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Medially Deviated Axes and Forefoot Varus Extentions

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Robertisaacs, Oct 21, 2009.

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    I put this one up as a puzzler on the SALRE study day and it provoked a lively debate. I promised I'd pop it up here for more general consumption and to see who was nearer to right.

    We have a foot with a medially deviated axis. We place a forefoot varus wedge in positioned thus.


    The Question was, is this modification, on an orthotic or in isolation, more likely to A: Pronate or B: Supinate the foot.

    I left the wording of the question deliberatly vague.

    I said B for the following main reason (Although there were others).

    The wedge is almost entirely lateral to the axis, therefore it will generate pronatory moments. HOWEVER. If it increases ORF under the thick bit of the wedge it must DECREASE them under the thin bit (because the total GRF/ORF won't change particularly). Therefore the COM is nearer to the axis.

    If the pressure is increased nearer to the the Axis the lever arm is shorter. Therefore we have less pronatory moment from forefoot GRF. If there is a supinatory moment from other tissues (like the Tibialis muscles) this may bring the total pronatory moment down to less than the supinatory moments and the foot will supinate. If not the foot will not supinate, but the residual pronatory moment (for eg compressive force in the sinus tarsi) will be smaller. No kinematic change but smaller supination resistance.

    So, no more supination moments, but potentially less pronation ones. I say B.

    There are obviously other elements at work, It might stuff up the windlass which would reduce supinatory moment later in gait. It will cause more inversion moment around the very debatable longditudinal axis of the mid tarsal joint and the ligaments constraining that joint WILL be medial to the axis so it might generate some supination moment there. Finally It will change the paralell planter position, which changes where we would find the axis. However setting these aside, what say y'all about the lever arm idea?

    In particular I would love to see some of those who came up with counter arguments on the day raise them here, because some of them were most cogent. and i feel it would make for interesting debate! But I don't want to paraphrase them for fear of misquoting.

  2. Donnchadhjh

    Donnchadhjh Active Member

    Thank you for posting this puzzle on the forum Robert I have still been pondering this one trying to reason it out in my head (Sunday has definitely got me thinking more like an engineer in terms of forces and moments) - I have even been brave enough to attempted some of Mr Kirby's thought experiments.

    Ok back on subject and sticking my neck out here - I say C. It would neither Pronate or Supernate the foot.


    I can accept that the wedge illustrated exerts a supinatory moment on the forefoot by moving the CoP and GRF closer to the axis, which would also in all likelihood stuff up the Windlass.
    However, the question asks if this wedge would supinate or pronate the foot. I would think that this wedge would not have an effect on overall foot position because any supination that occurs as a result of the moment exerted would be taken up by the long axis of the MTJt.

    However, I am now not sure the forefoot would actually change position at all because the thickest part of the wedge sits exactly half way over the axes - hence (I am assuming) it would be like trying to open a door by pushing on the door frame.

    A slightly confused/bemused and generally thoughtful Duncan
  3. I would say Pronate.

    1. reduced windlass effect and cause negative windlass which will elongate the foot. Navicular height will reduce.

    2.The wedge will also cause a dorsiflex moment on the distal end of the 1st ray, which will mean the proximal portion will plantarflex on the navicular.Therefore an unstable joint, this will allow more navicular drift and drop.

    3. Now that the navicular position is such as described above the PT must work harder to cause a Supination moment at the STJ.

    4.Due to the medial deviated axis TP, FDL,FHL etc also have shorter level arms so must work harder to create a supination moment.

    Thats the way I see it.
  4. Confused, thoughtful and trying to think like an engineer. Its like looking in a mirror:drinks. Full credit for having a crack!

    Well done M8.
  5. Robert,

    The wedge may or may not pronate/ supinate the foot about the STJ, although it will change the moments about said joint- reducing pronation moment about the STJ axis from the lateral forefoot. It will also increase the dorsiflexion moment on the metatarsals; increasing from lateral to medial. Lets assume that in static stance it does bring about a dorsiflexion of the 1st met by 5mm, the 2nd met by 3mm the 3rd met by 2mm and the 4th met by 1mm, what would this do to the height of the medial longitudinal arch, viz. the midfoot in this situation?

    As for longitudinal axis of the MTJ, the net MTJ axis is created by the forces acting about the joints of the midfoot, do you think adding this wedge is likely to increase the forces such that the MTJ axis is likely to be more longitudinally orientated or more obliquely orientated?

