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Development in casting technique 2014

Discussion in 'Biomechanics, Sports and Foot orthoses' started by fabio.alberzoni, Feb 16, 2014.

  1. fabio.alberzoni

    fabio.alberzoni Active Member


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    Hi everybody!

    After 3 faboulous day at Zaragoza's congress I come back really confused!

    In the congress Kirby, Fuller, Payne and Spooner spoke about:

    MTJ lockin isn't real...new mtj concepts
    zoos
    tissue stress theory
    orthosis's geometry
    and more...

    At the end of the congress I spoke with Simon Spooner askin him why I should continue to cast with MTJ in "lockin position" and STJ in neutral position...

    That' s because new biomechanics discoveries make the Root's criterions no more valuable.
    I asked him if these new discoveries mean that the new criterion for a normal function of the foot is that position of zoos(zone of optimal stress) for the tissues...

    He told me that sometimes he cast in different positions to change the foot orthosis's geometry and that is important to have a good anatomical and biomechanical knowledge.
    I thoght: wow! great! I'm gonna try it!....

    well...I feel "locked" like MDJ in Root's biomechanics!!

    Someone (or Simon please!! :bang:) have experience about how apply tissue stress theory to different way to cast?

    I really like that someone told me about one of his patient explaining me how he cast differently and why!


    cheers,
    fabio
     
    Last edited: Feb 16, 2014
  2. Craig Payne

    Craig Payne Moderator

    Articles:
    8
    The basic principle to start from for "casting" to to cast or model the foot in the position that will allow you to deliver the design features in the foot orthoses to achieve the goal that you want. I know that this is a somewhat lofty or theoretical claim but its all abut reducing the stress in the tissue that is problematic. You work out what the design features are that will reduce the stress in that tissue, then take a "cast" or model the foot to allow you to deliver those design features. Start from this point and work forward.
     
  3. Fabio:

    Glad you enjoyed the Zaragoza seminar.

    First of all, just because Simon and I said that the term "midtarsal joint locking" is not the best term to use in describing dorsiflexion loading of the lateral forefoot, this does not also mean that you would change all your negative casting techniques in your patients. It just means that the term "midtarsal joint locking" is a poor term to describe the spring-like behavior of the midtarsal and midfoot joints of the forefoot just as it would be improper to say that the leaf-springs in the rear axle of your pickup truck are "locked" when the truck is being driven with no load in the bed of the truck.

    Like Simon, I modify my negative casting techniques of the foot (I still use plaster casting of the foot in the supine position) depending on the patient I am treating. I may pronate the subtalar joint in the negative cast. I may plantarflex the medial column in the negative cast or I may dorsiflex the medial column of the negative cast. All of these negative casting modifications are done in order to change the shape of the negative cast so that a more therapeutic and comfortable custom foot orthosis may be made for the patient.

    I cover many of these ideas in my new book (Kirby KA: Foot and Lower Extremity Biomechanics IV: Precision Intricast Newsletters, 2009-2013. Precision Intricast, Inc., Payson, AZ, 2014) on how to design orthoses to better relieve the stress in the tissues in the more common pathologies that podiatrists see in their practices. Then, with that information in hand, I would consider how different foot geometries in the negative cast may influence the resultant foot orthosis and give some different casting positions a try.

    Good questions and good luck with your casting experimentation!:drinks
     
  4. blinda

    blinda MVP

    I won't say I told you so, Fabio. ;)
     
  5. efuller

    efuller MVP

    When my instructors talked about "locking" the midtarsal joint when they casted they described a position of the CC joint where there was some tensile load in the plantar lateral ligaments, specifically the long and short plantar calcaneal cuboid ligament. They didn't actually say that, but I think this is what they were doing. To get to this position you have to dorsiflex the cuboid on the calcaneus. Their description of casting method would seem to do this.

    The alternative to this position is to plantar flex the cuboid on the calcaneus. My instructors said that when you plantar flex the lateral forefoot on the rearfoot you often got orthotics that were painful. It made sense to me in that there would be sort of a lump in the orthotic under cuboid. I've never, on purpose, made a cast with the lateral forefoot plantar flexed on the rearfoot to see if it is true that an orthotic made from this does hurt. I have seen casts where there was a high lateral arch, when the lateral foot was fully dorsiflexed, that were sent to professional labs, have the lateral arch filled in to create a lower lateral arch in the finished orthotic. So, the people in the lab thought that having an excessively high lateral arch would have been painful. So, I will admit that I cast with lateral forefoot dorsiflexed to the point where the plantar lateral ligaments are tight, because that is what I was taught. Would it be ethical to make some orthotics from casts that have the lateral forefoot plantar flexed to see if it does hurt? I haven't come up with a good reason to create a cast with a higher lateral lump/arch. So, I haven't done it.

