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Shank dependent vs shank independent foot orthotics

Discussion in 'Biomechanics, Sports and Foot orthoses' started by admin, Dec 26, 2007.

  1. admin

    admin Administrator Staff Member

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    I am grateful to Kevin Kirby and Precision Intricast for permission to reproduce this July 1998 Newsletter (you can buy the 2 books of newsletters off Precision Intricast):​


    There are a large and diverse range of materials being used today in the construction of functional foot orthoses. These materials range from thermoplastic sheets (e.g. polypropylene) and the composites (e.g. graphite) to thermoplastic foams (e.g. Plastazote). Every year there are new materials being introduced which are claimed to have superior properties. Unfortunately, most of the claims, like most advertising claims in today's world, are not based solely on objective analyses.

    In order to appreciate both the beneficial and detrimental characteristics of each orthosis material, one must have a good understanding of how each material works to resist the deforming forces from weightbearing activities. One of the turning points in my thought process regarding orthosis materials came while attending a lecture by Dr. Michael Burns in the early 1980's on foot orthosis therapy ("Functional Forefoot Extensions and Accommodative Orthoses" in, Kirby, Kevin A. Foot and Lower Extremity Biomechanics: A Ten Year Collection of Precision Intricast Newsletters. Precision Intricast, Inc., Payson, Arizona, 1997, pp. 75-76). Dr. Burns made the point that the orthosis material by itself does not solely determine whether the orthosis will be "functional" or not. He claimed that the shape of the superior surface of the orthosis in combination with its resistance to deformation is what determines how well an orthosis will help prevent abnormal motions of the foot.

    Dr. Burns classified orthosis materials into two types, shank independent and shank dependent. Shank independent orthoses use materials within their construction which have enough inherent structural rigidity to resist deformation without needing to contact the shank, or insole board, of the shoe. In other words, shank independent orthoses do not need a filler material in the medial or lateral arch area of the orthosis to resist deformation under the body's loading forces. Examples of shank independent orthosis materials would include polypropylene, acrylics, graphite laminates, fiberglass laminates and stainless steel.


    Figure 1. If a beam of wood is supported on both ends with a weight in the middle of the beam, the upper layers of the wood beam are subjected to compressive internal forces, or stresses, and the lower layers of the beam are subjective to tensile stresses. Located somewhere in the middle of the beam is a layer, free of stress, separating the zones of compression and tension stresses called the neutral plane.

    On the other hand, shank dependent orthoses are made of materials which do not have enough inherent structural rigidity to resist deformation by themselves without needing to be congruous, or in contact, along its entire plantar length with the shank of the shoe. Plastazote, ethylene vinyl acetate (EVA), Birkocork, leather, and other thermoformable foams are all considered to be shank dependent. Orthoses made from shank dependent materials will function differently depending on the shape and contour of the shank of the shoe since these orthoses rely on their congruency with the shank of the shoe to resist deformation during weightbearing activities.

    Another important distinction between shank independent and shank dependent orthoses is that they utilize very different physical properties to achieve their goal of redirecting forces on the plantar foot. Since the shank independent materials utilize a relatively thin sheet of material which does not contact the shank of the shoe in the arch area of the orthosis, then the material resists deformation by relying on its resistance to bending forces.

    To demonstrate how a material resists bending forces, let's take an example of a beam of wood with a weight resting on the midsection of the beam and being supported only at each end of the beam (Fig. 1). Within the layers of wood which constitute the beam there are internal forces occurring called stresses. (Stress is defined as an internal force within a material which is described as the magnitude of force per unit cross-sectional area of the material.) In the upper layers of the beam of wood, there exist compressive internal forces, or stresses. In the lower layers of the beam, there exist tensile stresses. Somewhere in the middle of the beam there is a stress-free layer where the transition from compression to tension occurs called the neutral plane of the beam. The internal structure of a shank independent orthosis, whether it is made of a thermoplastic, metal or graphite laminate material, will be subjected to compressive and tensile stresses, just like the wood beam, due to the bending forces placed on its longitudinal arches during weightbearing activities. If the material has a high stiffness, or high elastic modulus (elastic modulus is defined as the relative measure of the stiffness of one material to another), it will be more resistant to bending forces. If the material has a low stiffness, or low elastic modulus, it will be less resistant to bending forces and, therefore, more flexible (Ozkaya, Nihat and Margareta Nordin: Fundamentals of Biomechanics, Equilibrium, Motion and Deformation. Van Nostrand Reinhold, New York, 1991).

