Brief Resume of the intended work : 6.1 Need for the study : Root canal instruments manufactured from Nickel-Titanium alloy are introduced to overcome the rigidity of stainless steel material. There is a general perception that Nickel-Titanium instruments have a high risk of fracture in use. Although excess torsion and cyclic fatigue have both been implicated as a reason for ﬁle fracture, the latter is probably the more prevalent cause of unexpected breakages.1
There are many engine driven rotary instruments available commercially that use the Nickel Titanium of various designs and dimensions to facilitate cleaning and shaping procedures. Despite this improvements, instrument separation is still a major concern.2
The repeated tension–compression cycles caused by rotation within curved canals, increases cyclic fatigue of the instrument over time and may be an important factor in instrument fracture.3
Recently the Twisted Files with R Phase technology represents advancement in the manufacturing of Nickel titanium instruments. This R-Phase heat treatment significantly increases instrument’s resistance to cyclic fatigue and flexibility.4
Hence the objective of this in-vitro study is to compare the Cyclic fatigue and Fracture characteristics of Grounded and Twisted Nickel Titanium Rotary Files.
6.2 Review of literature :
A study was conducted to compare the fatigue resistance of traditional, ground nickel-titanium rotary instruments with the Twisted File and to examine the fracture characteristics of the fatigued fragment. Size #25, 0.06 tapered, Twisted Files(TF), RaCe, Helix, and ProTaper F1(#20.0.07 taper) were examined with scanning electron microscope for surface characteristics before subjecting to a cyclic fatigue test. The time until fracture was recorded to calculate the number of revolutions for each instrument. The fragments were examined with scanning electron microscope both in lateral view and fractographically. The results concluded that Twisted Files showed a significantly higher resistance to cyclic fatigue than other nickel-titanium files that were manufactured with a grinding process. The path of crack propagation appeared to be different for electropolished (TF and RaCe) versus non-electropolished (Helix and ProTaper) instruments. Although all specimens showed similar fractographic appearance, which indicated a similar fracture mechanism, instruments with abundant machining grooves seemed to have a higher risk of fatigue.1 A study was conducted to investigate whether cyclic fatigue resistance is increased for Nickel-Titanium instruments manufactured by using new processes. This was evaluated by comparing instruments produced by using the twisted method and those using the M-wire with instruments produced by a traditional (Nickel-Titanium) NiTi grinding process. Tests were performed with a specific cyclic fatigue device that evaluated cycles to failure of rotary instruments inside curved artificial canals. Results indicated that size 06-25 Twisted Files instruments showed a significant increase in the mean number of cycles to failure when compared with size 06-25 K3 files. Size 06-20 K3 instruments showed no significant increase in the mean number of cycles to failure when compared with size 06-20 GT series X instruments. The new manufacturing process produced nickel-titanium rotary files (TF) significantly more resistant to fatigue than instruments produced with the traditional NiTi grinding process. Instruments produced with M-wire (GTX) were not found to be more resistant to fatigue than instruments produced with the traditional NiTi grinding process.2 A study was conducted to assess the resistance to fatigue of three Nickel-Titanium files after the immersion in sodium hypochlorite solution in conditions similar to those used in clinical practice. A total of 150 new Twisted Files, Revo S SU files , and M two file size #25 .06, were tested. Fifty files of the same brand were randomly assigned to five groups and submitted to the following immersion protocol in 5% NaOCl at 37°C for 16 mm: no immersion (control), 5 minutes statically, 1 minute statically, 5 minutes dynamically (300 rpm/min), and 1 minute dynamically. Resistance to cyclic fatigue was determined by counting the numbers of cycles to failure in a 60° curve with a 5-mm radius. Resistance to cyclic fatigue of the same NiTi file was not significantly affected by immersion in NaOCl. The Twisted File showed a higher resistance in all groups than Revo .Static or dynamic immersion in NaOCl for 1 minute or 5 minutes did not reduce the cyclic fatigue resistance of NiTi significantly. However, the type of instrument influences cyclic fatigue resistance. In this study, Twisted Files were more resistant followed by Mtwo and Revo S SU files.3
The purpose of this study was to determine if new Twisted (Nickel-Titanium) NiTi instruments were more resistant to cyclic fatigue compared with traditionally ground NiTi rotary instruments such as EndoSequence and ProFile . Size #25 Twisted Files, Endo Sequence, and Pro File and size #20 GTX with .04 and .06 tapers were tested in a simulated canal with 60 degrees angle of curvature and a 3-mm radius. The number of rotations until fracture was recorded for each instrument. Twisted Files was significantly more resistant to cyclic fatigue than Endo Sequence but not different from Pro File with the same tip size. The new manufacturing processes appeared to offer greater resistance to cyclic fatigue in a simulated canal model.4
