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Cast Corrections - Rearfoot

Discussion in 'Biomechanics, Sports and Foot orthoses' started by joejared, Dec 15, 2008.

  1. joejared

    joejared Active Member


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    As an attempt to keep the proper focus of scanners in the scanner thread, perhaps we could also discuss other stages of cast correction, keeping the focus first on the mechanical implimentations as they applied, old school, and variations of each method. While this thread would be of more interest to labs, I think it important to keep up with the times, and possible topics, intrinsic(plantar surface), extrinsic(bottom surface), accomodations, (addition or removal of material). From a developer perspective, I personally prefer to discuss cast corrections from the pespective of a postive mold, but that isn't critical as long as it's clear what is being discussed. For example, I learned of the kirby skive by watching John Burney of Ultrex labs actually file into the positive mold at a given angle and depth. Dr Kirby would be the best to describe this accomodation but I'm sure there are other ideas worthy of mention.
     
  2. Freeman

    Freeman Active Member

    Before doing rearfoot casts mods, I take a set of cailpers and measure weightbearing heel width to determine the exact amount of expansion per foot. They are not equal in many cases. I measure to the millimeter.

    If you apply a varus or valgus rearfoot post and do expand appropriately, that is laterally, 50/50 as I call it, or medially then your patient may very well not sit into the heel cup but rather on the edge and it will both negate the correction you intend and also feel uncomfortable. I wish I could give credit to who showed this to the PFOLA conference I attended, but I have to say I can't remember. You must also take into consideration the top cover material and whatever soft tissue supplement you might be running from the heel forward, in terms of heel expansion.
    Best wishes
    Freeman Churchill
     
  3. efuller

    efuller MVP

    I suggested this measurement in
    Fuller E, Hogge JD Measurement of the expansion of the calcaneal fat pad upon
    weightbearing. J Am Podiatr Med Assoc 1998 Jan;88(1):12-6

    A student suggested this research project to me, but I can't recall his name now. The point of the article was that you should measure fat pad expansion even if it was symmetrical, because some people had symmetrical expansion beyond the average. The purpose is to have the heel cup wide enough to prevent irritation at the rim of the heel cup. I saw Larry Huppin present this idea at one PFOLA conference.

    On other modifications, I often use a plantar lateral expansion instead of a medial heel skive. Apologies to those who've heard me tell this story before. When I first heard about the medial heel skive, I ran down to the lab to try it on casts of my feet without reading the paper. I carved a very small area and it created a painful device that hurt right where the skive was. I then thought that you can either raise the medial side of the heel cup with a skive, or you can lower the lateral side of the heel cup to achieve the same wedge effect in the heel cup of the orthosis. I'm still wearing the devices that I pressed that day.

    The medial heel skive is easier and faster for someone inexperienced in the techniques. However, the plantar lateral expansion maintains the arch height. As you grind plantar heel, you can grind enough of the original cast to lower the effective arch height. This is not hard to adjust for, if you are aware of the issue. Of course, you can do a plantar medial expansion, just as you can do a lateral heel skive, if you want to increase pronation moment.

    Cheers,

    Eric
     
  4. joejared

    joejared Active Member

    My thoughts are based on a cad/cam based system, so assume I'm thinking it through like a computer processing a request. I hope I can be clear at the same time. For your product requirements, an actual absolute heel width is required, unique per foot. Often, labs use a percentage increase to meet this requirement, but the good news is at least for my application, both methods are supported, an absolute heel/forefoot width as well as percentage or absolute increase over the anatomical reference points that represent the heel, or forefoot points. The only negative I see is that more often than not, those that can set widths usually set them as a pair, but orders such as you describe could be customizable easily, either as seperate orders with seperate prescriptions, or splitting orders with the same orthotic using relative additons and absolute additions. While the rearfoot expansion varies from foot to foot, is the relative expansion in a non-weight bearing cast vs semiweight bearing comparible from left to right, from a percentage value, or would left right widths be something that you'd rather provide the labs, precisely? It might warrant a small bit of research, to see if there are constants based on the original non weight bearing dimensions vs weight bearing dimensions for starters. For future reference, one lab wants to have both methods of scan per patient, so this may well be a a bit of research I can determine while he plays.

    For my own application I'm asking because I can treat each orthotic individually, perform weighted averages, assign absolute widths, absolute additive widths, and relative additive widths, and am trying to determine if I need to add absolute left/right rearfoot/forefoot widths.

    I'm sure the goal of any orthotic is to match to a foot first, and not to the shoe it goes in, at least from discussions I've had on the subject. For deep heel cups, the top cover is obviously more of a factor, but easily accountable using an absolute width, or absolute additive dependent on top cover materials. Generally, these heel width modifications are automatically factored in, based on heel cup depths. This topic has caused me to consider if it would be practical to calculate the 'takeoff' angle, plotting an imaginary line laterally and medially from where each side terminates, and using that angle to make the expansion a bit more precise. Depending on the steepness of the angle, it would be a simple matter to calculate just how much to add, based on a top cover thickness and calculating thickness at that angle. At near vertical, the effect would be 100% of thickness, whereas dress style orthotics with almost no heel cup would automatically be negligable. Careful, you're making me think. ;-)
     
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