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Functional F/O Rigidity in CAD Systems

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Arjen, Sep 9, 2009.

  1. Arjen

    Arjen Active Member

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    Functional F/O Rigidity Specification in CAD Systems

    In-house here at Amfit we have been trying to come up with a new paradigm to define the rigidity of a functional F/O. The reason we feel the current paradigm leaves something to be desired is its subjectivity and lack of a scientificly repeatable and pretictable result due to that subjectivity.

    Our motivation is that we are trying to add a simple and predictable stiffness definition capability to our software. In the event that a method results from this effort, we will propose it to the PFOLA as a standard for the industry.

    I'd really like this community's input.

    When making conventional hand laid-up vacuum formed F/O's, a scrip may specify a desired stiffness in the finished F/O. This may be defined in terms like "flex" "semi-flex" and "rigid".

    While we may all feel we have a handle on what a rigid device may "feel" like, it is in fact a complex array of variables that result in a given stiffness in a finished fabricated orthoses. In my experience, most labs will select a material thickness in an experiential manner to try to achieve the desired stiffness.

    Stiffness in the finished F/O is dependent on at least the following variables:

    • Flexural modulus of the material used
    • Thickness of the final material
    • The arch length ("beam" length in a structural engineering context)
    • The ratio of arch length to its height. This is the concept that all things being equal, a short foot (and a short arch length) with a high arch will produce a stiffer finished device than a longer foot (with a longer arch length)

    There are other issues including the width of the foot, but they are relatively minor compared to the ones noted above.

    We are also struggling with the generic terms for stiffness since they are very subjective and context dependant. For instance: rigid in a small foot may mean something quite different than in a large foot.

    We would like to create a definition that is far more scientific than the existing one. We would also like to solicit the community's input on how to best define the parameters that would be used in a scrip.

    It could also be a derived parameter: for example it could include the patient's weight, arch length and height and the historic generic terms for stiffness to better nail down a final device characteristic.

    Any thoughts?
    Last edited: Sep 9, 2009
  2. Arjen:

    I believe we had this same discussion on Podiatry Arena some years back and I have also written about it in one of my books. Maybe someone can find the discussion in the archives.

    Stiffness of the orthosis will be relatively easy to define. All you need to do is put the orthosis on a materials testing machine, place the load cell onto a certain area of the orthosis and then determine the load vs. deformation curve in Newtons per mm of deformation which will then give you a characteristic load vs. deformation curve, with the slope at any point along the curve being the stiffness of the orthosis at that location on the orthosis. This should be done both outside the shoe and inside the shoe since EVA and plastazote orthoses which are shank dependent will behave much more differently than inside shoes with different shank contours.

    The permutations available for the orthosis testing protocol are what are going to be confusing and difficult, not the defining what orthosis stiffness is.
  3. Agreed, Kevin. Arjen, if you are using CAD/ CAM why not just drop the device models into some FEA software and model the load / deformation? This will give a more complete impression of the orthosis stiffness than the material testing approach described by Kevin, but both are valid. I presented the results of the FEA approach at the PFOLA conference in San Diego a few years back, in addition to your list above the shape and degree of the rearfoot posting applied to the orthosis shell will also influence it's stiffness. If you are using visco-elastic materials in your device, then obviously the rate of loading will be significant too. If you look at the work I linked to in the orthoses thread yesterday, you should get an indicator of the stiffness range that should improve running performance and also the correct terminology to use. In one of the studeis they used midstance and 2.3 x body weight in the calculation. The trick is in modelling the direction and magnitude of loading that would occur during running and the inter-subject variability in this. BTW I suspect orthosis stiffness is less significant in walking than in running- do you agree Kevin? You also look to calculate energy storeage in the devices.

    You can also look to quantify supination resistance (navicular dorsiflexion stiffness) and optimise the arch height / stiffness of the orthosis. Which is why it would be helpful if you could adjust the stiffness of the pins in your contact digitizer, Arjen!!!

