ASTM F1820 – Standard Test Method for Determining the Axial Disassembly Force of a Modular Acetabular Device

This test method covers a standard methodology by which to measure the attachment strength between the modular acetabular shell and liner.  Although the methodology described does not replicate physiological loading conditions, it has been described as a means of comparing the integrity of various locking mechanisms.

Axial Disassembly

The axial disassembly of an acetabular device test method provides a means to measure the axial locking strength of the acetabular liner for modular acetabular devices.  The assembled device is attached to a fixture such that the cup opening is facing downward.  The acetabular shell is supported and an axial force is applied to the acetabular liner until it disengages.  The force required to disengage the acetabular liner from the acetabular shell is recorded.

Offset Pullout or Lever Out Disassembly

The offset pullout or the lever out disassembly method is intended to assess the resistance of the locking mechanism to edge forces that could occur when the neck of a hip prosthesis impinges on the edge of the acetabular liner.  An impinging force could cause the edge of the acetabular liner opposite the area of impinging contact to be pushed out of the shell.  The resistance of the acetabular liner edge to being pulled loose from the shell is a measure of the resistance to impingement causing loosening of the acetabular liner.  The assembled device is attached to a fixture such that the cup opening is facing upward.  The acetabular shell is constrained from moving at a minimum of four locations spaced uniformly around the top circumference of the acetabular shell.  For an offset pullout a force is applied to a liner contact point, a location near the top surface of the liner.  The line of action of the force is constrained to a direction that is parallel to polar axis of the liner.  The force required to disengage the acetabular liner from the acetabular shell is recorded.

Torque Out Disassembly

The torque out disassembly method is intended to assess the resistance of the locking mechanism to high friction events that would attempt to rotate the acetabular liner within the acetabular shell.  The assembled device is attached to a fixture such that the shell opening is unimpeded, allowing the acetabular liner to be pushed free of the shell.  The acetabular shell is constrained from moving at a minimum of four locations spaced uniformly around the top circumference of the acetabular shell.  A head of a diameter appropriate to the liner is attached to the liner at a minimum of four equally spaced locations or adhesively bonded. A torque is applied through the head along the polar axis of the liner. The torque required to disengage the acetabular liner from the acetabular shell or break the adhesive bond between the articulating surfaces of the acetabular liner and the head is recorded.

Photos of Test