K3XF and ProFile Vortex rotary instruments are both manufactured using heat treatment technology in order to maximise their flexibility and resistance (6,10). In the present study, even with a small sample size, results show a significant difference between the NCF when comparing K3XF to Profile Vortex. K3XF showed an increase in cycling fatigue resistance. The higher resistance to cyclic fatigue can be explained by the fact that K3XF are manufactured with a proprietary process, R-phase heat treatment technology differing from the Profile Vortex files using M-Wire NiTi raw materials. According to Hayashi et al. (15), R-phase technology modifies the crystalline structure of rotary NiTi instruments to become finer than traditionally processed materials, and maximises instruments flexibility. The results showed no difference in resistance to fracture when NiTi rotary instruments are used either in continuous CW rotation or CCW rotation. The absence of significance could be associated with the small sample size. In the recent past, there has been growing interest
© 2012 The Authors Australian Endodontic Journal © 2012 Australian Society of Endodontology
in reciprocation. Apart from available reciprocating instruments, new motors allowing the use of traditional rotary NiTi instruments in reciprocation have become commercially available. Because the currently available motors use different reciprocating angles of rotation, the purpose of studying the fatigue resistance in CW and CCW rotation was to investigate if there is any possible difference between the two commercially available movements. In this study, the rotary instruments rotate freely in the artificially curved canal. However, with the consideration of the torsional resistance and the existence of dentin chips, in actual clinical cases, the rotary instruments with more crack initiation sites at sharp cutting edges or corners would be more susceptible to the fracture in the mixed mode, either by fatigue or torsion (because of continuous weakening in strength against torsional fracture as fatigue cracks grow). Therefore, clinicians using instruments made of conventional super-elastic NiTi materials would have a greater chance of instrument separation under similar circumstances. Analysis of the data regarding the length of the fractured instrument revealed no statistically significant difference in the mean size of the instruments tested. The maximum overall difference among groups was
Aust Endod J 2013; 39: 151–154
ORIGINAL RESEARCH
Cyclic fatigue resistance of newly manufactured rotary nickel titanium instruments used in different rotational directions aej_353
151..154
Gianlucca Gambarini, DDS, MSC, PhD1; Richard Gergi, DDS, MSC2; Nicola Maria Grande, DDS, MSC1; Nada Osta, DDS, MSC2; Gianluca Plotino, DDS, MSC1; and Luca Testarelli, DDS, MSC1 1 Department of Endodontics, La Sapienze University, Rome, Italy 2 Department of Endodontics, Saint Joseph University, Beirut, Lebanon
Keywords clockwise rotation, counterclockwise rotation, cyclic fatigue, heat treatment. Correspondence Dr Richard Gergi, Department of Endodontics, Saint Joseph University, PO Box 166255, Beirut, Lebanon. Email: [email protected] doi:10.1111/j.1747-4477.2012.00353.x
Abstract The aim of this study was to investigate whether cyclic fatigue resistance is increased for nickel titanium instruments manufactured with improved heating processes in clockwise or counterclockwise continuous rotation. The instruments compared were produced either using the R-phase heat treatment (K3XF; SybronEndo, Orange, CA, USA) or the M-wire alloy (ProFile Vortex; DENTSPLY Tulsa Dental Specialties, Tulsa, OK, USA). Tests were performed with a specific cyclic fatigue device that evaluated cycles to failure of rotary instruments in curved artificial canals. Results indicated no significant difference in resistance to cyclic fatigue when rotary nickel titanium instruments are used in clockwise or counterclockwise continuous rotation. In both directions of rotation, size 04-25 K3XF showed a significant increase (P < 0.05) in the mean number of cycles to failure when compared with size 04-25 ProFile Vortex.
