An in vitro comparison of cyclic fatigue resistance of ProTaper universal and GT series x files

Objective: The aim of this study was to compare the cyclic fatigue resistance of two nickel-titanium (NiTi) endodontic instruments from ProTaper and GT series X files. Study Design: Cyclic fatigue test was realized with instruments from ProTaper: F1 and F3 (Denstply Maillefer, Ballaigues, Switzerland) and GT-X: 20.06 and 30.08 (Dentsply Tulsa Dental, Tulsa, Oklahoma, United States of America). A total of 320 instruments were rotated in 4 curved artificial canals with different angles and radius of curvature. The time and cycles to failure were calculated. The data was compared using a Mann-Whitney, Kruskall-Wallis, and Kolmogorov-Smirnov tests, with a significance level of p<0.05. Results: GT-X files rotated for a significantly longer period of time before separation occurred, thus GT-X files where more resistant to the cyclic fatigue compared with ProTaper. Conclusion: GT-X files have a greater resistance to cyclic fatigue, this fact can be caused by the use of the Ni-Ti alloy “M-Wire”. Key words:Endodontics, GT-X files, ProTaper files, cyclic fatigue.


Introduction
The introduction of the �ic�el-Titaniu� (�i-Ti) rotary files in the field of endodontics has led to greater efficiency in the preparation of root canal by si�plifyin� the process, because of the flexibility, higher fracture tough-ness and cutting efficiency (1,2), with less apical extrusion of debris (3) and allo�in� instru�entation of curved root canals �ith �ini�al transportation (4,5). Despite having a greater flexibility compared with stainless steel files (1) they show cyclic fatigue fracture (6), e534 that is described as the clinical failure caused by the continuous rotation of an instru�ent, in a curved space, in the absence of threadin� it, �here it under�oes alternat-in� cycles of pressure and tension. It is affected by the an�le, the radius of curvature of the canal, and by the size and taper of the instru�ent, bein� the �ost i�portant factor in the fracture of �i-Ti instru�ents (4,(7)(8)(9).
The Ni-Ti file shows no visible signs of permanent plastic defor�ation and separation occurs �ithout �arnin�, negating the validity of visual inspection of the files (7). The separation frequency of these instru�ents, co�pared to stainless steel files, is five to seven times higher (10). In the recent research, no standarization �odel has been found for cyclic fati�ue test, respectin� to the an�le of the canals, or the �aterial used for �anufacturin� (7). Recently, the GT series X files (Dentsply Tulsa Dental, Tulsa, O�laho�a, United States of A�erica) have been developed, and incorporated into its structure �i-Ti alloy "M-Wire" (12). It is obtained by cycles of heatin� and tempering during production, conferring the file a hi�her resistance to cyclic fati�ue and �reater resistance to fracture (13,14). The GT Series X files have a rounded cutting edge, with a greater number of flutes in the tip, with a variable distance bet�een the�, eli�inatin� the bloc�a�e of the tip into the canal. It has three different tapers and rotates at 300 rp� (14). The ai� of this study �as to co�pare the cyclic fati�ue of two Ni-Ti instrument files, GT Series X files with Pro-Taper Universal files (Dentsply Maillefer SA, Ballaigues, S�itzerland).

