ASTM F543

What the test does

F543 is four mechanical tests under one designation, three of them rotary. In the torsion annex the screw shank is clamped with roughly five threads left exposed, a driver is engaged coaxially in the drive recess, and rotation continues until the screw twists off, torque and angle recorded past the breaking angle. The other three use a rigid foam block: driving torque records what it takes to advance a screw under a very light axial load, pullout extracts a seated screw until it strips or breaks, and self-tapping drives a screw against a ramped axial force to see how readily it cuts its own thread.

What it measures, and why it matters

The torsion annex reports breaking torque, torsional yield strength and breaking angle, the screw being turned through to fracture. Breaking torque is the margin a surgeon has before shearing the screw off in the patient; too small a margin means a broken fragment in a hole that must then be revised. Breaking angle separates a screw that yields visibly, warning the hand, from one that fails abruptly. Torsional yield strength is the point past which the screw does not spring back: driven beyond it, a screw stays permanently wound up in the bone with nothing visibly failed, which is why insertion instruments are set against the yield figure rather than the fracture figure. Driving torque relates to insertion feel, and pullout force to how well the thread form holds in a substrate of known density.

Specimen

The specimen is a finished screw as supplied. Thread form, root diameter, drive recess and surface condition are not incidental — the test sees all of them. The controlled dimension in torsion is how much thread stands out of the clamp — roughly five threads — since clamping deeper shortens the twisting length and raises the apparent breaking angle. The foam blocks conform to ASTM F1839, the density chosen standing in for a bone quality and reported with the result. Several screws are needed per size and type, each foam site used once. Testing is in ambient air, with no conditioning atmosphere or temperature set.

What the machine must be capable of

The governing instrument is a torsion machine, not an axial frame. The torsion and driving-torque annexes turn the screw at single-figure revolutions per minute, the self-tapping annex considerably faster, and both driving cases hold a controlled axial force alongside the rotation — light and constant in one, steadily ramped in the other. Only pullout is conventional axial work: the screw is seated by rotation, then extracted at a slow crosshead rate. The annexes give the values. An axial-torsion frame covers all four; a single-axis frame covers one.

No capacity is prescribed, so equipment is sized from practice. The torque annexes sit well below 20 N·m — published insertion torques in low-density foam blocks run to a couple of newton-metres — while pullout from the same blocks is commonly a few hundred newtons, rising into the low kilonewtons in cortical bone. Resolution at the bottom of the range matters more than capacity at the top: a torque cell sized for a large frame will not resolve a sub-newton-metre trace.

The torsion annex fixes measurement quality instead: angular accuracy in both directions of rotation, torque resolution referred to the torsional yield strength, and an angular scale fine enough to resolve the small offset by which torsional yield is defined — so the rotary channel, not the force channel, is the one to specify carefully. No force- or torque-verification accuracy class could be confirmed, so none is quoted.

What goes wrong in practice

Cam-out is the classic. A worn, undersized or slightly skewed driver rides out of the recess and deforms it, and the torque trace then shows a rounded peak that is the recess failing, not the shank.

Off-axis clamping is the quiet one. A screw held slightly out of alignment sees bending superimposed on torsion, which lowers the measured breaking torque and scatters the angle, so check runout at the driver against the clamp.

Backlash matters more here than elsewhere, because torsional yield is defined at a small angular offset: lost motion in the drive train — a loose coupling, a compliant driver, a slack recess fit — inflates the measured angle and pushes the yield figure the wrong way. Measure the machine's own angular compliance with a dummy shaft and subtract it.

Foam is the last. Reusing a hole, pre-drilling wrongly, or trusting an assumed block density strips the thread early and gives pullout figures nobody can compare against.

Related and equivalent standards

ISO 6475 is the document F543 is routinely set against — breaking torque and angle of rotation at failure for metallic bone screws, dated 1989 and confirmed in 2021. It is the narrower document, addressing torsional properties alone; F543 adds driving-torque, pullout and self-tapping annexes and a specification. ISO 5835 for dimensions and ISO 5832-1 for material are companions rather than counterparts, as is ASTM F1839, which fixes the foam the force annexes rely on.