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Cast Corrections

Discussion in 'Biomechanics, Sports and Foot orthoses' started by DSP, Mar 5, 2008.

  1. DSP

    DSP Active Member


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    Dear All,

    After reviewing my orthotic laboratory’s prescription sheet I have a few questions for those of you reading along:

    1. How many of you assign a number when balancing the forefoot (intrinsically)?

    2. What is the piont of assigning a number to the corrected forefoot position when it is going to be influenced by the corrected calcaneal position anyway? For example, a foot has a 5 degree forefoot varus deformity, the cast is balanced 5 degrees inverted, as a result, the forefoot would now have to be balanced 10 degrees inverted.

    Personally, I balance most of my casts either inverted or vertical, leaving the forefoot correction at 0 degrees, which means that the forefoot is always balanced in accordance with the corrected calcaneal position.

    Does anybody do this differently, and if so why?

    Any thoughts please…

    Regards,

    Daniel
     
  2. Admin2

    Admin2 Administrator Staff Member

  3. I don't have to use prescriptions because i have my own lab.

    However this is something i've questioned before now so i'll be interested in some of the replies.

    I have always had concerns about "cast correction" that after we take our cast with exquisite and milimetric attention to detail and make our prescription the lab then "corrects" everything to the point that the finished article bears little resembalance to the initial cast.

    I suspect many people would be shocked to see how much alteration is done at the lab.

    I would be very interested in carrying out a little experiment here. Take 5 patients with the same shoe size. Cast each one with the standard STN root technique and send prescriptions to a lab. Then when you have the 5 pairs of insoles back see how much difference there is between them and whether you can do a cinderella job fitting the right insole to the right person.

    This is one reason i prefer foam box casting. Your fixed reference plane is the ground rather than the bisection line of the heel. You can "cast correct" yourself to the precise amount desired without even getting your hands wet. You can be sure the cast is balanced. You can post intrinsically or extrinsically.

    IMHO it puts you much more in control of how the finished item will come out.

    Regards
    Robert
     
  4. tarik amir

    tarik amir Active Member

    There are too many errors with plaster casting and having orthotics manufactured by a laboratory.

    I do it, but I don't like it.

    When we take x-rays we have patients standing in thier angle and base of gait or at least weightbearing. There are very important reasons why we do this. However, myself and many others take casts non-weightbearing in which the osseous and soft tissue structures of the foot and leg have no relation to the ground surface.

    I have to say i have abandoned assigning numbers to forefoot correction. I also find Roberts Challange very interesting. He is pointing out that our orthotics are far from prescription devices.
     
  5. Houston, Gangming et al did an interesting paper in japma on Changes in Male Foot Shape and Size with Weightbearing (Journal of the American Podiatric Medical Association • Vol 96 • No 4 • July/August 2006)

    They found that :-
    and that.

    Now so far as bony alignment is concerned i'm not so sure that we WANT the foot in its WB configuration. However i do think there is significant benefit to capturing the soft tissues in their WB configuration in order to more accuratly control the skeletal structure.

    Not entirly what i meant. My "challenge" pertains to differnces in foot morphology for a single prescription (STN balanced).

    However our prescriptions incorperate a good deal of other things which WOULD be easily distinguishable between orthotics. Heel raises, extrinsic wedges, Medial heel skives, PF grooves, Cluffy wedges :rolleyes: FF varus / valgus exensions, met / met head cut outs, the material we use, etc etc etc.

    My point is that IMO the morphology of the foot we cast so carefully is soemtimes "lost in translation".

    Regards
    Robert
     
  6. Phil Wells

    Phil Wells Active Member

    Daniel et al

    If we are looking at forefoot alignment, we also need to address sagitall plane alignment.
    IMO if this is not addressed or corrected then the potential for patient problems is far more than frontal plane alignment.
    At my lab we state on the RX form that all impressions - foam and POP- have the forefoot corrected to 'zero' and then any forefoot posts are taken from this.
    We use CAD software and very accurate scanners so we can usually assess forefoot alignments better than doing it by eye.
    The issues tend to be when there is a plantar flexed forefoot on the rearfoot - what are we supposed to do with it - correct it, add a heel raise or add a pitch and assume the footwear requires this.

    It all comes down to my biggest annoyance when making insoles - why don't practitioners assess their impression before sending them in and advise us accordingly or ask for help?

    Rant over

    Phil
     
  7. Scott

    Scott Member

    I think the thing to remember about balancing the positive cast is that this dictates the amount of intrinsic forefoot posting being placed in the orthotic. When we forefoot post, the typical forefoot post is addressing the forefoot to rearfoot position with the MTJ locked and not the forefoot to the ground position of our completed foot orthotic device. If you are posting forefoot to ground and also applying a rearfoot post in varus, then you will have to put in degrees everted for balancing when intrinsically posting for a forefoot varus. Otherwise balancing vertical will place an intrinsic forefoot post that matches the forefoot to rearfoot position captured in the cast or scan submitted to the lab.
     
  8. Trent Baker

    Trent Baker Active Member

    Daniel,

    I prescribe as you have indicated, leaving the forefoot at 0 degrees and there fore balanced to the corrected calc' position. To post a forefoot would suggest that you have a set rigid forefoot position such as a forefoot varus, which are a rare thing if they actually exist al all.

    That should spark some debate, lol.

    Trent
     
  9. DSP

    DSP Active Member

    Dear All,

    Thank you so much for all your repsonses. I've had a really busy couple days in the office, so when i get some time i will repsond.

