| Since the 1990 s Nickel-Titanium(Ni-Ti) rotary instruments have been used to improve the speed and the effectiveness of endodontic preparation, even in curved root canals. They are much more flexible and elasticity than traditional stainless steel instruments. And they were discovered to be beneficial owing to shorten preparation time and reduce procedural errors, for instance transportation, zipping, ledging compared to stainless steel files. So Ni Ti files are used in current endodontic practice commonly. Despite they have these advantages, Ni Ti instruments happen to have a risk of separation[1]. To avoid the fracture of instruments, one way of improving flexural fatigue resistance is by changing the phase of metal Crystal. Conventional Ni Ti rotary instruments are in the austenitic phase. With stress or thermomechanical processing, the austenitic phase of Ni Ti instrument turns to the martensitic phase. One of the most excellent properties of the martensitic phase is superior resistance to fatigue[2]. Another way of improving flexural fatigue resistance is by electropolishing the instruments.With improving the surface smoothness, these instruments delaying the initiation of surface cracks[3].Except the fracture, transportation can be found during curved root canal endodontic preparation.Due to the tendency of files to restore themselves to their original linear shape,they remove the canal wall structure on the outside curve in the apical half of the canal.The transportation is one of undesired iatrogenic errors,and it results in reduced success rates.So the ability of Ni Ti instruments to stay centred within the canal is a superior characteristic[4]..So many manufacturers have taken different designs to reduce apical transportation during canal preparation.The design of files has been identified as a potential factor for good instrumentation outcome[5].but some files have still been very little studied,if at all.Hence, the purpose of this study is to exam the properties of Ni Ti rotary instruments,such as Mtwo,Pro Taper and S3.This research was divided into three parts:the first one is to evaluate the deformation and fracture rate of instruments discarded,and to discover the mechanism of failure;the second one is to compare flexural fatigue fracture of three types of Ni Ti rotary instruments Mtwo,Pro Taper and S3 by making a stimulate models in vitro;the last one is to evaluate the canal transportation and centralizing ability of Ni Ti instruments afer root canal preparation by Micro-CT.In experiment 1,defects in discarded rotary instruments after routine clinical useA total of 906 Mtwo,603 Pro Taper,328 S3 Ni Ti rotary instruments were collected after clinical use from the clinic of endodontics over 6 months.The instruments were examed with stereo-microscope and SEM to determine the types of defects, such as unwinding, curving, bending and fracture, and micro-cracks on the surfaces of chosen bending instruments.The fractured files were photographed digitally and the pictures were analyzed by software Image-Pro Plus to measure the distance from the break point to the tip.The data were statistically analyzed by using SPSS 17.0 software package.The types of defects included unwinding,winding flutes and plastic bending.Mtwo rotary instrument separation occurred in 221 files(24.4%),and the most frequently fractured file was #10.04(39.3%);Pro Taper rotary instrument separation occurred in 189 files(31.3%), and F2 was the most frequently fractured file(40.6%);S3 rotary instrument has 101 fractured files(30.8%), and S2 was the most frequently fractured one(33.9%). The highest fracture level of the three ones were Mtwo #30.05, Pro Taper F3, S3, and the mean distance from the breakpoint to the file tip: Mtwo #30.05 was 3.65±0.80 mm, Pro Taper F3 was 4.20±1.45 mm, S3 was 3.99±1.28 mm. So it is important not to exceed the maximum number of usage recommended by the manufacturer and discard the instruments on a regular basis, Undestanding the defects types of Mtwo, Pro Taper, S3 Ni Ti rotary instruments is usefully way to avoid file breakage during root canal therapy.In experiment 2, flexural fatigue resistance of Mtwo, Pro Taper, and S3 Ni Ti rotary instruments.Mtwo, Pro Taper and S3 each has 20 new Ni Ti files were studied. These files were divided into three groups. All instruments were tested for flexural fatigue. Resistance to flexural fatigue was depended on recording the time in seconds to fracture,by using a model of Artificial tooth root in a 60°curve with a 5 mm radius. And to observe the section of fractured instruments by scanning electron microscope(SEM). The resistence to flexural fatigue was different from three types of Ni Ti files. The time to fracture of S3 and Mtwo were significantly longer than Pro Taper’s. S3’s and Mtwo’s had no significant difference. So the resistance to flexural fatigue of S3 and Mtwo were better than Pro Taper. With observation of the SEM, the cross-sectional design of the three Ni Ti rotary instruments were different, a large number of toughness nests were found in broken surface. Time to fracture was affected by the quality disfigurement. And the lifespan was affected by the design of cross-section.In experiment 3, comparative analysis of canal transportation and centring ability of Mtwo, Pro Taper, S3 rotary instruments, assessed by Micro-CT.Forty extracted premolar teeth randomly divided into four groups. Root canals were respectively prepared by using Mtwo, S3, Pro Taper Ni Ti instruments and stainless steel hand K file. A large number of preoperative and postoperative images were taken by Micro-CT. Canal transportation and centralizing ability were measured with reference to the distance between the noninstrumented portion of the root canals and the mesial and distal periphery of the root(double root canals only recorded the buccal one), compared with images obtained after the preparation. Then centring ratio value and the canal transportation were obtained by the formula proposed by Gambill. Among Mtwo, S3, Pro Taper groups, there were no significant difference relating to centring ratio value and canal transportation.So S3 can well maintain the original root canal path and center in the preparation as well as Mtwo and Pro Taper rotary instruments. Therefore, S3 has practical value for clinical. |
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