    Often, when we make orthotics, we want to increase load on the lateral forefoot to decrease load on the medial forefoot. So, I like the explanation that if you cast the foot with lateral forefoot maximally dorsiflexed you are more likely to make the lateral plantar ligaments tight, so that they can accept load sooner. (If the ligaments were loose, you would have to dorsiflex the lateral forefoot to make them tight before they would accept significant load.)

    Anyway, it is important to understand how changing your casting position might alter the shape of the finished orthotic. In one of my lectures I described how I dorsiflexed the medial column in a patient with a flexible cavus. At, rest her first ray plantar flexed and this increased the amount of forefoot valgus in the cast significantly. She did not have the eversion range of motion for any forefoot valgus intrinsic post. She was also laterally unstable and I wanted a lower medial arch height in her cast. So there are two reasons why I chose to dorsiflex her first ray when I was casting. However, I did keep her lateral forefoot fully dorsiflexed and "locked". So, try and understand how what you do alters the shape of the orthotic. Try to have a good reason as to why you are changing that shape.

    Eric
     
  6. fabio.alberzoni

    fabio.alberzoni Active Member

    THANKS TO EVERYBODY!

    I' m still thinking about OMTJA and LMTJA. I mean...they died!Seems to me impossible that a window with a different hinge could work with the same old handle.
     
  7. To add to Eric's comments, Mert Root actually described, in about 1986, a modified casting technique where he would plantarflex the forefoot on the rearfoot (what he called supinating the oblique midtarsal joint axis) during negative casting for orthoses made for higher heeled women's dress shoes. He lectured on it at one of his Root Lab Seminars that I attended on a yearly basis around that time.

    I remember this quite clearly since I thought it was so odd of him to say that the foot could be casted this way for dress shoes but that absolutely no other changes to the negative casting procedure could be done otherwise for orthoses made for any other types of shoes. I fact, Dr. Root repeatedly emphasized during his lectures that one of the "cardinal sins" of negative casting was "supination of the longitudinal or oblique midtarsal joint axes". Dr. Root was not practicing at this time but was still consulting for Root Orthosis Labs.

    John Weed lectured, when we were podiatry students in 1981, that if the negative cast was made "with the oblique midtarsal joint supinated", then the patient would complain of arch pain from the orthosis across the whole midtarsal joint (lateral to medial) and would sometimes describe the discomfort from the foot orthosis somewhat as if they were "standing on a Coke bottle". I have made this casting mistake a few times in my practice career and have had to recast the patient being careful to more forcefully load the lateral column during negative casting.

    The practical ideas that Root and Weed gave us, often times, were excellent. However, many of the biomechanical explanations for their ideas and observations need serious updating and revising due to the additional knowledge we have accumulated over the last 30+ years since these two men last actively taught and wrote.
     
  8. Fabio,
    I addition to the excellent responses from my colleagues I should add that firstly it is important to understand that casts made for the subsequent manufacture of foot orthoses do not in themselves treat patients, rather it is the foot orthosis itself which modify's the forces at the foot-orthosis interface. Thus all the cast ever does is capture a shape to initiate the process of design and manufacture.

    What you should have gleaned from the lectures in Zaragoza is that the shape of the foot orthosis will to a large extent determine the load/deformation characteristics of the foot orthosis at varying points across it's interface surface with the foot. This then modifys the rate in which the body's momentum is "given up" to the foot orthosis at varying points across the interface and viz. the reaction force distribution at the foot-orthosis interface.

    You should also have realised that certain design features make the orthosis differentially stiffer in certain areas which will tend to increase the reaction forces in these areas. So... If, for example, we wish to increase reaction forces medial to the subtalar joint, we may wish to include design features which make the device stiffer in this location, if we wish to decrease the reaction forces medial to the STJ the reverse might be true. By manipulating the casting position of the foot we can begin this design process- we know for example that more highly curved sections of the orthoses will tend to be stiffer than lower, flatter portions. It's just a case of thinking ahead when casting as to the curves you are wishing to create in the shell.

    That said, with a good rasp and plenty of time you could start with a cube of plaster and still carve it into the shape you want from your positive model.
     
  9. fabio.alberzoni

    fabio.alberzoni Active Member

    Thanks.
    I'll start with some experimentation on myself...
     
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