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What lies beneath? Foot orthotic stability in shoes

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Simon Spooner, Apr 10, 2011.


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    All, I just got back from presenting at the 6th Congresso Nacional de Podologia in Porto Portugal (thanks for your hospitality). On the Journey back I got to thinking about the orthosis-shoe interface.

    The fundamental purpose of a foot orthosis is to transmit loading from the body through it's structure to the shoe and so to the ground.

    If we take the first diagram, let the green and red areas be the rearfoot post of a device, and the light blue area be the shoe. It occurs to me that if the net force vector from the foot above is transmitted through the orthosis in the green, central part of the rearfoot post, then there will just be compression between the orthosis and shoe.

    But if the net force vector from the foot above is transmitted eccentrically through the orthosis in either of the red areas of the rearfoot post, as in the second diagram, there will be a tendency for the orthotic to rise up away from the shoe at the other end. So, in the second diagram the right hand side of the device will rise away from the interface with the shoe, unless a counter moment is available and of sufficient magnitude to prevent this. This should come from friction between the heel counter of the shoe and the outside of the heel-cup of the orthosis.

    Next time you take an orthosis out of a shoe, take a look at where the rearfoot post has "bitten-in" to the shoe and compressed the material at the orthosis interface. Often it will be deeper on one side. Yeah.....

    Then think about why the net force vector from the body might be eccentrically loading the interface between the orthosis and the shoe...
     

    Attached Files:

  2. Re: what lies beneath? Orthotic stability in shoes

    Simon:

    I remember Dr. John Weed lecturing in the early 1980s at CCPM on looking inside the shoe to see whether the orthosis wear pattern on the shoe insole at the anterior edge and rearfoot post areas of the orthosis was more indented medially or laterally to gain insight as to how the orthosis was functioning inside the shoe. I believe John Weed taught us this information in our 2nd year of podiatry school (1980-1981).

    More insole indentation medially was thought to indicate that the orthosis was resisting excessive pronation forces and more insole indentation laterally was thought to indicate that the orthosis was resisting excessive supination forces from the foot. In fact, I believe I remember John Weed saying that one of the reasons he preferred an orthosis with an intrinsic forefoot post versus an orthosis with an extrinsic forefoot post was so that he could see the anterior shoe insole wear patterns more clearly.

    Even though this is a 30 year old concept, I still believe it basically is true and could be developed further using modelling techniques.
     
  3. Re: what lies beneath? Orthotic stability in shoes

    Kevin, thank you for your reply. It seems to me that we have two situations which may load the orthosis-shoe interface eccentrically. One might be that the foot is pronating or supinating on top of the orthosis, this should shift the centre of pressure either medially or laterally respectively at foot-orthosis interface and could lead to a thrust line which tends to create instability at the orthosis-shoe interface. Or, that the foot orthosis shifts the centre of pressure at the foot-orthosis interface without kinematic change. Yet this too, may create instability at the orthosis shoe interface.

    If we have eccentric loading through the orthosis to the orthosis -shoe interface (lets say this is a medial centre of pressure between the orthosis and the shoe), then this should tend to cause the lateral portion of the rearfoot post to rise away at the lateral aspect of the orthosis-shoe interface, this should then create increased pressure at the lateral aspect of the foot-orthosis interface, shifting the centre of pressure laterally here. This may well stabilise the orthosis at the orthosis-shoe interface, but will potentially undo the desired effect of the orthosis to increase external supination moment and may actually drive further pronation of the foot.

    So, assuming we want to increase external supination moment acting at the rearfoot, it seems that we need to shift the centre of pressure medially at the foot-orthosis interface beneath the heel, without driving the centre of pressure too far medially at the orthosis-shoe interface. Otherwise, we get a self-stabilising effect. Rather, we actually want to direct the thrust line through the rearfoot post of the orthosis as far laterally as possible, since this would tend to cause the medial aspect of the orthosis to rise, shifting the centre of pressure further medially and enhancing the orthotic effect.

    Question: has anyone studied centre of pressure at the foot-orthosis interface and centre of pressure at the orthosis- shoe interface simultaneously? Nice little study, for anyone with in-shoe.... This would give us an idea of the thrust line through the orthosis. It's at times like this that I want someone to give me an f-scan to play with for a week or two.... anyone, anyone.. Norman Murphy, anyone? Oh to be rich enough to own one. If you own one, go ahead , do the research, pretend it was your idea.

