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  • Injuries and loading discussion

    Dear Biomech:ers
    Earlier (950124) I had posted a series of questions concerning injury types
    and loading in joints and tendons. I have received many responses that were
    interested in the results of the responses (13) and two that may serve as
    sources of information. Accordingly, I send the two responses and perhaps
    initiate a discussion.

    The general question is: assuming that certain injuries have a biomechanical
    etiology, is it loading or loading rate that is the injury-causing factor to
    a biomaterial? Motivation of this question is obvious for many, we wish to
    identify those conditions that may be harmful to our subjects (horses in my
    case).

    Let us begin with a discussion, which I do not claim to be complete or
    correct but a synopsis of my subjective perception of the knowledge in this
    area. Criticism is encouraged along with contribution.

    In the literature I have found many statements as to the effect of
    "increased popularity of running has resulted in an increase in
    injuries......degeneration of articular cartilage is SUPPOSED to be result
    of repetitive impulsive loading." This is potentially a very powerfull
    statement as there is a cause-effect relationship supposed. Based on this
    supposition, one need not search long to find articles on impact/impulse
    loading in various sporting events, surfaces, shoes and other variables
    considered important in impact. Much of this supposition is based on what I
    believe to be excellent experimental work of Dr. Radin and coworkers. The
    general hypothesis is that joint damage is brought upon by repetitive
    loading and oscillary movement. This is appealling and a strong
    cause-effect relationship is suggested in the animal studies of this group
    and many others. Yet, at which frequency or loading rate is injury
    induction, 40-70 Hz at relatively low joint moments or 7-10 Hz at higher
    joint moments?

    Another question which I find interesting is the direction of the loading,
    that is many experiments look at longitudinal loading or vertical and
    horizontal loading. If we assume the stiffness of the distal limb to be
    greatest in the longitudinal direction (column of bone), then one may
    continue to assume that this is sufficiently stiff to withstand the loads
    imposed upon it. However, the transversal direction of the system may not
    be as stiff and thus limb geometry and direction of loading may be of
    consequence. Recently, O'Connor and Johnston (not me!) ISB Congress Paris
    1993 suggested that compression (impulsive loading) and friction may be
    needed to approach the physiological limits of articular cartilage. Does
    impulsive loading have to occur in at least two directions to induce
    pathological changes or does the enviroment in the joint have to change to
    increase friction to then induce changes?

    In response to this area of interest, I received an E-Mail from Howard J.
    Dananberg, DPM (howiedbpg@aol.com). The response is within quotations.

    >"I read your posting today and would like to contribute to the discussion. I
    >am a podiatrist who has spent many years in gait analysis. While injuries
    >(eg. falling from a height) occur regularly, the more interesting subject is
    >the overuse type complaint (osteoarthritis). While my expertise is in feet,
    >I am involved in seeing many lower extremity complaints which appear related
    >to style of gait rather than direct injury. In other words, the degeneration
    >occurs slowly over time. One of the most common arthridities is in the great
    >toe joint (hallux limitus). The shock at forefoot strike does not appear to
    >be vectored correctly to provide the type of degenerative changes which are
    >visible. Instead, I believe that the process of injury is related to the
    >following. As the heel lifts from the ground (a motion created by the pull
    >of the opposite swing limb rather than the calf), there exists a momentary
    >failure of the MTP joint to immediately flex. Compression then occurs across
    >the joint, and eventual DJD type changes occur (after millions of cycles).
    > This would seem true in the hip as well, considering the lack of motion
    >which occurs in DJD is extension, a motion which occurs in the 2nd half of
    >the gait cycle and unrelated to heel strike impact loads. Even knee
    >arthritis, which is predominately (95%) in the medial aspect, fits this
    >paradigm. The lateral knee is loading initially at heel strike. The medial
    >compartment, (which does take 60% of the total load by midstance) is loaded
    >later in the step.

    I have written several papers on this subject and would be happy to send them
    to you. I would also be happy to discuss this in more detail if you wish."

    I think a more indepth discussion would be very interesting for all and
    please do give the references.

    The next question was that on loading and loading rates of ligaments and
    tendons. The information here is scant to say the least. Strain rate has
    been suggested as the determining factor of injury type in ligaments
    according to two articles I have found ( Noyes et al. Biomechanics of
    Anterior Cruciate ligament Failure:... J. Bone Jt. Surg. 56 (A-2) 1974;
    Crowninshield and Pope The strength and failure characteristics of rat
    medial collateral ligaments. Journal of Trauma 16 (2) 1976). The general
    opinion is that low rates of strain result in bone-ligament interface
    failure while higher rates result in mid-substance failure. My question
    was if there is a differential length change in ligaments (tendons) as
    related to loading rate?

    Response to this came from Sharon Dixon (s.j.dixon@uk.ac.lut). Response is
    in quotations.

    >I was very interested to read your posting on the Biomech listing. I am at
    >present in my second year of a PhD studying Achilles tendon loading during
    >running. I have used a two-dimensional rigid body representation to estimate
    >maximum Achilles tendon force, and have found that maximum force occurs during
    >the midstance phase.
    >Investigation into the influence of heel lifts on the max. force has suggested
    >that max. force may not be the most important factor when considering injury
    >occurrence.
    >The next stage of my work will be to examine the loading at impact, looking in
    >particular at the loading rate. Modelling techniques will be used for this.
    >I would be very interested in any evidence you come across on the cause of
    >tendon injury, specifically with reference to tendon loading rate.

    I hope these comments are of interest, please feel free to comment and
    contribute to the discussion.

    I can compilate the discussion at a later date

    Regards

    Chris Johnston, DVM
    Equine Biomechanics Lab.
    School of Veterinary Medicine
    Uppsala, Sweden
    Christopher.Johnston@ah.slu.se
    Chris Johnston, DVM
    Equine Biomechanics Lab.
    School of Veterinary Medicine
    Uppsala, Sweden
    Christopher.Johnston@ah.slu.se
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