Year : 2011 | Volume
: 14 | Issue : 2 | Page : 169--172
An in vitro evaluation of the effect of deep dry cryotreatment on the cutting efficiency of three rotary nickel titanium instruments
Gingu Koshy George1, Kavitha Sanjeev2, Mahalaxmi Sekar2,
1 Department of Conservative Dentistry and Endodontics, Pushpagiri Dental College, Thiruvalla, India
2 SRM Dental College, Chennai, India
Gingu Koshy George
Department of Conservative Dentistry and Endodontics,Pushpagiri Dental College,Thiruvalla, Kerala
Context : Cryogenic methods have been used to increase the strength of metals.
Aim : The aim of this study was to evaluate the effect of deep dry cryotherapy on the cyclic fatigue resistance of rotary nickel titanium instruments.
Materials and Methods : Twenty K3, RaCe and Hero Shaper nickel titanium instruments, size 25, 0.06 taper, were taken for this study. Ten files were untreated (control group) and 10 files were deep dry cryogenically treated. Both the untreated and cryotreated files were subjected to cyclic fatigue evaluation. Cyclic fatigue was evaluated as the number of cycles it took for fracture of the instrument within a stainless steel shaping block of specific radius and angle of curvature.
Statistical Analysis : Mean values were compared between different study groups by using one-way analysis of variance (ANOVA) with P < 0.05 considered as the level of significance.
Results : The results showed a significant increase in the resistance to cyclic fatigue of deep dry cryotreated NiTi files over untreated files.
Conclusions : It may thus be concluded that deep cryotherapy has improved the cyclic fatigue of NiTi rotary endodontic files.
|How to cite this article:|
George GK, Sanjeev K, Sekar M. An in vitro evaluation of the effect of deep dry cryotreatment on the cutting efficiency of three rotary nickel titanium instruments.J Conserv Dent 2011;14:169-172
|How to cite this URL:|
George GK, Sanjeev K, Sekar M. An in vitro evaluation of the effect of deep dry cryotreatment on the cutting efficiency of three rotary nickel titanium instruments. J Conserv Dent [serial online] 2011 [cited 2020 Sep 28 ];14:169-172
Available from: http://www.jcd.org.in/text.asp?2011/14/2/169/82627
Biomechanical preparation has been considered the most important step in root canal therapy. Advent of rotary NiTi instruments has aided endodontists in cleaning and shaping of root canals. Civjan et al. first suggested the use of NiTi alloy for fabrication of hand and rotary instruments. The extraordinary characteristics of super elasticity and shape memory of the NiTi alloy have made it possible to manufacture rotary instruments. 
Endodontic instruments upon rotation are subjected to both tensile and compressive stresses in curved canals. This stress is localized at the point of curvature. It has been demonstrated that the continuous cycle of tensile and compressive forces in the area of curvature of the canal to which NiTi rotary instruments are subjected produce a very destructive form of loading, causing cyclic fatigue and eventually fracture of the instrument.  It has been suggested that cyclic fatigue has accounted for 50-90% of mechanical failures.
Despite the advantages of rotary NiTi instruments, concerns have been expressed by many authors and clinicians about the potential for these instruments to fracture within the root canal system during endodontic treatment without any indication, whereas fracture of stainless steel files is preceded by instrument distortion serving as a warning of impending fracture.
Cryogenic methods have been used to increase the wear, abrasion, corrosion resistance, and to improve the strength of metals. Cryogenically treated NiTi instruments have shown increased microhardness. Deep cryogenic treatment involves suspending the metal over a super-cooled bath containing liquid nitrogen at −196°C or −320°F and then allowing the metal to slowly warm to room temperature.
A study evaluating the cutting efficiency and wear resistance of Profile rotary files after deep dry cryotherapy  had shown a significant increase in the cutting efficiency but not the wear resistance. However, the cyclic fatigue resistance of other NiTi instruments after deep dry cryogenic treatment has not been evaluated so far. So, the aim of this study was to evaluate the effect of deep dry cryotherapy on the cyclic fatigue resistance of three rotary NiTi instruments.
