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Results

Table 1: Time and cycles to failure of size 25 ProFile .04 taper NiTi instruments

Results are shown in Table 1.Difference in both time of instrument failure and cycles to failure were observed between the 100, 200 and 350 rpm groups. Time of instrument failure and cycles to failure decreased with an increase in rotational speed. Statistical analysis indicated a significant difference between the 350 rpm ad the other two groups. No statistically significant diferences were observed between the 100 and 200 rpm groups.

Discussion
The results of the present study showed that a higher rpm consumed the useful life of the instruments much faster than a lower rpm. Thus, a lower rpm would be beneficial and provide a greater clinical life , more slowly using the finite number of cycles to failure available. This is especially helpful in complex cases, where rotary instruments are used in anatomical conditions (severe curvatures, calcifications) suspected to induce stress fatigue. A lower speed could allow a longer but safer usage, especially if recapitulations are needed to facilitate negotiation to the apex..Moreover, if the rotary file risks to catch on a natural ledge and buckle, a slower rotational speed allows better tactile awareness to interpret and avoid these problems.

The results of the present study should not be interpreted as a fixed mean time or number of cycles to failure for size # 25. Many other factors ( position of the curvature,hardness of dentin, engagement of the cutting blades,etc.) may induce further stress within the rotating instruments and diminish resistance to cyclic fatigue.

However it's important to notice that,in the present study conditions, operating rpm was a significant factor affecting cycles to failure. These findings did not confirm those reported by Pruett et al (8) using Lightspeed instruments, who demonstrated that cyclic fatigue was not affected by the rpm. It's difficult to understand whether the different design of the two instruments could have any effect on the different findings.

A possible explanation is that a certain range of speed helps keeping the alloy as much as possible in the crystalline phase most resistant to failure. This can be more easily achievd in an experimental model, but is still not proven. Another possible explanation is that torque could be related to speed in the specific test apparatus, and therefore the use of a motor unable to maintain constant torque.

On the other hand,the data of the present study confirmed the findings of Dietz et al. (1) , who have investigated the effect of rotational speed on the breakage of nickel-titanium rotary files, using a contra-angle electric machine mounted on an Instron machine and curved canals (radius of curvature of 5mm) in bovine bone.. Results indicated that there was a significant difference between 150 and 359 rpm in all file sizes ( n.3-4 and 5), with files separating later at 150 rpm.They concluded that .04 taper NiTi rotary file breakage was less likely to occur if the file was run at the lowest recommended speed.

An excessive speed of rotation ( more than 350 rpm) is not recommended for these instruments.A slower speed is also not recommended in clinical practice, because it may reduce removal of debris,thus engaging too much of the file surface in the canal. In such cases failure is a threat.In conclusion.,this study suggest that further researches are required to improve the unerstanding of criteria thet may be used to predict nickel-titanium rotary instruments fatigue leading to separation. Without a complete investigation of these parameters, recommendation for a safer and longer use are only speculative. However a lower rpm (150-200 rpm) seems to be more beneficial in endodontic practice. It provides a greater clinical life and a better tactile awareness.

Bibliografy

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2. Horn A.Instructional video. Profile 0.04 taper series 29 rotary instruments. Tulsa Dental Products. 1994
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7. Walcott J,Himel VT. Torsional properties of nickel-titanium versus stainless steel endodontic files. J Endodon 1997:23:217-20
8. Pruett JP,Clement DJ,Carnes DL Jr. Cyclic fatigue testing of nickel-titanium endodontic instruments. J Endodon 1997;23:77-85
9. Patnam BW,Brown Jr CE, Legan JJ. Evaluation of distortion and breakage of nickel-titanium rotary instruments. J Endodon 1998;24:274 (abstract).
10. Wahl P. How to safely use nickel titanium files. Practical Endodontics Jan.1995;1
11. Dietz DB,Di Fiore PM, Bahcall JK, Lautenschlager EP: The effect of rotational speed on the breakage of nickel-titanium rotary files. J. Endodontics 1998;24: 273 (Abstract)