Testing standard
ISO 7206-4
Implants for surgery — Partial and total hip joint prostheses — Part 4: Determination of endurance properties and performance of stemmed femoral components
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ISO 7206-4 determines the endurance properties and performance of stemmed femoral components of hip joint prostheses. The stem is embedded to a defined level and cyclically loaded through the head, simulating the dynamic loading of gait, and the standard specifies both the test forces and the number of cycles required.
At a glance
- Test type
- Fatigue — a load is applied over and over until something fails
- Published by
- ISO
- Edition
- ISO 7206-4:2010
- Material
- Metals, alloys & welds
- Runs on
- Series 7200 and Series 9000
What the test does
The stemmed femoral component is embedded in a medium up to a level the standard defines, and loaded cyclically through the head at angles specified in two planes so that the loading reproduces the hip joint reaction during gait. The standard specifies both the test forces and the number of load cycles that must be survived. Displacement amplitude is logged through the run, and the component either completes the specified cycles or the cycle count and location of failure are recorded.
What it measures, and why it matters
Whether the stem survives a lifetime of walking. What distinguishes the method from most implant fatigue testing is that the requirement is in the standard: the endurance limit forces and the corresponding cycle counts are specified rather than chosen by the applicant, which makes results directly comparable between devices. The geometry carries the rest of the meaning — the stem is a cantilever, so the embedded level fixes the lever arm and therefore the bending moment, and the specified loading angles put the component into combined bending and torsion as a femur does.
The embedding level is the test
How much of the stem is held, and how much is free, decides the bending moment the implant sees. It is specified, not chosen.
- Embedding level
- Defined by the standard relative to the stemEmbedding deeper shortens the lever arm and reduces the moment. The result would look better and mean nothing.
- Loading angles
- Specified in two planesThey reproduce the orientation of the femur under gait loading rather than a convenient vertical.
- Test forces
- Specified, together with the number of cyclesUnusually, the requirements for the endurance limit are in the standard rather than left to the submission.
- Simulates
- The dynamic loading of a hip stem during gait
- Part 6
- Covers the neck region separately
- Check the embedding medium has not degraded
- Over a long runDakA medium that softens or cracks during millions of cycles changes the support and therefore the moment, mid-test.
This is a specified-force, specified-cycles endurance test. That makes it a pass or fail against a stated requirement rather than a characterisation, which is unusual among implant fatigue methods.
Test speed
- Frequency
- High enough to be practical, low enough not to heat the specimen or the medium
- Reported
- Survival to the specified cycles at the specified force, or the cycle at failure
- Monitoring
- Displacement amplitude through the runA rising amplitude usually means a crack has started well before the stem parts.
- Verify the applied force dynamically
- At the test frequencyDakA frame calibrated statically can under-deliver at frequency, and every one of several million cycles inherits the error.
Calculations
M = F × lever arm, set by the embedding level and the load angles
This is why the geometry is specified so tightly. The force alone does not define the loading.
Survival at the specified force for the specified number of cycles
Both values come from the standard, which is what makes this a requirement rather than a characterisation.
A progressive increase in displacement amplitude at constant force
Worth logging continuously — a stem that cracks at two million cycles and parts at four has failed at two.
How the test runs
- 01Identify the size and design of the stemmed femoral component.
- 02Prepare the embedding medium and fixture to the specified geometry.
- 03Embed the stem to the level the standard defines, checking it before the medium sets.
- 04Set the loading angles in both specified planes.
- 05Select a load cell suited to the specified test force.
- 06Verify the applied force dynamically at the test frequency.
- 07Apply the specified cyclic force.
- 08Log displacement amplitude continuously.
- 09Run to the specified number of cycles, or to failure.
- 10Record survival, or the cycle count and location of failure.
- 11Inspect the embedding medium afterwards for degradation.
