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Does a valgus rearfoot post cancel out a varus heel skive?

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Simon Spooner, Aug 20, 2011.

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    OK, this one came through as a private message but it's worthy of discussion and as a little work out for the brains trust:

    Question: would a valgus rearfoot post and a varus medial heel skive "cancel each other out"?
  2. it could if the external force vectors from the medial skive and lateral rearfoot post were equal and opposite.

    You would have to be very skilled or really lucky to do this, but as the vector from a device is a reaction force to have the heel strike the device with the same force or at same time or have the same shape and stiffness of fat pad, it gets much less likely that this cancellation could occur, impossible even.
  3. My short answer was: no.
  4. Petcu Daniel

    Petcu Daniel Active Member

    Which foot condition could require this prescription ?

  5. efuller

    efuller MVP

    It depends on what it does to the location of center of pressure. I know that I get less of a varus wedge effect when I remove rearfoot post from under the medial side of the medial heel skived cup. Conversely, you also get more varus wedge effect by adding a rearfoot post to an unposted medial heel skive device, because the orthosis is stiffer. I know this from altering my own orthoses.

    If the valgus rearfoot post tilts the heel cup enough, it will no longer look like a varus wedge. I maintain that the varus wedge effect can be canceled.

  6. RobinP

    RobinP Well-Known Member

    I would assume that theoretically it is entirely possible. However, the likelihood of doing it is slim. Given the inconsistency of surface terrain, device interface with footwear, etc etc does out really matter if they can absolutely cancel each other out?
    One would assume that it is with reference to treating a pathology and therefore some effect via the two modifications was being sought. Then, it becomes a case of how much the centre of pressure is influenced by each of the modification.
    So, an active pronator with pronation related symptoms may require a prescription that applied external pronation moments throughout the majority of stance phase but that, at a given point may require a limit to the external pronation moments. Although I am sure that a device could be designed in which we could accurately chart the centre of pressure, the force vectors will make it extremely difficult to do this on a gross level, therefore a valgus through wedge with a medial longitudinal arch support may be a design that would fulfill the criteria. Not exactly the question of the OP, but just trying to give a situation for opposing moments to be required at a given instant in time
  7. Hi Daniel

    I would not think in practical terms more of a discussion of theory.
  8. Bruce Williams

    Bruce Williams Well-Known Member

    I disagree. Why would you do this to an orthotic? Because you can't grind out a medial skive and don't want to pay for a new device. I have done this several times and it works at least as far as the patient being comfortable in the device again.
    Except thru pure experiment ther is no other reason to do this.
  9. I agree, it's a pure thought experiment. The key here is to understand how foot orthoses work and how the design variables are modified by heel-skives and rearfoot posts. Moreover to realise that they are very different beasts. I'll let it run a while longer... I got a busy week ahead, but if I get time I'll run this through FEA and post the results up here.
  10. Hi Bruce I agree to reduce the Supination effectiveness of a Medial Skive adding a rearfoot Valgus post is one option, but the original question was do they cancel each other out - so when I was answering Daniel I was suggesting that using a medial skive and a Valgus rearfoot post to cancel each other out it would better to think theoretical rather than practical

    Simon are you agreeing with Me or Bruce ?
  11. Both. Here's a nice little practical for y'all to try: draw a frontal plane section through the heel-cup area of an orthosis with a zero degree rearfoot post; now draw a similar section through the device with a medial heel skive (note the change in material thickness under the medial side of the device and the change in shape of the heel cup cross-section); now do the same thing adding varying degrees of valgus rearfoot post without the skive; repeat with valgus posts and medial heel skive- note how the cross-sectional shape is never the same. If the cross-sectional areas are different, the load/ deformation characteristics are always different too. Remind me again how foot orthoses work?

    1) altering the load/ deformation characteristics at the foot orthosis interface
    2) altering the topography characteristics at the foot orthosis interface
    3) altering the friction characteristics at the foot orthosis interface

    end of story.:drinks
  12. N.Smith

    N.Smith Active Member

    I'd imagine it wouldn't feel very comfortable (depending on the amount of skive used) and would put too much pressure on the medial heel. Probably cancel out the skive effect but not sure! If you got it that wrong, to do that, might want to start again as I'd imagine it would want to squeeze the heel out of the orthotic. If a heel is everted, it will thin out as it becomes more vertical so pushing it back the other way probably need more space/width. :drinks

  13. I got the same private message. I said that they would tend to cancel each other out since the internal varus heel wedge from the medial heel skive would have less varus if used with a valgus rearfoot post, so there is no really no reason to combine the two. However, using one with the other would produce an orthosis that is not the same as a non-medial heel skived-non-rearfoot valgus posted orthosis.
  14. gaittec

    gaittec Active Member

    Interesting question.
    Why would you do this. Only thing I can think of for designing this way would be to prevent
    lateral migration of the heel while maintaining some varus correction, where a deep heel cup and neutral lateral heel flare are not possible in the footwear.
  15. efuller

    efuller MVP

    I do it when the patient tells me that their medial heel skive device is giving them peroneal tendonitis. Or I could make a whole new device... Or more often when someone else has given someone with a laterally deviated STJ axis a medial heel skive device.

  16. gaittec

    gaittec Active Member

    Eric, thanks for a more elegant and precise way of saying what I was trying to indicate.

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