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The perfect position to "cast" the foot in

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Simon Spooner, Dec 17, 2010.


  1. Members do not see these Ads. Sign Up.
    All, something for the weekend...

    If there is a singular "perfect" position to cast (take a model of) the foot in for the manufacture of foot orthoses. What is it and more importantly why, oh why?

    For feet with pathology related to excessive pronation moments, I've been thinking about casting the foot at the point that supination resistance is overcome. Craig called this "tip-over". I've called this weightbearing casting position the "POTS position" (point of tip-over sign =POTS)...but I can see some potential flaws in this approach, not least that a static weightbearing assessment of the POTS position doesn't necessarily correlate to the dynamic position.:bash: But I thought I'd put it out there for your consideration.
     
  2. We're working on the assumption of the perfect position for that particular patient right? There obviously can't be one position which is right for everyone.

    So the "tip over point" is the point at which the supination moment exerted by the orthotic is equal or more than the residual moment.

    But what if that is not the point which gives the theraputic effect required? In terms of, for example, tibialis posterior pathology. Lets say the untreated "demand" is 300 AUs. Overcoming the residual moment will cause the visible kinematic change but that won't reduce the demand to zero. Say, 100 AUs. If the the muscle will be pathological at anything above 50 AUs then thats not enough. Or, if the muscle will be pathological at anything above 150 AUs then the orthotic won't need to exert force greater than the residual moment to have the desired effect!

    And as you say, its not as simple as how much supination moments an orthotic exerts. There is also the way it does is, the range it operates in, the points at which it operates during the gait cycle, the effects it has on the other joints of the foot, etc, etc etc. We're not "single axis theorists after all ;)
     
  3. Riddle me this. Take a completely flat pes pancakus foot with ever so much floppyness. Lets say that the residual moment is the same whether the navicular is on the floor, or 1 or 2 cm off it on an orthotic. So in either state the pressure under the nav is the same. Hypothetically.

    Will a 1 cm high arch and a 2 cm arch will exert the same supination moment?

    Will they therefor have the same effect on the foot?
     
  4. I don't know what an AUs is?
     
  5. Craig Payne

    Craig Payne Moderator

    Articles:
    6
    Simon - I think you know my answer!
    No. We used to think it was the "STJ neutral and the MTJ, locked and loaded", but I think we know enough now to know that that is probably not the right position for everyone and even when we cast in that position, the orthotic does not actually hold them in that position anyway.

    The "perfect" position will be that position that allows you to deliver the prescription variables that are needed for that patient. The problem is that we do not yet know enough or have deveploed enough criteria to help determine that postion.
     
  6. Are there some "casting" positions that won't allow you to deliver the prescription variables that are needed for certain patients then? Can you give me some examples?
     
  7. In the same foot, if the navicular is in differing position will the residual pronation moment be identical?
     
  8. No. But hypothetically. If it was. Say the other structures are not called into play and the axial position does not change by a substantial degree.


    Arbitary Unit.
    And that it will be different for every patient. And the position is only of the surface anatomy. Etc.
     
  9. Oh, OK. The S.I. unit for moment is the Newton Metre. In my opinion, it does no-one any favours in the longer term by pretending its so simple that you take it so far beyond the point of over-simplification that it becomes a complication. I understand physics, but I hadn't got a clue what you were talking about there. Just a point of view and maybe something to think about.
    http://en.wikipedia.org/wiki/Moment_(physics)
     
  10. That'll be "no", then.
     
  11. I know. But I don't know what order of magnitude supination moments happen in. So rather than say 100 and find it will be between 1 and 5 I thought I'd make it arbitary. Sorry, will stick to proper terminology next time.

    Indeed. Its a hypothetical construct to try to communicate my point, that being that there is a danger implicite in considering orthotics as a balance between supination resistance vs ORF supination moment. There may, hypothetically, be two positions wherin the supination resistance is the same, but meeting them with the same ORF in position A might produce a different effect than meeting them with the same ORF in position B.

    For me, the "tip over point" you describe in the OP is not neccessarily a quality of the cast position. The tip over point is a matter of kinetics, the position is of kinematics.

    Another example then. A fully flat flaccid foot with a very bony nav (little / soft tissue to deviate) on the ground. I pul a 3mm EVA arch under the nav with means it is now 3mm off the ground. I have thus affected a kinematic change. Have I crossed the "tip over point?" Must have done, because there has been a change in position (however slight). Might not be in the STJ, but I will probably have changed a joint position somewhere. Will that device be theraputically effective...?
     
