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Should the Medial Skive be parallel to the STJ Axis ?

Discussion in 'Biomechanics, Sports and Foot orthoses' started by mike weber, Feb 7, 2017.


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    Was thinking about this the other day while out for a head space clearing run, Now of course there is many holes in the idea, due to the 3 body planes 4 dimension concept of the axis, not least to the fact that the Subtalar joint axis is moving and some other stuff ( which I then started wanting to be able to google on my run )

    But in the pic I have drawn a very basic idea if the STJ axis, the calc, the point at which to mark the skive to define depth and a line parallel to the STJ axis at that given moment.

    The practical notion of being able to define the precise position of the STJ axis during weightbearing and when the exact time you want the skive to be most effective is of course an issue in itself, but as a though experiment would and parallel marking point to define depth of the skive be more effective than the current system ?
     

    Attached Files:

  2. Ian Drakard

    Ian Drakard Active Member

    Hi Mike
    Not sure if i can understand the diagram properly. Are you proposing something different to a MOSI?
    Cheers
    Ian
     
  3. The black line is the 2/3 marking for the medial skive, where you would decide the mm of depth

    Not the best diagram I agree, sorry all
     
  4. Petcu Daniel

    Petcu Daniel Well-Known Member

    I don't have an answer but a question: why you've raised this question firstly relative to the transverse plane and not to the sagital one?
    Daniel
     
  5. No real reason except that much of the drawing I think of when thinking STJ axis postion are in the transverse plan, if there is any merit in my random ramblings ideas the obvious next step would be the Sagital plane
     
  6. When I first started experimenting with the medial heel skive by modifying the plaster positive cast of a patient's foot in 1990, I basically just started the medial heel skive at the medial third of the plantar heel of the cast with a plaster file. I then skived a 15 degree varus angle, parallel to the sagittal plane, to achieve the initial plane of the varus heel cup produced by the medial heel skive (Kirby KA: The medial heel skive technique: improving pronation control in foot orthoses. JAPMA, 82: 177-188, 1992). Wow! I just realized my medial heel skive paper will be 25 years old in 2 months. It guess it has been a long time!

    Over the past quarter century of using and thinking about the biomechanics of the medial heel skive, however, I have come to realize that the actual angle of the medial heel skive relative to the sagittal plane or relative to the subtalar joint (STJ) axis is not that critical. As long as the ground reaction force (GRF) is shifted more medial to the STJ axis to increase the external STJ supination moment by the medial heel skive, then the job of the medial heel skive has been done. Therefore, contrary to what others have written on other similar varus heel-cup modification techniques for foot orthoses, if the "axis" of the medial heel skive is simply oriented somewhere between being parallel to the sagittal plane of the foot and the spatial location of the STJ axis, then I doubt that one medial heel skive orientation will be more mechanically effective than another on altering the kinetics of the foot and lower extremity.
     
  7. efuller

    efuller MVP

    I agree with Kevin. The point of the skive is to shift the center of pressure more medially to decrease pronation moment from the ground. Many heel cup shapes will shift the center of pressure more medially. Worrying about the angulation of the cut made for the skive really seems like splitting hairs.

    Why the transverse plane: The problem of calculating the moment from ground reaction force about the STJ axis is really three dimensional. However, to simplify, you need to know the perpendicular distance from the line of action of force to the axis. When the force is pointed directly toward the viewer it is easy to see the distance from the line of action of the force from the axis. Ground reaction force is close to vertical so the vector will be perpendicular to the transverse plane and pointed toward the viewer.

    Actually, we don't need to know the exact moment about the STJ, we just need to know which direction to shift it. This is best understood by thinking in the transverse plane because that gives us how a shift in the location of force will change the moment about the STJ axis.

    Eric
     
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