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Cast taking and balancing

Discussion in 'Biomechanics, Sports and Foot orthoses' started by issy1, Mar 16, 2010.

  1. issy1

    issy1 Active Member


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    Hi, I have never used the medial/lateral heel skive modification before but am keen to start using this in my orthotic prescriptions. I understand that the skive modification should never be added to a vertically balanced cast. Could someone explain to me exactly how they take their casts for this modification and how the positive cast should be balanced as I will be sending my negative casts to a lab. who are not in the habit of doing heel skives but with clear instructions are happy to do so. I have found good instructions of how to create the heel skive itself but not cast balancing for the skive.

    Do I need to bisect the calcaneus and if so I assume you can't use foam boxes - If you use slipper casts do you take the foot in STJ Neutral/ non-weight bearing/semi weight bearing??

    Balancing the Cast - Do you hammer nail into 1st/ 5th MPJ until the rearfoot bisection line is inverted/everted to you desired degree?

    I would appreciate it if anyone could give me clear instructions as to the method they use to take the negative cast and how they balance the positive cast for the skive modification. Many Thanks
     
  2. You are quite correct. You cannot bisect the heel of a foam box cast. Well technically you can but it's completely pointless. The point of bisecting is to get the "base plane". In a suspension cast this is tricky because the foot is suspended (as the name implies) so we have to work off the rearfoot and extrapolate. In foam you do the balancing, not the lab. You get the forefoot to be flat to the base plane (viz the ground) or if you want an intrinsic post you impress the foot with the forefoot in varus / supinatus then simply press out the medial forefoot with your fingers. Same effect as the nail and plaster method only you get to control it. So when you send the box the 3 cardinell points, 1st met, 5th met and heel should all be the same depth in the foam. Cast balanced.

    Then for your skive you just measure up where and how much on the positive and grind away. That's the labs job.
     
  3. Why not? I do this a lot. I also add skives (both medial and lateral) to casts that are inverted and to casts that are everted. It's all about the final shape of the shell that you are trying to achieve. Think about what the balance actually achieves in terms of the final shell geometry in a) a foot with an everted forefoot and b) a foot with an inverted forefoot.

    In a weightbearing cast the forefoot to rearfoot alignment is lost. The superior surface of the positive cast becomes the datum. You can still invert or evert the cast by a known number of degrees from this datum and balance to this position. If you want to increase the medial longitudinal arch height you can invert the cast in the balance, if you want to increase the lateral longitudinal arch height you can evert the cast and balance. Not my cup of tea, but I know Robert likes weightbearing casts.

    P.S. hammering nails- yep you're on the money. You can also manipulate the foot during the casting process to achieve similar goals and also tinker with medial addition / lateral expansion on the positive.
     
  4. efuller

    efuller MVP


    I agree with Simon. You can skive any cast and the most important thing is the shape of the finished product that the patient stands on.

    Do you understand how an intrinsic forefoot post changes the shape of the finished product? Do you understand how a skive changes the shape of the finished product? You also have to understand what measurements of the patient make you want those changes in the finished shape.

    STJ axis position in the transverse plane is one of the most important variables in making that decision.

    Regards,

    Eric
     
  5. issy1

    issy1 Active Member

    Do you take your foam impressions weight bearing/ semiweight bearing - could you give me more information as to how you actually hold the foot in the position you want to acheive for taking the impression - do you weight bear the toes at all?
     
  6. issy1

    issy1 Active Member



    So is the main reason for inverting/everting the cast for skive to change the arch height?

    If you want to say incorporate a forefoot valgus into your orthotic to off load the medial column do you take your slipper cast with the rearfoot held in neutral/inverted position? - and then manipulate the forefoot into valgus position so that this adjustment is intrinsic in your orthotic in which case does this do away with the need for extrinsic forefoot valgus posting. How do you balance this orthotic to stop the lateral forefoot sitting up in the air?? Hope you can understand what I'm asking here:confused:
     
  7. Yeah that's all pretty much right. Balance with an intrinsic forefoot valgus post and lateral plaster addition. As Eric and I have already said- explore and understand what these modifications are actually doing to the shape of the orthosis.
     
