Bone Screws are used as internal fixation devices to secure implants, fracture fixation plates, or to create an anchor into bones for sutures and other devices. ASTM F543-13 contains four testing annexes that outline tests to evaluate bone screws.
Annex 1: “Test Method for Determining the Torsional Properties of Metallic Bone Screws” requires the screw to be clamped and a rotational velocity between 1 and 5 rpm to be applied to the screw until screw failure. The measured torque profile and rotational angle are reported.
Annex 2: “Test Method for Driving Torque of Medical Bone Screws” measures 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.
Annex 3 “Test Method for Determining the Axial Pull-out Strength of Medical Bone Screws” measures the force required to axially remove the screw that had been inserted into the test block using the method from Annex 2. The machine is then required to apply a tensile load at a constant rate of 5 mm/min until the bone screw is removed from the test material.
Annex 4:“Test Method for Determining the Self-Tapping Performance of Self-Tapping Medical Bone Screws” specifies the procedure to evaluate the axial loading required to engage a self-tapping bone screw. The test requires a rotational velocity of up to 30 rpm, while the axial load is increased at a rate of 2 N/s. The goal of the test is to record the torque and axial load profile of the bone screw during insertion into the test material.
An Electromechanical system with a torsion add-on can be used to perform these tests. The torsion add-on can be added to any 5900 single column testing machine to add rotational capabilities. The system operates on Instron’s Bluehill® 3 Software with TestProfiler. Bluehill 3 is able to performed closed-loop control for axial testing and measures axial displacement, rotary displacement, axial load and torque. A biaxial Dynacell load cell is mounted to the base of the machine. Advanced screw action grips and a three-jaw chuck are pictured for ASTM F543-13 testing.