Thermoplastic Joints (metal)

Thermoplastic Joints (metal)

Tom Lunsford, CO

Paul Reith, CPO

    I have tested most of the mechanical ankle joints on the market for
fatigue life and signs of wear. I am happy to share with you the data but I
am a little confused by your question.
    Free motion mechanical ankle joints, whether they are made from metal,
plastic, or a combination, experience very little stress and hence the joint
or mounting screws would likely come loose before fatigue or wear were
noticed.
    Unfortunately endurance is not a mechanical property usually associated
with such an articulation.
    When these joints are used to restrain DF or PF in stance phase, then the
pathomechanical forces are transmitted to the joint screws/bushing and the
mounting screws (or rivets) which can elongate the mounting holes in the
plastic. This generally does not result in a catastrophic failure, but
degraded control of the gait deviation in question. Also, the stress can
tend to loosen the joint and mounting screws which leads to loss of gait
deviation correction and/or a catastrophic failure.
    The plastic DFA joint tends to experience early fracture of the elastic
member when used to prevent excessive PF in swing phase and the Camber axis
have chronic problems with mounting and joint screws loosening and failing in
shear when used for restraining excessive DF in terminal stance. The horse
joints appear to be the most prone to fail, except for the free motion
application.

Tom Lunsford, MSE, CO, LO