  8. Generally semi weight bearing. Very, very rarely weight beating.

    Although its not really weight bearing. No the patients weight at least.

    What I do is this.

    1. Decide what postion I want the rearfoot and the forefoot relative to the ground and each other. If the NWB exam shows a forefoot rearfoot relationship you don't want balanced out this CAN be incorperated into a semi WB cast.

    2. Position the box directly under the foot with the patient sitting so their lower leg is upright. with knee at 90 degrees

    3. Hold the foot using 3 point pressure positioning the stj where I want it and the forefoot however i want it relative to the rf. If I need to I may press the FF slightly into the foam to get it right.

    4. Apply firm steady pressure through the patients knee with one hand and a shoulder whilst using the other to hold the foot in the desired position.

    5. If desired, hold the rearfoot steady and pronate (evert) the forefoot to plantargrade

    6. If desired use fingers to "balance" the cast or apply intrinsic wedging.

    I usually leave the toes alone when pressing the foot in. Allowing the hallux to dorsiflex plantarflexes the 1st met nicely and allows the foot to adopt a position in which I know the plantar apeuneurosis is not maximally tensioned.
     
  9. issy1

    issy1 Active Member

    Simon, just to confirm - do you mean hammer nail into 5th met. head of positive cast and then contour with plaster fill in which case you are increasing lateral arch?
     
  10. issy1

    issy1 Active Member

    Robert - Can I ask you the same question as I did Simon for slipper cast technique - eg. if you want to hold rearfoot in slight inverted position and forefoot in valgus position how do you have a balanced cast - I think you said earlier you do the balancing at negative stage?
     
  11. Increasing lateral arch height and decreasing medial arch height. So this makes the lateral side of the orthosis relatively stiffer and the medial relatively more compliant. Which should result in a more lateral (less medial) centre of pressure (CoP) position, all other factors being equal. You can achieve a similar aim by using an external oblique extrinsic rearfoot post, this will also stiffen the lateral side of the orthosis shell, but obviously the superior surface of the shell will have a different (lesser) curvature. Also you can have a shell of greater thickness beneath the lateral arch- this will relatively stiffen the device in this area too. Obviously you can do both/ all if you really want to pull the CoP laterally and also add forefoot valgus extension to continue effect through propulsive phase.

    Similarly you can stiffen the medial side of the device by increasing arch height and using an internal oblique rearfoot post. Thicken the shell medially etc.

    P.S. remember when you significantly invert or evert the cast you will also change the frontal plane profile of the heel cup too.
     
  12. issy1

    issy1 Active Member

    Thanks Simon - the fog is starting to lift!
     
  13. Get a positive cast and play with it on your desk!
     
  14. Jeff Root

    Jeff Root Well-Known Member

    We have a real terminology problem in the biomechanics/orthotics world! The lack of standard and consistent terminology makes discussions like these laborious and confusing at best.
     
  15. Do you want to expand on this, Jeff?
     
  16. Jeff Root

    Jeff Root Well-Known Member

    Simon, I think you did a good job of explaining things. Balancing is simply the act of adding an extrinsic modification (balance platform) to the forefoot of a positive cast in order to orient the cast in the frontal plane. A heel bisection is the most common method of establishing a reference for the balancing position. In other words, the heel can be balanced with the bisection inverted, everted, or vertical. Some people use the term "posting the cast", which only creates confusion. Posting is an extrinsic modification added to an orthotic shell.

    Manipulating the foam impression, scan, or a suspension cast can alter the forefoot to rearfoot relationship, but it has nothing to with balancing of the positive cast. For example, we can cast the same foot three different times with the forefoot inverted, perpendicular, and everted relative to rearfoot (heel bisection). No matter how these casts are balanced, the forefoot to rearfoot relationship is fixed in the positive cast. The balancing position only alters this fixed relationship relative to the supporting surface (ie. the counter top). Any other modification to the positive cast, be it heel expansion, arch filler, or a heel skive is done to manipulate or alter the plantar contour of positive cast proximal to the balance platform.

