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Rearfoot position?

Discussion in 'Biomechanics, Sports and Foot orthoses' started by elainehoey, Mar 24, 2011.

  1. elainehoey

    elainehoey Member

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    Hi everyone,

    I'm a bit confused about this and I'm sure there's a simple explanation.

    At uni I was taught that the rear foot position is measured by assessing NCSP and RCSP when weight-bearing with the sub-talar joint in neutral and the patient standing in their angle and base of gait. This was deemed in a large number of cases to be a rearfoot varus when NCSP was greater than 4-5 degrees inverted. The type of rearfoot varus depended on the amount of compensation.

    However in a non-weight-bearing position and with the sub-talar joint in neutral the rearfoot in many cases is parallel to the ground ie not everted or inverted.

    Why this discrepancy? Or are they two different measurements.

    Would appreciate it if someone would remove this senior moment!!

    Elaine Hoey
  2. Elaine this may not remove the Senior Moment (I´m guessing pun unintended but very funny - you will get it after some reading)

    RCSP and NCSP alot of us have moved on for this type of thinking - I would suggest some reading for Re Subtalar joint axis.

    Subtalar joint axis threads.

    The above reading may mean you don´t look at NCSP or RCSP so much anymore, may also mean more questions.

    Good luck
  3. elainehoey

    elainehoey Member

    Many thanks for that Mike.

    It all looks very complicated. I now know what you mean by more questions!!!

    I knew I should have done a degree in physics too. :D


  4. Elaine it seems that way but work you way through it start new threads with questions if you need, it will click and then there are 10 000 more questions but thats for another day- most of us have started at Root and been in a similar place so your not alone.

    Have fun
  5. efuller

    efuller MVP

    You don't need a degree in physics. At undergrad there were physics classes for engineers, biology majors and finally poets. You only need to understand physics at the poetic level. More helps, but is not necessary. You have to know where you want your orthotic to push harder and where you want to push less hard. That's the poetic level.

    As to your orignial question. Look up the definitions of those measurements. That should help you out with that bit of confusion. As has been said, the bigger question is why is RCSP, NCSP and neutral position important. I don't think that they are.

  6. Yet, ultimately you need to understand how your orthotic design makes it "push harder" in certain places compared to others...
  7. Its not complicated at all. Well, let me put it differently. The physics of how the foot functions is infinitely complex and far more than we yet understand. However assuming you want to know how to use biomx to fix broken people rather than cracking the atom, it is far more simple that what you're use to.

    1. Find the structure which hurts.

    2. Say what it does.

    3. Design an insole which does the same.

    That, in broad (very broad) strokes is tissue stress biomechanics. As you learn more about the physics of it, you will get better at step 3 and perhaps insert your own spins on the other steps. However at core this is much more simple than what you're used to.

    Look at it this way. Lets say the patient has had an inversion injury and hurt their calc-fib ligament. The CF ligament limits supination. Therefore your tx is to limit supination. Strapping perhaps, or an insole with a lateral wedge. Plantar fascia makes the windlass work, or in simple (and slightly inaccurate) terms, tries to plantarflex the 1st met. Your insole, therefore, should be trying to plantarflex the 1st met.

    That, to me, is a lot simpler than balancing varuses, valguses, normals, and suchlike. :drinks Those things are well nigh impossible to measure with accuracy anyway so its a SWAG at best.

    Tissue stress is a simple framework within you can make biomechanics as complex as your understanding permits.
  8. elainehoey

    elainehoey Member

    Hi Robert,

    your 3-step plan is a great way to start and your example made a lot of sense thanks.

    I will read in detail the STJ axis threads and try to get a simple plan in my head about it. I was getting a bit bogged down in GRF, ORF, moments, axis etc.

    Just a quick question. Is there a difference in ORF's if one adds intrinsic control to a shell as opposed to extrinsic control?

    By shaving control into a device -for example for the 1st ray does that decrease ORF's at that point enabling plantarflexion of the 1st ray?

  9. Robert:

    It is very important that you add a #4 to your list:

    4. Make sure the insole doesn't cause other problems.
  10. efuller

    efuller MVP

    Hi Elaine,

    An orthotic cannot plantarflex the 1st ray. It can't reach up and pull the metatarsal down. However, as you said, it can enable 1st ray plantarflexion by decreasing the ground reaction force on the first ray.

    I'm not sure what you mean by intrinsic vs extrinsic control. The "control" is provided by the shape of the top of the device and the resistance to deformation of the device.

  11. Agreed, but I'd also add friction characteristics at the foot / hosiery-orthosis interface to the list.
  12. I'd also emphasize that the shape of the top of the orthotic is not always (I'd opine not often) the topography of the orthotic in vivo, in the shoe. Worth remembering that the inside of a shoe is very rarely a hard, flat platform for the insole to sit on.

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