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Need for precision/accuray in orthotic prescription

Discussion in 'Biomechanics, Sports and Foot orthoses' started by obeywan, Jan 30, 2006.

  1. obeywan

    obeywan Welcome New Poster

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    <ADMIN NOTE: This topic has been split of from the thread on the mid-tarsal joint>

    Thank you very much for the responses to my question. Once a clinician has studied and understood the implications of this issue, (and I understand fully the necessity for research related accuracy of biomechanical knowledge), how can this be applied to the patient? Is the prescription of orthoses, for example, such an accurate science that it can reflect this development in knowledge, and can this information be used in other ways in the clinical setting besides orthotic prescription?
    Best wishes
    Last edited by a moderator: Jan 31, 2006
  2. Craig Payne

    Craig Payne Moderator

    I think thats the $64 000 question and we do not really know the answer. Vested interests would want you to beleive that orthoses prescription is that precise and accurate ... at the end of the day the jury is still out.

    What we do know is that for a certain % of the population, it does not seem to matter what you put in the shoe they get better....just look the the symptomatic improvement in the placebo groups of the RCT's (I see this happening all the times as a researhers). Then there is a certain % of the population, it does really matter what you put in the shoe - it does need to be accurate and precise, especially if it is to change the specific parameters in the foot that need to be changed to achieve an outcome (I see this all the time as a clinician where the nth degree does count).

    The problem are (and we are working on it with no end in sight), are how big is each % and how do you pick which one is which in advance? .... I got absolutly no idea. :eek:
    Last edited by a moderator: Jan 30, 2006
  3. I have spent the last twenty years of my life trying to educate podiatrists on both the art and science of treating mechanical injuries of the foot and lower extremity. I am sorry to say, we are not to the "accurate science" phase of orthosis prescription. However, on the other hand, I don't know of a single branch of medicine where the best clinicians don't use some art along with science to achieve the best therapeutic results for their patients. So, why are we desiring podiatric biomechanics and orthosis therapy to be an accurate science?? This certainly is a desirable goal, but not probably a realistic goal in my lifetime and probably not a realistic goal within the lifetime of anyone reading this posting.

    The information gleaned from the knowledge of foot and lower extremity biomechanics, and neuromuscular science in general, can be used by any intelligent clinician to improve the well-being of the patient with musculoskeletal pathology. It can help the patient walk and run and play sports more comfortably and efficiently, help them prevent further injury, and help them heal faster from injury. What more could an healthcare professional ask for from a specific type of knowledge?
  4. Ian Linane

    Ian Linane Well-Known Member


    Thanks for the above answers. I think the difficulty with "accuracy" beyond the above is that the foot multitasks according to demand and environment. So an accurate type of device in one context for one individual may not be appropriate for that same individual in another context, even assuming that was possible. Add to that the variations in stiffness during a day etc.

    A well thought out approach and device together with soft tissue work or joint mobiisations / manipulations may well be the provider of some kind of "accuracy" for an individual - if that makes any sense?

  5. Phil W

    Phil W Welcome New Poster


    I work for an orthotic manufacturer and specialise in CAD orthoses. I recently performed a small study where I went through a process of making orthoses accuratley to a cast - by accurate I mean 0.1mm tolerances. Normal expansion were then added as standard. The amount of returns for adjustments increased by tenfold compared to my normally manufactured devices. The non-compliance of these devices could not readily explained but the following possibilities were identified -
    1. Was the cast accurate in the 1st place?
    2. If the above was not an issue, how relevant is one impression of the foot to how the foot actually functions - foot/orthoses interaction?
    3. A quick test of 10 of the devices made was done by comparing the cast (Non-weight bearing) against a fully weight bearing in-shoe cast. The discrepencies between the dimesnsion, shape etc of the feet wasin some cases very large.

    The question I have is how much of the responsibilty is the labs and how much the practitioners and how much is totally un-avoidable?

    Any opinions?