    Respectfully,
    Jeff
    www.root-lab.com
     

  17. Agreed. Re-reading the posts I see that misinterpretation hovering.

    Hope you are well Jeff.

    Best wishes,
    Simon
     
  18. Ann PT

    Ann PT Active Member

    Jeff,

    Thanks for clarifying some confusing terminology. Can you comment on "building" a cast inverted vs. "balancing" a cast inverted? I believe some labs make this differentiation.

    Thanks,
    Ann
     
  19. Jeff Root

    Jeff Root Well-Known Member

    Ann, good question and I would be happy to answer it to the best of my ability. I have never heard of the term “building a cast inverted”. There are several terms used by labs to describe the same cast modification process. Some labs refer to this as cast correction, cast modification, balancing, cast build-up, cast dressing, etc. Balancing is actually one aspect of the cast modification process. The other aspects include medial and lateral expansion (fill), accommodations, skives, etc.

    There are several different methods of cast correction (ie. cast modification) that can accomplish essentially the same result. The term “balancing” a cast means applying a plaster balance platform under the metatarsal heads to influence the frontal plane position of the positive model of the foot. For purposes of illustration, let’s assume that we have a cast in which the plane of the metatarsal heads is perpendicular to the bisection of the heel (ie. a cast with no forefoot varus or valgus).

    When we fill (pour) the negative cast with plaster to create our positive model, the heel will rest vertical and the forefoot will be parallel to the countertop (assuming that the negative cast is consistent in thickness and doesn’t alter how the cast rests on the countertop). When we remove the negative cast from the positive cast, the dorsal surface of the positive cast will sit parallel to the countertop (and parallel to the plane of the forefoot and perpendicular to the heel bisection, which was remove with the negative cast). Our new heel bisection reference is the top of the positive cast because we know that the top of the cast is perpendicular to the heel bisection.

    Now, let’s assume that we want to correct the cast with the heel ten degrees inverted. If we place a level on the top of the cast and invert the cast ten degrees, we know the heel bisection is now ten degrees inverted. When we add our plaster balance platform under the met heads, the top of the cast should sit ten degrees inverted. I would say that this cast is now balanced (ie. corrected) ten degrees inverted.

    Another option is to pour our negative cast with the heel bisection ten degrees inverted by placing a wedge under the medial aspect of the forefoot of the negative cast. After we pour our negative cast, the heel is ten degrees inverted when we bring the dorsal surface of the positive cast parallel to the countertop. Our balance platform for this cast will support the heel ten degrees inverted when the top of the cast is level. These are two slightly different methods of accomplishing the same thing.

    If I had to guess as to the answer to your question, I would say that two labs are using different terms to describe the same process. If you haven’t done so, you might want to look at the “technical topics” link at the Prescription Foot Orthotic Laboratory Association (PFOLA) website http://pfola.org. Perhaps this information will help you get a straight answer from the labs in question.

    I hope this helps.
    Respectfully,
    Jeff
    jroot@root-lab.com
     
  20. efuller

    efuller MVP

    I'm not sure you got all your questions answered.

    I think (but I'm not sure) the original intent of the inverted cast technique was to invert the heel cup of the orthotic. I'm not sure exactly what Richard Blake was thinking when he decided to try the inverted cast technique. Kevin, in his medial heel skive paper noted that when you have a round heel inverting doesn't do much. When you invert a circle you still have a circle. I believe Richard Blake wrote a chapter about the technique in the text edited by Ron Valmassy.

    Regarding forefoot valgus intrinsic post.
    The post should be made so that there is contact as far distal as possible under the fifth metatarsal shaft. I will often add a valgus cork extension under the metatarsal heads when I use a larger amount of valgus intrinsic forefoot post. Beyond a certain point the intrisic post will start to push the limits of the volume of the shoe. I usually have no problem with a 5 degree post.

    There is an interesting study there. What are the pressures at the 5th met head with varying amounts of intrinsic post? What are the pressures over the entire 5th ray with varying amounts of intrinsic post?