    Frustrated of Plymouth
     
  4. Re: what lies beneath? Orthotic stability in shoes

    In fact, I actually think that more insole indentation medially should be seen as a sign that the orthosis is failing to resist excessive pronation forces and vice versa. That is, more medial indentation is an indication the orthosis itself is everting at the orthosis-shoe interface under loading and relying upon the shoe to provide the additional necessary reaction force to achieve static equilibrium. Which begs further questions of the dichotomy of orthoses into "shank-dependent" and "shank-independent"- right? Since both are ultimately dependent upon the shoe to provide the necessary reaction force at the foot-orthosis interface:drinks ;)
     
  5. Re: what lies beneath? Orthotic stability in shoes

    Two things come to mind here

    - softer "shank dependent devices will not normally cause the indentation - which I guess will come from the different stiffness of the device added to the greater surface area - therefore pressure is spread over the whole device.

    -preferred motion pathways - I see this indentaion is some patients not others maybe this can be explained by preferred motion pathways of Nigg
     
  6. Re: what lies beneath? Orthotic stability in shoes

    Comes down to how much work is being done on each structure . If the orthosis is more compliant than the shoe, the shoe will do more work on the orthosis than the orthosis does on the shoe, if the orthosis is stiffer than the shoe it will do more work on the shoe.
    Naaaargh. It can be explained by physics, without any need for voodoo.
     
  7. Re: what lies beneath? Orthotic stability in shoes

    I see that - gives you something to think about.......


    I digress canĀ“t beleive this is 17 years old now - http://www.youtube.com/watch?v=-Fz85FE0KtQ
     
  8. Re: what lies beneath? Orthotic stability in shoes

    Not quite as simple as that... but close enough...
     
  9. Boots n all

    Boots n all Well-Known Member

    Re: what lies beneath? Orthotic stability in shoes

    Most modern footwear is force lasted and was never designed to sustain a shell/shank independent orthosis and this is were we see most of the these orthosis twisting in the shoe, leaving a more medial impression for a pronator.

    Add to that the common type of force lasted footwear does not have a heel counter that goes below the feather edge, making for a weak support system.

    Simon are you seeing the same in a shank dependent orthosis ?

    More importantly are you seeing this in a cement lasted footwear were the firmer insole material is used, a real shank and a solid heel stiffener that goes below the feather edge?

    The shell/ shank independent orthosis are designed to work with or for the foot, but the designers forgot that it has to work with the shoe also to get the best results.
     
  10. Re: what lies beneath? Orthotic stability in shoes

    You see variation in the depth of the orthosis "biting" into the shoes in everything from Goodyear welted Loakes, to carpet slippers. It is usually less obvious with shank dependent devices, but I have seen it with these too. As Mike pointed out, with the shank dependent devices the pressure is generally lower at the orthosis-shoe interface since the contact area is higher. And as I said, there is also the issue of the relative stiffness of the device versus the inside of the shoe. If the shoe is relatively stiffer than the device, the device will compress more than the shoe during loading, if the device is relatively stiffer than the shoe at the contact area, the shoe will tend to compress more than the device.
     
  11. CraigT

    CraigT Well-Known Member

    Sorry Simon- maybe I being a bit slow, but I am not sure about what you mean by 'transmitted eccentrically'... can you explain further?

    I would have thought that these factors would be quite minor compared with the fact that there is still the body weight of the individual on top of the orthosis, as well as the design of the orthosis to create an increased orthotic reaction force on the medial side...

    The stiffness of the orthosis is also significant as (assuming the forefoot and rearfoot base of the device is balanced) it dictates how the load is distributed to the shoe. If the device is flexible, then a direct force on the heel part of the orthosis is primarily trasmitted to the footwear at that point (the heel), whereas if the device is more rigid, it will have a also significant amount of force also being applied at the forefoot edge at the same time (which is generally also wider and therefore has a longer lever arm). I believe this can have a significant bearing on how much force an orthosis can effectively apply and its resistance to compression.

