ASTM F2502

What the test does

This is a specification carrying its procedures in annexes rather than a single method: the body defines the devices and their performance quantities; four annexes say how to measure them.

Four physical actions follow: a screw turned by a driver until it twists off; a screw driven into a foam block through four revolutions while driving torque is recorded; a seated screw pulled straight out; a plate bent in four-point loading through one cycle. Each is repeated on specimens soaked in warm saline for varying times.

What it measures, and why it matters

Torsion gives maximum torque, breaking angle and a torsional yield strength read at a small angular offset. Maximum torque is the margin before a screw shears off in the hole; breaking angle separates a screw that warns before failing from one that snaps; yield is where it stays wound up permanently, so insertion instruments are set against yield, not fracture. Driving torque describes insertion feel; pullout is the load a seated screw resists before stripping out; plate bending gives bending stiffness, bending strength and the structural stiffness EI, which sets the load shielded from healing bone.

The sequence is the point: each measurement repeated at successive immersion times, so strength and stiffness loss plots against degradation. No limits are set; the manufacturer declares its own values.

Specimen

Specimens are finished plates and screws of hydrolytically degradable polymer resins or composites, tested as supplied; devices degrading mainly enzymatically are out of scope.

Preparation is conditioning, not machining. Specimens sit in physiological or phosphate-buffered saline at 37 °C, withdrawn at intervals — day one, week one, week four and onward is usual — a fresh set at each. Insertion and pullout use rigid polyurethane foam blocks to ASTM F1839, standing in for cancellous bone, grade recorded with the result.

What the machine must be capable of

Two instruments are involved and no single-axis frame covers the document. Torsion and driving torque need a rotary drive with a torque cell and an angle encoder, plus a light axial hold-down keeping the bit seated; pullout and plate bending need an ordinary axial frame, in tension and compression. Either two machines, or one axial-torsion frame. Nothing here is cyclic, so no fatigue capability is called for.

No force or torque capacity is fixed and no accuracy class is specified, so the frame is sized from the device. Pullout from foam and single-cycle plate bending peak between the low hundreds of newtons and roughly two kilonewtons; polymer screw torsion usually stays well under ten newton metres. Resolution at the bottom of the range matters more than headroom at the top.

Rates are prescribed: one to five revolutions per minute for both torsion annexes, driving torque continuing four revolutions, 1440 degrees, under an axial hold-down of 1.14 kg or less, about 11 N; 5 mm/min for pullout; position control for plate bending, with the rate left to the referring specification. No extensometer is needed — the channels are torque against angle and load against displacement, so angular resolution and load-string compliance carry the burden.

Fixtures decide whether the result means anything. Collet or chuck grips must hold the shank without crushing a polymer far softer than metal; the driver bit must match the recess exactly, or it destroys the recess instead of the shank; and the bend rig's spans follow plate length, symmetry and hole spacing, so one left at another plate's spans changes the stiffness reported. All of it runs submerged in a heated circulating bath at 37 °C, not a dry cabinet, with pH watched: drift changes the degradation rate and ruins comparison between time points.

What goes wrong in practice

Specimens dry out between bath and frame. A degraded polymer stiffens as soon as surface water leaves it, so a screw blotted and tested in air minutes later reads high. Test submerged, or fix and record the interval.

Recess damage is the common torsion failure. A polymer recess tolerates far less cam-out than a metal one, so a worn or skewed bit rounds it out and the trace shows a soft peak that is the recess failing, not the shank. Press harder to prevent it and driving torque stops being the measured quantity.

Both yield offsets are small enough for the machine to swamp them, and bath fixturing adds compliance, so measure the system's take-up on a dummy specimen and subtract it. A reused hole or unrecorded foam grade spoils pullout figures the same quiet way.

Related and equivalent standards

ASTM F543, for metallic medical bone screws, is the closest match: same structure, same trio of torsion, driving torque and pullout, same foam. A laboratory equipped for F543 owns most of the screw fixturing; what it lacks is the bath and the discipline of repeating the battery at each time point.

ASTM F382, for metallic bone plates, is the parent of the plate bending work and carries cyclic bending too, so plate fatigue is its ground, not F2502's. ASTM F1839 defines the foam the screw annexes lean on; ASTM F2902 is a broader guide to absorbable implants. Neither substitutes for this document.