Material and Methods
The study �as carried out at the University of Valencia, Medical and Dental School, to co�pare the cyclic fatigue of ProTaper files (Dentsply Maillefer SA, Ballaigues, Switzerland) versus GT-X files (Dentsply Tulsa Dental, Tulsa, OK, USA). One hundred and sixty ProTaper Universal files and 160 GT-X files were used, and divided into four groups: group 1: 80 ProTaper files F1 (tip 0.20 and taper 0.07%); group 2: 80 GT-X 20.06 files (tip 0.20 and taper 0.06%); group 3: 80 ProTaper files F3 (tip 0.30 and taper 0.09%); group 4: 80 GT-X 30.08 files (tip 0.30 and taper 0.08%). All files were instrumented with an endodontic motor X-S�art (Denstply Maillefer SA, Ballai�ues, S�itzerland) 16:1 at 300 rpm, 20 files of each group were tested in all the four canals �ade of sintered stainless steel, to allo� the reproduction of the test (7,13). The four canals were 18 mm length, tip of 0.40 and 9% of taper. The dia�eter of the si�ulated canals �as hi�her than files, allowing a free rotation: canal 1: 60º angle, radius of curvature of 8 mm, canal 2: 45º angle, radius of curvature of 8 mm, canal 3: 60º angle, radius of curvature of 5 mm, canal 4: 45º angle, radius of curvature of 5 mm (15).
A modification of the cyclic fatigue test (7) was used, in �hich sa�ital sections �ere created in the canals, a quarter of its size, and then �ounted on an acrylic plate, obtainin� a visual control of the fracture �o�ent of the file and a bounded position in the acrylic plate. The handpiece �as �ounted, also, on an acrylic surface and cla�ped by a scre� and nut to hold it in the sa�e position (Fi�. 1) eli�inatin� the operator factor, �hich �a�es apical pressure in the instru�entation (7,(12)(13)(14)(15)(16)(17). Instru�ents rotated �ithout resistance and overheatin�, the simulated canals were lubricated with fluid petroleum jelly 180 (Millet-Fran�lin, BA, Ar�entina), throu�h the �indo� created on the surface, �hile each instru�ent rotated inside the canal (9,12,(15)(16)(17)(18)(19). Visual inspection of the �o�ent of separation �as performed, using SurgiTel (General Scientific Corporation, MI, USA) �a�nifyin� �lasses �ith an increase of 2.5 x. The instru�entation �as recorded in video, and subsequently the nu�ber of rotations �as calculated for each file before reaching the fracture. The number of rotations �as calculated �ultiplyin� the revolutions per minute (300 rpm) for the time each file rotated, until the fracture occurred. The data obtained �as or�anized in four �roups to co�pare �ith each other dependin� of the taper and the tip of the files. The results �here analyzed �ith SPSS 18 (SPSS Inc, IL, USA), usin� the Mann-Whitney, Krus�all-Wallis (for independent sa�ples), and Kol�o�orov-S�irnov test (for continuous samples), with a significance level of p<0.05.

Results
The avera�e nu�ber of rotations �as calculated by �ulti-plyin� the ti�e in �inutes per revolutions, calculatin� an overall avera�e of the analyzed sa�ples (Table 1) It �as sho�n that the canal �ith �ore co�plex anato�y (canal 1), �enerated a lo�er nu�ber of rotations (in the four �roups co�pared), bein� the avera�e nu�ber of the rotations in this canal �uch less than the avera�e nu�ber in the canal 4 (easier canal), �hich �enerates the largest number of rotations for all files, the differences where statistically significant (p<0.05) (Fig. 2).

Discussion
File separation is a �ajor concern durin� the endodontic treat�ent, despite the separation �ay occur by �ultiple factors, the cyclic fati�ue is one of the leadin� causes (4 with GT-X 20.06 files founding no significant differences among the two files; in this study the differences between ProTaper and GT-X files where significant. The incorporation of �i-Ti "M-Wire" alloy in the GT Series X files can improve the resistance to intracanal failure; in recent research, there are fe� articles that compare GT Series X files versus files without any modification in the fabrication process, with similarities in volu�e, size and taper (21). Ho�ever, this study compared two different file systems, with different geometries, which were used for finishing the instrumentation of the root canal. ProFile GT and ProFile GT Series X �here also co�pared in canals with curvature of 45º and 60º, with an apical �ove�ent of 10 revolutions per �inute (23). In that study, no significant differences were found. However, �ore recent studies (24) found differences �ith respect to the cyclic fati�ue, �hich is dependent of the ther�o�echanical treat�ent applied in GT-X. These results are si�ilar to this study, in �hich no apical �otion was used and the free rotation of the files was allowed.
The volu�e of the instru�ent affects the results of cyclic fati�ue, because as the instru�ent size increases, the ti�e to reach the fracture decreases (7,10). In this study �e found that ProTaper F3 �as the less resistant file to cyclic fatigue due to the volume that it own. The cyclic fati�ue �as also co�pared bet�een GT Series X 20.04 and 20.06 �ith T�isted Files, EndoSequence and ProFile, �ith a tip dia�eter of 25 (19), obtainin� a �reater number of rotation for the GT Series X files 20.04 and 20.06. This results are attributable to the �ass difference between the compared files. In the present study, the volumetric size of the files was the same. The cyclic fatigue resistance for F1 and F3 files is similar dependin� of the volu�e, but independent of the �eo�etry of the canal (16), bein� F3 the one that sho�s less resistance. These results are consistent �ith our results. The fracture test perfor�ed in this study sho�ed that the place of separation is located at the �idpoint of the curvature. These results �ere previously found by other researchers (7,15).
In conclusion, GT Series X 20.06 and 30.08 files, had a �reater resistance to cyclic fati�ue co�pared to ProTaper Universal F1 and F3 files. In the "in vitro" test, this could be attributable to the �anufacturin� process in which files are subjected to heating and tempering.