ISO 7206-4 Endurance Testing of Stemmed Femoral Hip Components

What the test does

The stemmed femoral component is embedded to a level the standard defines and loaded cyclically through the head at angles specified in two planes, reproducing the hip joint reaction during gait. The waveform is sinusoidal, the frequency between 1 Hz and 30 Hz. Forces and cycles are both specified: 5 000 000 cycles at an endurance limit force FD of 2 300 N for a stem of 120 mm to 250 mm centre-to-tip length, 1 200 N above and below that band, six specimens unbroken. Displacement amplitude is logged throughout, and the component either completes the cycles or the cycle count and failure location are recorded.

What it measures, and why it matters

Whether the stem survives a lifetime of walking. What sets the method apart from most implant fatigue testing is that the requirement is in the standard: endurance forces and cycle counts are specified rather than chosen by the applicant, which makes results comparable between devices. Geometry carries the rest of the meaning: the stem is a cantilever, so the embedded level fixes the lever arm and hence the bending moment, and the specified 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

1 Hz to 30 Hz, sinusoidal at the primary frequencyClause 6.1 a) and 8.9. The band is a choice, not a fixed rate: run as fast as the fixture stiffness allows while still holding the prescribed load to ± 2 % of the load range. That choice sets the calendar — 5 000 000 cycles takes about 46 h at 30 Hz, just under 12 days at 5 Hz and roughly 8 weeks at 1 Hz.

Frequency — modular stems in fluid

5 Hz or lessClause 8.7. Modular components run submerged in fluid test medium held at 37 °C ± 1 °C and continuously aerated, and that protocol caps the frequency.

Endurance limit force FD and cycles

1 200 N for CT ≤ 120 mm; 2 300 N for 120 mm < CT ≤ 250 mm; 1 200 N for CT > 250 mm — all over 5 × 10⁶ cycles, with six specimens unbrokenTable 2, Clause 9, where CT is the stem centre-to-tip length. The 1 200 N figures are stated as lower limits that may need adjusting upwards on design, material and clinical grounds.

Minimum load in the cycle

200 N to 300 NThe note to 8.8 gives this as the range found necessary for satisfactory operation of the testing machine.

Load accuracy

± 2 % of maximum load, verified to ISO 4965

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.

Deflection cut-off

Stop the test if deflection exceeds 1,25 × f or 5 mm, whichever is greaterClause 8.10. f is the initial vertical load axis deflection, and the threshold is set from a reading taken after about 300 cycles. Deflection is monitored to ± 0,2 mm.

Verify the applied force dynamically

At the test frequency and waveform actually usedDakA frame calibrated statically can under-deliver at frequency, and every one of five million cycles inherits the error.

What the machine must be capable of

Sinusoidal cyclic loading anywhere from 1 Hz to 30 Hz, held within ± 2 % of the load range, applied load error no greater than ± 2 % of maximum load to ISO 4965. The frequency is chosen rather than fixed, and the choice sets the calendar: five million cycles takes about 46 hours at 30 Hz, under twelve days at 5 Hz, roughly eight weeks at 1 Hz. Aim at the top of the band for a dry room-temperature test on a non-modular stem. Modular stems are the exception — submerged in fluid at 37 °C ± 1 °C, continuously aerated, capped at 5 Hz or less.

Force must also be verified dynamically at the test frequency, not only under static calibration. ISO 7500-1 states that its values are not necessarily valid for dynamic testing, and a frame that under-delivers at frequency passes that error into every one of five million cycles. Displacement logging to ± 0,2 mm is needed too: it identifies crack initiation, and the machine must stop itself when deflection exceeds 1,25 times the initial value or 5 mm, whichever is greater — a threshold set from a reading taken after about 300 cycles.

How the test runs

1. 01Identify the size and design of the stemmed femoral component.
2. 02Prepare the embedding medium and fixture to the specified geometry.
3. 03Embed the stem to the level the standard defines, checking it before the medium sets.
4. 04Set the loading angles in both specified planes.
5. 05Select a load cell suited to the specified test force.
6. 06Verify the applied force dynamically at the test frequency.
7. 07Apply the specified cyclic force.
8. 08Log displacement amplitude continuously.
9. 09Run to the specified number of cycles, or to failure.
10. 10Record survival, or the cycle count and location of failure.
11. 11Inspect the embedding medium afterwards for degradation.

Calculations

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 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. Running a non-metallic specimen near 30 Hz, where the standard warns frequency may itself influence the result, or a modular stem above the 5 Hz its fluid-immersed protocol allows. And omitting the loading angles from the report, without which the bending moment cannot be reconstructed from the force — a reviewer cannot check a result whose geometry is not stated, however complete the force data looks.