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Standardization: ankle joint

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  • Standardization: ankle joint

    Some comments on 'A Joint Coordinate System for the Ankle
    Complex', a proposal submitted to the ISB standardization and
    terminology committee (ISB Newsletter 59, pp.6-8).

    First of all, I admire the courage of the ankle joint
    subcommittee for stepping into this minefield of standardization.
    It is always much harder to construct such a proposal than to
    criticize it. So, please take these comments as constructive
    criticism, with the intention to improve the final outcome of
    the process.

    1. Anatomical landmarks and anatomical planes
    Of the anatomical axes defined in the proposal, only the X-axis
    of the tibia/fibula and the z-axis of the calcaneus are actually
    used in the joint coordinate system (JCS). The other landmarks
    are only required for definition of the neutral configuration.
    For this reason, considerable time and effort can be saved in the
    data collection protocol when another definition of neutral
    (e.g. a reproducible standing position) is acceptable. This, of
    course, depends on the purpose of the analysis. The analysis may
    become unnecessarily complex when using this standard.

    Another point is that the proposed definitions of neutral are
    possibly subjective and sensitive to errors in locating the
    anatomical landmarks. Specifically:

    "plantar aspect of the foot". Is that the same as the ground
    surface when the person is standing (full, or half body weight?),
    or a plane defined by three anatomical landmarks (which points on
    the calcaneus?).

    "projection in the transverse plane of line from M5 to O1".
    First, why not just use the y-axis of the tibia, as defined
    earlier? Second, this projection is very short, which makes the
    orientation quite sensitive to errors. This is probably not a
    very reproducible definition of neutral for internal/external

    2. Choice of rotation sequence
    The proposed rotation sequence is (at least in its terminology)
    inspired by the widely used standard of Grood and Suntay (1983)
    for the knee joint. However, some arguments can be made for an
    alternative choice in the ankle, as proposed by Cole et
    al. (1993):

    (i) If quantification of rotation of the distal segment about its
    *long* axis (rather than its "vertical" axis; which is never
    really vertical anyway during movement) is found useful (as in
    the Grood/Suntay standard for the knee), the rotation sequence
    for the ankle complex should be: plantar/dorsiflexion -
    internal/external rotation- in/eversion, with the latter axis
    fixed in the calcaneus.

    (ii) The orientation of the long axis is definitely less
    sensitive to errors in locating anatomical landmarks than the
    orientation of a (short) "vertical" axis. In the proposal, the
    orientation of the z-axis of the calcaneus depends completely on
    the definition of neutral, i.e. it is perpendicular to a not very
    well defined "plantar aspect" of the foot (see comments above).

    (iii) Historically, the horizontal projection of the long (or
    A-P) axis of the foot, has been defined by Inman (1976) as the
    line from the midpoint of the posterior aspect of the calcaneus
    to the gap between the second and third toe. The sagittal
    projection of this line still requires some definition of the
    "plantar aspect", (the axis can be defined to be parallel to the
    plantar aspect) but at least one projection is well defined. In
    my opinion, this makes this axis more reliable and more suitable
    for use in a joint coordinate system than the z-axis as proposed.

    3. Choice of XYZ
    The choice of which axis to label as X, Y and Z is somewhat at
    odds with the ISB proposal for a global reference frame. It
    would be desirable to have global and local reference frames
    roughly aligned during a neutral position. This allows easier
    interpretation of absolute segment orientation (small angles
    corresponds to close to neutral), and less risk for
    singularities. If fact, if I'm not mistaken, with the foot in
    neutral position, the ISB proposal for absolute orientation can't
    be used with this calcaneus reference frame! The absolute
    orientation matrix involves a rotation of close to 90 degrees
    about the Y-axis (angle beta = 90 degrees, see Part 4 of ISB
    proposal), which leads to division by zero when attempting to
    calculate the other two angles alfa and gamma.

    Rather than change the calcaneus reference frame, I suggest that
    this is another reason to reconsider the proposed standard for
    the global XYZ directions.

    4. Practical use of the standard
    The proposal does not make any suggestions for practical
    implementation of the standard. Is it proposed that markers are
    attached to all of the proposed anatomical landmarks, or are
    those landmarks only used during a 'subject calibration'? Some
    practical advice may be useful.

    5. The use of actual joint axes in a JCS
    Movement in the ankle joint complex occurs mainly as rotations in
    two joints: the talocrural joint and the subtalar joint. In
    unloaded conditions, it has been shown that these joints can be
    described as hinge joints with one degree of freedom. It may
    therefore be desirable, for some applications, to choose a joint
    coordinate system (JCS) in which the tibia-fixed axis coincides
    with the talo-crural joint (as in the proposal), and the
    calcaneus-fixed axis coincides with the subtalar joint. The
    rotation about the floating axis would then become very small,
    and a more meaningful interpretation of the other two rotations
    becomes possible (Scott and Winter, 1991). Another big bonus of
    such a JCS is that joint moments with respect to these axes can
    be directly related to muscle forces. This is not without
    problems, however. Hontalas and Williams (1995) conclude:
    "... the assessment of motion about the subtalar joint for
    individuals may not be accurate until valid in vivo methods are
    developed for determining subtalar axis orientation."

    The subtalar axis orientation can be determined in vivo (Bogert
    et al., 1994), but this is not an easy protocol, and the result
    is quite sensitive to relative movement between markers and bone.
    Therefore I personally feel that a JCS based on anatomical
    landmarks, not considering joint axes, is at this point more
    practical, and I agree with the Ankle Subcommittee that this
    should be standardized. My point is, that it may sometimes be
    advantageous to go through a complex procedure to determine a
    non-standard JCS which is more appropriate for the purpose of the
    analysis. It is therefore important, as already mentioned on
    Biomch-L by Cavanagh and Yeadon, that standards are not "cast in
    stone" and blindly enforced.


    Bogert, A.J. van den, G.D. Smith, and B.M. Nigg (1994) In vivo
    determination of the anatomical axes of the ankle joint com-
    ples: an optimization approach. J. Biomech. 27,1477-1488.
    Cole, G.K., B.M. Nigg, J.L. Ronsky, and M.R. Yeadon (1993) Appli-
    cation of the joint coordinate system to three-dimensional
    joint attitude and movement representation: A standardiza-
    tion proposal. J. Biomech. Engng. 115,344-349.
    Grood, E.S. and W.J. Suntay (1983) A joint coordinate system for
    the clinical description of three-dimensional motions: ap-
    plication to the knee. J. Biomechanical Engng. 105,136-144.
    Hontalas, K.L. and K.R. Williams (1995) Talocrural and talocal-
    caneal joint rotations during the stance phase of high
    heeled gait. Proc. 19th ASB Congress, Stanford CA, pp.223-224.
    Inman, V.T. (1976) The Joints of the Ankle. Williams & Wilkins,
    Scott, S.H. and D.A. Winter (1991) Talocrural and talocalcaneal
    joint kinematics and kinetics during the stance phase of
    walking. J. Biomechanics 24,743-752

    -- Ton van den Bogert
    Human Performance Laboratory
    Faculty of Kinesiology
    University of Calgary, Canada