CYCLIC FATIGUE TESTING OF NICKEL-TITANIUM ENDODONTIC INSTRUMENTS

Cyclic fatigue of nickel-titanium, engine-driven instruments was studi ed by determining the effect of canal curvature and operating speed on the breakage of Lightspeed instruments. A new method of canal curvatu re evaluation that addressed both angle and abruptness of curvature wa s introduced. Canal curvature was simulated by constructing six curved stainless-steel guide tubes with angles of curvature of 30, 45, or 60 degrees, and radii of curvature of 2 or 5 mm. Size #30 and #40 Lights peed instruments were placed through the guide tubes and the heads sec ured in the collet of a Magtrol Dynamometer. A simulated operating loa d of 10 g-cm was applied. Instruments were able to rotate freely in th e test apparatus at speeds of 750, 1300, or 2000 rpm until separation occurred. Cycles to failure were determined. Cycles to failure were no t affected by rpm. Instruments did not separate at the head, but rathe r at the point of maximum flexure of the shaft, corresponding to the m idpoint of curvature within the guide tube. The instruments with large r diameter shafts, #40, failed after significantly fewer cycles than d id #30 instruments under identical test conditions. Multivariable anal ysis of variance indicated that cycles to failure significantly decrea sed as the radius of curvature decreased from 5 mm to 2 mm and as the angle of curvature increased greater than 30 degrees (p < 0.05, power = 0.9). Scanning electron microscopic evaluation revealed ductile frac ture as the fatigue failure mode. These results indicate that, for nic kel-titanium, engine-driven rotary instruments, the radius of curvatur e, angle of curvature, and instrument size are more important than ope rating speed for predicting separation. This study supports engineerin g concepts of cyclic fatigue failure and suggests that standardized fa tigue tests of nickel-titanium rotary instruments should include dynam ic operation in a flexed state. The results also suggest that the effe ct of the radius of curvature as an independent variable should be con sidered when evaluating studies of root canal instrumentation.