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How much skive to prescribe?

Discussion in 'Biomechanics, Sports and Foot orthoses' started by footdoctor, Jan 5, 2008.

  1. footdoctor

    footdoctor Active Member

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    Hi all.

    Following on in concept from simon's post regarding extrinsic rearfoot posting angles.

    Does anyone have any rationale for the degree/mm of skive that they prescribe in there orthotics?

    Commonly most lab prescription forms that I have looked at come in increments of 2mm.(2mm,4mm,6mm and 8mm)

    How do we know when to prescribe 2mm,4mm,6mm ...........

    Could anyone who prescribes a set degree of skive to be included in there orthotic design scientifically justify its use???

    Do I use experienced guess work? yes I do.

    What about the rest of you???

    P.s sorry about the terrible english and terminology but i'm knacked........!!!!!!:boohoo:

    kind regards

    Last edited by a moderator: Jan 5, 2008
  2. Craig Payne

    Craig Payne Moderator

    Re: how much skive to prescribe

    Generally, the greater the force needed to supinate a foot (supination resistance), the greater the skive or rearfoot wedging needed.

    The hypothetical rationale is that the harder the foot is to supinate, the greater the lever arm that the foot orthotic needs on the medial side of the STJ axis. If we resolve all the forces in the rearfoot to one orthotic reaction force (ORF) vector, then this resolved vector will be more medial if a skive or medial wedge is presnt (ie the ORF will have a greater lever arm on the medial side of the STJ axis) .... so if the force needed to supinate the foot is greater, we hypothetically need that greater lever arm for the orthotic to overcome that supination resistance force.

    As for the scientific rationale to back up the hypothetical rationale, will get back to you in a few months.
    Last edited: Jan 5, 2008
  3. Admin2

    Admin2 Administrator Staff Member

    Last edited: Jan 5, 2008
  4. Bruce Williams

    Bruce Williams Well-Known Member

    Re: how much skive to prescribe

    From my anecdotal observations;

    I use the AMFIT digitizer to take my digital casts. The interesting thing regarding the use of medial skives from the foot/casts I see onscreen is that the more lateral arch height there is, in general the less I need to medially skive the device.

    Now, I / we can cast in a higher lateral arch height to a point. In general, the lower the lateral arch height, the more adbucted the FF on the RF, the more you will need to increase the # of mm, or degrees, of the medial skive. You may want to lengthen the skive anteriorly on the device as well.

    So, other than having the patient try to supinate their foot from a midstance position, or utilizing Craig P's Strap system ;), I would also guage the degree of lateral arch height and FF abduction as compared to RF abduction.

  5. Stanley

    Stanley Well-Known Member

    Re: how much skive to prescribe


    I am a little confused as to what you are saying.

    The definition of skive is to remove material. Classicially, if a rearfoot post was applied to an orthotic to hold it in neutral, some material was removed on the medial side of the post to allow for 4 degrees of normal pronation. It doesn't seem that this is what you are saying.


  6. Craig Payne

    Craig Payne Moderator

    Re: how much skive to prescribe

    I took the first question from footdoctor to mean the medial heel skive (material taken off the cast) as originally described by Kevin Kirby, not the material skived of the post to allow for motion.
  7. Stanley

    Stanley Well-Known Member

    Hi Craig,

    Thanks, now I can follow along.


  8. Craig Payne

    Craig Payne Moderator

    Re: how much skive to prescribe

    I actually think we may be doing the same thing! The force to supinate the foot (which is the criteria I use) is ~ 1/3rd related to the position of the STJ axis .... those with a more medial STJ axis (higher force to supinate by my criteria), tend also to be maximally pronated and would have more ff abduction etc (your criteria) ....
  9. Bruce Williams

    Bruce Williams Well-Known Member

    Re: how much skive to prescribe


    there is no doubt in my minde we are doing the same thing! :drinks

    Now if you would just start to adopt my advanced descriptive terminology, we could communicate so much better! ;)

    I think that this is where casting failure comes into play, to a point. DFion of the lateral column keeps or brings the cuboid in the same plane as the navicular and eliminates the positional advantage of the Peroneus Longus to stabilize the medial column.

    Further, PFing the medial column in your cast while still DFing the lateral column will not fully correct this positional discrepancy either. You have to get the cuboid to be inferior to the navicular at heel strike and into midstance or your medial column will collapse or be unstable adn lead to MTJ compensations.

    I find I can block some of the STJ medial position thru my change in this casting technique, but not always. Medial Skives definitely help, as do lateral FF wedges, 1st ray c/o's etc.

    Try to keep up from now on Craig!:drinks
  10. Scott:

    When you speak of the "skive" I am assuming you are referring to the medial heel skive which I originally described over 15 years ago (Kirby KA: The medial heel skive technique: improving pronation control in foot orthoses. JAPMA, 82: 177-188, 1992). Since I also use and have written about the lateral heel skive, then it would be helpful to be a little more specific with your terminology when referring to these type of modifications in future posts.

    The medial heel skive was originally described as being angulated 15 degrees from the transverse plane being varied by the depth of the skive (i.e. 2, 3 4, 6 mm). The deeper the skive, the larger is the surface area of the orthosis heel cup that is made into a varus plane and also the greater will be the medial heel cup force on the plantar foot. There is no reason why the angle of the medial heel skive cannot be varied, but I chose 15 degrees since, via trial and error, this angle seemed to produce the best results in my patient's orthoses when I was originally experimenting with the modification back in 1990.

