ASTM F543-17 Axial and Torsion Testing of Bone Screws

Bone screws are used in surgical procedures for securing implants, osteosynthesis devices, and fracture fixation plates to the skeletal system. In normal clinical use, a surgeon applies combined axial and torsional forces to a bone screw as it's implanted within the body. Manufacturers and scientists test bone screws to determine various mechanical properties in order to evaluate new materials and designs.

In ASTM F543-17, there are four testing annexes. Annex A1 requires the screw to be sufficiently clamped and for a rotational velocity between 1 and 5 rpm to be applied until specimen failure. This annex also requires that the torque profile and rotation angle be measured. Annex A2 is designated to measure the torque required to insert and remove the screw with a constant rotational velocity between 1 and 5 rpm, while maintaining an axial load of no more than 10 N in compression. The third annex, Annex A3, specifies a test to measure the force required to axially remove the screw that has been fully inserted in the test block using the method from Test A2. The screw should be loaded in a tensile test at a constant rate of 5 mm/min until failure of the bone screw, or removal from the test block. Finally, Annex A4, specifies a test method for testing self-tapping bone screws. The procedure evaluates the axial loading required to engage a self-tapping bone screw into a standard laboratory material. Although the self-tap action is considered a simple clinical procedure, reproduction of this in the laboratory involves a relatively complex motion. The test requires a continuous rotational velocity of up to 30 rpm, while the axial load is incremented during the insertion at a rate of 2 N/s. The objective of this test is to record the torque profile as the bone screw is inserted into the material and then removed.

The challenges of testing bone screws to this standard are:

• Determining what type of a biaxial system is right for your needs
• Low torque and force measurement

Instron® Solution:

• Instron offers three systems capable of testing bone screws, which include the MT Series torsion only systems, the Torsion Add-On 2.0 for 5900 and 6800 Series table model systems, as well as the ElectroPuls™ Linear Torsion systems.
• When testing to Annex A1, either the MT Series torsion only systems, the Torsion Add-On 2.0 systems, or the ElectroPuls Linear Torsion systems are recommended.
• When testing to Annex A2, Annex A3, or Annex A4, either the Torsion Add-On 2.0 systems or the ElectroPuls Linear Torsion systems are recommended. For laboratories testing a variety of orthopedic devices, the ElectroPuls E3000 or E10000 Linear Torsion systems may be more advantageous as these system are designed to perform fatigue testing. The Torsion Add-On 2.0 systems are controlled with Bluehill^® Universal software, while the ElectroPuls Linear Torsion systems utilize WaveMatrix software.
• Low force and torque measurements are capable with Instron’s biaxial Dynacell technology, which can be used on the 5900 Series, 6800 Series, and ElectroPuls Linear Torsion systems.