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Here are two FEA models that I did for polypropylene copolymer and high density polyethylene a few years ago. The geometry of the devices are identical, only difference is material. As you can see the deformation under load (1000N uniformly applied) is very similar in both models.
Thanks Simon,
some very useful stats on both materials. However I would like to know why subortholene is generally the preferred orthotic shell material in children? What is the reasoning behind it? I ask because I never really found a definite answer in the text books I was studying from as a student.
Nick
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Nick S. Caravaggio BSc. C Ped. (C)
Canadian Certified Pedorthist
Caravaggio Orthotic Therapy Clinic
Thanks Simon,
some very useful stats on both materials. However I would like to know why subortholene is generally the preferred orthotic shell material in children? What is the reasoning behind it? I ask because I never really found a definite answer in the text books I was studying from as a student.
Nick
Nick:
Who says that subortholene is the "preferred orthotic shell material in children"? I've never used subortholene.....ever. Polypropylene is what I have routinely used in children's orthoses for the past 25 years. Many ways to skin a cat when making orthoses......even for children.
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Sincerely,
Kevin
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Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College
I completely agree with you kevin, I was just searching for a specific reason that it was said to be used most often in children. That is what alot of the text books on the subject state, however they don't give a reason.
I even remember a question on my certification exam. Which type of orthotic material is used for children, i put subortholene because that was the popular concensus among my classmates and instructors.
Nick
__________________
Nick S. Caravaggio BSc. C Ped. (C)
Canadian Certified Pedorthist
Caravaggio Orthotic Therapy Clinic
Forget the data for a moment. Remember the patient for whom all this research is for (or is it the researcher). If you return the sub shell to the cast after 12 months it will have altered from its original shape, where poly retains its memory far better, remaining the desired shape to alter the patients biomechanics more accurately as intended by the practitioner. Often practitioners who use labs & don't have the casts to check for distortion, or if they don't often review patients long term. If you are looking for long term memory, my vote goes to poly.
As a lab owner, I have found that Subortholene and other high molecular weight polyethylenes (such as Sureform and JMS 500 here in the USA) are much easier to vacuum press without creating wrinkles in the heel cup for UCBL devices.
The intended result will be the same using either material for pediatric devices but fabrication of very deep heel cups is much easier with subortholene....at least for me.
it is not about what is easier for the lab (as a lab owner for 25 years & lecturer in orthoses for 10 years, I do agree sub is easier to mould without wrinkling), but considering what patients are often charged for a piece of moulded plastic, the decision on what to use should be what will give the best outcome for the patient. Remember the price of orthoses used to be justified because of the time podiatrists needed to make them & patients were charged accordingly, but now most use a lab, we still seem to justify this fee somehow.
the decision on what to use should be what will give the best outcome for the patient.
Agreed, but the type of plastic employed does not determine the outcome per se, ultimately the outcome is determined by the geometry, load/ deformation and frictional characteristics that the orthoses supplies at the foot-orthosis interface.
Take another look at those two FEA models, the deformation distribution is pretty much identical under a given load, the surface geometry is identical, we can top-cover these in the same material and hence the frictional characteristics between the foot and orthoses would be identical too. I wouldn't expect any difference in the outcomes these two devices achieved in a patient.
That the creep characteristics of polyethylene are not as good as polypropylene might be an issue over time. But we were talking about paediatric devices here, how long before they outgrow them and need a new set anyway?
The concept of creep also raises the question of "just how accurate does an orthoses need to be?"
Remember the price of orthoses used to be justified because of the time podiatrists needed to make them & patients were charged accordingly, but now most use a lab, we still seem to justify this fee somehow.
I say we increase our prices, our knowledge and time is worth it.
No question Suborth is easier to work and manage in the lab. PP requires much more care to avoid imbalance in the structure during molding and cooling, and finishing on machines is much nicer and easier in suborth. PP is much less likely to deform over time, but it more likely to crack and split in stress areas. Very small imprefections in edge finishes can cause problems in time.
Re cost, you have to be looking at $300 per hour to cast/modify/fit as a minimum. Then there is lab labour and materials and production overheads. Then as Labguy rightly points out, there is the knowledge factor. Its like the carpenter who charges $100 to fix a squeeky floor board. It only takes one nail and 2 minutes. Thats worth $5. The other $95 is knowing where to put the nail. My business is 99% what I know and you don't know. Your patients come to hear what you know about their feet, and if an orthosis is a consequence of that, no problem. Your opinion has huge value. Here is a tip for nothing, give your patient a brief report summarising your assessment. A 3 fold brochure format works fine for this. Makes the consultation more tangible and valuable.
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Also Labguy is right re manufacture. PP for UCBL's needs to be vacuum drawn, rather than pressed. Anything on the PP surface can result in a pretty ordinary finish, whereas suborth can take vacuum press. Oh yes, and it also re-heats and adjusts very much easier than PP which can be a bitch. Suborth can be hand worked with tools to adjust fit. Not so with PP.
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I recall as a student subortholen was generally preferred as it moulded more easily around small heels on the cast - less wrinkles. The heating time seemed to have greater window of use - so good when as students you don't always do everything in a timely manner. And lastly it is cheaper than PP - so the pod department preferred that.
I have used suborthalen in 3mm and 4 mm and PP in 3mm 4mm and 4.5mm. Suborthalen cost more in Aus and is less rigid for the same thickness in what I used any way......and lost it's shape whereas PP has never been an issue with that. The only device I have ever had back snapped in half was suborthalen. I agree it's easier to work.......but if it's only about that then why do I try to do the right thing ALL the time even when the alternative is easier. There was a practice in Melb that used to use 5mm suborthalen all the time and maybe at that thickness the long term results are better??
regards Phill Carter
I always though Suborth to be more flexible in like thicknesses but was reluctant to say in the face of Simon's impressive images! Always liked a little bit of flex in the device provided it was accomodated in the design. I guess there are good and bad for both really. Does anyone still use themoset resins and carbon/glass fibre laminates?
Subortholene vs Polypro
Linear Mode
