March 20, 2012, Accutek Engineering Team
By: Maciej Jakucki, Senior Project Engineer
A medical device designer recently asked Accutek, “What would you suggest for how best to tackle testing facet screws?” When researching companies with this type of system on the market, you’ll find that they have used a variety of test methodologies. This can be confusing when trying to develop a test plan for your own facet screw design.
Of course, as with all mechanical testing, the variation in test plans can be due to differences in interpretation of the test specifications. The situation can also vary depending upon the actual design of the implant. While the best course is always to confirm what testing is required with your regulatory department or consultant, Accutek has outlined typical guidelines for facet screw testing.
What screw should I test?
Worst case scenario should always be tested. If the screw can be used by itself, then it should be tested by itself. If the facet screw has additional components like washers, the FDA may require for it to also be tested with the additional components. However, if you can provide adequate justification for testing without the components, then it may be enough to test the screw by itself, especially if stand alone is the worst case.
What specifications do I need to test to?
• ASTM F543 – Axial Pullout, Driving Torque, and Torsional Properties
• ASTM F2193 – Static and Dynamic cantilever bend
• Additional Specifications as needed
How long of a screw is needed?
The specifications generally outline size and loading requirements. For example:
• ASTM F2193, states “the screw length needs to be sufficient, such that at least 5mm of threaded screw length is exposed and a 10mm test block be attached.”
• ASTM F543: Screw Testing
Torsional Properties, worst case scenario should always be tested. A shorter screw will break quicker than a longer screw, but the torque required to break the screws should be the same.
Axial Pullout, the shortest screws will typically be tested.
Driving Torque, a thread length sufficient such that 4 revolutions can be performed
How many screws?
N=6 per test is typically required. However, there are a few exceptions (static and dynamic testing in ASTM F2193: N=12). The axial pullout and driving torque screws can be re-used, if needed, as they are non destructive tests.
What about ASTM F1717, F1798 and ASTM F1264?
Both ASTM F1717 and 1798 are geared more toward plates and bilateral pedicle constructs. ASTM F1798 requires the use of a rod and screw, and since the facet screw doesn’t use a rod, this specification doesn’t typically apply. ASTM F1264 is a four point bend test that may be required, but both the ASTM F2193 and F1264 test the bending strength of the screw, so usually one or the other should suffice.
Disclaimer: These are just general guidelines – the appropriate testing program can vary depending upon many factors (screw design, regulatory requirements, screw material, etc.) If you do have a facet screw, or any other device you are unsure of how to test, a member of Accutek’s knowledgeable engineering team would be pleased to discuss your project with you.