    Regards,
    Eric Fuller
     
  21. Jeff Root

    Jeff Root Well-Known Member

    Rich's original justification for inverting the heel to was to allow for the varum attitude of the tibia and therefore the heel, in runners. He theorized that if the tibia was inverted during running, the heel had further to everted than when it was not so inverted. He thought inverting the heel could reduce the demand or range of pronation in runners with a varum gait.

    It's important to recognize that the term "inverting a cast" is a little misleading. When we invert a cast we are only measuring motion in the frontal plane. In actuality, we are make triplane changes in the position of the cast, and therefore the shape of heel (ie. heelcup).

    If you invert a cast in space, this would be a pure frontal plane motion. The axis of rotation of the cast would lie in the sagittal and transverse planes with motion occurring in the third plane (the frontal plane; ie. pure inversion). When we invert a cast that is resting on the countertop, the axis of rotation shifts as we increasingly invert the cast.

    When a cast is highly inverted as in the Blake technique, it rotates around an axis that runs from the plantar aspect of the heel to the plantar aspect of the 5th met head. The axis is abducted. This abducted axis results in triplane changes in the position of the cast.

    If you look at the changes that occur at the heel cup, the medial heel is medial to the axis of rotation. There are sagittal plane changes that occur which result in a significant increase in the slope (ie. inclination angle) of the medial aspect of the heel where it transitions into the medial arch. I do agree with you that the anatomical shape of individual heel can influence the effectiveness of cast inversion. If an individual has a greater anatomical inclination angle of their medial heel, inverting the cast will have a greater influence than it would on someone with a low medial inclination angle of the heel. The best way to appreciate this is to invert a positive cast and look at the anterior, medial aspect of the heel.

    Regards,
    Jeff
     
  22. Issy:

    A few points for you. When I was a student and Biomechanics Fellow at CCPM, the terms "balancing the cast inverted" and "pouring the cast inverted" and "building the cast inverted" meant basically the same thing. As the others have said, the heel bisection of the negative cast was "balanced" in an inverted fashion on the table with a wedge of material under the first metatarsal head, the liquid plaster was "poured" into the negative cast while the negative cast was resting inverted on the table and then the resultant positive cast was "built" so that the top surface of the positive cast was level to the ground (and the heel of the cast inverted to the ground).

    Even though these terms were commonly used at CCPM in the 1970s and 1980s, they may not currently be standardly used in other parts of the world.

    Regarding the medial heel skive, the cast construction technique was described in my original paper (Kirby KA: The medial heel skive technique: improving pronation control in foot orthoses. JAPMA, 82: 177-188, 1992). If you contact me privately, I will give you the password to my website where you can access a clean copy of the paper.

    Good luck.:drinks
     
  23. Ann PT

    Ann PT Active Member

    Thanks Jeff. Reading your answer made me realize the question I meant to ask was what is the difference between "building" and "pouring" not "building" and "balancing." I believe you did answer my question regarding each technique. Correct me if I'm wrong but I understand you to say that with one technique you are building the positive mold inverted (after pouring the positve with the negative sitting neutral on the counter)and in the other you are pouring the negative cast inverted right from the start. Is there a difference in terms of the resulting positive mold and orthotic (assuming all other variables are the same)?
     
  24. Ann PT

    Ann PT Active Member

    Jeff,

    After rereading your response I wonder if method number one (inverting the positive and building the balance paltform) results in the entire positive cast being inverted relative to the counter and method number two (pouring the positive with the negative supported in an inverted position then building the balance paltform so the met heads are parallel to the counter but the heel is inverted) results in a positive with only the heel inverted??

    Thanks,
    Ann
     
  25. Jeff Root

    Jeff Root Well-Known Member

    Ann, as a point of clarification, it is import not to use the terms vertical and neutral interchangeably. The bisection of the heel could be inverted, vertical or everted when the STJ is in the neutral position. It is impossible to tell the neutral position of the foot (i.e. the heel) by looking at a negative or positive cast of the foot.

