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Random thoughts/questions

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Ann PT, Sep 30, 2008.

  1. Ann PT

    Ann PT Active Member


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    Hi everyone,

    I'm wondering about your thoughts on a couple random thoughts/questions of mine...

    1. Don't use a Blake Inverted Orthosis to control pronation in a patient with tarsometatarsal DJD due to supination force on midfoot from increased forefoot varus correction. Agree or disagree?

    2. Can Xrays be helpful to determine if a first ray is plantarflexed and if so, what lines are you looking at? The bissection of the first metatarsal and what?

    3. Thinking about any causal link between leg length difference and hallux rigidus-
    Possibilities:
    a) Greater degree of hallux DF to attempt to keep the foot on the ground longer could result in DJD through "overuse."

    b) Inverting the subtalar joint on the short side may increase the range and speed of pronation causing DF of the first ray and ultimately DJD at the 1st MPJ.

    c) Likewise pronating the subtalar joint on the long side could cause DJD for the same reason.

    d) Increased vertical ground reaction force on the short side causes DJD at the 1st MPJ through greater 1st ray dorsiflexion.

    e) Greater out-toe on the short side to stabilize against lateral shear at heel strike leads to DJD at 1st MTJ.

    Agree? Disagree? Does anyone associate leg length difference with hallux rigidus?


    I appreciate any thoughts. Thank you!

    Ann
     
  2. efuller

    efuller MVP

    A Blake inverted does not necessarily create a forefoot varus correction. An unmodified Blake device will end behind the metatarsal heads and therefore cannot create a forefoot varus correction. A Blake device can have quite a high medial arch, but there is a lot of "lab discretion" on how high the arch is, so it would be difficult to make a blanket statement regarding its use.


    I think the location of the calluses would be a better indicator of 1st ray poisition in function.


    I see a lot of bi lateral hallux limitus, so I'd say you have about a 50% chance of having the hallux limitus/ rididus on the short side.

    What is the amount of measurement error in limb length descrepency? Your individual error and the error across all practioners.

    Can a 3mm limb length descrepency be diagnosed if your measurement error is +/- 2mm?

    Regards,

    Eric
     
  3. Ann PT

    Ann PT Active Member

    Thanks for your response, Eric.

    My understanding of an inverted orthosis is that the lab inverts the negative cast during pouring of the positive and then the positive is balanced so the heel rests inverted to the horizontal to the specified degree of inversion. Wouldn't this create an increased forefoot varus correction?

    I have never used Xrays to look for a planterflexed first ray. I was just wondering how one would assess for it on an Xray if possible.

    I agree that leg length is a very unreliable measurement in the absence of an imaging study like a scanogram. But if we just assume someone has a LLD, for example 1/2 inch, is there any research looking at the relationship between leg length and hallux rigidus or do you see a relationship clinically? I have patients frequently with unilateral DJD at the first MPJ and two this week had an obvious LLD.

    Ann
     
  4. pod29

    pod29 Active Member

    Hi Anne

    My thoughts, for what it's worth....

    A Blake inverted device will rarely hold the calacaneus in an inverted position. Just like a standard Root 4 degree rearfoot post will not hold the calc in 4 degrees inversion. What a blake inverted device will do is apply an increased supination moment to the subtalar joint. The aim of an orthosis should be to decrease pathological forces. There are many ways to do this to the subtalar joint ie. kirby skive, blakes device etc etc. Changing the position of the calcaneus may or may not occur (depending on the amount of the correction given). But the jury is out as to whether this is particulatrly relevant anyway.

    As for the arch height, i agree with Eric. This can vary according to your own prescription and the labs discretion.

    regards

    Luke
     
  5. Luke ,
    While I'm inclined to agree with you, I thought it might be worthwhile from an educational standpoint to examine the reasons why this is the case. To hold the calcaneus inverted (I'm assuming you mean from heel strike through the contact phase) the supination moment must exceed the pronation moment throughout this period, so what prevents the orthoses modifications you list above from achieving this?

    Also, if we did achieve this abnormal calcaneal motion via our orthoses, what might the consequences be?
     
  6. Ann PT

    Ann PT Active Member

    Hi...

    I'm not sure my question is understood. There are many ways to control pronation but that is not what I'm looking to do. I'm just wondering about the forces on the tarsometatarsal area with an inverted orthosis...

    Ann
     
  7. Ann, perhaps you need to re-word your question if you are not getting the answers you were looking for.

    My take on what I think you are asking: inverted positive cast = higher medial longitudinal arch profile of orthoses (all other prescription variables being equal) = stiffer medial longitudinal arch segment of orthosis = increased orthosis reaction force in this area of the foot.