Materials and Methods
Twenty RaCe (FKG Dentaire, Switzerland), K3 (Sybron Endo, CA, USA) and Hero Shaper (Micro Mega, France) nickel titanium instruments, size 25, 0.06 taper, were taken for this study. The taper and size of the instrument was standardized in all the groups (i.e., size 25, 0.06 taper and of length 21 mm). The files were randomly divided into two groups as follows:
Group A: untreated NiTi rotary files for evaluating the cyclic fatigue and Group B: cryotreated NiTi rotary files for evaluating the cyclic fatigue. These were further divided as the following:
Group A1: 10 untreated RaCe rotary files for evaluating the cyclic fatigue; Group A2: 10 untreated K3 rotary files for evaluating the cyclic fatigue; Group A3: 10 untreated Hero Shaper rotary files for evaluating the cyclic fatigue.
Group B1: 10 cryotreated RaCe rotary files for evaluating the cyclic fatigue; Group B2: 10 cryotreated K3 rotary files for evaluating the cyclic fatigue; and Group B3: 10 cryotreated Hero Shaper rotary files for evaluating the cyclic fatigue.
Protocol for deep dry cryogenic treatment
The cryogenic treatment was carried out in a vacuum-insulated treatment chamber which included a perforated platform extending parallel to and spaced above the bottom of the chamber (Cryotherapy Unit; A.C.I. CP-200vi, Applied Cryogenics Inc., MA, USA). Temperature measuring sensors, thermostat valve and helical heat exchanger were mounted in the treatment chamber. The information derived from temperature sensors was utilized by a process controller to direct the supply of cryogenic liquid to the treatment chamber in accordance with the desired temperature descent and ascent profiles for the sample within the chamber. Samples to be treated were supported on the platform and cryogenic liquid was introduced into the chamber. The cryogenic liquid passed through a helical heat exchanger and got converted into vapor and entered below the platform in accordance with a time-temperature program which reduced the temperature of the samples in stages to −184.44°C (88.6 K or −300°F) at a rate of 1.5°C/min for 2 hours and 18 min by cooling the samples with evaporating vapors from the cryogenic liquid pool in the space below the platform. After a soaking period of 36 hours at the −84.44°C (88.6 K or −300°F) temperature level, the temperature of the samples in the treatment chamber was raised to room temperature at 1°C/min for 3 hours and 30 min by controlled heating of chamber with heaters and blowers. The protocol followed was similar to that of Vinothkumar et al. 
Protocol for evaluating cyclic fatigue
The experimental set up for evaluating the cyclic fatigue of the nickel titanium instrument comprised an endodontic micro motor (Endomate DT, NSK, Japan) which operated with a 16:1 reduction gear hand piece attached to the descending crosshead of Universal Testing Machine (Lloyds Instruments, UK) and a stainless steel shaping block for endodontic files. The shaping block was fixed and consisted of a concave tempered steel radius and a convex tempered steel cylinder that guaranteed a curve of the instrument during rotation. This concave radius incorporated a notch for guiding the instrument into a canal which had a radius of curvature of 5 mm and angle of curvature of 45°.
The instrument was mounted on the reduction gear hand piece which was attached to Universal Testing Machine (Lloyds Instruments) with a crosshead speed of 2 mm/sec. The files were then guided into the notch of the radius and were allowed to rotate until fracture occurred. The instruments were rotated at a speed of 300 rotations/min with a torque set at 3 N m. The time to fracture was noted for each group. The time to fracture was multiplied by the number of rotations per minute to obtain the Number of Cycles to Fracture (NCF) for each instrument. Mean values were then calculated for each group.
Statistical analyses of the tests were done using SPSS software 11.5 for Windows. Mean and standard deviation were estimated from the samples for each study group. Mean values were compared between different study groups by using one-way analysis of variance (ANOVA). In the present study, P < 0.05 was considered as the level of significance.