Introduction Nickel titanium (NiTi) root canal files were first introduced in 1988 by Walia et al. (1) to overcome the rigidity of stainless steel instruments and thereby improve the instrumentation of curved canals (1). NiTi is far more flexible then stainless steel and its super-elasticity reduces the restoring forces (2), thereby allowing improved canal shaping and reduced transportation (3). Despite the many advantages of NiTi instrumentation, instrument fracture is still a major concern when using NiTi rotary files (4,5). Cyclic fatigue is reported to occur unexpectedly in the absence of any sign of previous permanent deformation (6,7). Cyclic fatigue is most likely to occur in a canal with an acute curve and a short radius of curvature as defined by Pruett et al. (8) and is the leading cause of NiTi instrument separation. Increasing the resistance to file separation has been a focus in new NiTi rotary instrument manufacture and design (9). Many variables that might influence the resistance to cyclic fatigue fracture of NiTi rotary files have been investigated, such as operational speed, design of the
© 2012 The Authors Australian Endodontic Journal © 2012 Australian Society of Endodontology
instruments, metal surface treatment and the effect of the irrigating solution. Possible strategies to increase the resistance to cyclic fatigue of NiTi rotary instruments include an improvement in the manufacturing process or the use of new alloys with superior mechanical properties. Recently, several manufacturers have developed special thermomechanical processing, with the aim of producing a NiTi super-elastic alloy that contains primarily a martensitic phase stable under clinical conditions. They are reported to have shown better physical and mechanical properties than instruments manufactured with traditional NiTi alloy (3,10). Recently, two new file systems, manufactured from this super-elastic alloy, have been introduced commercially. The K3XF (Sybron Dental Specialties, Orange, CA, USA) is a NiTi engine file identical to the K3 file (Sybron Dental Specialties), but with improved R-phase heat treatment (similar to the Twisted File). DENTSPLY Tulsa Dental Specialties introduced ProFile Vortex Rotary Files using M-Wire NiTi raw materials, which have been developed at SportsWire of DENTSPLY Tulsa Dental Specialties 151
Cyclic Fatigue Resistance
through a proprietary heat treatment process. Parallel to the introduction of newly heat-treated rotary NiTi instruments, a change in the movement kinematics is being introduced (11). The technique that was originally proposed by Yared (11) has recently led to the commercialisation of new specific reciprocating NiTi instruments and proprietary, undisclosed movements and angles of reciprocation (Reciproc, VDW, Munich, Germany; and Wave One, Dentsply Maillefer, Ballaigues, Switzerland). There have been no previous studies published investigating the mechanical properties of these new NiTi rotary instruments. The purpose of this study was to evaluate the cyclic fatigue life of 0.25 taper 0.04 K3XF and ProFile Vortex engine-driven files used with two different movement kinematics.
Materials and methods Two groups of NiTi endodontic instruments consisting of identical instrument sizes (constant 0.04 taper and 0.25 tip diameter) were tested under continuous clockwise (CW) and counterclockwise (CCW) rotation until fracture. Ten instruments from group 1 compared cyclic fatigue rotation of K3XF under continuous CW rotation with K3XF used in continuous CCW rotation until fracture. Resistance to cyclic fatigue fracture was also compared using 10 NiTi ProFile Vortex instruments (group 2) in CW and CCW continuous rotation. Ten instruments from each system were tested for cyclic fatigue resistance, resulting in a total of 40 instruments. The cyclic fatigue testing device used in the present study has been used for studies on cyclic fatigue resistance previously performed by the authors (6,12). The device consists of a mainframe to which a mobile plastic support is connected for the electric hand-piece and a stainless steel block containing the artificial canals. The electric hand-piece was mounted on a mobile device to allow precise and reproducible placement of each instrument inside the artificial canal (Fig. 1). This ensured three-dimensional alignment and positioning of the instruments to the same depth. The artificial canal was created by reproducing an instrument’s size and taper, thus providing the instrument with a suitable trajectory that respects the parameters of the curvature chosen. A simulated root canal with a 60° angle of curvature and 5 mm radius of curvature was constructed for each instrument. The centre of the curvature was 5 mm from the tip of the instrument, and the curved segment of the canal was approximately 5 mm in length. The instruments were rotated at a constant speed of 300 rpm by using a 16:1 reduction hand-piece (W & H Dentalwerk, Burmoos, Austria) powered by a torque-controlled electric motor (VDW, Munich, Germany). To reduce the friction of the 152
G. Gambarini et al.
file as it contacted the artificial canal walls, special highflow synthetic oil designed for lubrication of mechanical parts (Super Oil; Singer Co Ltd, Elizabethport, NJ, USA) was applied. All instruments were rotated until fracture occurred. The time to fracture was recorded visually with a 1/100 s chronometer. The time to fracture was multiplied by the number of rotations per minute to obtain the number of cycles to failure (NCF) for each instrument. The length of the fractured tip was also recorded for each instrument. Means and standard deviations of NCF and fragment length were calculated for each system. Data were subjected to one-way analysis of variance to determine significant differences between groups. When the overall F-test indicated a significant difference, the multiple-comparison Holm t-test procedure was performed to identify the mean that differed from the others. Significance was set at the 95% confidence level.