    For example, a patient could have the following STJ rom:
    Total range of inversion of the heel with maximum supination: 20 degrees of inversion
    Total range of eversion of the heel with maximum pronation: 4 degrees of eversion
    Total STJ rom: 24 degrees
    Neutral position: 24/3=8 so neutral is 8 degrees from maximum pronation or in other words, neutral position is 4 inverted (i.e. 4 degrees of rearfoot varus)

    Let's assume you cast this foot in STJ neutral. If you wanted to correct to neutral, you would have to place the heel of the positive cast 4 degrees inverted to the countertop when the cast is balanced. If you corrected the heel to vertical, this neutral position cast would be 4 degrees everted from the mathematical neutral position of the foot. The practitioner (not the lab) should decide which position the heel of the cast should be corrected to.

    Now back to your question. Yes, in theory both methods should result in identical orthotic shell shapes. In reality, there can be some differences related to the lab technician's different perspective of the foot. This seems to more of a factor when you start using high ranges of inverted correction, such as in the case of the Blake inverted technique (typically 15 to 45 degrees inverted). My lab pours Blake casts inverted because it makes the plaster correction work easier for our technicians. With high degrees of inversion, the corrections become more subjective and the two techniques can (not necessarily do) lead to noticeable differences.

    Respectfully,
    Jeff
     
  26. Ann PT

    Ann PT Active Member

    Thank you Jeff for clarifying the important difference between neutral and vertical. I should have said "after pouring the positive with the negative sitting with the heel bisection vertical"

    I think part of my confusion is in differentiating between techniques as they impact the heel vs. forefoot. Once the balance platform is applied to hold the heel inverted for example, doesn't this effectively evert the forefoot on the hindfoot? Wouldn't this create a similar effect to the Blake where a lot of medial arch fill is added to the positive to lower the arch height of the orthotic and decrease heel slippage laterally? When are you inverting the whole positive (heel and forefoot) vs. just inverting the heel?

    The more I think about this, the more confused I get!

    Thanks,
    Ann
     
  27. Ann PT

    Ann PT Active Member

    Kevin Kirby's newsletter in October 1991 talks about balancing the heel of a negative cast inverted six degrees and that this will result in the forefoot also being inverted six degrees. I thought balancing the heel involved using a platform across the met heads to hold the heel inverted which would result in only the heel being inverted. Therefore the patient with a perpendicular forefoot to rearfoot standing on this "balanced" orthotic would have an inverted heel and be pronated at the LMTJA rather than supinated as Kevin speaks of in his newsletter.

    Kevin...want to jump in to this conversation and help clarify?
    My original question of pouring inverted vs building inverted developed after switching to Precision and they differentiated the two!

    Ann
     
  28. Ann:

    If a patient with a perpendicular forefoot to rearfoot relationship stood on an orthosis made with a 5 degree intrinsic forefoot varus correction (which could be achieved by balancing their negative cast 5 degrees inverted) then this would tend to "supinate the longitudinal midtarsal joint axis (LMTJA)" not pronate it. And, by the way, the terminology of "LMTJA" is older terminology that should not be used any more, other than for interpreting older publications on midtarsal joint biomechanics (Nester CJ, Findlow A, Bowker P: Scientific approach to the axis of rotation of the midtarsal joint. JAPMA, 91(2):68-73, 2001).

    If a patient with a 7 degree forefoot valgus is casted and then that cast is balanced 5 degrees inverted, the resultant orthosis would have a 2 degree intrinsic forefoot valgus correction.

    If a patient with a 2 degree forefoot varus is casted and then that cast is balanced 4 degrees inverted, the resultant orthosis would have a 6 degree intrinsic forefoot varus correction.

    If a patient with a 5 degree forefoot valgus is casted and then a 25 degree Blake inverted orthosis is made, there would be no intrinsic forefoot valgus or varus correction since Blake positive cast balancing technique eliminates any intrinsic forefoot balancing.

    Hope this helps.
     
  29. Ann PT

    Ann PT Active Member

    Thank you Kevin. Your response was very helpful.

