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Vacuum formed versus direct milled orthotics

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Simon Spooner, Sep 1, 2010.

?

Which orthotic manufacturing process do you prefer?

  1. vacuum formed

    12 vote(s)
    44.4%
  2. direct milled

    11 vote(s)
    40.7%
  3. no preference

    4 vote(s)
    14.8%
  1. joejared

    joejared Active Member

    Thanks! :) That's a fair statement as it was based on subjective information as I've seen orders and only intended to be an example of wrk# variable usage, not the data itself. Mathematically, with derived endpoints of the slope of thickness vs weight, the equation is sound for most devices.

    Patient weight and arch height and whatever extrinsic posts there are on the device are probably the three most important factors in determining rigidity of a given device with cad cam based systems. A hard shell device will have different rigidity with an extrinsic rearfoot post than with an intrinsic, simply because the area that is flexible is reduced significantly. In one case, a device that pulls the forefoot extrinsic post proximal to a 1st metatarsal cutout, also would strengthen the device, but that is the intended goal anyway, so it's moot.
     
  2. Joe, your posts seem to me to be an attempt to promote your product. I think that you might be able to offer this forum much more, if you only left your product at home when you post here because quite frankly and again with the greatest respect, you appear to be a fine engineer, and you are good with computers, but your knowledge of the foot and foot orthoses is very, very limited.
     
  3. joejared

    joejared Active Member

    Let's try it a little simpler, implying normal mathematical priorities.
    Assuming x = a patient's weight, and y the desired material thickness.

    Let patient 1 be x1,y1
    Let patient 2 be x2,y2

    m= (y1-y2)/(x1-x2)

    Using one of the 2 patients, finding the yintercept (b)
    b=y1-m*x1 or b=y2-m*x2

    For future calculations, y=m*weight+b, or wrk0 and wrk1 with respect to OreTek. While it sounds complicated, a lab tech would ultimately refer to a chart for rigidity based on weight, and enter a value. What I do just eliminates that need and simplifies the design from a technician perspective. Conversely, in a wet lab environment, you depend on the skills of that technician to provide the flexibility you desire grinding here and there to meet your requirements.
     
  4. Joe, it's not complicated, but you are suggesting that we can predict shell thickness from body weight alone (according to the model posted previously shell thickness in mm? inches? = 2 x body weight). I don't believe it's that simple. Now, in order to build a regression model, you need data- you can't just make it up, as you appear to have done. I'll ask once again, where have you drawn the data from to build your regression model? You can fool some of the people some of the time with mathematical bull****, but you won't fool all of the people... Now if you did have a model, which I don't believe you do, there would be 95% confidence intervals for that model too.
     
  5. joejared

    joejared Active Member

    Discussion of any system promotes that product, which wasn't my intent, but is my experience. My intent was to show another method that makes it easier for a lab technician, at least with respect to material thickness. At least 2 systems were mentioned that modify lateral thickness for various reasons, among which, valgus posting and my own personal experiences come to mind. I'm guessing most systems enter the values by hand, and not by values that are calculated. There's nothing to say other systems can't adopt this method or some variant.

    In another message, someone mentioned breakage in a heel post, to which I offered a solution that is known to correct that problem without compromising material thickness distal to the heel post. In the end, if the reader understood the cause, he or she could contact the developer of that product and suggest slowing down the perimeter cut out of the heel post.

    Going further, and emulating that which is done by hand, one cannot consider overall thickness without considering heel thickness, lateral thickness, overall thickness, and feathering along the edges. Mathematically duplicating that process, or any part thereof has taken parts of this thread down quite a few paths and stimulated new ideas.

    While I'm not promoting OreTek, I am describing some functions of it that other systems could adopt, and in part by answering _your_ questions, have.

    I am, however, promoting my wife beater. :)

    [​IMG]
     
  6. And a lovely wife beater it is too, can I suggest a can of Stella to replace the orthoses?
     
  7. joejared

    joejared Active Member

    Let's wind this back a notch or 2, thinking like a wet lab.

    A material of uniform thickness is formed over a cast corrected positive. We know this material to be of uniform thickness, which means after forming, we could take a cross section cutout of it, and confirm it. There are no ridges, no valleys, it's the all same thickness. The material thickness doesn't change tangent to any part of the surface, no matter how high the arch or heel cup is. At some point, a technician may also grind into it, weakening it in specific areas. For the purpose of this discussion, let's further assume that nothing is added to this device.

