How to Guide- ASTM F543- Test Method for Metallic Bone Screws

This is a summary and how to guide for ASTM F543 Standard Specification and Test Methods for Metallic Bone Screws.

ASTM F543 is a testing standard used by the medical device industry to perform quality control assessments on the manufacturing and handling processes of metallic bone screws. The specification describes four annexes, each designed to test a specific set of mechanical properties. This is a general guide intended to help you understand the basic test procedure and equipment required to complete each test accurately. When you are ready to test products to this standard, we recommend purchasing ADMET’s Biaxial Testing System and following the steps in the official ASTM F543 document. ISO 6475 is a similar specification.

Annex 1: Test method for determining the torsional properties of metallic bone screws

Test Procedure:

1. Secure the specimen so that five threads or 20% of the specimen is exposed. Make sure that the clamping mechanism prevents the specimen from rotating.
2. Drive the specimen at the specified rate.
3. Record any axial load required to keep the screwdriver bit in the screw head.

Analysis:

1. Torsional Offset Yield Strength is determined at 2 degrees offset using the torque versus angle of rotation curve
2. Maximum Torque is determined by the largest value on the torque versus angle of rotation curve.
3. Breaking Angle is the point at which torque portion of the curve demonstrates its most rapid descent to total failure.

Equipment:

1. A torsion testing machine capable of applying the required torque.
2. A fixture capable of preventing screw rotation.
3. A testing machine controller capable of recording torque and torsional displacement.

Annex 2: Test method for determining the driving torque of metallic bone screws

ASTM F543 Annex A2 is used to measure the torque required to drive a bone screw into a test block made of rigid unicellular polyurethane foam. The results obtained from this test bears no direct correlation to the torque required to insert a bone screw in human or animal bone. This method is used as a quality control check for maintaining product uniformity.

Test procedure: Clamp the test block with a pre-drilled pilot hole into the machine.

1. Drive the bone screw into the test block at a rate between 1 and 5 rpm. The insertion torque will be the maximum torque measured within the first four revolutions of the bone screw.
2. Reverse direction and record the maximum removal torque during the four revolutions required to remove the bone screw.

Note: A 1.14 kg (2.5 lb) or less axial force should be applied to maintain engagement between the screw head and drive bit.

Equipment:

1. A fixture capable of preventing screw rotation.
2. A bushing to guide the specimen into the block with minimal friction.
3. A block clamp to hold the test block.
4. A torsion testing machine capable of applying the required torque.
5. A testing machine controller capable of recording torque and torsional displacement.

The embedded video demonstrates the ADMET E81 Torsion Tester recording the insertion and removal torque of a bone screw according to ASTM F543-A2. The test apparatus is a 20Nm capacity vertical torsion tester equipped with ADMET’s MTESTQuattro® PC based Torsion Testing Software.

Annex 3: Test method for determining the axial pullout force of metallic bone screws

Test Procedure:

1. The bone screw should be inserted into test block according to Annex 2. Screw inserted 3r/min to a depth of 20mm (see specification for accommodations for smaller screws)
2. Secure bottom test block to testing system. Secure top of screw in spherically recessed fixture.
3. 5mm/min until failure or separation from test block.
4. Record max load.

Equipment:

1. A fixture capable of preventing screw rotation.
2. A bushing to guide the specimen into the block with minimal friction.
3. A block clamp to hold the test block.
4. A torsion testing machine capable of applying the required torque.
5. A testing machine controller capable of recording torque and torsional displacement.

Annex 4: Test method for determining the self tapping performance of medical screw

Test Procedure:

1. Screw driver secured to crosshead and test block secured to bottom of test system.
2. Engage screw to block until 1.0Newton preload
3. Torque at 30r/min, increase axial force by 2n/sec until self-tap is achieved.
4. Torque 5 full revolutions and stop.

Equipment:

1. A torsion testing machine capable of applying the required torque.
2. A fixture capable of preventing screw rotation.
3. A testing machine controller capable of recording torque and torsional displacement