ASTM F543 Medical Bone Screw Torsion Test

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ASTM F543 Medical Bone Screw Torsion Test

The ASTM F543 medical bone screw torsion test is a specification for assessing the twisting torque of a torque-controlled, pre-loaded and unloaded, single-piece or double-headed orthopedic implant screw.

What Is ASTM F543?

ASTM F543 includes four different test methods for measuring the mechanical properties of metallic bone screws that are implanted into human bone. 

What Other Standards Apply to Medical Bone Screw Testing?

ASTM F543 was based in part upon other test methods that include ISO 5835, ISO 6475, and ISO 9268.

Why Perform ASTM F543?

ASTM F543 is used in the biomedical industry to characterize mechanical performance of metallic medical bone screws. Both adult and pediatric bone screws are used in surgical procedures for securing implants, osteosynthesis devices, and fracture fixation plates to the skeletal system. Surgeons, in that process, apply combined axial and torsional forces to implant a bone screw into the body. Implanted as such, bone screws are then Class 2 FDA devices, requiring testing and analysis of mechanical properties prior to approval and release.

Important Considerations When Testing per ASTM F543

Select which specific annex applies to particular device testing needs. The test methods or annexes may not all be appropriate for all types of implant applications. The user is cautioned to consider the appropriateness of the test methods in view of the devices being tested and their potential application. These tests can provide data for R&D, engineering design, quality control, acceptance or rejection under specifications, and for other purposes.

ASTM F543 Annex 1 Torsional Properties of Bone Screws

ASTM F543 Annex 1 measures the uniformity of bone screw products and enables comparison to other different, yet similarly sized, products. The annex measures torsional yield strength, maximum torque, and breaking angle of the bone screw. Torsional yield strength is an important engineering parameter to prevent screws from breaking during insertion or removal. Breaking angle is a measure of screw ductility as it undergoes a torsional moment. A large break angle provides an early warning to the surgeon that the screw is reaching its maximum strength.

ASTM F543 Annex 1 Test Equipment Requirements

Our 160T1 and 160T2 torsion test machines meet Annex 1 requirements. Capacities are available from 10 Nm (90 in-lbf) to more than 30 Nm (270 in-lbf), and capable of clockwise and counterclockwise modes. Each system includes Newton test controller, software, test bed, angular displacement sensor and torque cell. The digital angular position encoder is calibrated over the range of angles to be encountered in the test with an accuracy better than 0.3° (meeting ASTM F543). A Specimen Holder, or mechanical clamp, holds the bone screw to prevent its rotation while being stressed.

Our controller continuously records torque versus angle of rotation. The angular displacement scale has the sensitivity to measure an accurate offset for a 2° angular displacement. Graphical test reports plot torque vs angle.

ASTM F543 Annex 2 Driving Torque of Medical Bone Screws

Annex 2 measures the torque required to drive a bone screw into a defined standard material that simulates bone. This test method helps maintain product uniformity.

Insertion torque tests help avoid failure of the screw during insertion and ensure that the screw may be easily inserted by the surgeon. The insertion torque is much less than torsional yield strength of the screw and it’s screwdriver bit.

Removal torque tests help avoid failure of the screw during removal and ensure that the screw can be easily removed by the surgeon. The removal torque is also much less than torsional yield strength of the screw and of it’s screwdriver bit.

During both tests, the torsion motor is driven at a rate of 1 to 5 RPM.

ASTM F543 Annex 2 Test Equipment Requirements

160T2-10 torsion test machines are a good fit to Annex 2 requirements. Capacities are available from 10 Nm (90 in-lbf) on up, and capable of clockwise and counterclockwise modes. Each system includes a test controller, software, test bed, angular displacement sensor and torque cell. Options include test sample block per ASTM F1839, and deadweight axial loading fixture. Deadweight loading can be used to maintain the screwdriver bit in the screw head during both the insertion and removal procedures.

AT series axial torsion machine may be used as described in Annex 4. Using a two channel axial torsional system automated the process of loading the screw axially and eliminates the need for deadweights.

In both cases, our controller software records torque versus angle of rotation continuously.

ASTM F543 Annex 3 Axial Pullout Strength of Medical Screws

Annex 3 measures the axial tensile force required to fail or remove a bone screw from it’s application. Axial pullout strength tests are important if the screw is subjected to axial tensile forces, or if the screw is fixed into poor quality or osteoporotic bone.

ASTM F543 Annex 3 Test Equipment Requirements

Annex 3 requires a basic universal test machine capable of the force required to pullout or to break medical screws.

TestResources has multiple test machines for this application and each one conforms to ASTM E4 force measurement accuracy requirements. Using our controller, load versus fixture displacement data is recorded.

For the axial pull-out test, a custom fixture is needed to grab the head of the screw and pull it out after reaching the required depth of thread engagement.

A tensile load is applied to the test screw at a rate of 5 mm/min (0.19 in/min) until the screw fails or releases from the test block.

ASTM F543 Annex 4 Self Tapping Medical Bone Screw Performance

Annex 4 is the latest addition to ASTM F543. It tests the axial compression force required to engage the self-tapping feature of self-tapping medical bone screws. Self-tapping force test methods help ensure that the screw may be easily inserted by the surgeon, especially if the screw is screwed into poor quality or osteoporotic bone.

This test setup requires test machine controls that can measure and control both axial and torsional loads simultaneously. A properly configured system can perform all four annexes with the same single test machine.

ASTM F543 Annex 4 Test Equipment Requirements

Our axial and torsional test machines control both test speed and loading conditions in axial test, torsional test, or combined axial-torsion test modes. A combination biaxial load cell is mounted in the test machine.

Annex 4 requires a constant rotational rate of 30 RPM, which characterizes the clinical application of hand-driven screws. The axial compression force is to be applied at a constant rate of 2 N/s which applies to easy identification and reaction to unique self-tap events. A preload of 1 N maintains screw-to-screwdriver engagement. The axial force serves only to maintain driver-to-screw engagement.

All annexes can be tested with a 240AT-250-30 series axial torsional test machine. Models are available with 10 Nm (7.3 in-lb) and 25 Nm (18 in-lb) torsion force capacity.