  12. Another point, say you take the perfect tip over point cast, what do you do with the friction vector and change in stiffness of the medial arch, which may mean your perfect tip cast is infact an over correction.

    I think this is a similar point robert is making.

    Ps RI can you speak to your make guru bob and stop it snowing
     

  13. I should have perhaps added more about the casting technique, the POTS cast (maybe that should be "tip over point" (TOP) cast- Indeed, lets call it the TOP cast, apologies for the name change, I only dreamt this up yesterday :eek:) is taken with the patient standing, the casting jig pushes up beneath the medial longitudinal arch with a known force/ pressure, thus soft tissue is compressed and the force/ pressure required to arrive at the TOP is known (kinetics). So we now know how much force our orthoses must provide in this area if a kinematic change is sought. We should probably want our orthoses to achieve equilibrium with the foot at about this position (perhaps just beyond it). So you could calibrate the devices stiffness to achieve this given the known reference geometry obtained from the TOP cast within an finite element model.


    Putting a 3mm arch pad under the navicular does not necessarily change the position of the calcaneus / subtalar joint, i.e. induce TOP and increase it's supination. It depends on the relative stiffness of the pad vs the foot and how much "play" there is in the talo-navicular joint. By using the TOP cast technique you know exactly how much force is required to bring about the kinematic change you desire. So TOP cast is first and foremost a calibrated kinetic casting technique.

    Off to the patent office... Any commercial interest?

    Sorry Rob, you won't be able to use TOP cast with foam boxes ;-)
     
  14. I'll have a word But he may have forgotten his password :eek:.

    Nope. You lost me.

    I can see the deal in terms of calibrating orthotic stiffness in any given position. I can't see how having a value for that, defines the optimal position. I also can't see how the TOP point is neccessarily the best amount of force to apply!
     
  15. The TOP cast jig measures the force / pressure being applied to the medial longitudinal arch to bring about kinematic change at the time of casting. Thus it is a calibrated kinetic casting system.

    Depends on how you view residual pronation moment. In terms of pathology in structures resisting pronation, is residual pronation moment:
    A) good?
    B) bad?
    C) neither?
     
  16. Craig Payne

    Craig Payne Moderator

    Articles:
    6
    Nope.... as I do not have the answers!

    In a perfect world we will have a range of "general" positions that the rearfoot could be in and a "general" range of positions that the forefoot should be in relative to the rearfoot (in all 3 body planes) .... all we need are indications for which position(s).

    The one thing that MASS theory has open my eyes to is that there really is a range of positions that can be used, we just need to get the proper indications for each one.

    ...but it hard to shake and get over the mindset of STJ neutral and MTJ locked and loaded ... as we tend to use this as the 'norm' and judge different positions relative to that, which is probably not a good way to look at it.
     
  17. And at the position of casting. The force needed to bring about kinematic change will be different depending on which position you have the foot in surely (leslie neilson tribute gag cue). Put the foot in an inverted position and you will reduce the amount of force needed to get to the tipping point. I would imagine therefore that a higher arch insole would not need to be so stiff as a lower arch one.

    Or are we talking about the amount of force needed from the foots resting position? In which case the TOP position for a flat foot, will be pretty much flat would'nt it?

    Sorry if I'm being dense, I just don't get it!

    I'll take D) depends.
     
  18. Lets go back a step Robert. Why do you perform a supination resistance test, what does it tell you clinically?

    Define residual pronation moment, what is the clinical significance of residual moment?
    And, I didn't offer you a D) ;)

    Yes, but the orthotic in this case would also have to apply a very high force- right? So how might that be achieved? There will be a certain force above which the device would not be tolerated. And if we pre-angled the rearfoot during the TOP cast with a varus wedge, the force in the medial longitudinal arch required to achieve the TOP should be lower- right?. So if we measure the force required to TOP without the rearoot post in-situ, and then re-measure the force to TOP with the rearfoot post in-situ, we have now quantified the kinetic effect of the rearfoot post too. Moreover, if we cast the foot with the trial post positioned between the plaster bandage and the jig, we have also created an intrinsic rearfoot post within the TOP cast. Similarly by positioning trial wedging posts under the forefoot, their kinetic effects can be quantified and intrinsic posts simultaneously captured within the cast. I'm wasted here.;):D

    It's a method of casting that quantifies the kinetic effects of changes in the geometry of the device- You get it now?
     
