Rationale

The ASTM F543 standard is being updated. All changes are related to Annex A1 – Torsion test. The proposed changes are: • Standardize the length of the bit during the test – it will improve repeatability and reproducibility. • to allow the test to be performed without the bit (by clamping the screw’s head). • Acknowledge that bit failure it as a possible result and evaluating its failure mode. • Some changes on test method related to partially threaded or very small screws: instead of fixing a gage length of 20% of the threaded length, the screw will be tested with the maximum gage length possible that allows it to be firmly fixed. • To specify the screwdriver bit that will be used at the test must be representative of the instruments indicated to be used with the screw. • To remove acceptance criteria related to breaking angle. As discussed in the F04.21 Nov 2023 meeting in Washington, there are no documented reasons about why this criterion was included, and many doubts about the clinical relevance of this criterion. Best information so far (not confirmed) indicate that these values were proposed around 1989 based on real-life results of clinically successful screws made of stainless steel. The current breaking angle results are too dependent on pitch length (as 5 threads are exposed during the test) and apply only to HA and HB type screw. Finally, the FDA guidance “Orthopedic Non-Spinal Metallic Bone Screws and Washers – Performance Criteria for Safety and Performance Based Pathway” does not include “breaking angle” as an acceptance criterion. Due to this situation, during the Washington Nov 23 meeting, three alternatives were proposed: a) Standardize at least 90 degrees as acceptance criterion. For that, it must be defined a fixed exposed length during the test – the test cannot be based on number of threads exposed anymore. b) Remove breaking angle parameters from acceptance criteria. c) Define an equation that adjust breaking angle acceptance criterion according to the length exposed to the test, and maintain the test as it is The decision of Subcomittee F04.21 at that moment was alternative b), to remove it. At that meeting, it was also decided to change the Torque acceptance criteria from “Max. Torque-based” to “Yield-Torque” based. However, this is not being balloted yet.