    The physical characteristics of shank dependent materials, however, are quite different from the shank independent materials. Shank dependent materials do not resist bending forces effectively since they have a low stiffness or a low elastic modulus. However, this does not mean that the shank dependent materials do not make good orthosis materials, if used appropriately. Shank dependent orthoses must rely on their ability to resist compressive forces in order to resist deformation by the foot since they are designed to contact the shank of the shoe throughout their whole plantar length. For example, in an Ultra-Lite orthosis, made of Plastazote #3, the orthosis resists collapsing at the medial and/or lateral arches of the foot by its ability to resist compression of the material within the arches of the orthosis. Therefore, a correctly constructed shank dependent orthosis made of Plastazote #3 is subjected to minimal tensile stresses since it is subjected nearly solely to compressive stresses during weightbearing activities.

    As a result of the different physical properties of the materials used in orthosis construction, each material will react differently to different modifications to its structure and will react differently to each foot depending on the individual's body weight, foot type and activity. A basic knowledge of the physical principles governing the behavior of the materials used in foot orthosis construction is a very important consideration in the decision making process of trying to design the best functional foot orthosis for mechanically based pathology of the foot and lower extremity.

    [Reprinted with permission from: Kirby, Kevin A.: Foot and Lower Extremity Biomechanics II: Precision Intricast Newsletters, 1997-2002. Precision Intricast, Inc., Payson, AZ, 2002, pp. 44-45.]
    Last edited: Dec 26, 2007
  2. If I was to write this same newsletter today, instead of in June 1998, I would place more emphasis on the fact that shank dependent materials rely on their resistance to compression stresses as a method by which to prevent orthosis deformation inside the shoe, while the shank independent materials rely not only on their resistance to compression stresses but also to their resistance to tension stresses. I believe Simon Spooner's recent finite element modelling of certain foot orthosis shell designs, that he so nicely presented at the PFOLA seminar in San Diego last month, would bear this out.
  3. As ever, some important concepts are discussed within this newsletter. I think it is important to question whether one should differentiate the materials themselves into "shank dependent" and "shank independent" since the deformation of the orthoses which occurs is due to a number of factors, not simply the construction material.

    Obviously the load applied to the orthosis is highly significant in the resultant stresses and strains produced. A 2mm polyprop shell may deform little under the load of a 3 year old child, but would demonstrate much greater deformation in the shoe of an adult. The activity undertaken will also influence deformation since the ground (orthoses) reaction forces in running are clearly much higher than in walking. So the same orthosis, in the same shoe, of the same individual will exhibit very different stresses and strains during these two activities. As Kevin has noted in other newsletters the shell geometry is also significant, with a lower arched device deforming more than a higher arched device with all other factors constant.

    Another interesting point raised by this newsletter is the influence the shoes have upon the orthoses. Kevin intimates that the geometry of softer devices may be directly altered by the shoes they are worn within. We know that the geometry of the device will influence the orthoses reaction forces produced. Yet patients can still find relief of symptoms by wearing such devices in a wide variety of their chosen footwear. It's worth noting too that the mechanical effects of stiffer devices will also be influenced by the shoes they are worn in.

    So where does this leave us? The classifications of: rigid, semi-rigid, flexible, or alternatively: shank dependent and shank independent are awkward to define. For example, how much deformation is there in a semi-rigid device? Moreover, is it even necessary to categorize devices in this way? What effect does the stiffness of an orthosis have on the treatment outcome?

    I wanted to add to the final point I made here earlier hence this edit. If we look at the published literature regarding the efficacy of foot orthoses, not one (to the best of my knowledge) of the trials has attempted to control the stiffness of the orthoses employed. That is they have used the same thickness shell for all subjects within the trial, regardless of the subjects weight or the geometry of the orthoses themselves. And then we wonder why the results of the trials appear variable at times- we need to be more rigorous in our methods if we are to unravel the true value of these devices. If I were a PhD examiner or even a Journal reviewer and was presented with one of these trials, I would first want the author to explain to me why this is not a fundamental flaw in their research? Off my soap box now.
    Last edited: Dec 26, 2007
  4. Craig Payne