  19. N.Smith

    N.Smith Active Member

    I know exacly what your talking about! I sent a private message also.

    Cheers

    Neil
     
  20. efuller

    efuller MVP

    Lots to think about in this thread.

    I'm not quite sure I understand the definition of tip over point. Is it the point that supination resistance becomes zero or the point at which you would sprain your ankle if ground reaction force was applied? (This would be the position where the foot is inverted enough so the center of pressure is medial to the projection of the axis.) If supination resistance force is an upward force applied to the arch of the foot, how is this overcome?

    Does TOP top MASS?

    If the foot is really easy to supinate, is the height of the arch of the TOP orthotic lower than the standing arch height of the foot?

    The big question for me in high arched orthotics is what is your starting point. You've got a foot standing on the ground and it is a certain arch height. Now you put an orthotic on the ground and a person steps on it. The orthotic was made from a cast taken in TOP or MASS with minimal arch fill. As foot is lowered on to the device, what position will it reach in equilibrium and why?

    More questions than answers

    Eric
     
  21. I think it is about overcoming inertia, isn't it? If we pushed up under the navicular and plotted a load/ deformation curve then the gradient of the curve would increase to a certain point, as the soft tissue compressed under the navicular, and then the gradient should steeply increase up until the point when the foot begins to move (the TOP); at which point we should see a change (lowering) of the gradient- right? So if we cast the foot at the very TOP position, the net moments acting about the STJ from external moment is equal to zero at this point (assuming no muscular contraction). So we then have a position of the foot and know the loading required in a specific areas of the foot to remove all residual pronation moment.
    Theoretically TOP could be MASS in some individuals. But it's not very likely, how many people stand in MASS? The point of the TOP cast is that it eliminates the residual external moments acting about the STJ axis and provides a quantification of the orthotic reaction forces required to achieve this. MASS doesn't do this.
    No, because even in a foot that is relatively easy to supinate, the soft tissues beneath the arch of the foot should have been compressed during the weightbearing casting process, and even in a foot with low supination resistance there will still be some inertia. Theoretically you could have two feet which reach their TOP at the same arch height, but one takes much more force than the other to the TOP, so the orthotic should necessarily be stiffer in the foot with the higher resistance- right? You could employ a pronation TOP cast for pathology related to excessive supination moment: we could apply a load, say under the cuboid, until the pronation TOP was achieved and cast in that position, with knowledge of the force required at this point to achieve this TOP.

    The foot and orthotic reach equilibrium when the reaction force of the orthotic is equal to the foots force applied to it. The necessary orthosis reaction is generated by the stress caused by the action of the loads within the orthosis material, and by the ensuing strain within the elements of the orthosis structure. I'm guessing that ideally we would want the equilibrium to occur somewhere near to the TOP position (yet with dynamic loading within the equation). Theoretically you could cast a foot in say Neutral or MASS have both the devices reach equilibrium at the TOP position by manipulating their stiffness characteristics, but since you need to know the TOP position and the forces required to achieve it, why not just cast in this way to begin with?

    Thanks Eric.
     
  22. efuller

    efuller MVP

    Ahh, now I think I understand what the tipping over point is. It is the point at which you first see motion when applying an external supination moment in the arch. I don't think that this is just inertia. It's what Kevin calls residual pronation moment. I have to admit that I have never really liked this term. What is happening is that there is a pronation moment from ground reaction force. To reach equilibrium there must be an internal supination moment. As your fingers gradually compress soft tissue they are gradually reducing the external pronation moment. So, with enough pressure in the arch you eventually get an external supination moment from your fingers and the ground and then the STJ will supinate, increasing pressure latterally, and then equilibrium will be achieved in the new position. Essentially your fingers are shifting the center of pressure under the foot.

    As you increase external supination moment, there will be a decrease in internal supination moment. This is different than inertia.

    With additional force applied beyond the point of first movement, when there is motion of the STJ, you have to start taking into account the changing position of the STJ axis with STJ supination. A good question is whether the motion of the STJ will shift the center of pressure farther than the STJ axis is moved with the motion. I bet it will be different for different feet.


    The above was in response to the question of whether the TOP position will be lower than the standing arch height in easy to supinate feet. There are some rare feet that have to have constant peroneal activity to prevent their feet from becoming maximally supinated. These feet should not have pressure in their arch.

