J. Laszkiewicz
Institute of Dentistry,Medical University of Lodz, Poland

Gianluca Gambarini
University of Siena,Dept. of Dental Materials

The effect of rotational speed on cyclic failure of profile .04 taper, nickel-titanium rotary instruments.

Abstract
As a wide range of operating speeds has been advocated for NiTi rotary instruments,the purpose of the present study was to determine if rotational speed has an effect on the breakage of .04 taper nickel-titanium rotary files. A new model was developed where the files where tested at different rpm in artificial stainlees-steel curved canals of identical size and radius. Files size # 25 were tested at 150,200 and 350 rpm..They were rotated freely inside the curved canals until breakage. The time of instrument failure was determined by visual inspection and recorded.

Results showed that both 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 and the other two groups. No statistically significant differences were observed between the 100 and 200 rpm groups.These findings indicated 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 .

Fig.1 Artificial canal

Fig.2 Test apparatus

Introduction
Use of rotary nickel-titanium instruments is widely spread now. Several studies (1-4)have shown that NiTi rotary instruments cuts faster,removes debris better and more centered preparation and less apical transportation than stainless steel files. Excellent improvements in time-savings and in clinical results can be simply and quickly achieved by practitioners. Baumann and Roth (5) demonstrated that even students lacking any endodontic experience can easily learn to use profile .04 Taper Series 29 with success and achieve a good root canal geometry.

Niti is a peculiar alloy, which shows remarkable flexibility, a high resistance to torsional fracture and an extremely low modulus of elasticity.This is in direct contrast to stainless steel which exhibits Hookean elastic behaviour and therefore permanent change on loading.(6-7). These superelastic NiTi instruments allow easier preparation of curved canals while minimizing transportation.However, despite the excellent mechanical properties of alloy, it is important to know that nickel-titanium rotary files wear rapidly during usage and separtion can occur. All rotating instruments have their limits (cyclic fatigue), therefore's not stay long in canal rotation avoid over-use.

Separation is nowadays a concern with NiTi instruments and they have been reported to undergo unexpected fracture (8). Separation can occur without ny visible signs of previous permanent deformation, apparently within the elastic limit of the instruments. Visible inspection seems not to be a reliable method for evaluation of used nickel-titanium rotary instruments.Therefore,files do need to be discarded on a regular basis to prevent breakage. Putnam et al. (9) showed that when ProFile .04 instruments are used in simulated clinical trials ( extracted mandibular molars)a permanent deformation occurred before breakage prior to the twelfth mesiobuccal canal instrumented.One files from six fractured in the canal during use.,and all fractures occurred during or after twelve trials ( instrumented mesiobuccal root canals). To minimize the risk of separation ,used rotary NiTi instruments require frequent replacing.

It has been reported that in a rotation-to-breakage study conducted by the College of Dental Medicine at the Medical University of South Carolina the # 25 size stainless steel instruments broke after an average of approximately 55 revolutions, while the average revolutions before breakage for nickel-titanium U-files was nearly 800 revolutions for size # 30. According to the author (10), given this information and using a two-to-one safety factor , the nickel titanium instruments should probably not be rotated in canals for more than 30 to 65 seconds.

The ideal speed of rotation for nickel-titanium U-files ranges from approximately 150 to 350 rpm. A much lower or higher speed is reported to be a cause of instrument breakage (10).Each rotation within a curved canal subjects an endodontic instrument to both tensile and compressive stress, the most destructive form of cyclic loading. It has been postulated thet the phenomenon of repeated cyclic matal fatigue caused by canal curvature may be the most important factor in instrument separation (3). It is important to recognize that an instrument has a mean number of cycles to failure that is determined by specific parameters of root canal anatomical complexities and instrument diameter (8).

As a wide range of operating speeds has been advocated for NiTi rotary instruments,.the purpose of the present study was to determine if rotational speed has an effect on the breakage of .04 taper nickel-titanium rotary files.More precisely, if cyclic fatigue and the ultimate number of cycles to failure were affected or unaffected by rpm.

Materials and methods
Thirty size # 25 ProFile .04 rotary instruments (Maillefer, Baillagues, Switzerland) were used for the present study . All files were visually examined before usage and discarded if defects were detected.A new model was developed where the files where tested at different rpm in artificial stainlees-steel curved canals of identical size and radius for each file group tested (Fig.1). The method described by Pruett et al (8) for determine canal curvature was also used. The shape of the curve was depicted using two parameters: a 5mm. radius of curvature and a 60° angle of curvature.

In order to perform a controlled cyclic fatigue study on NiTi rotary instruments a specific test apparatus was developped.A 1:16 reducer contra-angle electric handpiece ( Anthogyr,Saillanches, France) was mounted on a specifically assembled motorized unit, which provided constant preset torque (approximately 5 N/cm), and speed.,allowing free rotation of the instruments within the experimental stainlees steel canal (Fig2).

Files size # 25 were tested at 100, 200 and 350 rpm. There were 10 files per group. Files were inserted at the same length (13 mm of working blades were engaged in the canal) and were rotated inside the curved canals until breakage. The time of instrument failure was determined by visual inspection and recorded. The number of cycles to failure calculated from the rpm data by multiplying the rpm by the time to failure.Statistical analysis of the results was performed with chi-square test.

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