    Craig Payne Moderator

    The whole concept of shank vs non-shank dependent foot orthotics is an important one. I often ask those who use shank dependent pre-fabricated orthoses (most prefabs) a lot what they know about the concept of "shank dependency" and if they check the shank of the shoe before deciding to use a shank dependent orthotic. I have yet to find anyone who knew about the concept, let alone checked the shoe for it !!
  5. What defines a shank dependent device, i.e. what is the stiffness which differentiates shank dependent from shank independent? What about placing orthoses in shoes that don't have shanks? Are soft devices that are placed inside a shoe without a shank, still shank dependent?;) In this situation do the "shank-dependent" orthoses fail to function? Don't like it- silly terminology. I would sooner we discussed in terms of force, deformation/displacement and stiffness.
  6. Craig Payne

    Craig Payne Moderator

    I am not going to disagree with you re the terminology. The sort of people I talk to about the concepts have not even heard of the term 'shank' dependent, yet use 'shank' dependent orthotics to affect peoples health without ckecking the 'shank' of the shoe :craig:
  7. So, playing Devil's advocate, why is this important given that many patients wear relatively soft devices in a wide variety of their footwear, i.e. they change the devices from shoe to shoe, and yet still find symptomatic relief irrespective of the shoes they are going about their activities of daily living in? In what type of shoes would the therapeutic effects of such devices be "optimized"?

    Moreover, what does this tell us about how orthoses work? Given that the orthotic reaction force is to some extent dependent upon the surface geometry and stiffness and that surface geometry and stiffness of these flexible devices will be influenced by the footwear they are placed in, what conclusions can we draw from the fact that some patients find symptomatic relief using a wide variety of footwear in combination with these flexible devices?
  8. Craig Payne

    Craig Payne Moderator

    The hypothetical assumption is that a shank dependent orthotic, needs the support of the shank of a shoe. If that shank support is not there, then the orthotic hypothetically will not work (and given we have no data, then this may well be the case). If the orthotic does work, then either its a placebo or the hypothetical assumption is wrong (ie maybe the orthotic is affecting something else) (and given we have no data, then this may well be the case).
    Orthotics can only work by one of three mechanims:
    1) Reduction in forces in the tissues
    2) Placebo
    3) They are not working, but natural history of cause of the symptoms is to get better

    I think what it tells us is that despite our theoretical assumptions, as long as there is some reduction in the forces (ignoring 2 & 3 above), then there will be some reduction in symptoms in some people, some of the time .... as to if that is an optimum change (based on the theoretical assumptions and modelling), then that is another matter.
  9. :good:Agreed.

    Questions for those following:
    1) What is the fastest rate at which a tissue e.g. the plantar fascia, can heal?

    2) What factors determine the rate of the healing process?
  10. A coouple of further questions for those who have gone this far: with all this knowledge in mind, how do arrive at an orthotic prescription? That is, do we really need to worry about posting angles?
  11. The terms "shank dependent" and "shank independent" refer to the shank of the shoe, which in this context, refers to the three dimensional shape of the superior surface of the insole board/fabric that lines the inside of the shoe (or close to definition #9 below).

    I'm sure another term could be used but I always thought the terms "shank dependent" and "shank independent" were very descriptive and very helpful terms when trying to broadly classify foot orthosis material characteristics. Do you have better alternative terminology, Simon?

    As I wrote in my newsletter, the main purpose of trying to broadly classify orthosis materials into shank dependent and shank independent is to suggest that the shape of the shank of the shoe, of which there is considerable variation from one shoe to another shoe, will have a much greater mechanical effect on those orthoses that rely on the shape of the shoe shank in order to resist deformation. Think of a flat shoe with no heel, where the shoe shank is flat, versus a shoe with a 1.5" heel height differential, where there is sagittal plane angulation between the plane of the heel and the plane of the midfoot of the shoe sole. A shank dependent orthosis will have much greater change in shape under weightbearing load when placed in both of these two shoes than would a shank independent orthosis. This is where this terminology is quite valuable, in understanding that shoe geometry will affect orthosis function and the amount by which the shoe geometry affects orthosis function is very dependant on the material with which the orthosis is constructed.
    Last edited: Dec 26, 2007
  12. When I worked in the footwear manufacturing industry a "shank" was a stiffening strip commonly manufactured from metal which was inserted within the sole of the shoe in the area roughly corresponding to the midfoot- the "shank" was not the "waist" of the shoe as the encarta definition seems to imply. But hey, what do I know?