    EDIT: I was typing the above while Michael was obviously thinking and posting similar thoughts to me regarding dorsiflexion of the metatarsals. I was talking static stance to make things simpler, windlass comes into it during walking gait, but probably less so during running gait. If we want to talk about dynamic function, we've got to start considering the excursions of the joint axes that should occur. Also, we are obviously limiting our discussion here to direct mechanical effects and not neuromotor effects.
  6. I love these games!

    I'm not sure about number 2. By that rationale dorsiflexing the 1st met will cause the base of the first met to drop.

    That particular contention should be easy to verify. Take a few feet, measure the nav height, pop a 5mm shaft under the 1st met then measure it again and see if its lower.

    I'll try that tomorrow but I doubt that is what will happen. For pushing the head of the met up to push the base down would imply a pivot mid shaft would'nt it?

    I'm with you on the potential for lost supination moments from jamming the windlass though.

  7. I would say it depends on the residual pronation moment in the Sub talar joint. If thi was (theoretically) very high, IE that the talus is fixed in space, then I would imagine the navicular would remain at the same height but the MLA would be lower (because the 1st met would be doriflexed.) If, however, the residual pronation moment is lower the MLA would be lower by the amount of hallux dorsiflexion - the amount of supination at the MTJ.

    Probably best we keep this to static. Gets horribly complex else.

  8. Or a rotation about its centre of mass. I have to say, you lot are all too clever for me these days.
  9. Not clever enough by half! Where would the COM be? Are we talking the centre of mass on the weight bearing area? or are we counting Z+/- force at the base of the met.

    Nope. Brain just fused. Help.
  10. Take a first metatarsal and nail it to a wall until you find out- that's what the Nazi's did. I was thinking in terms of a free body analysis with your pad and the first met.
  11. You're scary sometimes.

    Ah. One of those.

    Any chance you could rephrase your answer in the form of a diagram? Or a link to a site describing free body analysis aimed at less than PhD engineers?

    Sorry. I might be being slow this evening but I'm pretty sure I'm not the only one wondering!:eek:

  12. Robert, I don't think you are being slow, I was just thinking out-loud. If we took a metatarsal and draw the forces acting on it, we could start by saying that gravity acts down from the centre of mass (somewhere in the shaft). Now draw in the GRF under the head of the metatarsal, these two forces should create a couple- right, so in the absence of other forces the met would want to rotate- right? Thinking about it, the rotation would not be about the centre of mass but between the centre of mass and the GRF. If the met is to be in equilibrium, where do the other forces come from?
  13. BTW
    Yeah, how do you think our bio-engineering friends know where the centre of mass of the segments are? Sorry to say it, but much of what we know stems from less palatable approaches to science, from darker times.
  14. From the exact opposite for ever force there my be an equal and opposite.
  15. And...?
  16. Here's the start of the free body diagram that was in my head. Draw in the other forces that you can identify.

    Attached Files:

    • met.jpg
      File size:
      29 KB
  17. The only other force I can think of would be gravity but don´t that act equally on the whole object ?
  18. Griff

    Griff Moderator

    Would there be forces from the proximal phalanx and also the medial cuneiform?
  19. In the diagram, the two forces in isolation would create a clockwise rotation of the metatarsal- right? So, if the metatarsal is to be in static equilibrium there must be some other forces from some other structures, for example: compression between the superior portion of the base of the metatarsal and the medial cuneiform; tension in the plantar capsular tissues of the 1st met cuneiform joint; compression against the met head from the base of the proximal phalanx of the hallux (via tension in the plantar fascia, flexor hallucis longus etc) .... draw in all the forces you can think of!

    EDIT- you on the right track Ian- I seem to be out of sync with everyone tonight- it's the thought of growing my mullet again that is doing it.
  20. Nope, still struggling here.

    If the 1st met was a free body and we were looking at movement and mass across the met I could see the principle, but its not! At the distil end we have the GRF, no problem there. But the effect of gravity on the met itself will be negligable when, as ian says, there is half a body's worth of mass transmitted at the base of the met from the medial cuniform.

    I just can't conceptualise how the first met can pivot other than at either end of the met. There is GRF acting Z+ at the head of the met and Z- (with a bit of x+ and Y+ depending on planal dominance) acting at the base of the met.