Mean time to fracture in Group B1 (81.0 ± 0.6 sec) was significantly higher than that in Group A1 (70.4 ± 0.4 sec) (P < 0.0001). Mean time to fracture in Group B2 (141.6 ± 0.9 sec) was significantly higher than that of Group A2 (90.4 ± 0.6 sec) (P < 0.0001). Mean time to fracture in Group B3 (71.8 ± 1.9 sec) was significantly higher than that of Group A3(53.6 ± 1.4 sec) (P < 0.0001). Groups B1, B2, and B3 showed significantly higher time to fracture than Groups A1, A2 and A3 (P < 0.0001).
Mean number of cycles to fracture in Group B1 (405 ± 3) was significantly higher than that of Group A1 (352 ± 2) (P < 0.0001). Mean number of cycles to fracture in Group B2 (708 ± 4) was significantly higher than that in Group A2 (452 ± 3) (P < 0.0001). Mean number of cycles to fracture in Group B3 (359 ± 10) was significantly higher than that in Group A3 (268 ± 7) (P < 0.0001). Groups B1, B2, and B3 showed significantly higher number of cycles to fracture than Groups A1, A2 and A3 (P < 0.0001).
NiTi instruments are characterized by a greater degree of elastic flexibility in bending and torsion and superior resistance to torsional fracture compared to stainless steel files  due to the extraordinary characteristics of super elasticity and shape memory of the nickel titanium alloy. However, these instruments are prone to separation without warning. Two modes of fractures have been identified for NiTi files: flexural fracture and torsional fracture. Flexural fracture occurs due to the cyclic fatigue experienced by the files within a curved canal. Repeated loading and unloading applied to the NiTi files during instrumentation causes repetitive phase transformation between the austenitic and martensitic in NiTi, resulting finally in fracture of the instrument when it goes beyond the unrecoverable plastic deformation state.
Methods like boron ion implantation,  thermal nitridation,  physical vapor deposition of titanium nitride  and electropolishing  have been used for increasing the cutting efficiency of rotary NiTi instruments. Anderson et al. showed that electropolished instruments performed significantly better than non-electropolished instruments in both the cyclic fatigue testing and static torsional loading, which is likely to be caused by a reduction in surface irregularities that serve as points of stress concentration and crack initiation in nickel titanium.
Deep dry cryogenic treatment has been seen to affect the entire cross section of the instrument rather than just the surface. , Deep dry cryogenic treatment involves keeping the metal above a super-cooled bath containing liquid nitrogen −196°C or −320°F and then allowing the metal to slowly warm to room temperature. The cryogenic treatment was shown to have more beneficial effects than the traditional higher temperature cold treatment, which include increasing cutting efficiency as well as the overall strength of the metal.  Hence, in this study, deep dry cryotherapy of the NiTi rotary endodontic files was done.
Cyclic fatigue testing can be accomplished through static models, in which a file is flexed and then rotated until fracture occurs.  On the other hand, Dederich and Zakariasen  used a dynamic model and found that a cyclic axial motion significantly extended the life span of rotary files. This type of test more closely approximates a clinical brushing or pecking motion. Hence, this mode of design was used for our study. The crosshead speed used in our study was 2 mm/sec which was similar to that used by Svec and Powers  who had utilized a crosshead speed of 120 mm/min to assess the physical property and durability of rotary NiTi files. In this study, the instruments were rotated at a constant speed of 300 rpm with a torque set at 3 N m. This is in accordance with the study by Gambarini  who evaluated the cyclic fatigue of rotary NiTi instruments using high- and low-torque endodontic motors and concluded that a higher resistance to fracture was observed when the instrument was operated by a low-torque motor, and thus, low-torque motor was used in this study.
The results of this study showed an increase in fracture resistance of cryotreated NiTi files as compared to untreated files. This could be attributed to the fact that the complete transformation of the austenitic phase of the alloy to martensitic phase, which could have occurred at −195°C, would have decreased the internal stress within the alloy due to plastic deformation. Amini et al. have explained in their study that the presence of residual austenite phase in an alloy decreases hardness and also reduces the wear resistance of the tool. Thus, increasing resistance to wear, reduction of internal stresses can be regarded as the most important privileges of using cryogenic treatment. The less the temperature of cryogenic environment was, the better were the properties observed. The deep dry cryogenic treatment has been seen to affect the entire cross section of the instrument rather than just the surface, with no change in the elemental crystalline composition of the alloy.