    You could always fly me over as a consultant on this. Which reminds me, Phil Wells was interested in what I was doing with this stuff a while back- you still interested Phil? Would be nice to see my ideas put into production.
  4. admin

    admin Administrator Staff Member

    What Factors Affect Orthotic Rigidity?
  5. Arjen

    Arjen Active Member


    I'm sorry if I failed to make my goals clear. I can handle the FEA, the prescribing community cannot generally handle the FEA...and the point is that I do not want them to. I'd like to end up with a clear scrip protocol to define the desired stiffness that will allow the practitioner and lab to speak a common unambiguous language.

    Kevin...Thanks for pointing out the legacy thread. It has a thoughtful discussion on the differences in stiffness characteristics of shank dependent and shank independent devices. I'm not sure it suggests a solution to a common language in defining the desired stiffness characteristics of a shank independent F/O. Interestingly it does reinforce my contention that the prescribing community has better things to do than wrapping their brain around the FEA.
  6. Arjen:

    Actually, thanks go to me mate, Craig, who found us this thread from nearly four years ago that was lost somewhere deep in the Podiatry Arena archives.:drinks
  7. The prescribing community don't need to understand FEA, I didn't say the should. You should though. The prescribing community need to understand the significance of the load deformation characteristics of foot orthoses, and when to prescribe a stiffer or more compliant device. I doubt that the majority do. How do you determine the stiffness and moreover variation in the location of stiffness characteristics required for a given patient with a given pathology, Arjen? In other words how do you predict the orthosis stiffness that is required and where? We talked about this last week in the orthoses thread. In terms of language, I would use stiffness or load/deformation as this is the correct terminology.
  8. Arjen

    Arjen Active Member


    Ok. Now we are getting somewhere. Remember that my goal is to develop a common language that unambiguously defines the desired stiffness characteristics in a finished F/O in terms the prescribing professional can easily get their brain around.

    Let's spend a moment expanding the definition of the term you have suggested:

    Stiffness: (as you appear to be defining it) is deformation under a given load. I can accept that. The question then becomes, how do you map that to the legacy terminology of "flex" "semi-flex" and "rigid"?

    For argument's sake, let me throw out some measurements we have done here when trying to nail down this mapping. We started with a size 43 F/O with a 20mm medial arch height. We made them with different thicknesses of polypro and handed them around the facility to staff members and asked them if they would characterize them as "flex", "semi-flex" and "rigid". After they were characterized, we measured their deflection under load.

    The deflection test consisted of seeing how many Kgf were required for 8mm of deflection of the devices. The results are listed below:

    Flex ~2kgf
    Semi-flex ~5kgf
    Rigid ~13kgf

    I am not alleging a lot of science here. But it does show that the staff perceptions of these legacy categories show there is a fairly significant difference in the stiffness characteristics of these three categories.

    We are following this up with a similar test at small (38) and large (50) sizes to see what the subjective flex test that staff uses map to a finished device. For instance, I would expect that the size 38 flex will have to be made thinner than the size 43 to achieve the same staff reaction to its stiffness.

    This would suggest that a curve exists for these three categories at a given arch height. The curve for "flex" for instance would have size on one axis and material thickness on the other. What I do not know yet is how constant the kgf_for_8_mm_deflection will be across the range of sizes with a constant arch height.

    I am happy to derive these curves using a panel of expert users. I just think there should be some better way to define the three categories of stiffness. Once again I am soliciting suggestions.
  9. I understand this, I'm just not sure why you would want to create categorical data from a continuous variable, or map it to a meaningless system. The terms "flex" "semi-flex" and "rigid" are meaningless in this application since they imply degrees of deflection, deflection is load dependent thus two identical devices could be "rigid" under the foot of a small child, but "flex" under a fat lad. Regardless, the problem really arises when we reach the boundaries of the categories. Lets simplify it to say we have two categories: "stiff" and "compliant", lets pretend that the division between stiff and compliant using your numbers and system below is the force required to deform the device by 8mm and that "compliant" = 0-7 kgF and the "stiff" category is > 7.000 kgf. Does that mean that if we had two devices one deforming 8mm under 6.999 kgf and the other deforming 8mm under 7.001 kgf, one of these devices should be prescribed in preference to the other for a given individual with a given pathology because it will be more efficacious? Indeed, what if we used two devices from the same category: one deforming 8mm under 0.5kgf and the other deforming 8mm under 7kgf, will these two devices have the same mechanical effect on the foot?

