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Re: Joint stability, any standard definition?

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  • Re: Joint stability, any standard definition?

    I agree that there is no standard definition of stability as applied to joint function. Traditional engineering-based definitions (e.g. bounded input/bounded output) don't really apply. One of my old engineering textbooks describes a stable system as one that always gives responses that are appropriate to the stimulus (Franklin et al, Feedback Control of Dynamic Systems, 1987). The system we are talking about (a joint) is complex, consisting of a combination of joint geometry, soft tissue constraints, muscles/tendons and a neuromuscular control system that is subjected to wide range of externally applied loads. So, for joint stability, what is the stimulus, what response do we measure and what distinguishes an appropriate response from an inappropriate one?

    I propose that the stimulus should, as closely as possible, reflect the real-life loading that human joints are exposed to in daily life. For the knee (my area of greatest expertise), relevant activities would include walking or stair descent and perhaps running or jumping for athletes. It is during these activities that an individual with an injured knee is most likely to experience instability and damage other joint tissues, so the relevance of these movements to joint health is clear. I am no hand expert, but I assume that there are also some standard tasks that characterize functional use. By this definition, many measures traditionally used to assess stability are inappropriate. For example, simple laxity tests expose the knee to forces never experienced in daily living (e.g. uniaxial load, no compressive or muscle forces), and many studies that have shown little or no relationship between A/P knee laxity and functional outcomes in ACL-injured knees. Thus, though widely used, A/P laxity is poorly suited for assessing knee joint stability.

    As to what response is measured, I suggest it should be relevant to the tolerance of the joint tissues. We have traditionally relied on joint translations and rotations, which are easy to describe but have little direct relevance to tissue function. Most joints have no fixed axes of rotation, and translations are described relative to floating or arbitrarily chosen points. It is not surprising that defining a threshold for significant instability has been difficult using these measures, since they cannot be easily related to tissue function. Some of the recent joint injury literature has begun to focus more on tissue-specific measures, such as the amount of elongation of a ligament graft or the magnitude and location of cartilage deformation. These are directly relevant to joint health, since they are likely to be related to conditions we care about (e.g. graft failure or osteoarthritis development).

    If we choose tissue-relevant measures, then the question of what is an appropriate/stable vs. an inappropriate/unstable response becomes much easier to answer. An unstable joint is one that exposes joint tissues to damaging forces/deformations under functional loading conditions. Cadaver studies can be used to establish failure criteria, but the relevant thresholds may be much less than what would result in immediate tissue failure; chronic exposure to elevated loads can lead to gradual tissue destruction. We often don't know what the exact thresholds are for different tissues/measures, but we generally can get a good idea by examining the range of a particular measure during "high-demand" activities, or by relating the measure to degeneration or lost function in injured joints.

    In the context of your hand/finger studies, a geometric model could be used to relate your flexion/displacement curves to tissue-specific measures. If arthritis is the concern, then the shear motions at the joint surfaces might be relevant. If ligament damage is most important, then some estimation of how the finger kinematics affect ligament elongation might be more appropriate.

    I acknowledge that this definition for stability may be an academic ideal that is difficult to achieve in practice, especially in a clinical setting. But, I believe it also provides a framework by which simpler stability tests can be evaluated. For example, if we can show that a particular static laxity measure is a good predictor of dynamic stability (as described above), then we can establish it as a useful objective measure of joint stability. Right now, I don't think we really have a very good set of clinical tools for assessing stability for most joints.

    Just my thoughts - comments appreciated!

    Scott Tashman
    __________________________________________________ _____
    Scott Tashman, Ph.D.
    Director, Biodynamics Laboratory
    Associate Professor, Orthopaedic Surgery and Bioengineering
    University of Pittsburgh

    Orthopaedic Research Laboratories
    Rivertech, 3820 South Water St.
    Pittsburgh, PA 15203

    Phone: office 412-586-3950
    fax 412-586-3979
    mobile 412-260-7102

    On Aug 4, 2010, at 11:27 AM, Caravaggi, Paolo wrote:

    > Dear Biomech-l subscribers, I was wondering if any of you is aware of an objective test to assess joint stability. According to my literature research neither a standard definition of joint stability nor standard evaluation tests to determine the degree of instability at joints have been established. In most cases the level of instability is subjectively assessed by clinicians by applying dislocating forces to the joint. When more scientifically-objective approaches were taken, joint rotations to triplanar joint displacements (or rotations) are normally shown. However, although differences to the normal/stable joint are graphically presented, when/if the joint can objectively be considered unstable is not reported.
    > As far as our specific case, we are trying to quantify the level of instability at the proximal interphalangeal joint of the finger in-vitro through active flexion/extension of the joint following the release of supporting ligaments and the disruption of the joint by systematic resection of bone at its proximal aspect. We are indeed finding significant differences in the flex/ext rotation to joint displacement curves across different configurations (intact, disrupted..) but we are now facing the issue of establishing some kind of objective index for joint-stability. In other words, which variable is more relevant here and how far from the normal/non-pathological configuration a joint can still be considered to be stable?
    > Any suggestions and/or further comments on this matter are welcome.
    > Regards,
    > Paolo Caravaggi, PhD
    > _______________________________________
    > Joint Biomechanics Lab, Orthopedics dep.
    > University of Medicine and Dentistry of New Jersey
    > 185 South Orange Avenue, Newark, NJ 07103
    > Tel. +1 973 972 1426
    > _______________________________________
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