    In terms of units of measurement, I work on the american standard, which usually means both imperial and metric, interchangeably. For orthoses, it's easy enough to stick with metric, but tooling and cnc's are typically imperial and/or metric. We can leave stones and kilos out of the equation, or alternatively, use whatever terms you're comfortable with. In the initial drawing as a part of my first response, I translated for you, and generally, in drawings I show imperial unless otherwise mentioned, or if I'm showing a drawing of an orthosis.


    In a cad cam based system, typically, thickness is generally defined perpendicular to the plane formed by the device itself, and not tangent to the surface at any given point, which differs from molded devices. It also differs in terms of strength for the other reason I mentioned initially, dependent on how much step over there is from pass to pass, arch height, how much extrinsic posting their is, actual machine tolerances, how much the material stress relieves while cutting, how sharp the tool is, ad nausium.

    The example is not the data itself, but rather the thought behind it. Unstick yourself from the data for just a second. The example code I supply to my customers doesn't specify rigidity of any kind, but suggests one way to determine material thickness as a function of weight. Should they chose to use the concept, they insert data based on a specific rigidity and give this data a name to be used later, such as "flexible" and they implement it based on their specifications and the practitioner's request. They also have to consider arch height, which means if they use the idea, they will also have to create other variances of that design to reflect that difference as well.

    As I've written before, I only make samples and research test pieces, only. When I pasted in wrk#, the intent wasn't intended to define a standard, but rather to express a thought process. The labs I work with have the option to configure their system to suit their needs, not mine. In terms of regression of data, and assuming the lab had been entering patient weight all along into the system, it would actually be a simple matter to chart average thickness based on weight, but unfortunately, weight alone would not define the desired rigidity. The individual labs experience with their chosen materials provide the data needed to complete the equation.
     
  8. joejared

    joejared Active Member

    I wouldn't call it an orthosis. It's like a bit of marshmallow one stands on, praying not to hurt the stubborn and self-destructive diabetic alcoholic any more than they already have hurt themselves, and continue to when they don't follow their doctor's advice, stop drinking alcohol and lose a hundred pounds or so. Yes, that's what I really think of EVA devices. They're glorified insoles.
     
  9. DanthePod

    DanthePod Member

    hi Simon, As you know I have had a direct milled lab for over 15yrs now and have experienced many evolutionary changes in the process over that time. There are many miss conceptions in this area in terms of the capability of driect milled products. Namely they are can't be made ultra flexible that is just not true. Most peoples success or failure with a product is more based on appropiate design and less related to the manufacturing process. Direct milled delivers considerable advantages in terms of less human error in the manufacturing process, consistant application of specific design parameters, less chance of orthotic shell distorsion, better adjustability for the practioner post fitting; being that the device is generally more torsionally stable than a vac form device. The finish on a direct milled device is generally better and negates the need to use top covers to improve appearance. Lastly it is the present and the future of understanding of how orthotic design effects foot function, as the technology evolves so will this understanding. cheers dan
     
  10. Dan the Man:

    You're on quite a roll now Dan....4 posts!!! Now let's get to the serious stuff.....tell me again what the chicken in the freezer said???:rolleyes::drinks
     
  11. Yeah, Dan... I seem to recall we never got to the bottom of that one...

    Dan Everson ladies and gentleman, one of the funniest guys I've ever met. And someone who could add more to this site in the blink of an eye than many of us achieve in a year.... if only he could be bothered.
     
  12. RobinP

    RobinP Well-Known Member

    Direct milled 80% of the time.
    No 1 reason - ability to adjust shell thickness to suit in order to influence the Orthotic Reaction Force(ORF) in specific areas.

    No2 reason is repeatability.

    No 3 reason is speed of manufacture.

    No 4 reason is being able to use things like narrowed heel posts to improve fit into narrow based shoes like anti pronation runners. Probably possible in thermoformed orthoses but more difficult I would imagine?

    Like you Simon, I use a reasonable amount of Langer UK stuff and if it is not direct milled the turnaround time of 10 working days is too long.(This is not trying to make a point about Langer UK - I think they provide good products at very reasonable costs. 10 working days plus postage either side takes reappointing the patient for fitting up to 3 weeks which is fine for my NHS work but not for my private work)

    Phil 's company do a standard 5 days which I think is an acceptable time frame. Only down side is if I want something more flexible as Rob said, the toprelle splits the difference between a shell which can have the componentry refurbished(posts, covers etc) and a softer EVA device which cannot really be refurbished. Also, to have a low profile but very rigid device requires some of the materials that RX Labs use. Rigidity requires relative bulk for DM devices.