  6. Craig Payne

    Craig Payne Moderator

    Phil - Welcome to Podiatry Arena

    We already know the huge variability that exists between clinicians in taking a cast: Vivienne Chuter, Craig Payne, and Kathryn Miller: Variability of Neutral-Position Casting of the Foot J Am Podiatr Med Assoc 2003 93: 1-5 .... what we do not know is if that vairability is an issue or not. Given that most patients with foot orthoses do get better, it may not be an issue - I suspect it comes back to what I said above - in some it will matter and in some it will not ---- the problem is we do not know which is which and how to pick them in advance --> so therefore make every cast a good one.

    What is a good cast? I do not know (I used to know) --- we have been served very well for a very long time by the netural STJ and locked/pronated MTJ method and have the textbook descriptions of what makes it a good cast .... but we now know that you do not need to necessarily alter rearfoot motion to get an outcome and that the foot does not function around STJ neutral .... so more thought needs to be given to the positioning (eg it could be that the rearfoot should be in such a position that it lowers the force to establish the windlass mechanism - we do know that the position for each individual is different and in some, it is STJ neutral).. Having said that I still currently cast clinically in STJ neutral with MTJ pronated....but we evaluating other positioning in experiments.

    CAD/CAM offers incredible accuracy, but if the negative model (plaster cast, foam impression or optical scan) is not where is should be, then nothing more than a very accurate representation of the wrong position has occured.

    The solution is know your lab. Know what you are getting when you tick a certain box (and know that that lab's interpretation of what you are asking for will be different to anothers --- even different plaster technicians in the same lab may interpret it differently)

    For eg, one is plaster fill in the arch. Some labs and technicians add more than others as their "standard" - the more you add the greater chance the orthotic may not work, but the orthotic will be more tolerated by the patient. The less arch fill, the greater the chance the orthotic will work, but the greater chance the orthotic will be less tolerated by the patient ---- its a trade off and you need to know what your lab is doing. Labs that tend to get the crappier casts, tend to add more plaster to reduce their remake rate.

    I think labs have a huge responsibility in educating clients which one they are (more or less plaster fill as standard) and more labs should be returning the crap casts rather than try and compensate for them. Having said that, does it matter? ... in a lot of cases it may not matter (thats why many patients get better when a crap cast is taken and a crap orthotic is made from it), but in a lot of cases it does matter.

    At least with a CAD/CAM lab, when I ask for a 1.2mm modification of something, I will get a 1.2mm modification everytime I ask for it....we just need to work out if that is important or not :confused:
    Last edited by a moderator: Feb 1, 2006
  7. PF 3

    PF 3 Active Member

    WE have been using the FAS (Foot alignment system) for the last 2.5 years. For those who haven't heard of it (Think only Australian pod's using it at the moment) it is a system for taking corrected weightbearing casts.
    In short it works by the pt standing on two foam pads with plaster bandage wrapped around their feet/ankles.

    The pads are able to adjust so the pt is standing in AOG & BOG (can also adjust heel height). THe rearfoot is then pushed into the desired position by turning screws which tilt a platform under the rear of each foam pad. Essentially you just keep applying more force to the rearfoot till it moves to the desired postion. Once the rearfoot is postioned, the forefoot is then pronated by two screws forcing a plate under the forefoot to pronte the forefoot maximally.

    THe casts all come out with some form of Heel Skive (the size and angle is dependent on the amount of force applied to the foot by the machine)

    The beauty of this system is that no cast modification is required by the lab, as the pt is in weightbearing already. The cast produced by the practitioner is exactly what they get back. Heel cups always fit and the lateral borders of the orthotic match the lateral border of the foot (i.e you can't use the lat border of the orthotic as a ruler) It means far less skill is required by the manufacturer as the pressing and grinding of the orthotic is certainly more science than art (compared to plaster work anyhow)

    It's not perfect yet: The rearfoot can only be titled in the frontal plane (no different from tilting a Pos cast though). Because of the size of the pads, the force applied by the machine is much greater than the actual orthotic, which means you have to overcompensate when casting knowing the orthotic won't do as much work as the machine will.

    Anyone else in Oz using the FAS?



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