    Higher medial longitudinal arch of device should = decreased dorsal midfoot interosseous compression forces. Is this good for tarsometatarsal joint DJD? Depends on what the pathological forces are for tarsometatarsal joint DJD- right?

    Is this what you were looking for?
     
    Last edited: Oct 1, 2008
  8. Ann PT

    Ann PT Active Member

    Thank you Simon. Your feedback is helpful and I'll try to reword my question again.

    If someone has degenerative changes at the tarsometatarsal joints, I do not want to apply a torque to that area for fear of creating pain. I was wondering whether an inverted orthosis with a balanced forefoot would likely apply torque to that area. My thinking is that it would, and therefore in this patient I might choose to apply a supination force in another way (Ex: soft medial flange) versus using an inverted device.

    Here's the foot I'm thinking about. Most of the medial mass over the first tarsometatarsal area is soft tissue-maybe a lipoma? His joint is not actually touching the ground (as it may appear) due to the soft tissue mass. He has no pain in this area.

    Ann
     

    Attached Files:

  9. Ann:

    You may want to look at my newsletter on Dorsal Midfoot Interosseous Compression Syndrome (DMICS) since your patient may have this problem.
     
  10. efuller

    efuller MVP

    Hi Ann,

    The assumption made under the Root paradigm is that a forefoot intrinsic post actually works even though the support ends behind the metatarsal heads. I never understood this and asked all instructors and I felt I never got a satisfactory answer to the question, why doesn't the foot pronate after heel lift with an intrinsically posted forefoot varus orthotic? The "support" is no longer there. If it was held inverted by the intrinsic post it should pronate with loss of support with heel lift.

    It may have kept the foot supinated for a different reason than "the deformity was supported". A forefoot varus intrinsic post will tend to raise the medial arch height of the orthosis. This will tend to be uncomfortable unless the patient uses their muscles to supinate their subtalar joint and shift the force more to the lateral side of the orthotic. You will see some high arched orthotics with the lateral side of the top cover worn out and no wear on the medial arch of the orthosis. This observation supports the idea of muscles causing the foot to supinate and is contrary to what you expect if the intrinsic varus post was "supporting" the foot. I've seen high arched devices make a posterior tibial tendonitis worse.

    The Blake device will tend to invert the heel cup and raise the medial arch. So you get the shift of center of pressure effect at the heel and the muscle supination effect in the arch.

    For your patient you could use a medial heel skive that would give you an inverted heel cup without raising the arch. I would expect a high arched device would be very uncomfortable with that soft tissue mass. Looking at the picture it's very tempting to just use a varus heel wedge and avoid the arch pressure all together.


    Regards,

    Eric
     
  11. pod29

    pod29 Active Member

    Hi Simon

    In answer to your questions...

    The amount of force required to supinate the foot will be dependant on the STJ axial position. Assuming a medially deviated STJ axis position, the supination moment arm at the STJ will be relatively short. So a high amount of force will need to be applied to relatively small area of the calcaneus. In some feet a Blake's device will produce a large enough supination moment to cause a kinematic
    change. Whilst in other (more medial STJ axis position) feet, a combination of medial skive and Blake's device will still not effect any kinematic change. So in summary, if the blake's device doesn't effect any kinematic change, it doesn't mean the orthoses isn't doing it's job. It only means that it is not generating enough supination moment at the STJ to see a kinematic effect. The role of the foot orthoses is to reduce pathological forces, any kinematic change should be seen as a side effect of this goal. Your thoughts???

    If we were to hold a foot in an inverted position on a foot orthosis, i would assume that we have shifted the STJ axis to a more lateral position. So in this position the supination moment arm will be larger than the pronation moment. In this instance the peroneal muscles would be at a mechanical disadvantage. Problems with lateral instability may occur. As the calcaneus inverts, the force required continue inversion becomes less due to change in STJ axial position. This can be an important factor when an orthosis is causing considerable kinematic change.

    Question: Can kinematic change that is observed with the use of foot orthoses, be used as an indicator of overcorrection with the device? ie. has too much force been applied by the orthoses if we are seeing kinematic change? Or is kinematic change irrlevant, something that may or may not happen, and doesn't really make any difference either way?

    Kind regards:drinks


    Luke
     
  12. Nice work Luke.

    I don't think you can assume too much force has always been applied when we see kinematic change, only if we see secondary symptoms associated with that kinematic change.
     
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