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Pouring the negative cast

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Little Sesamoid, Apr 8, 2009.

  1. Little Sesamoid

    Little Sesamoid Active Member


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    Hi All,
    I have a query about pouring negative casts. Does it matter if the cast is inverted, vertical, or everted when it is being poured to make the positive mould? Does this make a difference to the orthoitc? Because the lab that i use pour the negative cast as it is. They don't do anything to it....they leave it sitting in the position that it was casted in.
    Thanks,
    LS
     
  2. Phil Wells

    Phil Wells Active Member

    Ls

    It is a really important part of the process.
    Hopefully Jeff Root can jump in here as he probably knows this subject inside out.

    Phil
     
  3. Jeff Root

    Jeff Root Well-Known Member

    When you pour a negative cast "as is", meaning you pour it exactly as it sits on the countertop, the cast rests in a position that is determined by the shape of the outside surface of the negative cast. In most cases, the outside of the negative cast closely resembles the shape of the interior of the cast. However, it is possible for the positive cast to rest on the countertop at a significantly different angle than the negative cast did. For example, the frontal plane position of the negative cast might be influenced by a surplus of plaster splint plantar to the toes. This may influence the angle of the cast. When the negative cast is removed from the positive model and the influence of the excess plaster sprints are removed, the positive cast may assume a different angle in the frontal plane. As a result, you really need to look at how the positive, uncorrected model rests on the countertop after the negative cast has been removed.

    The angle of the positive model might also be influenced by other things such as plantarflexed digits, rockerbottom deformity, surface defects in the cast, etc. Therefore, you should check to see where the positive model is contacting the supporting surface. You typically want to see contact plantar to one or more met heads and at the heel. As a result, if you have an inverted forefoot to rearfoot condition (i.e. inverted ff deformity: I try to resist calling these conditions deformities), the forefoot will sit relatively parallel to the countertop and the heel will rest everted by approximately the same degree as the angle of forefoot inversion. In other words, the cast will rotate to a state of equilibrium. Conversely, if you have everted forefoot condition (ie plantarflexed 1st ray, ff valgus, etc.) the heel of the cast will rest inverted because the cast will rotate to a state of equilibrium.

    After the positive cast is created, the lab has an opportunity to add plaster modifications to it. This can include a forefoot or balance platform, a medial soft tissue expansion, a lateral soft tissue expansion, a plantar fascia accommodation, and other local accommodations. In order to be considered a functional orthotic, the cast must be balanced or the orthotic shell must be extrinsically posted in the forefoot in order to influence the frontal plane position of the resulting orthotic device.

    If your lab is not balancing the positive cast or extrinsically posting the orthotic shell, and if they routinely make the orthotic shell to the cast "as is", then this is not a functional orthotic. We often manufacture our accommodate devices "as is" because we do not want intrinsic or extrinsic support built into the device. We also make hybrid orthoses that have both functional and accommodative characteristics.

    I would recommend reading a recent thread on varus and valgus posting http://www.podiatry-arena.com/podiatry-forum/showthread.php?t=27873 as these are related topics.

    Respectfully,
    Jeff Root
    www.root-lab.com
     
  4. You can pour the cast "as is", but then use the zero function on a digital angle finder on the positive when balancing the cast.
     
  5. An interesting point here (which Phil can probably jump in on) is: how scans are "poured"...
     
  6. Jeff Root

    Jeff Root Well-Known Member

    Simon, here is what we do:
    Plaster cast: We take the negative plaster cast, bisect the heel like usual, use a protractor to place the heel vertical in our laser scanner, and then scan the cast. The correction software allows us to invert or evert the balance platform, which allows us to invert or evert the virtual cast (scanned foot image).

    Biofoam: We have to wedge the biofoam in the frontal plane in our laser scanner to orient the foam impression. Biofoam does not capture the posterior anatomy of the heel. The loss of anatomical accuracy of the posterior surface of the heel makes bisecting biofoam less accurate (perhaps more consistent, but less accurate from an anatomical perspective). Biofoam only captures the plantar surface of the foot, so it is much more subjective to orient for scanning purposes and for traditional orthotic fabrication.

    Digital files directly from customer: We don't currently use them commercially, but we have used them for R&D purposes. We are at the mercy of the scanner operator to orient the foot to the plane of the scanner in space. The absence of posterior heel contour (data) make it very difficult to determine the proper ff to rf relationship or heel bisection. The digital files I have seen have poor resolution and some distortion as compared to a laser scan of a negative, plaster cast. Furthermore, it is virtually impossible for the practitioner to compare one scanner to the next since they never see the system from the labs perspective. In either case, these file can be manipulate in the frontal plane with the correction software.