    A really good question is whether or not TOP would be the position that most people would find comfortable. In looking at a lot of feet, it is my impression that most of them have there axis in a position where there is some pronation moment to the ground. I think this makes some evolutionary sense in that if there is too little pronation moment from the ground there will be a chance of trauma from a "twisted' ankle. Too much pronation moment from the ground and you get too much stress in structures that create supination moment. So, it would seem that a little bit of pronation moment from the ground is a good thing. If the TOP position, is a position, or an orthotic shape, that creates a situation where there is no pronation moment from the ground, then this might be a little too supinated. However, that is a prediction that should definitely be tested.

    Eric
     
  23. If we did achieve the TOP cast would there also not be a chance of pathology from not allowing for pronation. If its calibrated to start the foot at the TOP then it would not allow any pronation, ?.
     
  24. RobinP

    RobinP Well-Known Member

    I don't get it( no surprises there!)

    I've read this thread several times now and I'm struggling to envisage how one achieves/measures the TOP. I think I understand the load/deformation curve principle for the pressure under the navicular. The point at which a kinematic change is affected by the TOP jig will be close to the TOP? It is simply a point of equilibrium.

    What I don't understand(if my understanding is correct so far) is why that particular position is the least pathological. When I use a supination resistance test for a pronation moment related pathology, it gives me an idea of the force required by the orthosis to reduce the external pronation moments. But this is a continuum, not a specific point.

    I take account of the fact that STJ location alters and that, at certain points in the gait cycle, the force required to effect the external supination moment will be greater than it is statically.

    If you have "callibrated" your orthosis to the TOP which is a static position, how will this perform, assuming that the TOP is the least pathological position, as the orthosis deforms under load and it ability to apply the orthotic reaction force changes?

    If my initial understanding is flawed then clearly the rest of this lengthy, poorly articulated post is redundant.

    Regards,

    Robin
     


  25. It's simply a position of equilibrium in which there is no residual external pronation moment.

    Don't know if it is the least pathological position, but if there is zero residual pronation moment, it should mean less internal supination moment is required, thus stress in the supinatory tissues should be reduced. Provided that this places said tissues within their zones of optimal stress (ZOOS) the patients symptoms should resolve, assuming that other tissues are not concurrently placed outside of their ZOOS.
    It is a continuum, but the TOP should be the point of greatest supination stiffness. Thus it should also be the point at which the highest orthotic reaction force should be required to bring about a kinematic change.
    Indeed predicting dynamic loading is near impossible at present, since it will be activity and environment dependent. You could model the relationship between TOP and various other predictors to build a multiple regression model that should allow predictions of loading comparable with those exerted during walking or running etc. If you look at some of the research on leg stiffness during running, they make the assumption that loading is 2.5 times body weight. Which is a guess, but probably a reasonable guess.


    If you took the TOP measurement in single limb stance this should be a closer prediction for walking than bipedal stance. Similarly with a supination resistance test, most people perform this in static bipedal stance...
    In terms of modelling that into the orthosis via FEA it can be done and thus you can then manipulate the orthosis stiffness to bring about equilibrium at the TOP.
     
  26. Griff

    Griff Moderator

    I dabbled with this when doing my supination resistance study, but found that most individuals had a single leg stance which was so 'unstable' that there was far too much of a concentric muscle contraction of Tibialis Anterior (which I presumed may interfere with the accuracy of what I believed I was measuring).

    From memory Craig compared supination resistance in single Vs double limb support and there was not a significant difference? (Correct me if I'm wrong Craig)

    Ian
     
  27. Phil Wells

    Phil Wells Active Member

    Simon

    There is something bothering me about a casting technique that assumes that kinetic change = patient compliance.
    I assume that most orthoses have to deliver a force that is moderating dynamic variables and not the static ones that the TOP uses.
    For example ,two identical feet may have equal degrees of ROM but the velocity at which they arrive at there end ROM can be significantly different.
    The analogy is like stepping off a 12" step slowly or quickly - the difference in biological response and potential injury is significant.
    This velocity at which the foot reaches end ROM can sometimes explain why some orthoses are not tolerated by the patient.