    I think the compression of the orthosis into the shoe with a rearfoot posted shell is more due to more force being transmitted to a smaller area, and the flexible device will have less force applied to a greater area- less compression.

    Am I making sense or just rambling on...?
     
  12. off centre if you like, specifically outside of the central 1/3 of the contact area.
    It's not minor. The stability of the orthosis is key to the generation of orthoses reaction forces at the foot-orthosis interface. It is the loading from body weight which the orthosis has to withstand, if it unstable and rotates within the shoe under loading this will impact on the biomechanics.
     
  13. CraigT

    CraigT Well-Known Member

    Ok- I think I understand you now.
    In essence you could be talking about the effect a medial heel skive modification has? Or do I still not get it?

    Agree. Absolutely agree. However do you really think that "friction between the heel counter of the shoe and the outside of the heel-cup of the orthosis" is significant? Rather, more significant than other factors?
     
  14. I think with an eccentric centre of pressure lying outside of the middle third of the orthosis-shoe interface, there will be a tendency for rotation of the orthosis- equal counter moment has to be supplied from somewhere or it will rotate.... As I said yesterday, this could be from the friction on the outer surface of the heel cup and heel counter of the shoe, or it could be from the foot itself, in which case it will tend to shift the centre of pressure more laterally at the foot-orthosis interface, potentially negating some of the effect of the device, (if it is designed to increase supination moment).
     
  15. What might be interesting to look at Intrinsic vĀ“s extrinsic posting as well

    IĀ“m not sure of the maths but if the medial forefoot of a poly device digs in 2 mm with the length of the lever arm is must have quite a big effect on the magnitude of vector provided from a medial skive device.
     
  16. efuller

    efuller MVP

    Re: what lies beneath? Orthotic stability in shoes

    An observation and a comment.

    When I first started using varus wedge effect heel cup orthotics I would have people stand on the unposted device. I was amazed at how much more flex there was in the orthotic than with a symmetrical heel cup. My explanation was that there was a more medial center of pressure at the foot orthotic interface and a more lateral center of pressure at the shoe orthotic interface. That creates a force couple that will cause frontal plane flexion of the orthotic.

    When you use a rearfoot post there is a lot less frontal plane flex of the orthotic shell. Simon, in your diagrams you are assuming that the shoeo-orthotic interfaces stay in the center and the foot orthotic force is moved more medially. My observation above would support the idea that shoe orthotic force moves medially (posted device)with the medial shift of center of pressure at the foot orthotic interface. This would not "destabilize" the orthotic in the shoe. However, it would increase the force on one side of the rearfoot post and not the other. If the shoe had the same resistance to force on both the medial and lateral sides of the rearfoot post then it would be logical to assume that the area where the forces are higher would sink into the shoe more. So, it may not be a case of the lateral side lifting up (medial heel skive), but the medial side sinking more. It makes a good case for using dual density midsole shoes when you need your orthotic to create higher anti pronation moments.

    Kevin,
    John Weed taught those same concepts in 1984-5 and my recollection is the same as yours. I agree that the impression in the shoe is evidence of where the orthotic is applying higher pressures to the foot. (Deeper impression = higher forces)

    Eric
     
  17. Re: what lies beneath? Orthotic stability in shoes

    No, I'm not. I'm assuming that they both move together.

    Same difference, if the medial side has gone down, the lateral side is effectively higher. This should tend to reduce the varus wedge effect of the rearfoot post and decrease the supination moment exerted by the orthosis. If it's sinking and moving under loading, it isn't stable.
    I'm playing with dual density rearfoot posts at the moment, since not everyone wants to wear running shoes. But also because an analysis Dave Smith and I have been working on (far more Dave than I) seems to suggest that using a single homogenous material for the rearfoot post should not change the force distribution at the foot-orthosis interface, regardless of shape- more on that story later :morning::eek:.

    For those following, I've scanned in a couple of pages from a great little book called Structures by J.E. Gordon, let joint AB be the orthosis shoe interface. Since the orthosis and the shoe are not joined together, the tensile stress at B in the fourth diagram would not exist, rather the orthosis should start to separate away from the shoe at B, or sink into the shoe at A unless another "compressive force" was exerted on the B side of the orthosis-shoe interface.
     