    I'd like to see us adopting the language that other disciplines such as material scientists use to describe the mechanical properties of materials. I'm not sure that the "shank terms" really do classify material characteristics of orthoses for the reasons I've aready outlined above. Moreover, as you have often pointed out previously, the adoption of such parochial terms can only serve to segregate ourselves from other professions such as bioengineers.

    I have no problems with the fact that footwear influences the function of orthoses. At what point does a shank dependent device become shank independent? At what stiffness? If we are to adopt a dichotomous taxonomy such as the one suggested surely we must have a point of differentiation? And what of an orthotic that lies directly at this point, is it shank dependent or shank independent?One more question: is a shank dependent device, shank dependent in all footwear? If you can accurately answer these questions, I should happily adopt the terms without further protest.;)
  13. Simon:

    1. A shank dependent device becomes shank independent when the longitudinal arch of the device has the ability to resist bending moments sufficiently to accomplish the goals of foot orthosis therapy without the plantar aspect of the midfoot portion of the orthosis contacting the shank of the shoe.

    2. A shank dependent device becomes shank independent when the longitudinal arch of the device has sufficient stiffness to accomplish the goals of foot orthosis therapy without the plantar aspect of the midfoot portion of the orthosis contacting the shank of the shoe.

    3. An orthosis is either shank dependent or shank independent, there is no midway point. As soon as it relies on the shank of the shoe to resist deformation, then it becomes shank dependent. However, a single foot orthosis may be both shank dependent and shank independent for a given shoe and individual. For example, a foot orthosis may be shank independent in early midstance, only to become shank dependent in late midstance due to the increased bending moments exerted on the orthosis during the late midstance phase of gait. In this case, the orthosis would be considered an early midstance shank independent device and also a late midstance shank dependent device.

    4. A shank dependent orthosis will be shank dependent in all footwear if the goals of foot orthosis therapy have not changed as the orthosis is moved from one shoe to another. Whether the foot orthosis is shank dependent or shank independent will be affected by the foot type of the patient, their body weight, the type of weightbearing activities they participate in, and the shoe shank geometry, to name a few factors.

    I have answered your four questions, Simon. Now, please answer my question from the last posting,: do you have any better replacement terminology to describe to clinicians the fact that certain orthosis materials need to rely on the contact with the shank of the shoe in order to resist deformation? I certainly realize the terminology, shank dependent and shank independent, is not perfect, such as is also the case of any terminology in use relating to shoes and orthoses. However, unless you have better terminology to replace these very descriptive orthosis terms with, then, I suspect, we are just banging our heads into the wall for now.:bang::drinks
    Last edited: Dec 26, 2007
  14. I believe that the term "shank" is generally more commonly used to describe this shoe stiffener such as a "steel shank shoe". However, it also describes the waist of the sole of the shoe.
  15. efuller

    efuller MVP

    I certainly believe that there is a therapeutic range in orthoses. Some areas of orthoses can still be effective when there is a difference in several millimeters. In other areas of the orthosis 1-2 millimeters can be the difference between pain and no pain. A medial heel skive is an area where a one millimeter difference can be the difference between pain and no pain.

    That said we can examine how different shoes effect the height of the top of the orthosis with different materials. In the medial heel most shoes will provide enough force that a very soft device or very soft post will provide some support for a skived orthosis. I can think of an exception in the old Nike air shoes where there was a hole cut in the midsole near the edge so that you could see the air bag.

    Medial arch height of top of orthosis will be profoundly affected by shoe under it. Some shoes come with an "arch support" and others are flat and others use the upper to provide some arch support. Any material that bends or compresses with force applied will move until something stops it. The height of what stops the bending will vary across shoes. There is probably a wide range of medial arch heights that are therapeutic.

    Eric Fuller
  16. To make sure I understand, let me recap: what you appear to be saying is that a given orthosis could be both shank dependent and shank independent in the same individual at different times during the gait cycle; in different shoes; or during different activities. So obviously that same orthosis could also be shank dependent in one individual and shank independent in another. And that this may or may not change the outcomes of therapy. That this is a binary variable, without units.

    So if we had a bag of orthoses on a table, with a group of individuals and a range of footwear, the only way we would know when and if each orthosis was shank dependent or shank independent would be to use a pressure sensor between the orthoses and the inside of the shoe and have the patients, walk, run, climb stairs etc. and see if there is contact between the midfoot portions of the orthoses and the shoes.

