Effect of torsional loading of nickel-titanium instruments on cyclic fatigue resistance.
BACKGROUND
The aim of this study was to evaluate the effect of torsional preload on the cyclic fatigue life of nickel-titanium rotary instruments.


METHODS
ProFile (#25/0.06) (Dentsply Maillefer, Ballaigues, Switzerland) and ProTaper (F1; Dentsply Maillefer) were used. Each file was preloaded at 4 conditions (ie, no preloading and 25%, 50%, and 75% of mean ultimate torsional strength) of torsional prestress before the fatigue test. The torsional preloads were applied by securing 5 mm of the file tip while keeping the file straight, rotating it clockwise until the preset torque, and then returning to the original position. This motion was repeated until a preset number (10, 30, or 50) of repetitions were reached (n = 12). After torsional preloading, the number of cycles to failure was evaluated in a simulated canal. Data were analyzed using 2-way analysis of variance and the Duncan post hoc comparison. The fractured fragment surfaces were examined under a scanning electron microscope for the topographic features of fractured instruments.


RESULTS
For both instruments, there was a significant effect because of the extent of torsional preloads. The 50% and 75% torsionally preloaded ProFile and all ProTaper preloading groups had a higher number of cycles to failure than the other group(s). There was little difference in the lateral view appearance between new and torsionally preloaded files. After cyclic fatigue testing, all preloaded instruments showed numerous microcracks adjacent to the fracture site on lateral view examination. The microcracks did not seem to follow the machining grooves on the instrument surface but rather ran irregularly.


CONCLUSIONS
The torsional preloads within the superelastic limit of the material may improve the cyclic fatigue resistance of nickel-titanium rotary instruments.
Area of Endodontics, Faculty of Dentistry, the University of Hong Kong, Hong Kong SAR, China.
Gary Shun-Pan
GS
Cheung
Seung-Hei
SH
Oh
Jung-Hong
JH
Ha
Sung Kyo
SK
Kim
Se-Hee
SH
Park
Hyeon-Cheol
HC
Kim
D003722:Dental Alloys / Q000737:chemistry*; D004548:Elasticity; D004867:Equipment Design; D004868:Equipment Failure; D017276:Friction; D006801:Humans; D008422:Materials Testing; D008855:Microscopy, Electron, Scanning; D009532:Nickel / Q000737:chemistry*; D016062:Porosity; D018915:Root Canal Preparation / Q000295:instrumentation*; D012399:Rotation; D013314:Stress, Mechanical; D013499:Surface Properties; D013997:Time Factors; D014025:Titanium / Q000737:chemistry*; D019415:Torque; D054159:Torsion, Mechanical
D003722:Dental Alloys; C040654:titanium nickelide; D009532:Nickel; D014025:Titanium
Cyclic fatigue resistance; nickel-titanium rotary file; preloading; superelasticity; torsional load