What the report has to contain
- Reference to ISO 7206-4, the edition and whether Amendment 1:2016 applied
- Component identification, size, material and surface finish
- Embedding medium and the level achieved
- Loading angles in both planes
- Test force and frequency
- How dynamic force was verified
- Number of cycles completed, or the cycle at failure
- Location of any fracture
- Displacement amplitude history
- Condition of the embedding medium after the run
What the machine must be capable of
Cyclic loading at forces the standard specifies, sustained for millions of cycles, with the force verified dynamically at the test frequency rather than only under static calibration. ISO 7500-1 states plainly that its values are not necessarily valid for dynamic testing, and a frame that under-delivers at frequency passes that error into every cycle of a very long run with nothing in the record to show it. Continuous displacement logging is needed too, since it is what identifies crack initiation.
What goes wrong in practice
Embedding to the wrong level, which flatters the result silently. Verifying force statically and running dynamically. Reporting only final fracture, when the displacement trace shows the crack started far earlier and that earlier cycle is the meaningful figure. Running at a frequency high enough to heat the implant or the embedding medium, which changes both. And omitting the loading angles from the report, without which the bending moment cannot be reconstructed from the force — a submission reviewer cannot check a result whose geometry is not stated, however complete the force data looks.
ISO 7206-4 or ISO 7206-6
| Part 4 | Part 6 | |
|---|---|---|
| Region | The stem, embedded | The neck region |
| Question | Does the stem survive gait loading? | Does the neck survive it? |
| Geometry | Embedded to a defined level | Supported to expose the neck |
| Run together | Commonly | Commonly |
A stem and its neck fail in different places for different reasons, and a component can pass one and fail the other. Submissions normally carry both.
Questions we are asked about this test
What is ISO 7206-4?
It is the ISO endurance test for the stemmed femoral component of a hip prosthesis. The stem is embedded to a defined level, loaded cyclically through the head at specified angles to simulate the dynamic loading of gait, and required to survive a specified force for a specified number of cycles. The current edition is ISO 7206-4:2010 with Amendment 1:2016.
Why does the embedding level matter so much?
Because it sets the bending moment. The stem is a cantilever: the embedded portion is the support and the free portion is the lever arm, so embedding deeper shortens the arm and reduces the moment for the same applied force. A stem embedded a few millimetres too deep would survive a test it should fail, and nothing in the force record would show it. That is why the level is specified rather than left to the laboratory.
Why are the loading angles specified in two planes?
Because a femur is not loaded vertically. The hip joint reaction passes through the head at an angle in both the frontal and sagittal planes during gait, putting the stem into combined bending and torsion rather than simple bending. Reproducing those angles is what makes the test represent walking rather than a convenient laboratory arrangement.
Is this a characterisation or a pass-fail test?
A pass or fail. Unusually among implant fatigue methods, the standard specifies both the endurance limit test forces and the corresponding number of load cycles, so the requirement lives in the standard rather than in the submission. That makes it more directly comparable between devices than a method where the applicant chooses the loading.
Why monitor displacement rather than just wait for fracture?
Because a stem that has cracked has already failed, even if it has not parted. A fatigue crack reduces stiffness, so the displacement amplitude at constant force rises progressively before final fracture. Logging it continuously identifies the cycle at which the crack initiated, which is the meaningful number — a stem that cracks at two million cycles and breaks at four failed at two.
Why check the embedding medium afterwards?
Because it is part of the load path for the whole run. A medium that softens, creeps or cracks during several million cycles changes how the stem is supported, which changes the effective lever arm and therefore the moment — mid-test, and invisibly. Inspecting it afterwards is what confirms the loading was what the report claims it was.
Why verify the force dynamically?
Because a static calibration does not describe a machine in motion. ISO 7500-1 states that its values are not necessarily valid for high-speed or dynamic testing, and a frame that under-delivers at test frequency passes that error into every one of several million cycles. Nothing in the record afterwards reveals it, so the verification has to happen at the frequency and waveform actually used.
Running ISO 7206-4 on the Series 7200 and Series 9000
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 | Moderate — hip stem endurance forces are specified in the standard and run to a few kilonewtons | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | Dynamic force verification at the test frequency | ISO 7500-1 Class 0.5, verified to ASTM E4, DIN 51221 and BS 1610 |
| Gripping | An embedding medium holding the stem to a defined level, with the load applied at defined angles | Our a fixture built for this method, built to the specimen |
| Environment | 23 ± 2 °C standard laboratory atmosphere | 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.