Procedures for the mechanical testing of hip replacement
The tests according to ISO 7206-4 and ISO 7206-6 focus on the stem and neck regions of the femoral component of hip replacements. Under specified cyclic loads and numbers of cycles, the fatigue strength of the hip stem is evaluated. The fixation method and applied loads vary depending on the design and length of the stem, which is classified as short stem, standard stem, or revision stem.
Additional requirements arise depending on whether the design is anatomical or straight. If the product features taper connections in the stem or neck area, the tests are performed in a saline bath at 37°C.
Beyond the prescribed standard conditions, we offer, in consultation with our customers, testing with higher loads, increased cycle numbers, and load increments.
The static burst test evaluates the strength of the ball head in connection with the stem taper. After assembling the head onto the stem taper with a defined force, a static axial load is applied to the ball head at a specified rate. One of the failure criteria is fracture of the head. For a successful test, five samples must withstand the defined maximum load without failure.
The ball torsion test provides information about the torque required to loosen the taper connection between the hip stem and the ball head after the specified assembly technique. This is a static test involving a single application of load.
The procedure specified in ISO 21535 measures the range of motion (ROM) of hip replacements. This can be performed either on the actual component or within a CAD environment. The ranges of motion up to impingement are measured in the directions of flexion/extension, adduction/abduction, and internal/external rotation.
The standard defines minimum rotation angles that the product must be able to achieve before impingement occurs.
This test evaluates the fixation of inserts in the acetabular shell to compare the effectiveness of different insert fixation mechanisms. It includes three different tests. The shell is fixed in all tests using a suitable fixture to prevent movement of the shell while allowing the insert to be released. Before each test, the insert is fixed in the shell with a force of 2000 N (ASTM F2345).
Axial push-out test
A displacement is applied to the insert along the polar axis through a hole in the pole of the acetabular shell until the insert detaches from the shell. The resulting force is measured.
Offset pull-out test or lever-out test
For this test, a slot or hole is first created inside the insert at approximately 80% of its depth. Then, an axial displacement parallel to the polar axis is applied to the insert via a rod (offset pull-out test). Alternatively, the insert can be levered out using a lever arm that engages the slot or hole (lever-out test).
Torsion test
For this test, grooves parallel to the polar axis are introduced into the insert, into which springs on a prepared ball head engage. Alternatively, a ball head can be glued into the insert. Then, torsion is applied around the polar axis via the ball head until the insert is released.
The corrosion testing of the taper connection according to ASTM F1875 can be performed using two different methods:
Method 1:
The taper connection of the hip replacement is subjected to at least 10 million load cycles in a test chamber filled with electrolyte (0.9% NaCl) following ASTM F1140. The isolated fluid is chemically analyzed for particles and ions. Afterwards, the components of the taper connection are examined microscopically and with an electron microscope. Additionally, weight loss can be measured as an indicator of wear. This method quantitatively measures particles and ions and qualitatively assesses surface damage of the taper due to micromotion and corrosion.
Method 2:
The hip replacement is placed in a test chamber filled with electrolyte solution (0.9% NaCl), so that the electrolyte covers the taper and part of the stem. The contact surfaces are isolated from the electrolyte. The taper connection is loaded as in Method 1 according to ASTM F1140, but forces and test frequency may be lower.
- Procedure A:
A calomel electrode is used as a reference for measuring the corrosion potential with a voltmeter. The test ends when a stable potential is reached. - Procedure B:
A large-area counter electrode is placed in the solution to simulate the stem surface. An ammeter is connected between the hip replacement and the counter electrode. The difference in current before and during loading serves as a measure of wear corrosion between the taper and ball of the hip replacement.
The ball pull-off test, like ISO 7206-13, assesses the strength of the taper connection between the hip stem and ball head according to the specified assembly technique. Instead of measuring the torque required to loosen the connection, ASTM F2009 measures the axial pull-off forces.
This is also a static test involving a single application of load.
