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joint forces in a pelvis CS

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  • joint forces in a pelvis CS

    Dear Biomech-l readers

    I've been working recently with the FE model of the human pelvis developed
    by Michel Dalstra (1993), using the joint reaction forces he calculated.
    Starting with hip joint force data from Bergmann (1990) he transformed
    into the pelvis CS accounting for flexion-extension angles only (measured
    on a different subject).

    I've compared these with more recently published work: Witte H et al.
    (1997) Acta Anatomica 160 (4) 269-280 used the Bergmann force data
    (EBL277) but transformed it using kinematic data (Zebris) from 4 different
    subjects, choosing one for detailed analysis.

    Pedersen DR et al (1997) J Biomech 30 (9) 959-965 used instrumented hip
    implant data and kinematic data from the same subject. They used inverse
    dynamics and non-linear optimisation routines on force plate measurements
    taken during collection of the kinematic data to calculate a hip joint
    force. This technique was validated by comparison with the measured hip
    joint forces. In this paper they transformed the calculated hip force into
    a pelvis co-ordinate system using the same kinematic data.

    There are some differences between these hip joint reaction forces, though
    they all point medially and superiorly .

    The Dalstra forces all point anteriorly and move monotonically more
    anterior during the stance phase.

    At heel strike the Witte forces point posteriorly, but at the first
    (largest magnitude) force maximum they point anteriorly, at the second
    maximum posteriorly and then at toe off anteriorly.

    All the forces calculated by Pedersen point posteriorly, moving
    monotonically anteriorly from heel strike to toe off. Pedersen also finds
    that the second peak force is larger than the first.

    The monotonic anterior motion of the joint reaction force in a pelvis CS
    seems intuitively correct - the joint reaction force stays relatively
    constant in direction in a femoral CS and the femur-pelvis relative motion
    seems to be smooth during stance phase.

    Obviously the simplicity of the transformation Dalstra used will introduce
    some errors, but it is hard to see how two groups starting with similar
    data can produce such different results.

    I know very little kinematics but am trying to decide upon a scheme of
    loading (in a pelvis CS) for the above mentioned FE model of an implanted
    pelvis. I understand that the variability of component position, hip
    geometry and gait patterns will of course produce a wide variation in
    joint reaction force, but would like to use loads that at least represent
    the case in one individual with some idea of the worst cases.

    I would appreciate any comments on the differences between the papers I've
    mentioned and will post a summarised version of responses.

    Thank you very much

    Mark Thompson

    ================================================== =======
    Mark S Thompson PhD Student
    IRC in Biomedical Materials, QMW College, London E1 4NS
    ================================================== =======

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