    BTW, I'm calling it "stiffness" because that is what it is: http://en.wikipedia.org/wiki/Stiffness

    Why three categories, why not two or ten? Like I said the problem arises when we reach the boundaries of the categories. What if we have a much bigger number of devices with a wide range of stiffness characteristics and a greater number of testers, would they all put the same devices into the same categories or would some put them into different categories- that's the study you need to do.

    The span length of arch to height ratio will be significant in the device stiffness. I'll ask again, what is the significance of orthosis stiffness in patient outcomes? This question has to be answered before we can justify a need for categorisation in the first place! Then you need to justify the use of three categories...
  10. Arjen

    Arjen Active Member

    I was using the legacy terms of flex, semi-flex and rigid since they are in common use in the industry by the prescribing community which I regard as the audience of this effort. Clearly with terms that are so ambiguous there is a range of stiffness that would be expected. The kgf of force data I shared clearly shows a significant difference in kgf for each category by in-house expert users. My goal is to empower the end user to get an expected result on a consistant basis.

    For example...If the prescribing professional were to send over two foot data sets, both the same shoe size and both of similar weight, but one with a fairly cavus foot and the other more planus, if the material thickness were constant, one would expect the cavus device to be stiffer owing to its more contoured shape. If the prescribing professional asked for "semi-flex" on both devices and we vacuum formed them out of the same material, they would exhibit vastly different stiffness in the finished F/O. If the software would compensate for the variables it would automatically reduce the thickness of the cavus device vs the more planus one to produce a similar stiffness in the final device. I doubt that using our best efforts that we could make these two devices identical in measured final stiffness...but we would be far more consistant that present technology. I think your point on whether 7kgf vs 6.99kgf would throw you over a threshhold misses the point of the exercise.

    I am happy to explore more options. Hell it could be a slider bar of stiffness with infinite adjustment. I just want a common language for end users to get a given result. The study you seem to be advocating would only come after a solid spec exists for stiffness in the first place. Then you use the new consistant technology to find out what works best for patients. Science starts with repeatable results, not before then.

    I completely disagree for the reasons cited in the prior paragraph.
  11. Lol. Yes because you appear to have used three devices with three categories. If I gave you three apples and asked you to put them in three categories relating to size : small, medium and large its fairly easy- right, now get 200 people to put 20,000 apples into the same categories. Take two hundred devices of varying morphology and ask a 100 podiatrist to categorise them into three groups. There will not be 100% agreement. Viz., there will not be discreet boundaries in the perceived stiffness of the devices.

    So you want "semi-flex devices" to have similar stiffness regardless of their surface geometry- great, I can think of some fantastic research projects for this if you can make devices of differing surface geometry but identical stiffness, but until we have the data from these projects, this helps me as a practitioner because? As for missing the point of the exercise, if pointing out the flaws in the exercise means I'm missing the point, so be it- I'd sooner miss the point. However, if you can explain to me how this helps me as a practitioner, maybe I can understand your desire to do this better. So we now have two devices of differing surface geometry but similar stiffness, this is helpful because...?

    I can already get repeatable stiffness data for foot orthoses; I can put them in an Instrom, or enter the model into FEA software and get repeatable results, without categorisation or archaic terminology. A "solid spec" for stiffness already exists, it's measured by the slope of the load-deformation curve. What "new consistant [sic] technology"? Science starts with questions and hypotheses. How do you test to see if your results are scientifically repeatable, if "science starts with repeatable results, not before then."? This statement by you regarding science is just nonsense and adds nothing to this discussion. Moreover, it demonstrates a lack of understanding of the points I have made regarding artificial delineation and the ability to reliably categorise. Do some science and show me the repeatability of your categorisation system using a large number of orthoses, each of slightly differing stiffness characteristics and then get a large number of individuals to each categorise them. Then we can talk about inter-observer error and see if it worth pursuing your idea any further.