Results Mean values standard deviations expressed as NCF are displayed in Table 1. A higher NCF is caused by a higher
Figure 1 Artificial canal with 60° curvature and 5 mm radius.
Table 1 Fatigue resistance of size 25, taper 0.04 instruments used in CW and CCW rotation NCF
FL
Instruments
Mean
SD
Mean
SD
K3XF CW K3XF CCW ProFile Vortex CW ProFile Vortex CCW
1079 1164 877 896
151.17 147.77 114.43 120.65
6.15 6.20 5.85 5.90
0.43 0.32 0.24 0.17
CW, clockwise; CCW, counterclockwise; FL, fragment length; NCF, number of cycles to failure; SD, standard deviation.
© 2012 The Authors Australian Endodontic Journal © 2012 Australian Society of Endodontology
Cyclic Fatigue Resistance
G. Gambarini et al.
resistance to cyclic fatigue of the tested instruments. K3XF instruments, size 25 and 0.04 taper, showed a significant increase in the mean NCF when compared with size 25 and 0.04 taper ProFile Vortex files (1079 vs. 877 NCF in CW rotation and 1164 vs. 896 NCF in CCW rotation). No statistically significant difference (P < 0.05) was noted between instruments from group 1 and between instruments from group 2 (Table 1) used either in CW or CCW continuous rotation. The mean length of the fractured segment was also recorded to evaluate the correct positioning of the tested instrument inside the canal curvature and whether similar stresses were being induced. No statistically significant difference (P > 0.05) in the mean length of the fractured fragments was evident for all of the instruments (Table 1).
Discussion The grinding of file blanks during the manufacture of NiTi rotary instruments causes many machining defects. Kuhn et al. (13) reported in their microstructural analysis that ground- and work-hardened NiTi rotary instruments had a high density of defects that might disturb the phase transformation. Defects left on the instrument surface can increase the level of stress during instrumentation (14). This means that the resistance to fatigue failure can be enhanced by a smooth defect-free surface. One of the many promising approaches to improve fatigue resistance of rotary instruments is using better NiTi raw materials through optimised thermomechanical processing or new manufacturing technology (3,10). K3XF and ProFile Vortex rotary instruments are both manufactured using heat treatment technology in order to maximise their flexibility and resistance (6,10). In the present study, even with a small sample size, results show a significant difference between the NCF when comparing K3XF to Profile Vortex. K3XF showed an increase in cycling fatigue resistance. The higher resistance to cyclic fatigue can be explained by the fact that K3XF are manufactured with a proprietary process, R-phase heat treatment technology differing from the Profile Vortex files using M-Wire NiTi raw materials. According to Hayashi et al. (15), R-phase technology modifies the crystalline structure of rotary NiTi instruments to become finer than traditionally processed materials, and maximises instruments flexibility. The results showed no difference in resistance to fracture when NiTi rotary instruments are used either in continuous CW rotation or CCW rotation. The absence of significance could be associated with the small sample size. In the recent past, there has been growing interest
© 2012 The Authors Australian Endodontic Journal © 2012 Australian Society of Endodontology
in reciprocation. Apart from available reciprocating instruments, new motors allowing the use of traditional rotary NiTi instruments in reciprocation have become commercially available. Because the currently available motors use different reciprocating angles of rotation, the purpose of studying the fatigue resistance in CW and CCW rotation was to investigate if there is any possible difference between the two commercially available movements. In this study, the rotary instruments rotate freely in the artificially curved canal. However, with the consideration of the torsional resistance and the existence of dentin chips, in actual clinical cases, the rotary instruments with more crack initiation sites at sharp cutting edges or corners would be more susceptible to the fracture in the mixed mode, either by fatigue or torsion (because of continuous weakening in strength against torsional fracture as fatigue cracks grow). Therefore, clinicians using instruments made of conventional super-elastic NiTi materials would have a greater chance of instrument separation under similar circumstances. Analysis of the data regarding the length of the fractured instrument revealed no statistically significant difference in the mean size of the instruments tested. The maximum overall difference among groups was
