Bone screws used in surgical procedures experience torsional forces during medical implant installations. Therefore, bone screw manufactures and suppliers are often required to perform the torsion testing requirements found in ASTM F543 and ISO 6475. The torsion test determines mechanical properties such as Torsional Yield Strength, Maximum Torque, Breaking Angle, Insertion Torque and Removal Torque. These standards also provide information regarding the basic testing apparatus for performing the torque tests.
For this torsion application, we selected a 55MT1 torsion test system. The bone screws we tested had a maximum torque rating of 15Nm; so, the 22Nm full capacity rating of the 55MT1 was appropriate. The system also included Instron’s 5500 control electronics and Partner™ Torsion Testing Software. The adjustable filter frequency on the control electronics in conjunction with Partner’s high data acquisition rates, allowed us to generate very accurate torque vs. angle data plots. The precise alignment of the drive and torque measuring sides of the machine ensured repeatable test results across each sample of screws. The machine’s axial preload assembly applied a compressive load to the screw in order to drive the screw during the test. Finally, Partner Software automatically calculated the necessary results such as Maximum Torque and Breaking Angle while the software’s Destructive Torsion Test Wizard made it easy to set up the torsion tests according to the standards.
When required to perform torsion testing on metallic bone screws to ASTM F543 or ISO 6475, we recommend considering a 55MT1 machine. If some of your screws require less torque, you may want to consider a lower capacity torque cell as an option. Instron can supply standard drill type chucks for holding the drive bits and special chucks for gripping the test block material required by ASTM F1839.