Functional F/O Rigidity Specification in CAD Systems
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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?
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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. -
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. -
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Gentlemen,
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. -
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 -
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Simon,
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. -
BTW, I'm calling it "stiffness" because that is what it is: http://en.wikipedia.org/wiki/Stiffness
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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.
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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.
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Arjen;
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.
Cheers.
bruce -
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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 -
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.
Cheers,
Eric -
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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. -
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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. -
Extremely useful input. Thank you. This does suggest a systematic method to derive the stiffness characteristics of current product. -
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. -
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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.
Respectfully,
Jeff Root
www.root-lab.com -
Re: Functional F/O Rigidity Specification in CAD Systems
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Re: Functional F/O Rigidity Specification in CAD Systems
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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. -
Re: Functional F/O Rigidity Specification in CAD Systems
Last edited: Sep 13, 2009 -
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.
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