K3 rotary files showed significant resistance to fatigue over RaCe and Hero Shaper in this study. It is plausible that K3 files were the most fatigue resistant as a result of their ability to better distribute bending forces over their entire length. The file design of RaCe and Hero Shaper might act to concentrate stress at specific points, rather than distribute it along the entire file length. Subramaniam et al. have shown in their study that instrument design plays a role in enhancing its fracture resistance.
Parashoes and Messer  who reviewed the factors of rotary NiTi instrument fracture and its consequences conclude that along with the prevalence and relation of metallurgy and fracture, other factors like manufacturing process can also alter the fracture resistance and cutting efficiency of NiTi files. So, deep dry cryotherapy of NiTi files may be adopted by manufacturers to improve the cutting efficiency and fracture resistance of rotary NiTi instruments.
Deep dry cryotherapy of NiTi endodontic files can improve the cyclic fatigue resistance of NiTi files, but further studies are required to evaluate these cryogenically treated NiTi files clinically.
Dr. Mohan Lal and Mr. Jaswin are gratefully acknowledged.
|1||Civjan S, Huget EF, DeSimon LB. Potential applications of certain nickel titanium alloys. J Dent Res 1975;54:89-96.|
|2||Walia H, Brantley WA, Gerstein H. An initial investigation of thee bending and torsional properties of Nitinol root canal files. J Endod 1988;21:346-51.|
|3||Pruet JP, Clmenat DJ, Carnes DL. Cyclic fatigue testing of NiTi endodontic instruments. J Endod 1997;23:77-85.|
|4||Vinothkumar TS, Miglani R, Lakshminaraayanan L. Influence of deep dry cryogenic treatment on cutting efficiency and wear resistance of NiTi rotary endodontic instruments. J Endod 2007;33:1355-8.|
|5||Lee DH, Park B, Saxeba A, Serene TP. Enhanced surface hardness by boron implantation in Nitinol alloy. J Endod 1996;22:543-6.|
|6||Rapisarda E, Bonaccorso A, Tripi TR, Fragalk I, Condorelli GG. The effect of surface treatments of nickel-titanium files on wear and cutting efficiency. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2000;89:363-8.|
|7||Schäfer E. Effect of physical vapour deposition on cutting efficiency of nickel titanium files. J Endod 2002;28:800-2.|
|8||Baumann MA. Reamer with alternating cutting edges-concept and clinical application. Endod Top 2005;10:176-8.|
|9||Anderson EM, Price JW, Parashos P. Fracture resistance of electropolished rotary nickel-titanium endodontic instruments. J Endod 2007;33:1212-6.|
|10||Huang JY, Zhu YT, Liao XZ, Beyerlein IJ, Bourke MA, Mitchell TE. Microstructure of cryogenic treated M2 tool steel. Mater Sci Engin 2003;339:241-4.|
|11||Molinari A, Pellizzari M, Gialenella S, Strafellini G, Stiansky KH. Effect of deep cryogenic treatment on the mechanical properties of tool steels. Mater Process Technol 2001;118:350-5.|
|12||Dedrich DN, Zakariasen KL. The effects of cyclic axial motion on rotary endodontic instrument fatigue. Oral Surg Oral Med Oral Pathol 1986;61:192-6.|
|13||Svec TA, Powers JM. A method to assess rotary nickel titanium files. J Endod 2000;26:517-8.|
|14||Gambarini G. Cyclic fatigue of nickel-titanium rotary instruments after clinical use with low- and high-torque endodontic motors. J Endod 2001;27:772-4.|
|15||Amini K, Nategh S, Shafiey A, Soltany MA. To study the effect of cryogenic heat treatment on the hardness and the amount of residual austenite in 1/2304 steel. Metal 2008;13:1-7.|
|16||Subramaniam V, Indira R, Srinivasan MR, Shankar P. Stress distribution in rotary nickel titanium instruments - a finite element analysis. J Conserv Dent 2007;10:112-8.|
|17||Parashos P, Messer HH. Rotary NiTi instrument fracture and its consequences. J Endod 2006;32:1031-43.|