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Re: BIONET Controversial topics: boundary conditions in FE or EXPmodels

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  • Re: BIONET Controversial topics: boundary conditions in FE or EXPmodels

    Dear All

    Regarding to the discussion started by Marco Viceconti on the topic 1
    "boundary conditions in bone stress analysis", we want to give our small
    contribution.

    The aim of a recent work of ours, submitted to Journal of Biomechanics, was
    to investigate the effect of load simplification on the results predicted
    by a finite element analysis of the bone-implant biomechanics during stair
    climbing.

    This investigation was made using a finite element model, fully validated
    in a previous study, representing a human femur implanted with an
    anatomical cementless stem.
    Two load cases were compared: the reference load profile, derived from
    literature (Bergmann, 1999), explicitly modelled the actions of the
    principal muscles acting on the proximal femur during stair climbing; the
    simplified load profile included only the hip joint reaction as measured in
    vivo by an instrumented hip prosthesis (Bergmann, 2001).
    In order to avoid boundary effects near the implant, the femur was modelled
    down to the end of the diaphysis, which was assumed fully constrained.
    The effects of the above load cases were evaluated by comparing:
    bone-implant relative micromotion, bone-implant interface stresses (contact
    pressure and frictional stress) and Von Mises equivalent stress (implant,
    cancellous bone and cortical bone).
    For each parameters the difference between peak values, the maximum
    element-wise difference and the average (mean square) element-wise
    difference were computed.

    Results showed that under the action of the simplified load case peak
    micromotion and peak Von Mises stress in the implant were substantially
    unchanged (differences < 2%), while the peak Von Mises stresses in the
    cortical bone increased of a 6%. The peak interface stresses increased of a
    20%.

    When compared element-wise, the relative micromotion differed of a 16% on
    average. The average change for all three Von Mises stresses was 13% and
    only 7% for interface stresses.

    Data here obtained tend to confirm the acceptability of using only the hip
    joint force, neglecting the muscle forces if the objective is to predict
    the primary stability of a cementless stem, intended as the peak elastic
    micromotion induced by a certain motor task.
    On the contrary, if the study is aimed to investigate small differences in
    the biomechanical behaviour of the bone-implant interface, with attention
    to interface stresses or to the micromotion in very specific locations,
    such load simplification may be used only to achieve a very gross estimate.

    Because no methods are available to measure directly and non invasively the
    force exerted by each muscle group during daily activities, from this point
    of view the availability of a simplified model allows us to exploit from
    literature the large data of hip contact force in-vivo measuraments and to
    perform importan simulation studies.

    Thus, we agree with all authors that stated that model complexity has to be
    appropriate to the question investigated.

    Best regards


    *********************************
    Dr. Ing. Alberto Pancanti, MSE
    Laboratorio di Tecnologia Medica
    Istituti Ortopedici Rizzoli
    Via di Barbiano 1/10,
    40136 - Bologna, Italy
    tel: +39 51 6366554
    fax: +39 51 6366863
    e-mail: pancanti@tecno.ior.it
    Home Page: http://www.ior.it/tecno/

    *********************************

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