The purpose of this study is to evaluate the cyclic fatigue resistance of three types of Nickel-Titanium rotary files in a model using reciprocating axial movement. The influence of file size and taper was also investigated and fracture patterns were examined under SEM. The 10 experimental groups consisted of ProFiles, K3s, and RaCe files, size 25 in .04 and .06 tapers, as well as ProFiles and K3s, size 40 in .04 and .06 tapers. Each file was rotated freely at 300 rpm inside a stainless steel tube with a 60 degree and 5 mm radius of curvature. A continuous 3 mm oscillating axial motion was applied at 1 cycle per second by attaching an electric dental handpiece to the most inferior load cell of an Instron machine using a custom-made jig. The number of rotations to failure was determined and analyzed. Overall, K3 25/.04 files were significantly more resistant to cyclic fatigue compared to any other group in this study. In the 25/.04 category, K3s were significantly more resistant to failure than ProFiles and RaCe files. Also in the same category, ProFiles significantly outlasted RaCe files. In the 25/.06 group, K3s and ProFiles were significantly more resistant to failure than RaCe files, but K3s were not significantly different than ProFiles. In the 40/.04 and 40/.06 groups, K3s were significantly more resistant to cyclic fatigue than ProFiles. SEM observations demonstrated mostly a ductile mode of fracture. The results suggest that different cross-sectional designs, diameters, and tapers all contribute to a nickel-titanium instrument's vulnerability to cyclic failure.
6.3 Aims and Objectives of the study :
The purpose of this in-vitro study is to compare the Cyclic fatigue resistance of traditional, ground Nickel-Titanium Rotary instruments with the Twisted File and to examine the fracture characteristics.
Material and Methods: 7.1 Source of data : Four brands of Nickel Titanium instruments with similar cross sectional geometries are grouped as follows: - Eight Files from each group will be tested
GROUP 1- Twisted Files #25/0.06 Taper (Sybron Endo)
GROUP 2- RaCe Files #25/0.06 Taper (FKG Dentaire)
GROUP 3- ProTaper Files #20/0.07 Taper F1 (Dentsply)
GROUP 4- Helix Files #25/0.06 Taper (Dia Dent)
Twisted and RaCe both have an equilateral triangular cross section, Whereas Pro Taper and Helix have similar cross section of a convex triangle. The Pro Taper F1 (tip size #20 with 7% taper for the apical few millimeters)will be selected for having the same diameter (0.55mm) as for the other files at D5 (i.e 5 mm from the tip),thus a similar cross sectional area at that length.
7.2. Method of collection of data: Two new Files from each brand will be examined for machining grooves and surface texture under a Scanning electron microscope(SEM) before Cyclic fatigue testing. The fatigue testing will be conducted in a custom made device with simulated root canal in an attempt to eliminate variations of canal anatomy as a source for torque and force differences.
The instrument will be rotated at 300 rpm by using an electric motor at maximum torque. The instrument will be aligned to a specific mark(to allow for repositioning the same location),and then the File will be set to rotate, synchronized with timing by a stopwatch. The instrument will be allowed to rotate freely until fracture. Timing will be stopped as soon as fracture is detected visually and audibly. The time will be then converted into number of revolutions to failure.
The broken fragments will be ultrasonically cleaned in absolute alcohol for approximately 120 seconds before examination with the SEM. They will be examined in lateral view at various magnification. Then a random selection of the fragments will be remounted on the microscopic stage ,with the fragment facing upward for fractographic examination
Statistical Analysis : The number of rotations to failure for various groups will be analyzed using the one wayanalysis of variance . Scheff’e post hoc test will be applied to identify the groups that will be significantly different from others.
7.3 Does the study require any investigations or interventions to be conducted on patients or other humans or animals? If so, please describe briefly.
7.4. Has ethical clearance been obtained from your institution in caseof 7.3?
List of References :
Hyeon-Cheol Kim, Jiwan Yum, Bock hur, and Gary shun- pan cheung. Cyclic fatigue and Fracture characteristics of Ground and Twisted Nickel-Titanium rotary files. J Endod 2010; 36(1): 147-52
Gambarini G, Grande NM, Plotino G, Somma F, Garala M, De Luca M, Testarelli L. Fatigue resistance of engine-driven rotary Nickel-Titanium instruments produced by new manufacturing methods. J Endod 2008; 34(8): 1003-1005
3. Pedullà E, Grande NM, Plotino G, Pappalardo A, Rapisarda E.Cyclic fatigue
resistance of three different Nickel-Titanium instruments after immersion in sodium
hypochlorite. J Endod 2011; 37(8): 1139-42
4.. Larsen CM, Watanabe I, Glickman GN, He J.Cyclic fatigue analysis of a new