Acrylic materials used for orthotics

Thanks Bruce, but it's about how orthotics work In all seriousness, it's a discussion of the primary properties of foot orthoses viz. surface topography, load/ deformation and frictional characteristics and how these influence the reaction force at the foot-orthosis interface based on the FEA modelling and in-vitro experiments that I have performed. It's all there in my head and in the data, I have the majority of it mapped out and the pictures to illustrate it- just need to sit down and actually write it. I know it will probably be my opus majus for the profession in the same way that SALRE was for Kevin. As such, I want to get it right.

 

For those who are not familiar, when heating solid materials, there are two temperatures that need to be considered, Tm -- melting temperature, and Tg - glass temperature. I'm no expert in material themodynamics, however as I understand it, Tg is the point where the material is no longer brittle and will not experience a catostrophic event, however it will deform in a viscous like manner. Tm is the point at which the material turns to a true liquid. Polypropylene exists at room temperature above its Tg and below its Tm. That's why it doesn't fracture. Acrylic exists at room temperature below its Tg, which is why it can fracture with enough strain.

I'll try to use fewer abbreviations in the future. I'll make a few short remarks on the new thread about MTJ stability.

Thanks,
Daryl

 

Nobody has attempted to answer this question, so I'll attempt to answer it for you. It means that if the goal of the orthotic therapy is to alter the distribution of relative reaction forces, you should consider altering the orthosis topography rather than altering the orthosis material. If you wish to alter the magnitude of forces you should consider altering the orthosis material (or it's thickness).

 

Simon,

How do YOU make a determination on which to alter, reaction or magnitude, with certain specific presenting pathologies like fasciitis /fasciosis, etc?
Without an in-shoe pressure device, or something similar, how can we really know which one to focus on as well?
I'm not trying to do a commercial for in-shoe pressure, I am curious as to your thoughts on this since you have practiced with and without such a device.
Sincerely,
Bruce

 

to the bolded part would that include timing ?

 

Bruce, that's exactly the question I hoped to attract. In reality, I don't have the answer- sorry. I don't think in-shoe pressure necessarily gives us all the answers either- we both know the limitations of the technology. But in certain pathologies it might help. I do know that if you change a device from polyprop to acrylic and keep everything else the same, you are just increasing the magnitude and not the distribution of reaction forces, so if the goal is to manipulate centre of pressure pathways, it's more about the topography than the material/ thickness (in a single thickness shell) per se; if there is a specific threshold pressure which causes a pathology (say ulceration) then manipulation of material without altering topography might be the way to go. In reality though, both topography and material are usually modified to achieve therapeutic goals.

 

Uncertain from the FEA I have performed. Great question though. This is where the natural frequency of the material will come into play.

 

The only thing I can measure right now is the magnitude of force being exerted through the gait cycle. I cannot measure the direction of the force because I don't have a scanner to map the shape on top of the loaded orthotic. Topography primarily changes the direction of the force because the vector at any point is normal to the surface; however it also can change magnitude when it increases the compression of the soft tissues between the orthotic surface and the bone structure. Material resistance to deformation changes the magnitude of the force and is a function of both molecular makeup as well as its physical measurements (length, thickness, curvature). Of these three, when I'm playing in the orthotic lab, thickness is the easiest to manipulate.

Best wishes,
Daryl

 

Not so fast, Daryl. Any frictional forces between the foot and the foot orthosis will change the force vector away from being normal to the surface of the orthosis so that your statement "Topography primarily changes the direction of the force because the vector at any point is normal to the surface" is false.

Or are you saying that there is no frictional forces between the foot and a foot orthosis?

Here is a great paper on that same subject.

 

Thanks for directing me to the paper, Kevin. For some reason I didn't have it, but I'll get it tomorrow.

Certainly all the horizontal force vectors exerted by the shoe, top bottom and sides, on the foot have to equal the net horizontal force that brakes and propels each step, however the ratio of normal vs. shear force should average the same as it does w/o wearing orthotics, which means that normal forces are considerably more than shear forces. I'm sure that topography changes both!

Current technology with Novel or Tekscan sensors between the orthotic and the foot surface only measure normal force. Do you know what the coefficient of friction is of any of the orthotic materials? Coefficient of friction IS very important to me because of the number of diabetic high risk feet that I treat. Yavuz at OCPM (now Kent U) is working on shear measurement technology, but it's not ready for any type of commercialization yet.