    Three Options
    Balance the cast 4 degrees inverted- either by pouring the negative 4 degrees inverted or building the positive 4 degrees inverted
    4 degree intrinsic forefoot post
    4 degree forefoot cast correction

    Do we know where in the foot the majority of the force from the forefoot correction occurs?
    Are these all synonymous and will they all create effectively the same orthotic all other things being equal?

    Thank you for all your help-present, past and future!

    Ann
     
  30. Ann:

    I was taught using Subtalar Joint Neutral (STJN) Theory proposed by Root et al during podiatry school from 1979-1983 at the California College of Podiatric Medicine (CCPM). I was taught that the prescription foot orthosis should be balanced to heel vertical in nearly all feet. The STJN Theory proposed that by making an orthosis with the amount of forefoot valgus or forefoot varus correction that was measured in the foot with the subtalar joint (STJ) held in neutral position would prevent "compensation" of the foot into either an undesirable pronated position or an undesirable supinated position. It was taught also that a properly made foot orthosis would hold the foot in the STJ neutral position by "preventing compensation for forefoot varus or forefoot valgus deformities".

    Today, we know that the STJN Theory has numerous problems with it. One of these problems includes the fact that even though we cast the foot in the STJ neutral position, the foot orthosis, in most cases, does not "hold the STJ in the neutral position" and, in fact, often does not change the relaxed calcaneal stance position of the foot when comparing standing with the orthoses to standing without the orthoses under the foot.

    In addition, I was taught by the CCPM biomechanics professors that foot orthoses worked by "locking the midtarsal joint". I still don't know what "locking of the midtarsal joint" means but this term was thrown around CCPM daily in the biomechanics clinic with nobody, to my knowledge, ever giving this term a precise definition. Eric Fuller can probably also chime in here as to what he recollects since he was one of my former students.

    With this brief review of the journey that I have made over the past 25+ years from being a "disciple" of STJN Theory to where I am now, it is important that you realize that I believe that the STJN Theory makes things much too complicated and confusing especially considering our current level of foot and lower extremity biomechanics knowledge. When I teach orthosis biomechanics in seminars and at the California School of Podiatric Medicine, I emphasize that the way that foot orthoses produces kinetic and kinematic changes in the foot and lower extremity is by simply altering the location, temporal patterns and magnitudes of ground reaction force (GRF) that act on the plantar foot during weightbearing activities. If these simple principles that I have tried to promote are adhered to by the professor of biomechanics, even an intelligent high school physics student could be taught the basics of how foot orthoses work and how many foot and lower extremity pathologies may be effectively treated with custom foot orthoses.

    To specifically answer your question, the majority of the force from a foot orthosis with a forefoot correction will be dependent on the plantar forces acting on the foot during weightbearing activities from the orthosis. Changes in orthosis structure will change the temporal patterns, magnitudes and plantar locations of GRF acting on the plantar foot, which will, in turn, alter the moments acting on the ankle joint, STJ, midtarsal and midfoot joints and metatarsophalangeal joints during weightbearing activities. Knowledge of how these orthosis shape and stiffness alterations can affect foot kinetics and kinematics is the key to becoming an expert clinician in custom foot orthosis therapy.

    Hope this helps.
     
  31. Jeff Root

    Jeff Root Well-Known Member

    Ann, your original question to me was a pure technical question about orthotic manufacturing techniques. As a result, I intentionally attempted to avoided dialogue that would draw us into a potential theoretical discussion or a debate. Let me continue to try an answer the technical aspect of your line of questioning before we get involved in other topics.

    I will try to give you a historical perspective to help you better conceptualize the influence of cast balancing. Prior to Dr. Roots development of intrinsic cast correction (ie. cast balancing), a non-corrective forefoot platform was added under the met heads of the positive cast and the orthosis then received an extrinsic forefoot post in order to orient the shell in the desired position in the frontal plane.

