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Depth vs angle of Medial heel skive

Discussion in 'Biomechanics, Sports and Foot orthoses' started by alwinliew, Mar 27, 2013.

  1. alwinliew

    alwinliew Member


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    Hi guys,
    I have been practising for a few years now. I am still a little confused on how do you prescrive medial heel skive. Do you use depth (in mm) or angles (eg, 10degs MHS?)
    How do the depth of the heel skive and the angle relate to each other. Is it always 15degs skive with a specific (2-8mm) depth?
    Thanks for sorting out my confusion.

    Regards

    Alwin
     
  2. alwinliew

    alwinliew Member

  3. alwinliew

    alwinliew Member

  4. alwinliew

    alwinliew Member

  5. joejared

    joejared Active Member

    YES.

    Over the past decade, 3 different labs have requested 3 different variations of a kirby skive. The first implementation was angle and depth,, the second was angle and lateral starting position, and the last was a by the book Kirby skive, trisecting the heel, filing to the appropriate depth, and applying fill on the lateral and medial edges of the device. By the book, a kirby skive is depth based only. The heel cup height should be at least twice the height of the skive, as similar triangles would prove. Typically, it's 2 1/2+ times the height of the skive.
     
  6. I first started using the medial heel skive technique in 1990. After a few hours of drawing diagrams of the technique where I planned out the shape of the modification, I went to the lab to do practice cast modifications and practice orthoses. After a considerable amount of experimentation, I decided to use depth of skive at the intersection of the medial third and lateral two-thirds of the plantar heel to be the prescription variable for the medial heel skive.

    By varying the depth of the skive I found I was, in effect, increasing the percentage of the plantar heel that was shaped into a varus angle. In other words, a 6 mm medial heel skive would create a much larger percentage of the heel cup of the orthosis in a varus attitude than would a 2 mm medial heel skive. When I tried to vary the angle of the skive I found that this would, at the higher angles of skive, significantly narrow the heel cup of the orthosis which would increase the chance of orthosis heel cup irritation. It was for this reason that I decided to use the depth of medial heel skive, rather than the angle of the skive, as the prescription variable of the medial heel skive technique.

    In addition, I set the angle of the skive at a constant 15 degrees for my 1992 paper since I thought that having only one prescription variable would simplify things for the paper in describing the technique for the first time in the scientific literature (Kirby KA: The medial heel skive technique: improving pronation control in foot orthoses. JAPMA, 82: 177-188, 1992). Of course, any angle of skive will work to some extent but 15 degrees seemed like a steep enough angle to create a mechanically effective varus angular change within the resultant orthosis heel cup without being so steep of a varus angle that it would narrow the heel cup of the orthosis, which could possibly cause heel cup irritation from the orthosis.

    There are many variations of the medial heel skive being used now 23 years after I first started experimenting with the technique. However, I have always felt that the most important part of my 1992 paper was not the description of how the medial heel skive technique should be performed, but rather was my description of the biomechanical theory behind how the medial heel skive works to increase the magnitude of external subtalar joint supination moments during weightbearing activities and how it can be used in a foot with a medially deviated subtalar joint axis to address the abnormal subtalar joint pronation moments seen in these feet.

    Hope this helps.:drinks
     
  7. efuller

    efuller MVP

    I agree that the shape is what is important. I had just started teaching at CCPM when I heard of the medial heel skive and immediately went into the lab to modify the casts, before I read the whole paper. I did the skive over too small of an area that iteration hurt my heel. After that I tried what I called the plantar lateral expansion. The medial skive raises the medial portion of the heel cup. The plantar lateral expansion lowers the lateral portion of the heel cup. For my foot this was an improvement. When I make my own casts I often do a little skive a little plantar expansion. One of the problems with this is that I'm eyballing the relative amount of varus wedge that will end up in the finished orthotic.

    Perhaps we could take a frontal plane cross section of the heel cup and look at the relative area/ volume medial to the center line versus the area/ volume lateral to the centerline and use this as a measure of the amount of heel skive. Problem: doesn't help the guy in the lab. Any suggestions for other measures. A 4mm skive in a 40mm wide heel looks a lot different than a 4mm skive in a 70mm wide heel.

    Eric
     
  8. BarryD

    BarryD Member

    I have always manufactured the skives at 15 degrees, but I find the different foot casting mediums make an accurate depth measurement problematic. A foot cast in plaster will have a different plantar heel shape to a foam impression cast. The newer foot sock casts tend to give the heel a rounder shape. If we were to use the standard method of measuring the depth for the skive on all 3 of these methods we would get 3 different skives. A more simple but perhaps not as technically accurate method is to offer a small, medium or large skive (all at 15 degrees). this also assists customers who are unsure of which depth to prescribe.

    Barry Dent

    FootCraft Orthotic Laboratory
     
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