Of course, when looking at the normal vector field across the surface of the orthotic, you will still have a net vertical and net horizontal vector. Maybe Simon could help us out here. Simon, start with an assumption of 1 unit of force/mm^2, with direction of force normal to the suface, and an average orthotic shape, and tell us what the sum of the X, Y and Z vectors are. (You'll remember Kevin that I discussed this idea many years ago at a Weed Seminar)

So I am not saying that there are no frictional forces.
Now the question is, what is the answer to the question that I posed to you about how your casting for orthotics differs from what John Weed taught? If you have changed, why? If not, why?

Best wishes,
Daryl

 

I did a simplified 2-dimensional version of this in our paper, Daryl. If you have any problems getting hold of it let me know and I can send you a copy.

With regard to co-efficient of friction of top cover materials, you may recall that I presented on this subject in Florida. I've attached a slide from this lecture in which the angle of friction was presented between two types of hosiery: cotton and nylon and a selection of orthoses materials. I'm sure you can convert the angle of friction to coefficients, Daryl.

One of the issues I discussed in Florida was that due to the variation in the point to point surface angulation of the orthosis at it's interface with the foot, there exists potential for some areas of the interface to have angles which exceed the angle of friction and some of the interface may have surface angulations that are below the angle of friction. Thus, we may need to consider both static and dynamic friction co-efficients within the same insole within analyses. Clinically, this may impact upon the coupling between the foot and the orthosis at discreet areas of the interface, upon the direction of reaction forces and may contribute to microtrauma of the skin and subcutaneous tissues.

 

Uhm, are we off on a tangent (interesting one though Simon)?

The in-shoe pressure paper is a great read regarding the limitations of this technology ( two thumbs up).



I don't think JMS500 (subortholen) is necessarily any better than PP or Rhohadur or EVA or whatever material(s) one chooses to use. It is important that we understand the personality or quirks of each material used for orthotic fabrication. Ok, maybe I should not personify here and I should could say rather how each material changes ground reactive forces and joint moments. Also, important to know how it melts, grinds, and presses too. For example, JMS-500 grinds alot nice than PP in my experience.

Getting back to acrylics, how about Polydur? Does anyone use that and what clinical situations call for its use over PP? I think it comes in multiple colors kind of a reddish and a bright green? Does it still crack like its predecessor? I know some labs use this for pediatric orthotics, but it is not popular.

 

Hello Dr. DeBrule,
I apologize if I've met you, that I don't remember such meeting.

I started this post because everyone has their own opinion and no one has any data about orthotic materials. I think I've prescribed many orthotics from all the materials, and I find myself now back at square one, using either acrylic or cork/leather. I'm doing some materials testing on my own, and hope to have publishable data in the next year.

As I have maintained, an orthotic has to push up against the foot, and can only do so by actually making contact with the skin. That's why original shape is so important. Then one puts force down on the orthotic, and it flexes from the original shape, and in so doing it begins to create force against the bottom of the foot. So if you make the orthotic original shape to match a "neutral foot shape", as you stand on the orthotic, the more it deflects out of that neutral shape, the more force it creates against the foot. The amount of force it creates can be easily calculated if you know its modulus of elasticity and its deflection. Since all of the materials have different moduli of elasticity, you will either get different amounts of deflection or different amounts of force from the various orthotic materials. Thus I have found over the years differences in how patients feel with different orthotic materials, some small and some major. In one experiment we ran at Des Moines U, we made a single patient the same orthotic out of 6 different materials and then did pedobarographs on each material. Sure enough, the velocity of the CoP was different for each orthotic. Wish we had had more time and money to do a good publishable study.

Polydor has not been a bad material. You have to make it a little thicker than the old rohadur, and it cracks more often, but it's not that bad. I've been wearing a pair for several years, and have had one breakage myself. You just have to tell your patient that all materials either crack or fatigue, and have to be replaced sooner or later.

Hope that helps you understand my thinking.
Best wishes,
Daryl

 

Daryl,

Daryl, thanks for your input. Nice experiment: 6 different materials and the same orthotic positive. Consider publishing what you have as long as the data is reproducible. Why not? There was a concise paper written on just one patient (I think Spooner did one in JAPMA not too long ago) looking at just one patient varying the rearfoot posts looking at COP variation and some other variables. Maybe you could perform a similar analysis? Just a thought.