    Regards,
    Jeff
    www.root-lab.com
     
  7. Phil Wells

    Phil Wells Active Member

    Simon

    I am currently writing a Root style protocol for the design of CAD orthoses (A colleague said that CAD can't replicate hand made protocols so I thought I would give it a go).
    The big problem has been the initial cast evaluation from a scan.
    The best approach has been to use a Photogrammetry based scanner which outputs as an object file. (You get a photo of the foot)
    In some cases the scan can be orientated to show good heel depth and the practitioners bisection of the heel clearly seen. This is then used as normal.
    Another method is for the practitioner to extend the bisection line onto the plantar heel. This can then be used to reference the foot as normal.
    The fall back approach is to use the lowest part of the convex heel profile - less than perfect but so far all seem pretty robust.

    The actual pouring technique is pretty simple and can be applied to the shell itself while referencing the scan or direct to the scan and then producing the shell to the new profiles.

    How are you getting on with your CAD system?

    Phil
     
  8. Jeff Root

    Jeff Root Well-Known Member

    Phil,
    If a heel bisection line is drawn on the posterior surface of the heel it can be used as a reference to orient the foot or a cast in the frontal plane. If the line is extended onto the plantar surface of the heel, it can be used to orient the cast or foot in the sagittal plane but not the frontal plane. I don't see how one can use a plantar extension of a heel bisection to orient a cast or digital image of the foot in the frontal plane.

    The other issue I have with your comments is using the lowest point of the convex heel as a reference. As a cast is inverted, the lowest point of the heel moves laterally. The more inverted the cast, the more lateral the lowest point of the heel becomes. Conversely, if a cast is everted, the low point of the heel moves medially.

    In some feet, the lowest point of the heel is midsagittal. However, in feet that function more supinated or pronated there is frequently some adaptation of the plantar fat pad. In a severely pronated foot, compression of the medial fat pad moves the lowest point of the heel more laterally. In a supinating foot, compression of the lateral fat pad moves the lowest point of the heel medially. As a result, you can't use the low point of the heel to determine the frontal position of the heel. Unfortunately, the more abnormal the foot, the less reliable this technique becomes. In my opinion, the accuracy of the heel bisection becomes even more critical for orienting more abnormal feet than it is in “normal” feet.

    I would love to be able to suggest an in office foot scanner to my customers but I can't do so until I can find one that accurately captures the plantar, non-weightbearing contour of the foot and gives us some criteria to evaluate the proper frontal plane orientation of the scan. Many scanners make the assumption that the anatomical bisection of the heel (vertical bisection of calcaneus) is perpendicular to the plane of the scanner. This is completely erroneous.
    Respectfully,
    Jeff
    www.root-lab.com
     
  9. Phil Wells

    Phil Wells Active Member

    Jeff

    You are right re the projection of the heel bisection. I forgot to add that the practitioner also had to add a perpendicular bisection of the plantar heel - again based on an assumption about the lowest point of the heel. This cross hair works ok.
    The other issue is the orientation of the forefoot to rearfoot in the sagital plane. If the forefoot is plantarflexed, then again digital orientation can be difficult.

    Although I partially agree with your comments, I believe that the digital approach is just as accurate as the existing manual methods as it increases the practitioners ability to review and assess the foot. Using simple 3D software, the practitioner can assess the cast of the foot in ways that the POP method is unable to do. An example is sectional viewing where the user can 'cut' through the scan (similar to an MRI view). If not right, they can 'cast' the foot again. They can do this 3-4 times in the same time frame required for a POP cast.

    Please don't see the 3D image capture as the most important aspect of this process. A good scan is better than a poor cast. The important part is to give the practitioner more control over the process via the method above (and others) along with limited design capabilities.

    Cheers

    Phil
     
  10. Jeff Root

    Jeff Root Well-Known Member

    Garbage in, garbage out. Quality in can lead to: a)quality out or b) garbage out. We need quality in, be it a digital or physical cast to make a good product. There are advantages and disadvantages to both methods.

    The thought of giving practitioners more control over the design process brings up another issue. Does it result in better or worse outcomes on average? It is very possible that giving the practitioner more control over the design process may actually result in worse outcomes, since they do not necessarily have the orthotic design and manufacturing experience or knowledge that a well trained lab technician has.

