ASTM F382

What the test does

A finished bone plate is laid across four cylindrical rollers: two outer rollers support it, two inner rollers load it near mid-span. The inner pair sits so at least two screw holes lie between them, placing the holes in a region of constant bending moment and zero shear. The crosshead drives the inner rollers down and the plate bends. Annex A1 does this once, in displacement control, recording load against deflection; Annex A2 repeats it as a sinusoid at a load ratio of 0.1 until the plate cracks or reaches run-out.

What it measures, and why it matters

The single-cycle test yields bending stiffness, bending structural stiffness and bending strength. The two stiffness figures answer different questions — how the section resists bending, and how the plate as a whole deflects in the fixture it was tested in — so neither is usable without the roller spans alongside it. Stiffness governs how load is shared between plate and healing bone: too stiff and the bone is shielded from the stimulus it needs, too compliant and the fracture gap moves. Bending strength sets the margin against a single overload, such as a fall during rehabilitation. The fatigue annex reports cycles to failure at a given load, and testing at descending loads brackets a run-out level; that figure decides whether a plate survives the months to union under repeated gait loading. F382 sets no fatigue strength a plate must achieve, and cautions that four-point bending is not the in situ loading configuration, so the numbers rank designs rather than predicting clinical life.

Specimen

The specimen is the device itself, finished as supplied — no coupon is machined, and surface finish, hole geometry and contouring all form part of what is measured. The standard classifies five plate types: cloverleaf, cobra head, reconstruction, straight and tubular. Roller spacing is not fixed by the standard alone; it follows the screw-hole pattern, so the fixture must be reset for each design. A stiffness figure needs a small group of plates, a fatigue characterisation considerably more, spread across load levels. No conditioning atmosphere or test temperature is defined.

What the machine must be capable of

The requirement splits. Annex A1 runs on an ordinary static frame in displacement control, with the rate left to the user; laboratories commonly work at around 5 mm/min. Annex A2 needs a dynamic frame able to hold a load-controlled sinusoid for a million cycles — 5 Hz and a 1,000,000-cycle run-out are the standard's suggested figures rather than absolute mandates, and R = 0.1 is near-universal practice.

No capacity is prescribed. Small-fragment plates run out at a couple of hundred newtons while heavy femoral plates go well past a kilonewton; one published study performed to F382 identified a run-out load of 1,200 N. A dynamic frame of 5 to 10 kN covers the range with headroom. The reported quantity, though, is bending moment, which follows from applied load and roller spacing together — two laboratories quoting the same newtons can be reporting quite different moments, so fixture geometry must be recorded alongside the load.

No extensometer is used; deflection is taken from the crosshead or a deflectometer. Rollers are cylindrical and commonly 6 to 12 mm in diameter — a figure that reflects fixture practice rather than a dimension the standard fixes. Which force-verification practice F382-24 names could not be confirmed from public sources, so no accuracy class is quoted here.

What goes wrong in practice

Contoured plates are the usual nuisance. A curved or tubular plate wants to roll or creep on the support rollers, quietly changing the span and therefore the moment, and the result still looks reasonable on the plot. Roller contact marking is the next trap: a worn or undersized roller indents the plate and starts a crack at the contact line rather than at a screw hole — a fixture artefact, not a device property.

In fatigue the costliest failure is the false run-out. As the plate accumulates damage its compliance rises, and if the machine is not holding amplitude cleanly in load control the specimen sees less load than the report says. Check the recorded amplitude at the end of a run-out, not only at the start. Alignment errors that put the plate out of plane add a torsional component, and the crack then initiates on one side of a hole rather than symmetrically — a visible clue worth acting on.

Related and equivalent standards

The nearest counterpart is ISO 9585, which covers the bending properties of metallic osteosynthesis plates. F382 is the broader document: it wraps a static annex and a fatigue annex around the same fixture and adds material, labelling and handling requirements. Stiffness results are broadly comparable provided the roller spans are matched, and not comparable at all if they are not.