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Inverting the neg cast 5/10/15 degrees

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Berms, Apr 9, 2008.

  1. Berms

    Berms Active Member

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    Can anyone help me with a little explanation of the effects that inverting the neg cast 5 - 15 degrees has on the biomechanics of the foot? (not Blake style inverted device).
    Aside from creating a device (theoretically) with a greater supination moment, does it affect the 1st ray function or hinder 1st met plantarflexion?

    Apologies if this a very basic question.
    Thanks for any advice.
  2. http://www.apodc.com.au/AJPM/Contents/Full text/Vol40/abstracts/Vol 40_1_14-18abstract.pdf

    Given the findings of the above study, one would assume that this should influence 1st ray function. However, the effects of the cast inversion in isolation are likely to be blurred (in a clinical situation) by other orthoses prescription variables, i.e. medial addition, extrinsic rearfoot post, 1st ray cut out etc.
  3. efuller

    efuller MVP

    Beyond what Simon mentioned the lateral expansion plaster is important as well. As the cast is inverted the medial lateral distance of the heel cup can be decreased. To correct for this you have to add additional expansion plaster. If you have significant experience with plaster casts you will know that there are a lot of possible shapes of heel cup that you can create as you add the expansion plaster. It is possible to add expansion plaster in such a way that you could negate the effect of inverting the cast. The increased supination moment comes from the medial side of the heel cup being higher off of the supporting surface than the lateral side of the heel cup. (Take a frontal plane slice of the heel cup and make this measurement about 1/4 of the width of the whole heel cup from either side of the center. This is not the height of the edge of the heel cup.)

    As you invert the cast, the height of the medial arch (of the original cast) becomes higher off of the surface supporting the cast. If you do not add more medial expansion plaster then the medial arch height of the orthosis will be higher. In his chapter in Ron Valmassey's text Blake says to add medial expansion plaster to make the finished height of the orthosis the same as it would have been if the cast had not been inverted. Again you can cheat in the lab to get what you want. I tell the lab how high I want the medial arch to be in mm.

    Hope this helps,

  4. Actually, I originally posted this in a bit of a hurry. On reflection, I've misread abstract for this paper- they are saying that inverted devices did not significantly change 1st MPJ dorsiflexion -sorry. However, as Eric has mentioned the Blake rationale suggests "back filling the arch", so it's not the same. Nice little study to be done then.

    I've found the same abstract here: http://www.ncbi.nlm.nih.gov/pubmed/16415281
    that gives author details etc.

    Check out the mean dorsiflexion values of 80+ degrees though!!!!!!!!!! If they got pronated feet and getting a mean (I presume this was mean value, but they actually don't say in the abstract) of over 80 degrees dorsiflexion, no wonder the orthotics only changed the mean by a few degrees.

    Also no sample size/ subject demographics reported in the abstract.
    Last edited: Apr 9, 2008
  5. Craig Payne

    Craig Payne Moderator

    Also keep in mind that orthotics are really good at holding an inanimate plaster cast in the right position, but not so good at holding an foot in the right place
  6. Berms

    Berms Active Member

    Thanks Simon. So when inveting the cast say 10 degrees - what pescription variables would you add in order to prevent neg effects of 1st ray function? A simple 1st ray cut out? or something else?
  7. Berms

    Berms Active Member

    Yes, very good point... this is something I need to always keep in mind.
  8. CraigT

    CraigT Well-Known Member

    When I make a device and invert the cast, I look at it as 'dialling in' the amount of force that you wish to apply... much the same way as you increase the Kirby skive by making the skive deeper. Often people freak out with the concept of a 30 degree inversion, but you need to remember that you are not actually inverting the foot 30 degrees (as Craig said above)- it is a quanification of the force you are applying (at least that is my theory...)
    As well as applying an inversion, you also are applying inclination of the cast... which may be another usefull effect to resist calcaneal declination.
    The thing to remember is that when increasing the inversion, you have to be pretty creative with your plaster additions- there is a lot of fill, and it is pretty easy to lose the shape of the foot. I used to do these myself, and any time I asked a lab to do them, I was not happy with the result.
    You must have a lot of fill for tolerance and to avoid issues such as jamming of the first ray.
  9. Depends what you mean by negative effects on first ray function. For example, if we have a patient with painful structural hallux limitus, if we limit 1st MTPJ dorsiflexion by increasing dorsiflexion moment beneath the already dysfunctional first ray, this will reduce the patients pain during propulsion. So is this a positive or negative effect on 1st ray function?

