All, something for the weekend...
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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.
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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 ;) -
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? -
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Simon - I think you know my answer!
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. -
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http://en.wikipedia.org/wiki/Moment_(physics) -
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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...? -
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 -
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 ;-) -
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! -
A) good?
B) bad?
C) neither? -
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. -
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!
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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) ;)
It's a method of casting that quantifies the kinetic effects of changes in the geometry of the device- You get it now? -
I know exacly what your talking about! I sent a private message also.
Cheers
Neil -
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 -
Thanks Eric. -
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.
Eric -
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, ?.
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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 -
It's simply a position of equilibrium in which there is no residual external pronation moment.
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. -
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 -
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 -
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? -
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. -
Sorry should have said kinect, not kinetic
http://www.youtube.com/watch?v=7QrnwoO1-8A
http://www.youtube.com/watch?v=zS5Mb-CtoZk&feature=related
etc... -
Hey Spooner just noticed that you have over taken Kevin as the person with the most posts, are you MVP now.
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Thanks for a educational and entertaining year Simon. Merry Christmas to you and Yours. -
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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: -
Peace on Earth and goodwill to all men
http://www.youtube.com/watch?v=J8OYvHPpGDY -
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! -
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 -
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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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