Testing standard
ASTM F2502
Standard Specification and Test Methods for Absorbable Plates and Screws for Internal Fixation Implants
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM F2502 is a specification for bioabsorbable plates and screws used in bone fixation, carrying its test procedures in annexes. Four physical actions are defined — a screw torqued to destruction, a screw driven through four revolutions, a seated screw pulled out, and a plate bent in four-point loading — each repeated on specimens soaked in warm saline for varying times.
At a glance
- Test type
- Flexure & bend — the specimen is bent
- Published by
- ASTM
- Edition
- F2502-24
- Runs on
- Series 7200, Torsion Tester and Series 9000
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.
The soak is the specimen
What separates this from a metallic implant specification is that the material changes while it is being tested. A property measured dry is not the property the device will have in a patient a month later.
- Devices covered
- Bioabsorbable plates and screws
- Conditioning
- Soaked in warm saline for varying timesThe whole point. Absorbable polymers hydrolyse, so strength falls with immersion time and the DEGRADATION CURVE is the product of the test rather than any single figure.
- Four actions
- Torque to destruction, driving torque, pull-out, four-point bend
- Time points
- Several, across the soakOne time point gives one number and no curve.
- Report the soak time
- With every figureDakA pull-out strength without its immersion time describes nothing about an absorbable device.
Test conditions
- Saline
- Warm, at the specified temperatureTemperature drives hydrolysis rate, so it is controlled rather than ambient.
- Bend
- Four-point, one cycle
- Driving torque
- Four revolutions
- Rates
- From the annexes
What comes out
Each measured quantity plotted against soak duration
The curve IS the result. A device that is strong at zero days and has lost most of it at four weeks may be entirely unsuitable for a fracture that takes six to unite.
From the four-point bend, as a moment
As with metallic plates, reported as a moment because a plate with holes has no single meaningful section.
How the tests run
- 01Define the soak time points across the period of clinical interest.
- 02Immerse specimens in warm saline at the specified temperature.
- 03Remove specimens at each time point and test promptly, before they dry.
- 04For torque to destruction: rotate the screw until it fails, recording torque and angle.
- 05For driving torque: advance four revolutions, recording throughout.
- 06For pull-out: extract a seated screw at a constant axial rate.
- 07For bending: load a plate in four-point through one cycle.
- 08Plot every property against soak time.
- 09Report the curve, not a single figure.
Test promptly after removal from the saline. An absorbable specimen left to dry on a bench recovers stiffness it will not have in the body, and the result flatters the device.
What the report has to contain
- Reference to ASTM F2502 and which annexes were run
- Device identification, polymer system and sterilisation route
- Saline temperature and composition
- SOAK TIME for every specimen
- Time between removal from saline and test
- Torque to destruction with angle, driving torque, pull-out and bending results
- Each property plotted against soak time
- Number of specimens per time point
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.
Absorbable against metallic fixation
| ASTM F2502 absorbable | Metallic plate and screw standards | |
|---|---|---|
| Property over time | Falls by design | Essentially constant |
| Conditioning | Soak in warm saline | Ambient |
| The result | A degradation curve | A single set of values |
| Design question | Does strength outlast healing? | Is it strong enough? |
The design question is genuinely different. A metallic plate has to be strong enough; an absorbable one has to stay strong long enough — and a device that loses its strength before the bone unites has failed even though every initial figure passed.
Questions we are asked about this test
What is ASTM F2502?
It is the ASTM specification and test methods for bioabsorbable plates and screws used in internal fixation. The specification defines the devices and their performance quantities, and four annexes describe how to measure them — torque to destruction, driving torque, pull-out and four-point bending — on specimens soaked in warm saline.
Why are specimens soaked in saline?
Because absorbable polymers hydrolyse, so their strength falls with time in the body. A property measured on a dry device is the property it has on the day of surgery and not the property it has a month later. The soak reproduces that degradation so the change can be measured rather than assumed.
Why is the result a curve rather than a number?
Because the question is not how strong the device is but how long it stays strong. A plate that is impressive at zero days and has lost most of its strength at four weeks may be quite unsuitable for a fracture that takes six weeks to unite. Only the curve across several soak times answers that.
Why must specimens be tested promptly after removal?
Because an absorbable specimen left to dry on the bench stiffens up again — recovering properties it will not have in the body. Testing promptly, before it dries, is what keeps the measurement representative of the implanted state rather than of a laboratory artefact.
How does this differ from testing a metallic implant?
The design question changes. A metallic plate simply has to be strong enough, and its properties do not move. An absorbable one has to remain strong for long enough — so the standard is built around a time axis that metallic standards do not need, and a device can pass every initial figure and still be wrong for the indication.
Why does a bioabsorbable implant need its own specification?
Because its properties are a function of time in the body, which no metallic implant standard accounts for. A resorbable plate is designed to lose strength as bone takes over the load, so a single strength figure at time zero describes very little. F2502 therefore builds degradation into the procedure through saline conditioning, and the meaningful output is how the properties change rather than what they start at.
How is the soaking regime chosen?
From the intended in-service life of the device and what the specification requires. Longer immersion at body temperature advances the degradation further, so the sampling points are chosen to bracket the period during which the implant must still carry load. What matters as much as the durations is that they are stated with the results, since a strength figure without its soak time is uninterpretable.
Running ASTM F2502 on the Series 7200
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | F2502 fixes no force capacity at all, so the frame is sized by the device: absorbable screw pullout from rigid polyurethane foam and single-cycle plate bending typically peak in the low hundreds of newtons up to about 2 kN, which a 5 kN axial frame covers comfortably, while the two torsion annexes are limited by torque rather than force and usually sit well under 10 N.m for polymer screws. | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | unknown | ISO 7500-1 Class 0.5 — the method sets no class of its own |
| Gripping | Collet or chuck grips gripping the screw shank; a matched hex or cruciform driver bit on a torque-measuring rotary drive; rigid polyurethane foam test blocks conforming to ASTM F1839 (commonly 20 pcf, standing in for cancellous bone) for insertion, driving-torque and pullout; and a four-point bend rig for plates whose roller diameter and inner/outer spans are set from plate length, symmetry and hole spacing. All of it has to be immersible or run inside a 37 C bath. | Our chuck grips, built to the specimen |
| Environment | Specimens are degraded and tested wet: immersion in physiological saline or phosphate-buffered saline at 37 C, with mechanical properties measured at a series of time points (typically day 1, week 1, week 4 and onward) to chart the loss of fixation strength and stiffness as hydrolysis proceeds. Bath pH must be watched, because a drift changes the degradation rate and corrupts the comparison between time points. Practically this means a heated, circulating, pH-monitored bath surrounding the grips and bend rollers — not a dry temperature cabinet — and corrosion-tolerant fixtures. | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.