ASTM F1820-22 Standard Test Method for Determining Forces for Removal of Modular Acetabular Devices

The “ASTM F1820-22 Standard Test Method for Determining Forces for Disassembly of Modular Acetabular Devices” standard, developed by the American Society for Testing and Materials (ASTM) , provides a standard methodology for measuring the bond strength between a modular acetabular shell and liner. While the described methodology does not replicate physiological loading conditions, it is defined as a way to compare the integrity of various locking mechanisms.

For the purposes of this standard, the following terms and definitions apply:

• The acetabular liner refers to the portion of a modular acetabular device that has an internal hemispherical socket intended to articulate with the femoral prosthesis. This may include acetabular liners used for bilateral mobility or restricted applications. The external geometry of this component interfaces with the acetabular shell via a locking mechanism that may be an integral part of the design of the acetabular liner and shell or may rely on additional components (e.g., a metal ring or screws).
• The acetabular shell refers to the external, hollow (usually metal) structure that provides additional mechanical support or reinforcement for the acetabular liner and whose external features interface directly with the bones of the pelvic socket (e.g., by means of bone cement, firm pressing, coatings bonded to bone cement or tissue, integral threads, fixation screws, or pins). The acetabular shell may be solid or may contain holes for fixation or may have a hole for instrumentation.
• A locking mechanism is any structure, design feature, or combination thereof that provides mechanical resistance to movement between the acetabular liner and the shell.
• The polar axis is the axis of rotation of the rotationally symmetrical portions of the acetabular liner or shell.

While the test is being applied,

• The axial disassembly test method of an acetabular liner and shell system provides a means of measuring the axial locking strength of the acetabular liner for modular acetabular devices.
• The offset pull or leverage removal method is designed to evaluate the resistance of the locking mechanism to edge forces that may occur when the neck of a hip prosthesis impinges on the edge of the acetabular liner. The impact force may cause the acetabular liner edge opposite the impact contact area to be pushed out of the acetabular shell. The resistance of the acetabular liner edge to loosening from the acetabular shell is a measure of the resistance to impact causing loosening of the acetabular liner.
• The torque-removal removal method aims to evaluate the resistance of the locking mechanism to high-friction events that would attempt to rotate the acetabular liner within the acetabular shell.

This test method is designed to evaluate the locking strength of the acetabular liner in a modular acetabular shell when subjected to three different force application conditions. This test method may not be suitable for all implant applications. The user is cautioned to consider the suitability of the method in terms of the materials and design tested and their potential applications.

While these test methods can be used to measure the force required to disengage modular acetabular devices, comparisons of such data for various device designs should take into account the size of the implant and the type of locking mechanism being evaluated. The location of the locking mechanism relative to the load application may depend on the size and design of the acetabular device. In addition, the locking mechanism itself may vary with size, particularly if the design is circumferential (e.g., a larger diameter implant will have a larger acetabular shell/acetabular shell and liner interface area than a smaller diameter implant).

Material failure is possible prior to locking mechanism failure during push or offset pull/lever conditions. This is due to the possibility of the material shear strength being exceeded before the locking mechanism is fully tested.

These tests require only an apparatus capable of supporting the acetabular shell and allowing the acetabular liner to be freely removed from the acetabular shell. The testing apparatus must comply with the requirements of ASTM 4 (this standard describes the force verification procedures for tensile or compressive, or both, static or quasi-static testing machines). The loads used to determine the bond strength must be within the range of the testing machine as defined in ASTM 4. The testing machine must be capable of delivering a compressive or tensile force at a constant displacement rate. The testing machine must have a load monitoring and recording system.

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