    You can disagree all you like, but unless orthosis stiffness in isolation makes a difference to patient outcomes who gives a monkeys nuts? The reasons you stated (not cited) are invalid, for the reasons wot I just wrote.
  12. Jeff Root

    Jeff Root Well-Known Member

    Add in another variable: temperature. Polypro in a work boot on a 105 degree day with a road construction worker on asphalt in California vs. a -10 degree day in winter on ice in Wisconsin. Bill Olson wrote an excellent chapter on material properties in Ron Valmassy's book Clinical Biomechanics of the Lower Extremities.
  13. Bruce Williams

    Bruce Williams Well-Known Member


    From my perspecitve this is a very interesting discussion.

    I think the point that Arjen may not be spelling out is what examination parameters from a clinical perspecitve will help the FO manufacturer adn the prescribing clinician create a device that will be fully functional and comfortable.

    Ultimately it makes no difference how stiff the device is or is not if the patient cannot tolerate it and therefore will not wear it.

    Casting positioning plays a large part in this discussion from my aspect. I DF the toes when I partial weight-bear cast to stabilize the midfoot as much as I thnk the patient needs and can tolerate. From my clinical data wih in -shoe pressure, this makes a big difference in function, F/T curves, etc.

    The problem with this positioning is that often the EVA, and especially my pulled poly devices, are too stiff in conjuction with the patients shoe gear. Usually I can thin the poly at the midfoot arches (medial and lateral) adn the same in the EVA devices adn this will take care of the problem from the patients perspecitve and still usually allow relatively little change in functional feedback, usually.

    From my aspect what do we as clinicians need to document to give us an indication of what the patients foot will tolerate so that we can arrive at a properly prescribed stiffness in a device.

    Patient weight is obviously one factor. Casting positioning of the foot / midfoot. Navicular height / drop? Contour of both medial and lateral midfoot arches in reference to lateral / medial heel and lateral / medial MPJ? (open to more description on this one obviously) Also what about functional changes in many of these parameters?

    Hope this helps Arjen. I think this is part of the process, but obviously not all that you are looking for.

  14. Agreed on all counts. Bill's chapter is an excellent starting point for anyone interested in this subject.
  15. Perhaps I am missing the point of the exercise. Is the point of the exercise to make prescription orthoses that are only available in three "stiffnesses"? Just like the pre-fabricated devices that come in three densities. Then we can start thinking about making custom foot orthoses that only come with three arch heights: high, medium, and low so that we only have nine types of custom foot orthoses to select from the menu. Wait a minute, that's just like prefabricated foot orthoses. Now that's easy, I just measure the patients weight, their arch height and their shoe size, and give them the "right" device- so easy anyone can do it. You can't beat a bit of dumbing down to increase your sales potential.
  16. Arjen:

    The terms "rigid", "flexible", "semi-flexible" and/or "semi-rigid" are useless terms for describing the stiffness of orthoses since, as Simon pointed out, they attempt to pidgeon-hole a complex subject (i.e. orthosis stiffness) to the point of innaccuracy and ambiguity. If our collective purpose is to create a foot orthosis stiffness classification system that is useful, modern and that will last a long time for the medical professions that use custom foot orthoses, then we definitely do not want to use the terms "rigid", "flexible", "semi-flexible" and/or "semi-rigid".

    I suggest, as a starter, you should make a number of orthoses made up of different materials, different thicknesses and different widths that are made off a cast/3d digital file to fit a size 9 man's foot with an average medial arch height. Then, put each orthosis on an Instrom (i.e. materials testing machine) that measures the load-deformation curve of the device at two locations of the orthosis, at the apex of the medial longitudinal arch and the apex of the lateral longitudinal arch. These material tests should be done at a standard temperature and a standard strain rate and/or loading rate that simulates walking or running mechanics.