    Not sure I understand your point No. 4 though, as it seems to contradict itself: how can it be shank dependent in all footwear, if shank dependency is affected by shoe shank geometry?

    I'm sorry Kevin, I thought I had here:
    But to be more specific, I think what the shank terms are in some way attempting to qualify is the relationship which may be quantified as:

    deflection = force / stiffness

    Which could be employed to measure the stiffness of the orthosis in isolation, or probably of greater importance the stiffness of the orthosis in conjunction with the shoe-gear at a given instant in time during a functional activity.

    So I guess I would replace the shank terms with a measure of stiffness or it's inverse compliance.

    Obviously there are many more terms commonly in use by material scientists and mechanical/ biomechanical engineers which could and should be used when describing the mechanical properties of foot orthoses. Some of these can be found at:

    I'm not clear regarding your point that:
    Is it the material per se, is it the orthosis, or, is it the orthosis + the load applied?

    Also should we then conclude that in a situation such as the one you outlined in the example:
    that the deformation has been completely unresisted in this orthosis during late midstance until it contacted the shoe? And that orthoses that contact the shanks resist further deformation?

    To me these terms seem to imply that shank dependent devices work in conjunction with the shoe, while shank independent do not- this just isn't true. And since this is really about stiffness and deformation under load, surely the important factor is the net stiffness of the orthotic + shoe + supporting surface? I just don't see how the shank taxonomy helps.
    Last edited: Dec 27, 2007
  17. And this is pretty much what Karl demonstrated with his very soft "sham" orthoses.
  18. Simon:

    If I were to report in a scientific journal on the load vs deformation characteristics of an orthosis material and how that interacts with the shoe, then I would not report the numerical values of the load vs deformation as "shank dependent" or "shank independent" but would rather use values of stiffness or compliance, as you have suggested.

    However, if I was to lecture to a group of clinicians about how they may not need to use a plate type orthosis material (i.e. graphite, polypropylene) to make an orthosis that effectively resists pronation moments in a shoe and that they also may use a material that resists compression but not bending loads, then I would use the terminology "shank independent" and "shank dependent".

    This terminology contains very important concepts for the clinician that may serve as a valuable stepping stone toward them better understanding more complicated concepts such as stiffness or compliance of an orthosis in a shoe and how that depends on material, shape, body weight, activities, etc. In the 20+ years I have been lecturing using the terminology "shank dependent" and "shank independent", I have found these terms to be easily understood and accepted by those attending my lectures. Using this terminology in my lectures has allowed the students/clinicians to better grasp these important concepts that they may have otherwise not been able comprehend had I used more complicated terms. I won't likely be changing my use of these valuable terms any time in the near future unless someone can present to me better terms that are not only easily understood but also have the ability to broadly classify types of orthosis materials as to how they can function to resist deformation of the foot even though the orthoses themselves bend easily when taken out of the shoe.
    Last edited: Dec 27, 2007
  19. Boots n all

    Boots n all Well-Known Member

    Interesting debate guys, the term "Shank" is not always going to be true even though the shoe exhibits the same features/function as a shanked shoe.

    Possibly a better term might be needed or a more broader definition of the term "shank".

    Many brands have achevied this by going to a wedge type sole (shank not required) rather than a heeled sole (shank required)as fashion requires and the strange notion that a shank will make the shoe too heavy(?), most runners l see dont have a traditional shank either

    Maybe the the term "shank"...although very discriptive to me but not all, should be changed from "shank dependent" to
    "Surface dependent"
    "Cantact dependent"
    "Shoe dependent"
    or one l like the best.. leave it out
    "Dependent type"

    Of more concern to me is the destructive nature of the Independent type Orthosis.

    l have noticed over a number of years, more noticeable in January when “literally” hundreds of school children come into our store, just how destructive the polypropylene shell/ Independent type orthosis have been over the past year of wear on their school shoes and runners and how much of the "correction" has been completely lost.

    Happy New Year all !