    On another note, I think (based on 3 cases in the last 3 weeks) that the direction of milling has quite an effect on the rigidity. A company, who I will not name, started off doing direct milling with the milling pattern running largely parallel to the line of progression. I have ordered and supplied repeat devices in the last three weeks where the milling pattern has been changed to running perpendicular to the line of progression(for expediency I would imagine) and the devices are significantly weaker. Spec identical and material thickness should be identical(they certainly felt like they were)Anyone else found this?

    Robin
     
  13. A parrot and a chicken in the freezer are the only part of Dan's joke I remember....the only reason it was funny was because he actually sounds like a parrot when he tells it......BRAAAWWWKKK!!
     
  14. BarryD

    BarryD Member

    Thought it may be worthwhile putting my 2 cents worth in after manufacturing many thousands of vacuum formed orhoses over the last 10+ years.
    I have been told / or read many times re the death of the vacuum formed device .
    I have also done a lot of research, and trialled various cad systems but have not found any (yet!) that give the same result as formed orthoses. My biggest concern is that the majority of the cad software fall back to simple 'generic' shell shapes and only use the foot scan for basic measurements. The manufacturers will dispute this but more detailed investigation usually proves this point.
    This may be perfect for many pods because the get the same style of orthosis every time, and they get adequate results.
    I think the discussion over shell thickness, and damage/ alteration of shell structure is valid, but should be secondary to the discussion of the prescription and initial plaster/ cad work.
    Of course it all seems irrelevant when I get positive feedback from a customer, whose patient advised her that she had never felt so comfortable in her new orthoses - which were on the wrong feet http://www.podiatry-arena.com/images/smilies/wacko.gif

    Barry

    Never attribute to malice that which can be explained by stupidity.
     
  15. joejared

    joejared Active Member

    generic = someone else's patient data = glorified prefab. Within my own network, the features that make it easy to template an order off of another patient's foot is a luxury that I can chose to disable if abused, simply by changing a bit in the permissions of the security key of my software. One exclusion I'll be installing shortly will be a crc (cyclic redundancy check), which, when applied correctly, will allow those features (copy and rename) to be used in a limited fashion as they were intended.

    One argument for library devices from a lab perspective might be that they aren't willing to lose a doctor by asking them to recast a patient due to badly damaged casts. When this happens, however, the device can no longer ethically be called a prescription device. What scares me is the number of labs that absolutely ignore this point or that don't use patient impression data at all, even in molded devices.

    While some concepts are standard, such as posting amounts, I've found more commonly that arch fill and expansion definitions vary from lab to lab. Overall, styles of devices from each lab vary even more. Any cad cam based system needs to be flexible to the needs of what the lab is already doing. In theory, the device made on a machine should have little if any difference between it and its wet lab counterpart, ultimately resting on the actual cast corrected positive mold, if anything, with greater precision than its hand made counterpart.


    [/quote]
    Of course it all seems irrelevant when I get positive feedback from a customer, whose patient advised her that she had never felt so comfortable in her new orthoses - which were on the wrong feet [/QUOTE]


    Something like this happened to me a couple weeks ago in an experimental set of EVA devices. At least my test case had the good sense to ask, "Which one's the left?", but it did suggest a bit of drug induced brain damage or too much inbreeding.
     
  16. This has been my experience of CAD CAM devices, although that experience has been limited.

    Like most people the orthotics I see besides my own seem to be mainly other peoples failiures. I see a far few cad cams in there, although I suspect not proportionatly more than vacuums.

    Thing is, when I see a failiure, be it one of my own or someone elses, it is rarely because the shell was not of uniform or calibrated stiffness, nor that ridges run in a certain direction. Its usually some glaring and horrible error in prescription or design.

    I'd agree with Dan

    A better system won't make a bad pod good, and I've seen a good pod make a wildly successful orthotic using a £1.84 freelan, two bits of offcut felt, a folded up matchbox and a pebble. *

    Regards
    Robert
    * I lied about the matchbox and the pebble, but you get the idea.
     
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