    In general, I agree with Craig T and manipulate the amount of medial addition to achieve the results I'm looking for. You could also use a cut-out or reduced or increased (depending on what you are trying to achieve) shell thickness in the area of the 1st ray.
    Last edited: Apr 10, 2008
  10. Craig:

    Excellent posting!:good:....could have taken the words right out of my mouth!


    Now onto your very good question. When the positive cast is inverted, a few changes occur in the resultant orthosis:

    1. The heel cup becomes more inverted.
    2. The medial longitudinal arch becomes higher.
    3. The lateral longitudinal arch becomes lower.
    4. There is either an increase in forefoot varus correction, a decrease in forefoot valgus correction or a change from a forefoot valgus to a forefoot varus correction in the orthosis.
    5. The orthosis becomes slightly more narrow.

    These changes in the frontal plane positive cast angle during positive cast balancing will certainly tend to increase the external subtalar joint (STJ) supination moment since now, with orthosis inversion, orthosis reaction force (ORF) will be directed more toward the medial aspect of the plantar foot and less toward the lateral aspect of the plantar foot. However, this does not also mean that STJ supination and improved gait function will always result with this type of modification. One of the problems with excessive STJ supination moments being applied (and this will be a focus of my complaints with Ed Glaser's "Sole Supports" excessively high arched orthosis designs in our panel discussion) is that other pathologies and symptoms may be created if the external STJ supination moments applied by the orthosis are not "just right".

    For example, excessive STJ supination moments from an inverted or excessively high-arched orthosis design may create the following problems for the individual wearing these orthoses:

    1. Increased medial longitudinal arch irritation.
    2. Increased late midstance pronation.
    3. Medial hallux pinch tylomas.
    4. Lateral ankle instability/inversion ankle sprains.
    5. Pereoneal tendinitis/tendinosus.
    6. Peroneal muscle fatigue.
    7. Lateral midfoot pain.
    8. Tylomas plantar to lateral metatarsals.

    Therefore, contrary to what some individuals think, the optimum orthosis design is not always the highest arched, most aggressive inverted orthosis, but is one that best eliminates the patient's symptoms, that optimizes their gait function and doesn't cause other pathologies/symptoms to occur. I believe that Howard Dananberg, Simon Spooner, Eric Fuller and I are very much in agreement on this philosophy....but I'm sure they will correct me if I'm wrong.
    Last edited: Apr 10, 2008
  11. efuller

    efuller MVP

    Craig T,

    I agree up to a point. And if you are going to invert a cast over 30 degrees you had better inspect the cast work. I've seen 45 degree inverted orthotics that ended up with a "V" shaped heel cup. The original cast of the heel had a relatively square appearance and the lateral expansion addition did nothing to lower the lateral portion of the heel cup, thus creating the "V" shape. The heel bisection on the cast was 45 degrees inverted, but effect in the shape of the heel cup would probably be less than if it had been inverted 10 to 15 degrees.

    Craig, from your post I think you agree that the belief that the position of the heel bisection of the cast determines the amount of increase in inversion moment is just voodoo biomechanics.


    Eric Fuller
  12. Adrian Misseri

    Adrian Misseri Active Member

    I'm lucky enough to have a guy who comes in and does all of my orthotic work for me on site, so I get great flexibility with the devices I prescribe. I modify and construct as I need. My idea behind this is simple, know where you want the force and how much to apply. By inverting the cast 30 degrees, and playing with the plaster fill and additions, you can specificlaly target areas, i.e. sustanaculm tali for more calcaneal inverson, talo-navicular joint for more midstance, excessive midtarsal motion. Aggressively inverting the device and being creative with the plaster also allows me to have a play with creative additions to devices, like an exaggerated medial flange on an EVA device that will run up medially along the side of the navicular and cuineform, which makes more use of the shoe in those difficult patients ith excessive transverse plane motion through the midfoot. As we all seem to agree, correct force to the right area for correction to the right area. Plus it's quite enjoyable to get one's hands dirty and play with a bit of plaster and experement!


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