    After this test has been performed on, for example, 50 different orthoses made of different materials, different thickness and/or different widths, then one may decide to use a numerical value to rate one orthosis material relative to another orthosis material or rate one orthosis mofication relative to another orthosis modification in how it affects the orthosis stiffness. One way of simplifying stiffness for the clinician would be to list how much each type of orthosis deforms at the medial and/or lateral longitudinal arch under a 50, 100, 150 or 200 N load (approx 12 - 50 lb load). Then once these tests are performed so you can see the ranges of the material deformation characteristics in the materials commonly used to manufacture foot orthoses, numerical values may be assigned to the load-deformation characteristics of the orthoses using a orthosis stiffness system of 1 - 10, with 1 being the most compliant material and 10 being the most stiff material. I would suggest we also come up with a new name for the standard testing procedure such as Standard Foot Orthosis Stiffness Test (SFOST) that should become an industry standard.

    Hope this helps.

    Now, back to seeing my remaining 15 patients for the afternoon with the Kaiser 3rd year Surgical resident following me along in my clinic.:drinks
  17. efuller

    efuller MVP

    You have two different problems. One is defining/measuring "stiffness" the other is getting the orthotic ordering customer to understand stiffness and have them know what they want when they order. The only way I see to do this is to distribute orthotics made with the various materials so that the practitioner can "feel" the stiffness and then make an educated guess as to what to select when they order the orthotic.

    A major problem is that most practioners will not have experimented with devices with different stiffnesses, and shapes, for a single patient. A large portion of the practioners won't have any idea how stiff a device should be.

    I've thought about it a fair amount and I am pretty shure that if you had two patients with identical feet one would like the first configuration better and the other patient would like the second better. This area needs a lot of research.

  18. Jeff Root

    Jeff Root Well-Known Member

    I agree. Owning an orthotic lab, I find practitioners with significantly contrasting opinions and/or experiences as to what is ideal when it comes to orthotic stiffness. While some might claim that a material is too stiff and that their patient's could "never tolerate it", other demand stiffer orthoses. Given that they use the same lab, the main variable seems to be the practitioner and their training, methodology, casting, bias, etc. I would suggest that for most patients, there is a stiffness range that is acceptable. But stiffness alone is not the issue. The combination of orthotic shape and stiffness is important. I beleive that the stiffer an orthosis is, the more critical its shape then becomes.
  19. I couldn't agree more here with Eric. My last posting described only a method of doing material testing on foot orthoses so that we can develop a Standard Foot Orthosis Stiffness Testing procedure which will then allow various orthosis materials and orthosis design permutations to have their load-deformation characteristics tested so that valid and scientific comparisons of orthosis materials and orthosis designs may be tested against each other.

    As Eric pointed out, the necessary next step would be to try to educate clinicians on how orthosis stiffness may be modified to create more positive therapeutic effects from foot orthoses. Trying to educate clnicians may actually be the more significant and difficult task since many clinicians either don't want to take the time to be educated or don't have the background knowledge in rotational equilibrium theory, tissue stress, physics and modelling techniques to understand when and how to use these changes in orthosis stiffness for the benefit of the patient. Hopefully the book chapter that Eric and I completed nearly five years ago, "Subtalar Joint Equilibrium and Tissue Stress Approach to Biomechanical Therapy of the Foot and Lower Extremity" will finally be published within the next six months. I believe this chapter will go a long way toward explaining our approach to orthosis biomechanics and foot and lower extremity biomechanics which should then allow the intelligent clinician to understand when, for example, different parts of their patient's foot orthoses should be more stiff and other parts of the orthoses should be more compliant, in order to achieve optimal therapeutic results from their foot orthoses.

    Good discussion. Arjen, thanks for starting it.
  20. Arjen

    Arjen Active Member

    This point is well taken, but we were not planning on controlling for it with one exception. I think that labs would do well to quote rigidity at a specific temperature. At present the stiffness of a finished device is so inconsistent that this is a small factor now.
  21. Arjen

    Arjen Active Member


    I think I clearly stated earlier that the three stiffnesses are rather besides the point and are only in this discussion by reference. In fact I say "...it could be a slider bar of stiffness with infinite adjustment". I have made the point of the exercise very clear in the multitude of postings above.
    While the immediate motivation is to get professional input on an engineering User Interface problem we are confronted with, the fact that I am doing this in a public forum shows clearly that we want to improve the state of the art for all. Nothing in any of my postings is sales related. You are out of line.
  22. Arjen

    Arjen Active Member

    I agree completely...hence this thread.