    Why don't you stop messing about and apply your intellect to a digital, dynamic , 3d camera platform that tracks the foot over a series of steps and gives total ROM. Then using FEA, cross referenced with subjective clinical test, FEA of patient footwear and estimated GRF, prescribed a FEM of the insole required. Mill it, test it and away you go.

    Easy peasey, lemon squeezy!!! Weekend work at most!!

    Cheers

    Phil
     
  28. All casting techniques are pretty much static, unless you have your patients walk across a foam box. I don't think I ever said kinetic change = patient compliance, but there probably is a relationship between pressure at the foot orthosis interface and comfort. Obviously unless we are designing an orthosis for people to stand around in, it is the dynamic loading which is key. We just don't have any good models of dynamic loading and it's correlates to static assessments that might be useful in the manufacture of foot orthoses yet do we? Using the techniques described in the TOP cast, we might at least begin to quantify the potential kinetic effects of our orthosis on the static foot at the time of casting, which other casting techniques currently allow for this?

    It's early days for the TOP cast, until I've done a few and tested them for comfort it's all speculation. Like Eric, I think it's worthy of further study.

    I will ask this again though: at the moment when a supination resistance test is performed what does it tell you clinically and moreover, exactly how does it influence the orthotic produced?
     
  29. This fella's on the right lines for the first part of the problem:
    http://mi.eng.cam.ac.uk/~qp202/

    Been following with interest some of the hacks of the x-box kinetic- it just doesn't seem to have the resolution though.
     
  30. Hey Spooner just noticed that you have over taken Kevin as the person with the most posts, are you MVP now.
     
  31. It's clearly all about quality not quantity Mike.
     
  32. Maybe there should not be a MVP but 2 VP´s , or Highly Valuable Poster HVP. Gets my Vote

    Thanks for a educational and entertaining year Simon. Merry Christmas to you and Yours.
     
  33. joejared

    joejared Active Member

    1 = "To the moon Alice"!
     
  34. Simon:

    Now that I'm getting old and over the hill, and you finally have more posts than I do here on Podiatry Arena, maybe Ed will start calling you "God". Something to look forward to....:rolleyes::cool:
     
  35. Thanks for your kind words Mike, but I'm really not bothered about a title here, that is not my mission drive. I am pleased that you feel that I may have helped with your laughter and learning this year. You have certainly helped with mine. Merry Christmas.

    Some of my friends already call me "Dog" and others have said that I am your "bulldog" as Huxley was to Darwin. I'm sure Ed has other names for me. God bless you Kevin for the inspiration, enthusiasm and understanding that you continue to provide us with. If you stopped ruining your body with this crazy running stuff, perhaps you wouldn't feel so old and over the hill, old man ;):drinks

    Peace on Earth and goodwill to all men
    http://www.youtube.com/watch?v=J8OYvHPpGDY
     
  36. I think "the big dog" has a nice ring to it. "

    Wrote that on the side of your longbow. Which, BTW, is still sat in the kitchen because the delivery was canceled by the snow and there is no point in trying to rebook it at the moment. You'll have to wait til next year.

    I'm closing in on Craig on the post count... But at present rate Michael will overtake all of us within a year or two!
     
  37. Phil Wells

    Phil Wells Active Member

    Simon
    These guys have already done 4D capture -
    http://www.di3d.com/products/4d_systems/
    Also if we are talking about the ideal casting set up, how about a base platform system that is capable of mimicking the effect of the Rx on the TOP system.
    Something like the Dynastat could apply frontal and sagital plane variables which would allow the force measure to be more appropriate to the Rx needed.

    Phil
     
  38. Cool. Already doing the rest (more to come on that).
     
  39. Phil Wells

    Phil Wells Active Member

    Simon

    Here is the list what the supination resistance test means to my Rx-

    Low resistance
    1. Low lateral heel cup to reduce irritation
    2. More flexible shell in the MLA only with 1st ray additions from the Navicular forward.
    3. Very supportive uppers of the shoes to 'hold' the foot on the device.
    4. Lateral column support and strength in the shell - to reduce the potential for over inversion

    High resistance
    1. Medial flange and increased strength to the MLA
    2. Medial Skives
    3. Propulsive phase posting - depending of pathology and stjt axis
    4 Heel raise - I try to accelerate the COP past arch (tenuous but seems to help with arch irritation issues)
    5. Footwear must have some sagital plane rigidity - strong shank and solid sole unit with a good pitch + 8mm is ideal.

    Just my two penneth

    Cheers

    Phil
     
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