    Thanks again,

    Daniel
     
  10. DSP

    DSP Active Member

    Hi Scott,

    Agreed.

    Scott, could you please elaborate a little further. Are you implying that the corrected calcaneal position should be balanced everted in this instance? What would the benefit be of balancing a foot with a forefoot varus everted? Based on what you have just written, if the aim was to balance the forefoot to the ground, what would be the point in prescribing an orthotic then? If the intent was to prescribe a functional orthotic, then I would seriously doubt the effectiveness of this device.

    This is basically the point I am trying to make. What is the purpose of asking for a measurement when the forefoot is ultimately going to be dictated by the calcaneal position? Unless the forefoot is balanced in accordance with the rear foot, how can this measurement differ? If it does, then the corrected calcaneal position becomes inaccurate.

    Regards,

    Daniel
     
  11. Scott

    Scott Member

    Daniel,

    Scott, could you please elaborate a little further. Are you implying that the corrected calcaneal position should be balanced everted in this instance? What would the benefit be of balancing a foot with a forefoot varus everted? Based on what you have just written, if the aim was to balance the forefoot to the ground, what would be the point in prescribing an orthotic then? If the intent was to prescribe a functional orthotic, then I would seriously doubt the effectiveness of this device.

    You are correct. I did not explain myself as well as I would have liked. You would in fact end up balancing the positive cast inverted if you were attempting to intrinsically post the forefoot to the ground and not just the rearfoot in a patient with forefoot varus. However, if you are utilizing extrinsic forefoot posting for a forefoot varus you would want to balance the positive cast everted to eliminate any intrinisic forefoot posting. Again, provided you are posting forefoot to rearfoot and not forefoot to ground. I think that when you say the corrected calcaneal position you are somewhat misleading yourself. The vertical bisection of the calcaneus is the reference line in balancing, but the ultimate area influenced by balancing the positive cast is the forefoot.



    This is basically the point I am trying to make. What is the purpose of asking for a measurement when the forefoot is ultimately going to be dictated by the calcaneal position? Unless the forefoot is balanced in accordance with the rear foot, how can this measurement differ? If it does, then the corrected calcaneal position becomes inaccurate.

    Let me see if I can perhaps illustrate the difference. Most positive casts are poured vertically, meaning the vertical calcaneal bisection is perpendicular to the ground when the positive is poured and allowed to dry. The next step will be balancing the positive cast. This dictates the type and amount of intrinsic forefoot posting you are going to use in the orthotic. Let's say you have a patient with a 5 degree forefoot varus deformity. The positive cast is poured vertical, the positive cast is balanced vertical and a typical 4 degree varus rearfoot post is applied. When that orthotic is placed on a table the forefoot will be inverted 4 degrees to the ground due to the rearfoot post influence. If you balance the positive cast 4 degrees inverted instead of vertical then you will have more foot control as the forefoot has been posted to the ground and you will not have the same amount of motion available with the orthotic. If you were to use an extrinsic forefoot varus post of 5 degrees varus you then want to balance the positive cast 5 degrees everted to allow for the application of the 5 degree varus extrinsic post. I hope that example has made sense. A thought I like to maintain when it comes to orthotics is that they control foot function, they do not correct foot function. Again, hope this helps

    Scott
     
  12. efuller

    efuller MVP


    Intrinsic forefoot varus posting just raises the arch height of the device and medial expansion fill is added to bring the medial arch height of the orthosis back down to a tolerable level. Intrinsic forefoot varus posting makes no sense, because after heel off the deformity is not supported. Intrinsic forefoot posting makes some sense if you wish to increase the medial arch height of the orthosis.

    Scott's point about posting is quite valid when thinking about intrinsic forefoot valgus posting. When John Weed taught prescription writing for orthoses the (unstated) bottom line was to not evert the foot farther than it can go. A forefoot valgus post that is too high will attempt to evert the foot farther than it can go and cause high pressure on the lateral column or sinus tarsi pain.

    I have combined what John Weed attempted to do with forefoot to rearfoot, heel bisection and leg bisection measurements into a more real world attempt to look at how far the foot can evert. Standing, ask the patient to evert their foot without moving their leg. The height achieved, by the lateral column, with this maneuver is the maximum height that the forefoot valgus post. So, when I make my orthoses I ask for a height of intrinsic forefoot valgus post. (An uncompensated varus foot wil have a height of zero)

    When you ask for certain height of intrinsic forefoot valgus post you will change the position of the heel bisection of the cast. I fix the shape of the heel of the orthotic by adding a medial heel skive or lateral heel skive so that I get the desired effect in the finished orthotic. I base the desired effect on STJ axis position. A medially deviated STJ axis gets and inverted "appearing" heel cup. Average axis gets a "vertical" heel cup and a laterally deviated STJ axis gets and "everted" heel cup.

    Just because you capture a forefoot to rearfoot relationship in a cast does not mean you are bound to that relationship in the finished orthosis.

    I feel that an intrinsic forefoot valgus post is very important in selected conditions. For example peroneal tendonitis. In these feet you often want to increase the pronation moment and pronated position of the STJ. A forefoot valgus wedge can do this. Also patients with medial column overload also deserve an attempt at shifting the weight more laterally.

    I hope this helps,

    Eric
     
  13. DSP

    DSP Active Member

    Thank you very much for elaborating Scott, I found that very helpful. Thank you Eric for your detailed explanation - much appreciated.

    Regards,

    Daniel
     
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