    Q1 How are your students assessing if a shoe has a shank or not?
    Q2 Is the statement of "shank" a reference to location as well support?
    Last edited: Dec 31, 2007
  20. The function of all foot orthoses are dependent upon the footwear they are used within. That is, if we take a given device and place it in varying footwear the net mechanical effect will differ from shoe to shoe whether they are relatively stiff or compliant devices. Better that we should think in terms of shoes + orthotics, not shoes & orthotics as seperate entities. The wide variety of footwear many patients swap their orthoses into AND still find therapeutic benefit suggests that either the zone of optimal stress is broad and/or small changes may have large effects.
    Last edited: Jan 1, 2008
  21. David:

    I don't see how any of your proposed terms "Surface dependent", "Cantact dependent", "Shoe dependent", or "Dependent type", helps explain the difference between shell type orthoses and orthoses that rely on the shank of the shoe to prevent their arch deformation. All orthoses are affected by the surface they rest on. All orthoses rely on contact with the shoe sole to work. And I don't have a clue what you mean by "dependent type".:bang:

    By the way, David, if you had read the postings earlier in this thread, you would have realized that the usage of the word "shank" in "shank dependent" and "shank independent" does not refer to the stiffener placed within the shoe sole but rather that portion of the sole between the heel and forefoot region of the shoe.

    Now, whenever somebody can provide me good replacement terms for "shank dependent" and "shank independent" orthoses, then possibly I will consider a change in my orthosis terminology.
  22. This being the case, why is it necessary to differentiate?
  23. Phil Wells

    Phil Wells Active Member


    I like your definitions as they allude to which part of the plantar surface of the orthosis is in contact with the shoe.
    A typical EVA device can be in full contact with the shoe insole and be highly dependent on the shoe for it mechanical properties. A scoop can then be added to the underside of the shell and the properties changed significantly.
    By varying the size of the scoop, the ORF can be changed easily which helps with patient compliance. These reason why we do things this way at our lab is that we can then use very high density EVA's on the at risk foot and not worry about the ORF causing problems.

    However all of this is a moot point if the shoes are not working in harmony with the orthosis.

    Maybe the answer to the terminology issues is to define the the shoe/orthoses contact areas in some way - a percentage or a more qaulatative way such as full contact, partial contact etc. These would have to be agreed on by everyone - bugger I new there would be a problem!!!


  24. Bruce Williams

    Bruce Williams Well-Known Member

    Kevin and Simon;

    I actually like Kevin's terminology, as it works well for me due to the large amount of EVA devices I utilize.

    I do agree with Simon that it can be vague, and he has a point that we should try to match definitions across the board, much as Kevin has or is attempting with dorsiflexion stiffness, etc.

    On the AAPSM website we use the terminology "flexion stability" and also "torsional rigidity". I'm not trying to muddy the waters further, only showing that their are multiple ways of describing this issue regarding shoe stiffness.

    A more valid term for the forefoot and midfoot area evaluations might be flexion stiffness? Obviously I think we'd like to see less stiffness at teh forefoot adn more at the midfoot.

    Happy New year!
    PS: Craig, you need a happy new year icon now! ;)
  25. Shank dependent orthoses need to have full contact across their length with the shoe sole in order to effectively prevent orthosis deformation during weightbearing conditions. Shank independent orthoses need to only have contact at their heel cups and anterior edges (i.e. non-arch contacting) with the shoe sole in order to effectively prevent orthosis deformation during weightbearing conditions.

    Therefore, if one wanted to use the terms "full contact orthoses" and "non-arch contacting orthoses" then these terms would have similar meaning and would be acceptable replacement terminology for "shank dependent orthoses" and "shank independent orthoses". However, one should also ask the question of whether this new terminology would be any better than the current terminology that I first heard Dr. Burns lecture on about a quarter century ago?? I don't think so.
  26. Thanks for that Bruce. Actually, the terminology isn't mine. "Shank dependent" and "shank independent" were first described to me by Michael Burns, DPM, during a lecture he gave at a podiatry meeting in San Francisco while I was a 3rd year podiatry student back in about 1982 (if my memory serves me correctly) .
  27. I don't understand what you are saying here Kevin since both types of device do deform during weightbearing conditions. You appear to be saying that they don't?

  28. I'll change my wording to make my statement more accurate:

    "Shank dependent orthoses need to have full contact across their length with the shoe sole in order to effectively help prevent orthosis deformation during weightbearing conditions. Shank independent orthoses need to only have contact at their heel cups and anterior edges (i.e. non-arch contacting) with the shoe sole in order to effectively help prevent orthosis deformation during weightbearing conditions."

