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Medially Deviated Axes and Forefoot Varus Extentions

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Robertisaacs, Oct 21, 2009.

  1. Tell me what it is you don't understand and I'll try to help you.
     
  2. So medial or lateral deviation is only half the picture high and low will be as significant when deciding how many mm a skive should be ?

    This may also mean that 15 degrees skive could be modified from device to device once the spartial height of the axis can be located clinicially ?
     
  3. I don't understand your questions, Michael.
     
  4. When we discuss stj axis it´s mainly about medial lateral deviation you and Kevin have written papers on spartial location in relation to medial or lateral deviation. My thoughts/question are that if we know the height of the STJ axis it should also adjust the prescription variables..........

    But ive just realised something that the talus position ( which is in the same paper I beleive) will show the height so we already know the position to some degree which we already use to set our skive position and amount in mm.

    Sorry I think I just answer my own questions.
     
  5. efuller

    efuller MVP

    I think your question is should you change the ammount of skive if you see a high or low angulation of the axis relative to the transverse plane.

    Generally, I would say no, I don't consider the height of the axis. Ground reaction force is mostly vertical and when we attempt to treat the foot by shifting the center of pressure under the foot we are mainly changing the distance of the center of pressure to the projection of the axis in the transverse plain.

    With an axis closer to parallel to the ground, the moments will be higher than when there is a more vertical axis. However, we are trying to change the moment in a certain direction and you should get the same percentage change in both high and low axes with an equal medial shift in the center of pressure.

    Regards,

    Eric
     
  6. Asher

    Asher Well-Known Member

    Me too! Thanks for asking the question Ian.

    Rebecca
     
  7. The bottom line with this example, which I thank Robert for bringing to us, is that a varus forefoot wedge, even if it is acting only lateral to the STJ axis, will reduce the magnitude of STJ pronation moment, which, in effect, is the same as increasing the magnitude of STJ supination moment. This is the principle of rotational equilibrium and is an excellent teaching example.
     
  8. Donnchadhjh

    Donnchadhjh Active Member

    Thank you Kevin for the summary - that is basically the conclusion I came to on Sunday but was still trying to reason it in my head.

    Simon - I get the general idea of what you are saying - the free body movement work is what went over my head, and I dont like the idea of nailing my 1st met to the wall to try things though... I am keen to learn but not THAT keen.

    Having said that... any volunteers...? Bone pin trials are so 2009 :D
     
  9. Thanks Eric thats what I was trying to ask. Thinking too hard about things sometimes confuses the situation.
     
  10. mgrig

    mgrig Active Member

    does anyone feel that they could explain how windlass and STJ axis effect eachother....possibly could windlass move the axis more laterally? increasing the supinatory moment?
     
  11. The windlass mechanism should theoretically cause arch raising and supination, with supination comes a lateral shift in STJ axial position. However, try this: dorsiflex each of the toes one at a time what, observe the results...
     
  12. Everyone:

    First of all, the proper words are "supination moment" not "supinatory moment" and "pronation moment" not "pronatory moment". This is a point that Dr. John Weed emphasized to me over 20 years ago about proper terminology after he read my first paper on STJ axis location.

    Marc:

    Not a problem explaining windlass and STJ axis interrelationship.

    The windlass, when activated during propulsion, causes an increase in posteriorly directed force on the first metatarsal head and an increase in anteriorly directed force on the medial calcanal tubercle due to an increase in tension in the medial fibers of the central component of the plantar aponeurosis (i.e. plantar fascia). Depending on the spatial location of the STJ axis, the increase in tensile force in the plantar fascia will cause an increase in first metatarsal compression force which will be transmitted through the bones of the medial column as an increase in joint compression force within the 1st metatarsal-1st cuneiform joint, 1st cuneiform-navicular joint, and talo-navicular joint. The increase in posterior directed force on the talar head will, with normal STJ axis location, cause also an increase in STJ supination moment. Eric Fuller did a nice paper on this effect some years ago (Fuller, Eric A: The windlass mechanism of the foot: A mechanical model to explain pathology. JAPMA, 90:35-46, 2000).

    The activation of the windlass will, then in a normal STJ axis location, tend to cause STJ supination, but this will not occur if there is an increase in STJ pronation moment occurring simultaneously that tends to resist STJ supination. Feet with medially deviated STJ axes have such large magnitudes of external STJ pronation moment that they will often not show any STJ supination even with 20-30 pounds of hallux dorsiflexion force whereas only 1-2 pounds of force may easily dorsiflex the hallux in feet with laterally deviated STJ axes.

    Medial longitudinal arch (MLA) height also has a huge effect in that the higher the MLA, the smaller will be the plantar hallux force required to cause hallux dorsiflexion and the lower the MLA, the larger will be the plantar hallux force required to cause hallux dorsiflexion. My first Thought Experiment from 3.5 years ago went through this concept of how MLA height affects plantar fascial tension.

    Hope this helps.
     
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