    Respectfully,
    Jeff
    www.root-lab.com
     
  11. efuller

    efuller MVP


    The answers you got may be a whole lot more esoteric than the question you asked. Do you understand intrisic posts? If not, read on.

    The historic method of preparing casts was to bisect the heel and set the heel vertical and then pour liquid plaster into the negative cast to produce the positive cast. Thus the top of the cast was perpendicular to the heel bisection of the cast. This is not really that important when compared to what the plantar surface of the positive cast looks like. Now, if you had a forefoot valgus in your cast, the heel bisection of the cast would sit inverted before any modification. If you asked the lab, (or the lab does by default) to balance the cast vertical, an intrinic forefoot valgus post would be added to bring the heel bisection of the cast to verrtical.

    So, yes it does matter. What is more important is what you ask the lab to do and why you ask the lab to do it. You will get a different device if you ask the lab to balance the heel inverted as compared to vertical. There are reasons for choosing one over the other. That's where the prescriber earns their extra money for their expertise.


    In relation to the scanners and heel bisections. It would be a lot easier, if the prescriber just asked for a 3 degree forfoot valgus intrinsic post and symetric heel. (Symettric = no skive). That way the lab would not need a heel bisection.

    Regards,

    Eric
     
  12. Jeff Root

    Jeff Root Well-Known Member

    Eric, you lost me here. An extrinsic post is attached to the distal aspect of an orthotic device. You can accomplish the exact same forefoot and rearfoot support with intrinsic or extrinsic correction (posting). Are you saying something contrary to that or do you agree?

    It's also important to remember that an extrinsic post can be added to any orthosis, including one that was intrinsically corrected (balanced). For example, if someone has 10 degrees of forefoot valgus in their cast, I could balance 5 degrees of valgus in the cast and then extrinsically post the remaining five degrees of valgus on the orthosis, for a total of 10 degrees of valgus support. As a side note, it is much less confusing to use the terms extrinsic forefoot posting and intrinsic correction or balancing. The word post should not be used to refer to intrinsic correction as it creates too much confusion.
     
  13. efuller

    efuller MVP

    I agree with you Jeff, I was referring to an intrinsic post only, and was not including how an extrinsic post could give you the same shape of shell, but with valgus extrinsic post.

    That might be a better terminology. I'll have to go back and look at what John Weed taught. Or was that the lecture you gave to our class in 1985. I'm a little concearned about how the terminology effects understanding of the concept of intrinsic correction. If you look at the height of the top of the orthotic at the styloid process off of a table that the orthotic is sitting on, it should be the same height for an intrinsically corrected as for an extrinsically posted device. That heigth is acieved by the "posting". That height will alter the position of the foot on top of the orthotic as opposed to standing on the flat ground. The term valgus support may aid in that understanding.

    Regards,

    Eric
     
  14. Little Sesamoid

    Little Sesamoid Active Member

    Hi Jeff,
    Thank you for the reply. I understand what you're saying above.
    But in the case of a forefoot supinatus for instance where you cast the foot in the STJ Neutral (lets just say the STJ Neutral position is 0 degrees, neither inveted or everted)postion and cast out the supinatus and ask the lab to make the orthotic then what you have is an orthotic that is poured vertical, the positive cast's heel bisection is vertical, no extrinsic/intrinsic posts are added (because they're not needed) and we have a Functional orthotic, no?
    When the patient stands on the orthotic the excessive STJ pronation that was perpetuation the forefoot supinatus is controlled, no?
    Thanks again for the reply, and i look forward to answers regarding my new query.
    LS
     
  15. Little Sesamoid

    Little Sesamoid Active Member

    Thanks Simon,
    I think you best understood what i was asking.
    Pouring the cast inverted, vertical or everted doesn't make that much difference (except for what Jeff was saying, which is all very important). It can be poured "as is" and the important part is balancing the positive cast. Correct?
    Thanks again,
    LS
     
  16. Little Sesamoid

    Little Sesamoid Active Member


    Thanks for the reply,
    My question was purely related to the pouring process of the negative cast.
    Thanks again for the reply.
    LS
     
  17. Jeff Root

    Jeff Root Well-Known Member

    Forefoot supinatus is a condition of acquired (non-congenital) inversion of the forefoot relative to the rearfoot. Forefoot supinatus is thought to be secondary to excessive STJ pronation. Because forefoot supinatus is an acquired, inverted condition of the forefoot that is maintained, in part, by soft tissue contracture, it is possible to reverse the condition by reducing the degree of rearfoot eversion. If the orthotic device decreases rearfoot eversion moments (pronation), then the closed chain inversion moments that acted to produce the forefoot supinatus will be eliminated, and the soft tissue contracture that helps sustain the inverted condition of the forefoot will be eliminated.