    Maybe my referencing of these legacy terms has been counter-productive. I was trying to point out the inadequacy of them for the specification of a prescriptive device. The exact terminology to be used is absolutely within the scope of this thread. It could be as simple as a "stiffness" or "rigidity" value with a range of 1-10 for instance.

    Extremely useful input. Thank you. This does suggest a systematic method to derive the stiffness characteristics of current product.
  23. Arjen

    Arjen Active Member


    As you stated, these are two separate problems. Since the present paradigm produces product that is all over the map from a stiffness perspective, it is probably not possible to properly educate the practitioner on what a proper stiffness for a given pathology might be. I allege that once a consistent standard is created, the study and education of what is appropriate for a given patient will follow.
  24. Good, I've never wanted to toe the line http://en.wikipedia.org/wiki/Toe_the_line . You may believe that you have made the point of the exercise clear, however while it may be clear in your mind, that does not mean it is clear in the minds of anyone else. That's why that post began with the question" is the point of the exercise..?." Good luck with your altruistic venture, I look forward to the results of it being freely available. In the same way that my colleagues and I have freely given professional advice to you so that you can put it into your "user interface" (frankly, I don't really care where you put it) and "solve a problem you are confronted with" in your system that people pay to use, or do you give that away freely too? Now how about answering the points I raised in my previous post to you?
  25. Jeff Root

    Jeff Root Well-Known Member

    Re: Functional F/O Rigidity Specification in CAD Systems


    Here is the problem as I see it. Polypropylene comes in natural and various colors. The more the colorant, the less pure the poly. Do you establish a standard for each color and for each manufacture, since the chemical composition is different? In addition, we use TL-2100, TL-Silver, high density polyethylene, and Polydur. If you have a system to rate stiffness, it should apply to all types of functional orthoses.

    Our direct milled CAD/CAM orthoses are not uniform in thickness and are not vacuum formed. This influences the thickness to stiffness ratio. Many people believe that direct milled orthoses are more rigid than their vacuum formed counterpart.

    I think your intentions are admirable, I'm just not sure they’re very practical.

    Jeff Root
  26. Re: Functional F/O Rigidity Specification in CAD Systems

    Or practicable.
  27. Re: Functional F/O Rigidity Specification in CAD Systems

    BTW, your vacuum formed devices will not be uniform thickness either due to the flow of the plastic as it is pulled. Indeed, I have recently developed a new technique that I'll talk about soon which purposefully builds in variation in shell thickness in vacuum formed and CAD/CAM devices. I take your point though CAD/CAM devices may be engineered to have variations in their thickness and thus their stiffness characteristics at different points within the shell.
  28. Arjen

    Arjen Active Member

    Re: Functional F/O Rigidity Specification in CAD Systems


    My intention was to only cover machined F/O's. The different base materials can be dealt with. It might be nice to characterize other devices but I agree with your contention and I don't really see how you could control any vacuum formed part for finished device stiffness to that degree.
  29. joejared

    joejared Active Member

    Re: Functional F/O Rigidity Specification in CAD Systems

    You may want to consider specific factors, fill level, patient weight, and desired flexibility. The less fill, the greater the arch height, and rigidity will be, even with the same thicknesses. As for patient's weight vs flexibility, that's pretty much a straight line in terms of thickness. A simple y=mx+b type of equation for thickness vs weight, coupled with a factor related to thickness should cover most of it, followed up with a minor correction related to fill level of not more than 10% one way or another. Of course, if your machine is loose and incapable of producing the appropriate thickness +-0.3mm, no matter how you think, the process wont be repeatable.
    Last edited: Sep 13, 2009
  30. joejared

    joejared Active Member

    Re: Functional F/O Rigidity Specification in CAD Systems

    In my own software, there are thickness controls in the rearfoot, overall thickness, and feathering margins lateral, medial and distal. It might be the case slightly, because the milling lines effectively rib the devices, but if it's a lateral to medial rib, it's actually only as strong as the equivilent at its thinnest point. I'd be inclined to say that my own patented design would lean towards the thicker side, because the lines are longitudinal throughout most of the part, and elliptical along other sections.

    [​IMG] Closeup shot

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