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STJ axis spatial location and rotational equilibrium

Discussion in 'Biomechanics, Sports and Foot orthoses' started by music124, Dec 9, 2016.

  1. music124

    music124 Member

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    Student here,

    Whats the difference between 'spatial location' and 'rotational equilibrium'? Lost.

    Thanks in advance!
  2. music123

    music123 Member

    rotational position I meant, sorry.
  3. Spatial location of the subtalar joint (STJ) axis refers to the three-dimensional location of the STJ axis relative to the plantar foot or other reference. Rotational position of the STJ refers to where the STJ is within it's range of motion (i.e. pronated, supinated, neutral, 2 degrees pronated from neutral, etc). Even thought STJ spatial location and STJ rotational position are separate and distinct mechanical entities, they are mechanically interrelated in that alterations in one can mechanically affect the other.
    Last edited: Dec 9, 2016
  5. one of the greatest teaching aids re STJ axis theory that video
  6. efuller

    efuller MVP

    First you need to know what an axis of rotation is. It is an imaginary line the describes rotational motion. A lot of the time the surfaces of the joint determine the motion that occurs. Think about a lateral view of the trochlear surface of the talus. As the ankle plantar and dorsi flexes the top of the talus will slide along the bottom of the tibia and the ankle joint axis of motion will correlate well with the dorsal surface of the talus.

    On the other hand you can try to move the ankle in the frontal plane and you can see inversion (stress lateral x-ray) that does not correlate with the joint surfaces.

    Most of the time when we refer to axis of rotation we will be referring to the motion determined by the joint surfaces. The STJ joint surfaces, when compressed together will tend move around a small bundle of discrete axes that can be clinically considered to be one axis. The location of this bundle relative to some reference point eg. floor, talus etc, will be the spatial location.

    By definition, the stj axis does not move relative to the talus or calcaneus even as the joint goes through its range of motion (Pronation supination) However, the axis can move relative to other reference points e.g. ground, leg.

    Rotational equilibrium looks at the moments or torques about that axis of motion. Moment = force x distance. When you apply a force to the foot, you will most likely be creating a moment about the STJ axis. There are many different sources of moment about the STJ axis. When they are all summed you will get a net moment about the STJ and this will determine whether, and/or in which direction, the STJ will move. (Moment = moment of inertia x angular acceleration)

  7. Just for you, Music. I have created a video and discussion of STJ axis location vs STJ rotation for you and others interested on my Facebook Page.

  8. music124

    music124 Member

    Thank you Kevin. Is my understanding correct in that spatial location refers to the patient's stj axis non weight bearing whether that's pronated, neutral or supinated and so is singular and the rotational position referring to the changing position of this stj axis during weight bearing activities. Physics wasnt my strong point
  9. STJ Axis Spatial Location.jpg
    It would be helpful, Music, if you gave me your real name. That may also make others more likely to help you with your questions.

    The STJ axis spatial location refers to the three-dimensional (3D) location of the STJ axis relative to another reference frame, whether the foot is weightbearing or non-weightbearing. In the illustration above, note how the the STJ spatial location passing posteriorly through the area of the posterior lateral calcaneus and anteriorly through the area of the dorsal talar neck, being angulated from posterior-lateral-plantar to anterior-medial-dorsal. You wouldn't need to use the terms "pronated", "supinated" nor "neutral" to describe the 3D location of the STJ axis relative to the ground. All you would need would be the angulation of the STJ axis from the sagittal plane and from the transverse plane in addition to the anatomical landmarks where the STJ axis actually exits externally from the foot.

    However, STJ rotational position would be, for example, saying "the STJ is maximally pronated" or "the STJ is in neutral position" or "the STJ is 3 degrees pronated from the neutral position". Notice, there is no mention of the STJ axis since these are two very different mechanical description of STJ function.

    A mechanical analogy to STJ axis spatial location and STJ rotational position would be a door which you can open or close. The axis of the door hinges being directed vertically upward and located parallel and attached to the door frame would be the "Door Hinge Axis Spatial Location". However, the rotational position of the door could be, for example, "fully closed", "open all the way" or "opened to 15 degrees from full closed".

    Hope that helps.

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