Energy required to fracture an endodontic-instrument and its stiffness in-twisting

Objective: ANSI/ADA-Standard-No.28(ISO-3630) contains a test-method for resistance-to-fracture by twisting and angular-deflection. From this method, the maximum-torque an endodontic-instrument can withstand before fracture by twisting along with the corresponding angular-deflection at fracture can be determined.  The objective of this work is to report a method to calculate the energy required to fracture an endodontic-instrument, along with its “stiffness” in twisting, using the resistance-to-fracture by twisting standard-method, and to use this method to obtain data on endodontic-instruments from seven-different-manufacturers.

Methods: For each-manufacturer, ten endodontic-rotary-instruments (five-sterilized/five-unsterilized) were tested for each specified-size(seeTable1) and for each gripping-distance. Sterilized-instruments were autoclaved(pressure-0.21 MNm²,136±2ºC for 30±0.5minutes) and dry-heat-sterilized(180±5ºC for 30±0.5minutes). The resistance-to-fracture by twisting test was performed by gripping the last 3-mm(or1-mm) of the tip of the endodontic-instrument in brass jaws, which were attached to a torque-meter, and then rotating the shank-end of the instrument clockwise at 2-rpm until fracture occurred(Fig1). The test-equipment has the ability to collect continuous resistance-to-rotation values(N•cm) through use of an analog-to-digital converter. Figure2 shows an example of collected data plotted as a torque(N•cm) versus angle(degrees) curve. The total energy required to fracture the endodontic-instrument can be calculated from the area under this curve by converting angles to radians and using left Reimann-sum-equation. Also, from the slope of the linear-region of the curve in Figure2(shaded green), a measure of the stiffness of the endodontic-instrument when twisted can be obtained (i.e.,the greater the slope then the stiffer the instrument in twisting). These stiffness-values are compared to values obtained from performing tests according to the “Stiffness” method described in ANSI/ADA-Standard-No.28.

Results: See Figure3/Figure4.

Conclusion: Using the resistance-to-fracture by twisting standard test-method, a method to calculate the energy required to fracture an endodontic-instrument and its “stiffness” in twisting is demonstrated along with the successful use of this method to obtain data on endodontic-instruments from seven-different-manufacturers.