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Re: Joint stability, any standard definition?

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  • Re: Joint stability, any standard definition?

    Paolo and Biomech-l,

    Let me start my comments with an example to illustrate the ideas of
    stability and control for the sake of subscribers who aren't sure.

    Many people believe the Wright Brothers invented the airplane in 1903.
    They did not. In 1901, Wilbur Wright noted there were plenty of flying
    machines and lots of engine designs to go with them. The ability to
    control these components, he explained, would usher in the age of
    heavier-than-air flight. The Wright Brothers own patents on devices to
    control the motion of their flying machine.

    In biomechanics, we know the skeletal system and the muscles that affect
    joint position as the Wrights understood their systems. Controlling the
    dynamic processes in gait to prevent falls, negotiate obstacles, carry
    loads, adjusting to terrain are analogous to maintaining altitude and
    course, maneuvering, take off and landing. Instability in any area of
    gait (or flight) may result in system failure or loss of function.

    A biological system has a functional envelope: homeostasis. In
    engineering terms, this is steady state. In the uncontrolled condition,
    Newton's Laws are appreciated. A body in motion or at rest keeps doing
    that until acted on by a force changing position and speed setting a new
    steady state. A feedback control system responds to the outside effect
    to restore the steady state. In the airplane, the pilot is a crucial
    part of the feedback control although electronics and computerized
    systems are part of this system as well. When system failure occurs
    (crash), the structural and mechanical parts are examined but so are the
    command and control components. In joint motion, stability comes from
    the structural (skeletal) and mechanical (muscles, tendons, ligaments)
    parts and from the command and control components (central and
    peripheral nervous system).

    As an example of joint stability, consider the ankle. In the case where
    a clinically, excessive passive range of motion is noted and the subject
    is asymptomatic, a dynamic control system is at work, the ankle joint
    axis functions where it should and proximal and distal joints work
    within normal limits. Another ankle with normal range of motion
    subjected to a severe sprain which tore the lateral joint capsule has a
    disturbed proprioceptive feedback: functional instability. Adjacent
    joints are affected by the uncertainty in placing the ankle and the
    involved muscles function outside their normal phasic activities.

    I suspect you are correct in finding no stability criteria. The
    clinical assessment is too subjective and not dynamic. An investigation
    into the airworthiness of a bee or a stealth fighter based on form alone
    would reach a conclusion that neither could fly. I would recommend you
    continue to investigate dynamic challenges to your system. The Wrights
    performed extensive wind tunnel experiments (in vitro) before they were
    able to get their system operational.

    I think you'll find tracking the instantaneous axis of motion an
    important component of stability in general and that different criteria
    are appropriate based on joint size, type and function.

    At the PIPJ in particular, motion in the joint is affected by insertions
    distal to the joint and the extensor sling apparatus so it's no surprise
    you can find significant differences in flexion or extension indifferent
    configurations. What results would you expect (as compared to the in
    vivo system) if you kept each separate muscle connection intact and
    tested the range of motion it produces when other soft tissue is
    disconnected? Is the muscle still able to perform its function? Which
    conditions allow hyperextension or cause medial or lateral deviation?

    As a podiatrist, let me point out: the feet also have PIPJs and their
    functional requirements may be quite different from those in the hand.
    Hammer toes, claw toes, etc., may be the result of instabilities at
    these joints due to failure of neuromuscular components. When you know
    something about finger instability, maybe that can help understand toe

    James A. Furmato, DPM, PhD
    Chief Engineer, Gait Study Center
    Assistant Professor, Department of Orthopedics and Medicine
    Temple University School of Podiatric Medicine
    TUSPM Gait Study Center
    148 N. 8th Street
    Philadelphia, PA 19711
    Phone 215-625-5370
    Cellular Phone 609-933-2017

    -----Original Message-----
    From: * Biomechanics and Movement Science listserver
    [mailto:BIOMCH-L@NIC.SURFNET.NL] On Behalf Of Caravaggi, Paolo
    Sent: Wednesday, August 04, 2010 11:28 AM
    Subject: [BIOMCH-L] Joint stability, any standard definition?

    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.


    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