    In other words, I realize that all materials deform under load, even the most rigid shank dependent orthoses deform under loads. The important next step needs to be testing of various orthosis designs inside the shoe in order to produce load vs. deformation curves of orthoses which can be further analyzed. This is something that I have been recommending for over five years now, but no one has done it yet, to my knowledge.
  29. I think this is quite a difficult undertaking, while we could attach a rosette strain gauge (for those that don't know what this is, see here: http://www.efunda.com/formulae/solid_mechanics/mat_mechanics/strain_gage_rosette.cfm) Wouldn't this only measure the deformations in the areas bounded by the gauges? It would be difficult to measure deformation across the whole of the orthosis. Moreover, given the variation in the geometry of custom foot orthoses + the wide variety of shoes + the variability in loading patterns within and between individuals it would be difficult to obtain clinically meaningful data- don't you think?

    I will however be at rugby training with a chap this evening who is doing some work with strain gauges at the moment- I may bend his ear and see if I can play with his gauges :drinks
  30. Using a materials testing device , various orthosis designs could be set up into various shoe designs and then certain areas of the orthosis could be tested as to their load vs. deformation characteristics. For example, the plunger of the materials testing device (MTD) could be put on the apex of the medial arch of the orthosis, on the lateral arch of the orthosis or on the medial heel cup of the orthosis, depending on the area of interest. Shank dependent vs. shank independent orthoses could be tested. Rearfoot posted vs. non-rearfoot posted orthoses could be tested, and different shank independent materials (i.e. polyproplene vs. acrylic vs. composite) could be tested against the other. In addition, out of shoe vs in-shoe deformations of the orthosis could be tested. I don't think that a strain gauge would give you as much useful clinical information as the MTD would since the MTD would allow actual vertical displacement of the orthosis to be measured under varying load whereas the strain gauge would give you more elongation/shortening of the surface of the orthosis which, I believe, would be more difficult for the clinician to interpret.
  31. Discreet loading of areas of a device may provide a basic insight, but in-vivo loading doesn't occur like this. All sounds very time consuming to me and more easilly achieved using finite element methods. Regardless, how does this information help clinically?

    Lets say we have force/ deformation curves for the medial longitudinal arch of two orthoses of identical geometry, but manufactured from two different materials; such that one device is very stiff, the other very compliant- what you would call "shank independent and shank dependent";). The data is collected for the two devices sited in a given shoe with known sole stiffness. How do I, a clinician, use this information?
  32. I don't know of any good research ever accomplished that wasn't time-consuming. Of course, good orthosis research will be time consuming, but I believe it will produce much clinically useful data. I would just like to see it accomplished by someone since, to me, it seems like such basic research considering the vast numbers of foot orthoses that are prescribed every year throughout the world.

    First of all, this research has never been done, and should have been done twenty years ago. This is basic materials property testing of a medical device that is standardly done for all implants used within the body. This type of research will give us an idea of how different materials may respond utilizing all the parameters that are known to affect orthosis shell stiffness. And since orthosis shell deformation under load was the topic of your PFOLA lecture, Simon, then wouldn't you think that both FEA and materials testing device data of orthosis shell properties would be clinically useful? What was the clinical usefulness of your PFOLA lecture using FEA?

    As a clinician I would be interested in how different orthosis materials compared to the other regarding the medial and lateral longitudinal arch stiffnesses, how rearfoot posting affected longitudinal arch stiffness, how a plantar fascial accommodation affected longtudinal arch stiffness, how grinding down the medial arch of an orthosis affected longitiduinal arch stiffness, how placing arch filler in the medial arch affected load vs. deformation patterns of the orthosis, and the list goes on and on and on.......:drinks
  33. I guess my PFOLA lecture was supposed to demonstrate a number of things:
    1. That Finite Element Method (FEM) is now within the realms of the clinician without specialist expertise in the finite element method.
    2. That FEM can be applied to foot orthoses to help understand their behaviour under loading. Since trying to do this in-vivo is difficult and very time consuming.
    3. That the stresses and strains within orthoses are dependent upon the material selection, shell thickness and geometry, posting angles and loading patterns applied.
    4. That material failures in foot orthoses can be predicted prior to manufacture using the FEM approach.
    5. That due to 3. much of the research into foot orthoses is fundamentally flawed.
    6. That there exists a theoretical framework which suggests that surface stiffness is complicit in the aetiology overuse injuries.
    7. That there is evidence that manipulation of surface stiffness may be important in reducing injury rate/ improving performance in runners.
    9. That it has been suggested that the mechanical properties of foot orthoses may be significant in treatment outcomes.
    10. That it may be possible to manipulate orthosis design to obtain surface stiffnesses which would be potentially benefical in reducing injury rate and or improving performance.