    As long as the orthotic device provides sufficient control of rearfoot pronation, then one can cast our forefoot supinatus. So yes, I agree with you and I am impressed with your comprehension of this condition! You made my day!!!

    Respectfully,
    Jeff
    www.root-lab.com
     
  18. Correct, balancing is one of the important parts, pouring position is less so. Traditionally, casts were poured in their balanced position, enabling the top surface of the cast to be parallel to the forefoot balance platform. I think this may have caused some confusion among others.
     
  19. Jeff Root

    Jeff Root Well-Known Member

    I would like to comment to help clarify this situation. You have to pour the negative cast to make a positive model of the foot. Let's assume that we have a negative cast with a ten degree everted forefoot to rearfoot relationship. Let’s look at two manufacturing options.

    If you pour the cast "as is", the plane of the mets will parallel the tabletop and the heel will sit at a ten degree inverted angle when you pour it. The dorsal surface of the poured cast will also parallel the tabletop. After you remove the negative cast and place the positive cast back on the table, the top of the cast will parallel the tabletop, the heel will be ten degrees inverted, and the forefoot will parallel the tabletop. It will sit "as is" or in other words, just as it was poured. When you balance this cast, you will add a ten degree valgus plaster wedge under the met heads. As a result, the corrected cast will sit with the plantar surface of the balance platform parallel to the table top, the heel vertical, and the dorsal surface of the cast and the plane of the met heads will be ten degrees everted to the tabletop.

    Another option that has exactly the same effect is to pour the negative cast with the heel vertical by wedging the cast laterally under the forefoot during the pouring process. After you remove the negative cast and set the uncorrected, positive cast on the tabletop, the heel and the top of the cast will sit ten degrees inverted to the tabletop, and the plane of the met heads will parallel the tabletop. When you balance the cast, you will add a ten degree valgus wedge of plaster under the met heads. When you place the corrected (balanced) cast back on the tabletop, the top of the cast will parallel the tabletop, the plane of the met heads will be ten degrees everted to the tabletop, the heel will be vertical, and the plantar surface of the balance platform will parallel the tabletop.
    Both of these techniques result in exactly the same plantar contour of the positive cast. They are two slightly different techniques to achieve the same end result. Either technique is completely acceptable.

    Respectfully,
    Jeff
    www.root-lab.com
     
  20. In other words:
    No, it doesn't matter.
     
  21. Little Sesamoid

    Little Sesamoid Active Member

    Hi Simon and Jeff,
    You both have made my day........THANK YOU SO much for clarifying this for me.
    Cheers,
    LS
     
  22. Alex Adam

    Alex Adam Active Member

    The pouring of the cast should always refect the intent of the Podiatrist and the condition presented in the clinic. If the professional has taken the cast while the sub talar joint is in neutral then any forefoot alignment pathology should be reflected in the pour and so balancing can be carried out. In cases of uncompensated or partially compensated rearfoot varus the calcaneous should reflect the the degree that the subtalar joint is at RCSP.
    In cases of equinus then the forefoot to rearfoot alignment should be neutralised by the pour technique.
    The essesnce is the continuation of the frame of reference throughout the manufactuing process, It's all in the physics of gravitational effect and lines of force through the foot.
    Hope this helps
    Alex Adam
     
  23. DaVinci

    DaVinci Well-Known Member

    Did you keep a straight face while you wrote that? :drinks I assume you have not seen CP's data on how this approach works really well on inanimate objects (ie a positive cast) but dosen't work on animate objects (ie a foot).
     
  24. Jeff Root

    Jeff Root Well-Known Member

    Is CP's data the final authority this subject? Is no more research requried? If so, does CP's data tell Alex how to treat the foot differently that he currently does? Although I don't agree with some of Alex's conclusions or opinions, what evidence is there to suggest that he is not necessarily treating the foot correctly? What is the constructive value of your posting and how might it help others better their treatment of foot related pathology?

    Respectfully,
    Jeff
    www.root-lab.com
     
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