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Combination forefoot posting

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Simon Spooner, Sep 19, 2011.

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    All, this is really a thought experiment. So if your not into theoretical stuff, tune out now.

    Huson (1999) believed that the metatarsal rays are part of a non-constrained tarso-metatarsal mechanism in which each metatarsal ray has independent stiffness (resistance to dorsiflexion) and range of motion. He suggested that the second metatarsal ray is the stiffest and described it as a "fixed spoke". The biomechanical modelling performed by Salthe et al. (1986), and the dynamic forefoot pressure analyses performed by Hughes et al. (1991) supports this conjecture.

    I was thinking, if the second metatarsal is generally our stiffest and longest ray and thus acts our "pivot" ray, what would happen if, rather than using a single frontal plane angulation for our forefoot posts, we instead had two antagonistic posts converging on the second metatarsal? For example, if we had a valgus post 2-5 and a varus post 1-2 with the convergence of these two posts directly beneath the second metatarsal, how might this influence kinetics and kinematics? Conversely, if we had a varus post 2-5 and a valgus post 1-2 with the apex of the convergence of these two posts directly beneath the second metatarsal, how might this influence kinetics and kinematics?

    Has anyone ever tried this kind of thing?

    Your thoughts, please....
  2. Simon:

    A drawing might help here. I'm having difficulty imagining what this orthosis would look like.
  3. Some quick sketches Kevin. Frontal plane section just proximal to the MPJ's: on the left is varus 1-2 with valgus 2-5; on the right is valgus 1-2 with varus 2-5. Obviously a quick sketch or two, in reality the mets would adopt positions dependent upon the reaction forces. Just a thought experiment. How might such posting influence propulsive phase biomechanics?

    Attached Files:

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  4. markjohconley

    markjohconley Well-Known Member

    Excellent, am looking forward, like many i assume, to the responses, way over my head but very keen to follow, mark
  5. To answer the above question don´t we need to know CoP or path of CoP ?
  6. Isn't that one of the main points for consideration, how might such posting influence the centre of pressure?
  7. Yes, but if the CoP is under the 1st, 2nd or 5th Met heads 3 different types of moments will occur and therefore have different effects on CoP due to the opposed angles of the wedge , so don´t we need a starting point to know which way the wedges will cause the CoP to deviate ?

    I might not be thinking straight - tired and broken at the minute
  8. markjohconley

    markjohconley Well-Known Member

    "a valgus post 2-5 and a varus post 1-2 " same as a forefoot varus post with a 1st ray cutout?
  9. markjohconley

    markjohconley Well-Known Member

    ...the other one, "a varus post 2-5 and a valgus post 1-2"
  10. markjohconley

    markjohconley Well-Known Member

    And I don't hate all cats, mainly domestic cat owners who let their beasts out without multiple bells!
  11. markjohconley

    markjohconley Well-Known Member

    ...apologies Simon et al....
  12. OK Mike, lets see if we can get you started. Lets take figure 1. with varus 1-2 , valgus 2-5. If we stood on this wedge where might we feel high pressure and where might we feel less presssure? My guess is that higher pressures would be felt beneath 1st and 5th metatarsals- agree? But where would the centre of pressure lie? Now think about the second example, where would we feel high pressure and lower pressure and where might the centre of pressure lie?

    Get some sticky wedge and try it!
  13. Figure 1 Varus 1-2 Valgus 2-5 standing - increased pressure 1st MTPJ and 5th MTPJ agreed CoP if all things are equal and the increased pressure under the 1st and 5th is Equal middle distance between this 2 points.

    Figure 2 Valgus 1-2 Varus 2-5 standing increased pressure under the 2nd MTPJ and CoP should be close to this point, and in your initial question would have increased bending moments.

    But during gait the Proximal - Distal and medial - lateral changes in CoP this Position of CoP for the 2 figures will potentially/most likely be different due to GRF and Internal muscle firings
  14. Phil Wells

    Phil Wells Active Member


    I have tried this type of approach on my own orthoses - i.e. a flexible lateral column with a dorsiflexed 1st ray foot type - 1-2 medial posting and 3-5 lateral posting.
    On the polyprop device it changed transverse motion at the mets but on the EVA it felt more frontal plane dominant.
    Still working my way around it but it reality it is very material dependant.

  15. efuller

    efuller MVP

    Remember that COP is an average. You could have high pressure sub 1 and average pressure under the other mets and get a cop under the 2nd met, or you could have a long 2nd met with high pressure under it and average pressure under the other mets and get the center of pressure in the same location. You really want to know which mets have high pressure. I've considered a 2-5 varus wedge for a patient with 0 maximum eversion height and 1st MPJ pathology. The foot with the long second gets the 1st varus and 3-5 valgus (or you could just put a hole sub 2).

  16. And that was one of the points I wanted to raise with this little brain workout. That you could end up with the centre of pressure in pretty much the same place relative to the plantar foot with very different posting designs, like the ones I provided as examples here. But even if the centre of pressure was in exactly the same spot with both examples, would they really have the same functional influence upon the body above?
  17. efuller

    efuller MVP

    Thinking about that I got a lot more questions than answers.

