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Reduced arch push using a medial skive ?

Discussion in 'Biomechanics, Sports and Foot orthoses' started by mike weber, Feb 1, 2011.


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    Eric has mentioned this before and Robert mentioned it when he added a medial skive to a device in this thread - tissue/physical stress theory and using 1 device

    So we have a patient that we decide requires the following prescription.

    Medial skive

    full contact arch and we want contact under the Navicular at a height of x mm.

    when we add a medial skive the arch push provided from the device will be reduced the greater the skive the less arch push.

    Usually the height of the arch is from the ground - how does the medial skive in the device affect where the arch height is measured from or how many mm gets removed from the positive to get the required arch push.

    in the picture below the new arch height would move from X mm to the Y mm which in reality is lower.

    So how does the medial skive effect the medial arch contour in your design process.

    Hope that makes sense ?
     

    Attached Files:

  2. Ian Drakard

    Ian Drakard Active Member

    Great topic for a new thread Mike. I've been thinking a bit about this recently as there is a big element of guesstimation in how much I change the arch height relative to the skive.

    Usually seem to get it right but had to redo a pair of orthotics the other day because I undercooked the increase in arch height required, so would love to know if other people have a more reliable method.
     
  3. It been one of the things I´ve been playing with lately as well seem to be having better feed back on the higher arch, but have not gone to a 1:1 ratio yet , what ratio are you using Ian ?
     
  4. Ian Drakard

    Ian Drakard Active Member

    I'm afraid I've been muddying the waters as I've also been experimenting with the inversion and inclination angles of the skive at the same time:dizzy: I need to start getting a bit more scientific and play with one variable at a time before I combine the lot.

    The pair I had to redo not only had a sizeable skive, but fairly steep inclination angle to it, so not entirely surprised that the arch felt low. The reissued pair had what must have been getting on for a 1:1 ratio increase and could maybe have gone higher.
     
  5. Interesting question. I suspect the answer is the usual one. It depends.

    Lets say the intention of the orthotic is to increase external supination moment / decrease pronation moment. Thats well and good, both the arch and the skive will do this. But there is a limit to how much we want to do this, otherwise we'd be MASS THETANS (like scientology but groovier and makes less sense).

    So if we have reduced the ORF on the arch with our skive then do we necessarily want to increase it again with an arch raise?

    Depends how much increase in external supination moment we are going for, and how much the two segments of the foot can take. If trying for a maximum amount then yes, if I can take 100 lb/sq" under my arch then I should increase the arch height so I am STILL taking 100 lb/sq" under my arch after the skive. If I'm just looking for so much supination moment and I get lots from the skive then I don't NEED 100lb / sq" in the arch.

    Like I say. It depends.
     
  6. And as Robert knows, we'll be all over this topic at Megalab... comes back to the TOP cast thread.
     
  7. Was thinking about TOP but thought it was TIP my bad.

    But sometimes I think we forget the the MTJ is independent so will require different ORF's.

    So will depends is probably correct I do believe we should not just focus on the effect at the STJ.

    You up for a go around this topic now ?
     
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    Ready now. What was the question?
     
  9. Craig Payne

    Craig Payne Moderator

    Articles:
    8
    One of our hons students last yr looked at arch pressures +/- medial heel skive and found no diffferences in pressure.
    However, they only included those with FPI>4 (I think) and not specifially those who had a clinical indication for a medial heel skive.

    Clinically, I tend to prescribe more arch fill as more medial skive is used.
     
  10. markjohconley

    markjohconley Well-Known Member

    Craig, hi, I thought it would be 'less' arch fill?
     
  11. Craig Payne

    Craig Payne Moderator

    Articles:
    8
    The rationale is: More medial wedging/skive etc --> indicated when the STJ axis is more medial --> a higher arch profile is going to be pushing right on the axis (not very comfortable), hence prescibe more arch fill to lower arch profile.
     
  12. efuller

    efuller MVP

    The above data makes sense if you think about how much the medial heel skive will actually change STJ position. In my experieince the medial heel skive doesn't change the position of the STJ that much. It is probably not enough to supinate the foot enough to decrease the arch pressure.

    There is another way to look at this. In smaller feet, with higher amounts of skive you will be shaving off enough heel to effectively lower the arch height. (draw a frontal plane slice of a heel. Make the 2 mm cut then the 4mm cut then the 6 mm cut and draw a line that is 15 degrees from the horizontal through the deepest point of the cut. You will see that the highest point of the heel is changed when it is skived. You can shave off enough of the bottom of the heel so that this will effectively lower the arch of the cast.

    I get around this by adding plantar expansion plaster to the lateral side of the heel. You can either raise the medial side or lower the lateral side. I don't think it makes any difference in the varus wedge effect at the heel and it may make a difference in the finished arch height. There are many ways to get very close to the same finished shape.

    Eric
     
  13. markjohconley

    markjohconley Well-Known Member

    I misunderstood it as a positive correlation between heel skive and arch height completely forgetting the 'hidden' variable, STJ axis transverse plane thingy
     
  14. RobinP

    RobinP Well-Known Member

    Eric makes a good point about the magnitude of the correction being relative to the size of foot. A 6mm skive(which I would consider to be quite aggressive)will be quite different in terms of orthosis reaction force in a size 4 ladies foot as opposed to a gents size 14. Similarly, arch infill or perhaps arch removal(in the positive model) will be quite different.

    In that respect, the TOP principle negates this arbitrary assignment of linear measures, if my understanding of it is correct.

    What is the latest on the Megalab BTW?
     
  15. Craig while comfort is key,

    what do you do when Navicular drop is a problem and a medial skive is prescribed, but we also need to increase the "push" under the navicular?

    Also by just looking at the transverse plane translation of the STJ axis and using more arch fill you are then not looking at the sagittal plane translation of the STJ axis.
     
  16. I play around with the inclination angle of the skive as well, be great with your set up, I´m jealous.
     
  17. Ian Drakard

    Ian Drakard Active Member

    This has been knocking around in the back of my skull for a while now- have you got a bit more info about how this was conducted Craig?
     
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