    Whether I actually achieved any of this is a moot point.

    Could you explain please why you believe the stiffness of the medial longitudinal arch section of the orthosis is significant in positive clinical outcomes? Indeed, could you speculate as to how clinical outcomes may be improved by manipulation of orthoses stiffness in general?
  34. In those patients that have symptoms related to increased subtalar joint (STJ) pronation moments or medial forefoot dorsiflexion moments, an orthosis with increased medial longitudinal arch stiffness would be more capable of increasing STJ supination moments and decreasing medial forefoot dorsiflexion moments. However, in those patients that need more medial longitudinal arch flexibility, such as in orthoses made for side to side sports, decreased medial longitudinal arch stiffness would generally be desired to improve clinical outcomes.
  35. Thank you for this Kevin. I guess we could assume that the opposite also holds true: that in those patients that have symptoms related to increased subtalar joint (STJ) supination moments or lateral forefoot dorsiflexion moments, an orthosis with decreased medial longitudinal arch stiffness (or increased lateral arch stiffness) would be more capable of increasing STJ pronation moments and decreasing lateral forefoot dorsiflexion moments.

    That seems to be midfoot area of the orthosis covered, of course what we don't know is how stiff is the right stiffness for these areas. What about the heel cup? What do you think the significance of heel cup stiffness is? I presume this is going to be similar to the responses you gave for the medial longitudinal arch area?
  36. Asher

    Asher Well-Known Member

    Hi Simon,

    Is it accurate to say that 'an orthosis with decreased MLA stiffness is more capable of increasing STJ pronation moments'?

    I would think that varying the stiffness of the orthosis shell in the MLA will always lead to increased stiffness in that area, either to more or less of a degree. And that it can't reduce stiffness in the MLA to increase STJ pronatory moments.

  37. Rebecca:

    It would be more accurate to state the following: An orthosis with decreased medial longitudinal arch stiffness has decreased ability to increase external subtalar joint supination moments.

    Altering the moments and ultimately the motion across the subtalar joint (STJ), just like altering moments and motion around any other axis of motion between two objects, can be done by either increasing or decreasing moments in either direction of motion about that axis of motion.

    For example, how might the moments in both the supination and pronation directions be altered so that STJ supination motion is ultimately produced in the foot? To produce a net change in STJ moments that results in STJ supination motion, then either the STJ supination moments must be increased or the STJ pronation moments must be decreased. Here are many ways to produce STJ supination motion in the foot:

    1. Increase the external STJ supination moments (e.g. increase ground reaction force medial to the STJ axis with orthosis, use ankle-foot orthosis).

    2. Increase the internal STJ supination moments (e.g. increased contractile activity from posterior tibial muscle).

    3. Decrease the external STJ pronation moments (e.g. reduce the ground reacton force lateral to STJ axis).

    4. Decrease the internal STJ pronation moments (e.g. decreased contractile activity from peroneus brevis muscle).

    The same analysis then could be made as to how STJ pronation motion might be produced, by either increasing STJ pronation moments or decreasing STJ supination moments. In other words, to produce a motion in one direction, the moments in that direction do not always have to be increased. Another option to produce motion in one direction is to reduce the moments in the opposite direction of motion. This is why the concept of rotational equilibrium is so important to understand; it helps us better comprehend the many options available to us in the mechanical treatment of foot and lower extremity pathologies.

  38. Perhaps the terminology was not the best, but as Kevin has stated, it is the net moment which is key. I'd just used the opposite of the terminology that Kevin had originally used.

    How do orthoses work? The answer frequently given to this question is that they reduce tissue stress. Let's assume that this is correct. But how do they do this? What are the mechanical processes that are occuring within the orthosis and at the foot/orthosis interface under loading that enable a reduction tissue stress within the body?
  39. Asher

    Asher Well-Known Member

    Understood. Very thorough explanation Kevin!

  40. Rebecca,
    Any thoughts on the question I posed:

    What are the mechanical processes that are occuring within the orthosis and at the foot/orthosis interface under loading that enable a reduction tissue stress within the body?

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