    A non-corrective forefoot platform is a plaster platform that has a contact point under the 1st and the 5th met heads. As a result, this cast will sit with the plane of the met heads parallel to the countertop. A Rohadur shell was then pressed over the cast. A wedge was then placed under the forefoot of the orthotic shell to bring the shell into the desired angle (the corrected position) in the frontal plane. With the shell at the proper angle, the heel cup was ground to the appropriate height and the shell was ground to the desired width and length.

    The next step involved the addition of the extrinsic forefoot post. A wedge of methyl methacrylate (dental acrylic) was attached under the distal edge of the shell to support the orthotic shell in the corrected angle.

    Fore sake of example, let’s assume we are working with a cast that exhibits a ten degree everted forefoot to rearfoot condition (10 degrees of forefoot valgus). The positive cast will rest with the heel ten degrees inverted because the non-corrective forefoot platform will support the cast with the mets parallel to the counter top. A non-posted orthotic shell made from this cast will also rest ten degrees inverted. After application of the extrinsic post, the orthosis will now rest with the heel vertical and the plane of the forefoot ten degrees everted to the countertop.

    The anterior, lateral edge of the orthosis will have an abrupt drop-off due to the extrinsic forefoot valgus post. In fact, the distal edge of the shell will have a vertical drop off with the highest point lateral. As a result, the distal edge of orthotic shell must be ground to create a smooth radius because the vertical drop-off would probably not be well tolerated.

    When most labs balance a positive cast, they put a slight amount of filler proximal to the balance platform to create the same type of radius that would have been created had the shell been posted extrinsically. Therefore the goal of an intrinsically or extrinsically corrected orthosis is to support the forefoot in its angle of deformity. The angle of deformity of the forefoot changes relative to the countertop when you invert or evert the heel. Kevin gave some excellent examples of this.

    I think your line of questioning demonstrates that you have an excellent level of knowledge because you couldn’t have asked these questions without it. I hope this description helps you achieve a better level of conceptualization for what you are attempting to achieve with your orthoses.

    Respectfully,
    Jeff
     
  32. efuller

    efuller MVP

    I just realized I left out a step in the thought process. There is an inheirent assumption made in inverting the heel because there is a running limb varus. The assumption is that the calcaneus has a flat bottom. Although it is not often explicitly stated, many of the cast balancing protocols taught by John Weed seemed to be based on the calcaneus having a flat bottom that was perpendicular to the heel bisection. Weed's protocols, when boiled down, were to not evert the heel bisection of the calcaneus further than it could go. I think the overall concept is important, but there are a few problems with using calcaneal bisections in practice. (If anybody asks I'll expand upon that.)

    So, there is an assumption that when you balance the heel bisection of a cast inverted you will cause the foot on top of the finished orthotic to invert. To his credit, Blake actually looked at heel bisections of feet on top of casts and found that there was not a 1:1 relationship of cast inversion to foot inversion. I attended different lectures where he gave different numbers for the relationship that he found. In reality, I would bet that there is quite a range of potential resposnses to an inverted cast. There are two different potential causes for the variation.

    One potential cause is the variability of the shape of the finished orthotic with different amounts of inverted orthotics. I once saw a 45 degree inverted orthotic that essentially had a "v" shape. If the calcaneus had a flat bottom, it could go either way, or just sit balanced on the slopes of the "v". I believe that inverted devices work by creating a varus wedge effect. When you stand on a varus wedge you will shift the center of pressure under the foot and this will change the moment from ground reaction force.

    The other potential source in variability across individuals to an inverted device lies in the variation across feet. A shift in the center of pressure in a foot with a laterally positioned STJ axis will be much more lilkely to crate a supination moment than in a foot with a more medially positioned axis. In the medially positioned axis foot, the shift in center of pressure will reduce a pronation moment from the ground, but will not create a net supination moment.

    The original question was about how different cast modification techniques change the shape of the finished orthotic. A good lab technician could use each technique and make the exact same finished orthotic. It wouldn't be easy, but could be done. It is important to remember there is a lot of "art" that can happen after the cast of the foot has been taken. The end result is the piece of plastic that the patient stands on. There are many ways to reach the same shape of that piece of plastic.

    Regards,

    Eric Fuller
     
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