    Attached Files:

  18. Re: what lies beneath? Orthotic stability in shoes

    Or the shoe should sink into the orthosis at A if it is stiffer than the orthosis- in reality it will be both, but the greater deformation will occur in the more compliant of the two, until equilibrium is reached.
     
  19. N.Smith

    N.Smith Active Member

    Are you saying, that if an orthosis applies the right amount of force to the foot, putting it into its TOP (Tip Over Point), the foot will be in equilibrium, with no excessive Pronation or Supination force which causes the indentation, Medially or Laterally inside the shoe?

    Neil
     
  20. Neil, not really. I hadn't even thought about that to be honest. I was thinking about why orthoses might be unstable at the orthosis-shoe interface.

    If the centre of pressure at the orthosis-shoe interface remains exactly in the centre of the rearfoot post, then there should be equal distribution of compressive stress at the orthosis shoe interface, in any other position there will be greater compressive stress on one side or the other at the interface and increased strain here too- see the picture I attached last night.
     
  21. efuller

    efuller MVP

    Then there would be no force couple causing rotation. Unless, the shoe under the higher force area deformed more.

    I've always thought the 4 degree "motion" bevel shifts the center of pressure more than just a flat rearfoot post. I've certainly seen, and felt, increased supination moment effects from placing a varus wedge on the bottom of a rearfoot post. I don't see how that does not change force distribution at the foot orthosis interface.

    Eric
     
  22. Re: what lies beneath? Orthotic stability in shoes

    Simon:

    As you know, all materials will deform when external loading forces are applied to them. The hardest materials will deform the least and the softest materials will deform the most. Part of what we see by the imprint of an orthosis inside the shoe is the difference between the indentation hardness between the orthosis and the shoe insole (i.e. dorsal aspect of shoe sole).

    http://en.wikipedia.org/wiki/Hardness

    The harder the orthosis and the softer the insole, the larger the indentation left by the orthosis into the shoe insole. If the shoe insole is made of plastic and the orthosis is made of plastazote #3, then there will likely be little shoe insole indentation regardless of how the orthosis and foot is functioning inside the shoe. If the orthosis is made of polypropylene and the shoe insole if made of 3 mm soft EVA, then the orthosis will create quite a significant indentation on the shoe insole.

    In addition, if the orthosis is everting inside a shoe then it is possible that the orthosis is designed properly for the patient but the the shoe sole is too soft for the patient's specific foot mechanics, body weight and activities. If the orthosis is "wearing evenly" inside the shoe, this may simply be more a function of the hardness of the shoe insole and hardness of the orthosis being identical so that they both are undergoing compression deformation at the same rate.

    Likewise, in the foot with a an increased medially located center of pressure (CoP), this will tend to evert the shoe sole and in the foot with a laterally located CoP, this will tend to invert the shoe sole over time. When the orthosis is added inside the shoe, if the CoP under the orthosis is located medially, the indentation from the orthosis will be located more medially and if the CoP under the orthosis is located laterally, the indentation from the orthosis will be located more laterally. However, if the CoP under the orthosis is located under the midline bisection of the orthosis, then shoe insole indentation from the orthosis will become more even.

    I have never been a big fan of spending much time analyzing the inside of the shoe for shoe insole indentation patterns from the orthosis as Dr. Weed suggested to us nearly 30 years ago. I think there are so many factors that cause different insole wear patterns (as I have outlined above) and I am not convinced that depth or unevenness of shoe insole indentations from the orthosis is diagnostic of anything more than the difference in hardness of orthosis and shoe insole and the prevailing shifts of CoP under the orthosis that are dependent on the foot function, body weight, and patient activity types and levels.
     
  23. Ben

    Ben Member

    Simon,

    In regards to the diagram, evem though the compressive force at B will be less, there surely is still going to be a compressive force on the lateral aspect of the device, and with the transfer of load through to 'foot flat' happening relatively quickly, is the device going to move 'significantly'?

    Also with wear on the insole of the shoe, my experience is that the medial side absolutely compresses - to a point. But even with lots of wear, once that compression has reached its maximum then it becomes a relatively stable platform for the device to operate on.

    Ben
     
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