    As far as rationale, I suggest that clinicians use the medial heel skive any time that extra subtalar joint (STJ) supination moment is desired in their patient's orthoses in order to relieve the patient's symptoms or improve their gait pattern. I most commonly use 2 - 4 mm medial heel skives but will also occasionally use 6 mm medial heel skives in severe flatfoot or in children's flexible flatfoot deformity. I always combine the medial heel skive with a minimum medial arch fill in the positive cast and with inverted cast balancing of between 2-8 degrees in order to increase the medial arch height of the orthosis also. In addition, the medial heel skive should be used with a firm rearfoot post and with deeper heel cup depths than normal. I will order a 16 mm heel cup with a 2 mm skive, a 18 mm heel cup with a 3-4 mm skive and a 20 mm heel cup with a 6 mm skive to prevent lateral heel cup irritation from the deeper skives.

    As far as scientific justification, how about this as justification for the medial heel skive:

    1. Used in over 3,000 pairs of orthoses over the past 17 years in my practice to successfully treat symptoms caused by excessive STJ pronation moments.

    2. Used by hundreds of podiatrists and other foot-health practitioners in over 10 countries throughout the world to treat symptoms caused by excessive STJ pronation moments.

    3. Modelling of STJ moments using STJ axis location/rotational equilibrium theory (Kirby KA: Subtalar joint axis location and rotational equilibrium theory of foot function. JAPMA, 91:465-488, 2001) supports the mechanical actions of the medial heel skive.
  11. robcox

    robcox Active Member

    Hi Kevin, when you mentioned: "I always combine the medial heel skive with a minimum medial arch fill in the positive cast and with inverted cast balancing of between 2-8 degrees in order to increase the medial arch height of the orthosis"- do you mean that if writting a prescription you would request both the scive and a rearfoot varus post? If not can you please clarify!?


  12. Rob:

    I do include a rearfoot post with nearly all foot orthoses I order with medial heel skives. However, possibly you don't understand my other terminology:

    Minimal medial arch fill: A minimum thickness of medial expansion plaster is added to the positive cast of the foot (i.e. a minimum thickness of plaster is added to the medial longitudinal arch of the positive cast) so that the medial longitudinal arch of the resultant orthosis is higher than normal.

    Inverted balancing position: The positive cast is balanced so that the calcaneal bisection is inverted to the ground when the balancing nails are added to the forefoot of the positive cast so that the orthosis will be made around a positive cast that is more inverted to the ground than normal. A positive cast with an inverted balancing position will increase the height of the medial longitudinal arch, decrease the height of the lateral longitudinal arch and increase the forefoot varus and/or decrease the forefoot valgus correction in the resultant orthosis.
  13. markjohconley

    markjohconley Well-Known Member

    Re: how much skive to prescribe

    Good day Bruce, Admin has got me rereading this thread (plus others on medial heel skives) and I got stuck on this,
    as I thought a heel skive, whether medial or lateral, acted upon the posterior plantar calcaneal surface only. I can't understand to what anatomical (osseous) structure would an anteriorly extended skive work? Hope that makes more sense than a few of my latest efforts (English was by far my worst subject) Thanks and a merry christmas, mark
  14. Re: how much skive to prescribe

    Mark, by removing plaster from the cast you are effectively increasing pressure at the foot-orthosis interface at this point- in theory anyway.
  15. efuller

    efuller MVP

    Re: how much skive to prescribe

    There really isn't an osseous structure for an anteriorly extended medial heel skive to push against. Distal to the medial calcaneal tubercle you start getting into a lot of soft tissue that is not really designed to bear weight. In doing anatomical dissections I've seen as much as an inch of soft tissue between the skin of the medial arch and the inferior surface of the navicular and cuneiforms. Yes, if you increase pressure on the soft tissues of the medial arch you will shift the center of pressure more medially (which increases supination moment.) The trick is applying enough force in this location to shift the center of pressure without causing discomfort. Arch discomfort can lead to different muscle firing patterns that could include increased posterior tibial muscle activity that will help hold the STJ in a more supinated position. (This can be a good thing in something like sinus tarsi syndrome and can be a bad thing in posterior tibial dysfunction.) Increased muscle activity is not a generically bad thing.


  16. Bruce Williams

    Bruce Williams Well-Known Member

    Re: how much skive to prescribe

    I have to agree with Eric's remarks above. I use the AMFIT system and the skive is a built in automatic modification, only if you assign a numerical value to the size or degreee of the skive. I will sometimes lengthen the skive to match the patients arch height at the midfoot along the medial column. I find this gives more control for many patients, but I do agree it sometimes can be uncomfortable.
  17. joejared

    joejared Active Member

    Over the years, I've designed a few different skive methods, first for the Manager of Ultrex, defining an angle and depth and producing a wedge tangent to the surface, 2nd, for M-Tech, prescribing an angle and lateral starting postition, and finally, Dr. Kirby's documented method, specifying depth only,. From a technical perspective, assuming the latter, and assuming you have a defined angle you're trying to kick the heel over, you could calculate the skive depth as

    w=heel width
    angle= angle of skive
    depth =1/3w*tangent(angle).

    Another point in skive design that could be obvious is that the absolute minimum heel cup height for any skive of a defined depth is at least twice that of the skive itself. (similar triangle) Because of fill, however, it should be somewhat higher to provide adequate blending (fill) into the heel cup. For CadCam applications, care should be taken in producing a kirby skive not to punch through on the deeper side of the skive, but this is only a minor concern on skives exceeding material thickness. When this occurs, and prior to wasting materials, occasionally a 1 mm or so heel lift is added.

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