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Cadaveric flatfoot model: Ligament attenuation and Achilles tendon overpull

Discussion in 'Biomechanics, Sports and Foot orthoses' started by NewsBot, Jun 18, 2009.

  1. NewsBot

    NewsBot The Admin that posts the news.

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    Cadaveric flatfoot model: Ligament attenuation and Achilles tendon overpull.
    Blackman AJ, Blevins JJ, Sangeorzan BJ, Ledoux WR.
    J Orthop Res. 2009 Jun 15. [Epub ahead of print]
     
  2. Pop quiz, starter for ten. When referring to abduction and adduction here, are the authors using the midline of the body or the midline of the foot as their reference?

    It's not a trick question and I'm not about to "club" the first respondent. There is a point to this, humour me...
     
    Last edited: Jun 18, 2009
  3. Not that difficult, surely?
     
  4. Johnpod

    Johnpod Active Member

    Hi Si,

    The authors are using 'midline of body' reference, not 'midline of foot' reference. I suppose they felt obliged to do this because of the 'outside' relationships - notably to the tibia.

    Pes planus is traditionally seen as weakness of the invertors and long flexors: T.ant, T.post, FHL, FDL.

    I think it interesting that every reported effect can be undone by contraction of Tibialis anterior.


    I tend to think of P.tertius mostly as a stabiliser. Is it perhaps weakness of T.ant alone that allows AT to overpull?
     
    Last edited: Jun 20, 2009
  5. Johnpod

    Johnpod Active Member

    Just wanted to observe that if we take the references in the paper as being to foot midline, we have a recipe for the normal foot - even supination.
     
  6. I was thinking about navicular "drift" and "drop" which I suspect should be re-named talar drift and drop. With regard to tibialis anterior, it depends of the spatial orientation of the STJ axis.
     
    Last edited: Jun 21, 2009
  7. Johnpod

    Johnpod Active Member

    Drift and drop?, abduction/dorsiflexion, or adduction/plantarflexion of the talar head- what's the difference? Just another way to say it? In fact the Rootian explanation is more specific in that it is not the talus that 'drifts and drops' but specifically the talar head which lies in and moves with the navicular acetabulum.

    Given the lateral tibial origin and the medial navicular insertion of T ant., its contraction will always cause abduction and dorsiflexion of the talar head, dorsiflex the 1st met. base, adduct the foot and invert the calcaneum. Pull on the navicular and it will haul the talar head upwards and outwards.

    My understanding is that the talus being free of muscular attachment is free to take whatever position it is forced into by by the tibial base above and the calcaneal articulations below. 'Navicular drift and drop' and ''drift and drop of the talar head' are therefore so similar as to be one and the same and neither term is particularly specific or helpful.

    Rootian teaching is that this accommodation of the talus influences the talonavicular and calcaneocuboid articulations (midtarsal joint).
     
  8. John, the point being what exactly has moved and in which direction? The measurement is termed navicular drift or drop, suggesting that the navicular has moved medially (adduction) and plantarly. Your explanation above, concurs with my thoughts, that it is the talar head that drifts medially and is relatively adducted, while the navicular is relatively more abducted on the talus. Hence that which is commonly termed navicular drift is probably a misnomer.

    Re: tibialis anterior, please read the analysis presented here for peroneus brevis http://www.podiatry-arena.com/podiatry-forum/showthread.php?t=29075, the same applies for tibialis anterior, only the internal and external moments are reversed.
     
    Last edited: Jun 21, 2009
  9. A further thought, as the navicular and first metatarsal move into further abduction relative to the talus the insertion of tibialis anterior (tib. ant.) is also shifted further laterally relative to the STJ axis, ie. the supination lever arm is decreased/ pronation lever arm is increased as the forefoot is abducted. Combine this with an already medially deviated STJ axis (indeed the inability of the ligamentous tissues attaching to the navicular to resist navicular abduction may be the cause of the medially deviated axis) and we have even greater potential for the net moment from tibialis anterior being one of pronation, not supination. Kevin has already talked about change in the lever arm of tib ant. in several of his papers. I'm pretty certain he has not discussed the external pronation moment from the CoP that may be generated in this situation from the contraction of tibialis anterior (I could be wrong). BTW this will be increased due to the forefoot abduction. Indeed if the tib ant. insertion lay lateral to the STJ axis, the counter moment from CoP that tib. ant. contraction could create would be minimal due to forefoot abduction. Isn't it time we wrote this up, Kevin?
     
    Last edited: Jun 21, 2009
  10. Question: if we took a dorso-plantar x-ray of a severely (maximally) pronated foot, what proportion of the talar head would be incongruent with the navicular?
     
  11. Only just spotted this, as far as I am aware most anatomy texts refer to tibialis anterior inserting into the medial cuneiform and the base of the first metatarsal. John, can you give a reference for this "medial navicular insertion", please? Or is this "Rootian teaching" too?

    Given the commonly held belief that tibialis anterior inserts into the medial cuneiform and base of the first metatarsal rather than the navicular, what moment should contraction of this muscle generate about the talo-navicular joint (TNJ)? Put another way, what axial orientation is likely to result at the TNJ when tibialis anterior contracts?

    The above statement is wrong for a number of reasons; I counted six or seven.
     
    Last edited: Jun 21, 2009
  12. Simon is right, the anterior tibial muscle inserts on the medial aspect of both the medial cuneiform and first metatarsal base, not the navicular. It's hard to properly understand muscle function if muscle anatomy isn't first appreciated.

    Simon is also right that, contrary to many anatomy textbooks, the anterior tibial muscle can be either a pronator or supinator at the subtalar joint (STJ), depending on the spatial location of the STJ axis relative to the anterior tibial tendon. It is not uncommon to have the talar head so medially positioned relative to the path of the anterior tibial tendon that contraction of the anterior tibial muscle will cause an internal STJ pronation moment, not an internal STJ supination moment. Here is my newsletter on the subject.

    One last point that Simon is making here, is that every extrinsic muscle of the foot will produce both internal STJ moments [that occurs as a result of the tensile force vector from the muscle tendon pulling either medial or lateral to the STJ axis] and external STJ moments [that occurs as a result of the change in the center of pressure on the plantar foot from the effect of the muscular contractile force]. Honestly, a whole book could be devoted to detailing the internal and external moments that are generated by contraction of each of the muscles of the lower extremity and how this combination of moments may lead to both normal and abnormal function of the foot and lower extremity.
     
  13. Johnpod

    Johnpod Active Member

    OK Simon - I was wrong about the insertion of T ant into the navicular and agree that the insertion is into medial cuneiform and 1st met base. I can now appreciate that the navicular is not directly controlled by T ant.

    For the first time I can see that pes planus is not simply a 'pronation' deformity, but also comprises catastrophic collapse/disruption of the medial column. This exchange has proven valuable to me.

    Whilst checking it out I came across this that might be useful to someone else:

    http://www.blackburnfeet.org.uk/flash/flatfootBiomechanics1.swf

    The paper speaks of attenuation of selected ligaments - do we know which ligaments?
     
  14. Good. Thanks for sticking your head up to get involved!
     
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