ASTM F1717

What the test does

The construct — anchors, longitudinal rods and any connectors — is assembled as the manufacturer intends into two polymer blocks separated by a gap standing in for a removed vertebral body. The blocks connect to the machine through hinged rod-end joints, so force along the machine axis reaches the construct as a bending moment set by the block geometry. Four methods use that arrangement: the gap is closed in compression, opened in tension, twisted about the construct axis, or cycled in compression until the construct fails or reaches run-out.

What it measures, and why it matters

The static methods give a load-displacement or torque-rotation curve, from which stiffness, the yield point and the ultimate load are taken; compression bending runs out to roughly 20 mm of travel and torsion to about 60 degrees of rotation. Those figures say how much bending or twisting the construct carries before it takes a permanent set — the point at which surgical alignment is lost. The fatigue method reports cycles endured at a given load; the load survived to run-out is what most work turns on, since a construct must last until bone fuses across the gap. The standard is explicitly comparative: it sets no level a construct must reach, so a result means something only beside another from the same model.

Specimen

There is no material specimen. The article under test is a complete construct built from production components and assembled to the manufacturer's instructions, tightening torques included, since a set screw is often where a construct gives way. It is anchored into paired ultra-high-molecular-weight polyethylene blocks bridging the vertebrectomy gap; the blocks are single-use and are not carried over to a second test.

Two lengths govern everything: the active length of the longitudinal element and the moment arm built into the blocks, both chosen for the level represented — cervical, thoracolumbar, lumbar or lumbosacral. How many constructs the standard itself calls for is not settled by any public source; in practice group sizes fall out of the fatigue plan. An ultimate load is found statically, then groups are cycled at descending fractions of it, commonly about three-quarters, one-half and one-quarter, to place the load-life curve.

What the machine must be capable of

Capacity is close to the wrong question: the method specifies fixture geometry and a load protocol, and forces follow from the construct bolted into the blocks. Lumbar and thoracolumbar pedicle screw constructs reach ultimate compression bending loads of roughly 0.6 to 2.0 kN, and cycling runs at fractions of that, so most fatigue work sits between a few hundred newtons and about 1.5 kN — a 5 kN dynamic frame covers it with headroom. Cervical constructs sit an order of magnitude lower, where low-force resolution matters more than capacity.

The static methods are slow — up to 25 mm/min in bending, commonly 30 to 60 degrees per minute in torsion — so rate sensitivity is not the issue; control quality at low speed is. Fatigue is a different machine: force-controlled sinusoidal compression-compression cycling at a load ratio of ten or greater, at no more than about 5 Hz, to a run-out of five million cycles. That is some eleven and a half days of unattended running per specimen, so cycle counting and automatic failure detection matter as much as the actuator.

One method is torsion, in angular-displacement control, which a single-axis fatigue frame cannot run: torsional or biaxial capability is needed. Nothing is instrumented on the construct itself, because the fixture is the test: the hinged joints must move freely, since friction there changes the effective moment arm and every number reported with it. Testing is dry and ambient by default, with saline at body temperature where the implant materials warrant it.

The static rates, load ratio and run-out count come from testing laboratories rather than the purchased text; the 5 Hz maximum is ASTM's own recommended figure for cycling in saline or simulated body fluid. Which force-verification practice the current edition names could not be confirmed publicly.

What goes wrong in practice

Anchor loosening at the block interface is the commonest spoiled test: the screw works in the polymer, the gap closes further than the construct has deflected, and what is recorded is the behaviour of the model, not of the implant.

Hinge binding is the subtle one. A rod-end joint that has stiffened carries part of the moment itself, so the construct sees less bending than assumed; results come out flattering and repeat well, which is what makes them dangerous.

Permanent set is easy to miss: a construct whose rods have bent has failed in any sense that matters, yet it keeps cycling, so a displacement limit must be set and watched rather than waiting for separation.

Related and equivalent standards

ISO 12189 is the ISO-side comparator, but it loads a construct supported anteriorly rather than bridging a vertebrectomy gap, so the moment distribution differs and figures do not transfer.

ASTM F1798 is the standard this one is most often mistaken for. It examines a single interconnection — one anchor-to-rod or rod-to-rod joint — in isolation, showing whether that connection slips or fractures, where F1717 shows how the assembled construct behaves.