    How does the foot influence the body above?
    How does wedging under the foot affect the body above?
    How does wedging under the foot effect muscle firing and how does that effect...
    If there was no change in muscle activity, how would wedging effect.....
    How much of gait is pain avoidance?

    In someone who already has pain sub 2, one wedging design will be a whole lot more comfortable than the other. That will certainly effect gait when someone has intact sensation.

  18. Eric, I agree it's highly complicated although I'm also sure you're on my wavelength with this one. We could have two orthoses designs which put the centre of pressure under the same anatomical location relative to the foot, yet the way in which they achieve this will influence the internal stresses quite differently. So... can we segregate and place joint moments and tissues stresses into some sort of hierarchy in terms of clinical significance?

    All, if you don't like Socratic learning, tune out now:
    Which of the two posting designs should create greater foot stability?
    Is foot stability desirable?
    What might be the consequences of instability of the foot during propulsive phase?
    What determines the propulsive strategy adopted by an individual? ("low" or "high" gear, if you like those terms (me I think they're rubbish terms, but never mind that for now)).

    P.S. when I said "body above" I was including the foot in that.
  19. One more, for the kid's back home: How significant is the interface topography? In the diagrams I sketched earlier in the thread we had the inclined planes at the interface with the foot. What if we flipped them over so that the bases (adjacent sides in trigonometry) were next to the foot and the angled planes (hypotenuse) were facing downward toward the ground. How might this change things?
  20. Hi Phil - Sorry I just don´t get what you wrote.

    Motion in body planes in relation to what?

    and while I get the change in stiffness has effects the moment is dependent on shape and as the shape as not changed then ..........?

    Confused of Stockholm.

    Simon I´m thinking about your questions get back to unless some one else gets in 1st. - Mark maybe ?
  21. You and me both Kid, confused of Plymouth.
  22. Simon:

    Typically, with intrinsic or extrinsic forefoot varus or forefoot valgus posting, the flat plane of the anterior edge of the orthosis is still maintained, but the full width of the anterior orthosis edge is either tilted into varus or valgus. In your examples, you are basically modifying the anterior edge shape in order to either increase the orthosis reaction force (ORF) plantar to the 2nd metatarsal (orthosis with raised area at 2nd met) or decrease the ORF plantar to the 2nd metatarsal (orthosis with depressed area at 2nd met).

    If you consider the effects of a metatarsal pad added to the dorsal aspect of the orthosis plate centered at the 2nd metatarsal area, then this would be very similar to the mechanical effect of having a raised area at the 2nd metatarsal. I have done this orthosis modification many times. However, I don't think I have ever made the orthosis with a depressed area at the 2nd met but would think this may create areas of tissue overload at the 1st and 5th metatarsal shafts, but have used a Morton's extension along with a pad sub 4th and 5th to relieve pain plantar to met heads 2 and 3 numerous times.

    I did analyze the pedal stabilizing effects of combining a rearfoot varus and forefoot valgus wedge in an orthosis in a few of my previous newsletters, "Mechanical Effects of Varus and Valgus Wedging" - June 2001, and "Mechanical Effects of Varus Rearfoot and Valgus Forefoot Wedging" - July 2001 (Kirby KA: Foot and Lower Extremity Biomechanics II: Precision Intricast Newsletters, 1997-2002. Precision Intricast, Inc., Payson, AZ, 2002, pp. 57-64), which somewhat reminds me of your examples. I have including a drawing I did for the July 2001 newsletter which looks very similar to your drawing.

    In addition, I have included a recent drawing I did on the spring function of each individual metatarsal ray to point out the fact that the concept of "locking" of the metatarsal rays have really no basis in fact. Each metatarsal ray has the capability to dorsiflex and plantarflex independently of the other metatarsal rays so that the human foot may better adapt to uneven surfaces. This drawing was used to emphasize the spring function of each metatarsal ray and disprove the concept of midtarsal joint or midfoot joint "locking".

    Attached Files:

  23. markjohconley

    markjohconley Well-Known Member

    Thanks Mike very nice of you, but cacking myself!
  24. markjohconley

    markjohconley Well-Known Member

    Simon, just clarifying in the first diagram 1-2 varus, 2-5 valgus, the hypoteneuse (both of them) were facing up, whereas in the 2nd diagram the 1-2 valgus, 2-5 varus the one hypoteneuse was facing down. Is this what you say?
  25. Phil Wells

    Phil Wells Active Member

    Just to clarify by ramblings.
    In Simon's diagram he showed that the plantar surface of the post was flat and the surface topography was changed.
    In my test orthoses the opposite was true creating a post under the 5th met head (Lat post) and under the 1st met head (varus post). This created a 'tripod' between the flat heel post and the 1st and 5th met heads.
    These dug into my shoes and created instability at late midstance - again not sure why as the device was balanced in the frontal plane - probably due to CoP changes caused by the change in material stiffness due to the extrinsic posts.

    It was easy to fix as I changed the surface topography instead and all is now well.

    I do see this prescription request every now and again and it seems to be effective - no returns as yet.


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