    Perhaps I don't know enough about free body analysis.


  21. Griff

    Griff Moderator

    Some other forces drawn in - Proximal phalanx, medial cuneiform and plantar aponeurosis/FHL...

    I'm along for the ride but not entirely sure I'm sure where I'm going or what do to when I get there...

    Attached Files:

    • met.jpg
      File size:
      9.7 KB
  22. The journey is the end in itself. You know you love it! ;)
  23. Robert, I think you may be missing the point, or reading too much into what I was originally trying to say. I was basically in agreement with your original point, I was just pointing out that the metatarsal could rotate about itself. Take the free body diagram which I posted, if these were the only forces acting on the metatarsal, i.e. the mass of the metatarsal x gravity acting through its centre of mass and the GRF acting through the metatarsal head, would the metatarsal magically float in space as I have drawn it? Or, would the base of the metatarsal fall to the ground? It would fall to the ground- right? So if the head is already in contact with the ground and the base moves from its original position so that it is now in contact with the ground too, it must have rotated about a point to get there- Agreed? But in real life the metatarsal does maintain an angle to the ground in the manner in which I have drawn it, so there must be other forces acting upon it to keep it suspended. These forces as you and the other's have pointed out come from various sources including compression at the met head and base, ligamentous tension etc. Now, when we add in all of the forces, by definition for static equilibrium they must all balance. The point being that if we increase GRF under the 1st met head and elevate it with the varus pad, in order to maintain that equilibrium increased force (counter-clockwise moment) must be coming from somewhere else OR the metatarsal would rotate clockwise . Viz. the system will move to a new position of static equilibrium.
  24. Griff

    Griff Moderator

    You know that feeling when you are chilling out watching the champions league whilst eating tangy cheese doritos and searching google images for epic mullets and you accidently get involved in a conversation involving free body diagrams.... thats me that is.

    I'm going to have to sleep on this one and re-read it in the morning at work when I'm feeling a bit more in the zone chaps

  25. pgcarter

    pgcarter Well-Known Member

    I know I've seen discomfort generated by extra dorsal jamming at base of 1st met by wedges like this. But if some one has foot anatomy requiring large medial collapse in order for the plantar medial surface of the foot to reach the floor how else do you reduce the compensation other than bring the floor up to it one way or another?
    regards Phill Carter
  26. Good discussion.:drinks

    The varus forefoot wedge should shift the ground reaction force (GRF) more medially on the forefoot. If the GRF is shifted more medially, then, by definition, there will be an increase in external subtalar joint (STJ) supination moment due to the corresponding medial shift in the center of pressure (CoP). Therefore, the correct answer for static stance is that the increase in external STJ supination moment caused by the varus forefoot wedge would tend to cause STJ supination rotational motion, unless there is a corresponding increase in internal STJ pronation moment that is of greater magnitude than the increase in external STJ supination moment from the varus forefoot wedge.

    The question of what happens during the dynamics of gait is much more complicated and cannot be answered currently since the response will be variable. As Simon stated, the neuromotor influence of foot orthoses must be carefully considered here since the direct mechanical influence of foot orthoses is definitely not the only affect of an orthosis. In my many experiments in using varus forefoot wedging over the past 25 years, I have noticed that the varus forefoot wedge will cause many feet to have a more supinated posture in early midstance and an increase in late midstance STJ pronation during late midstance. This is a good example of the neuromotor effect of foot orthoses that I have mentioned in an earlier thread on the same subject where the orthosis causes the central nervous system to respond to the increase in external STJ supination moment with increased efferent stimulation of the peroneal muscles during late midstance to cause an increased magnitude in internal STJ pronation moment and an increase in late midstance pronation.

    Direct Mechanical vs. Neuromotor Effects of Foot Orthoses

    Prefabricated vs Custom Made Foot Orthoses

    As far as 1st metatarsal free body diagrams are concerned, this has been done already before...30 years ago....by Stokes et al.

    Stokes IAF, Hutton WC, Stott JRR: Forces acting on the metatarsals during normal walking. J. Anatomy, 129:579-590, 1979.
  27. Who says men can´t muti-task !!!
  28. Sorry up to speed now when I read free body diagrams I read just the met no other joints.

    I think I´m with what you say above now Simon. So If the met inverts there must be eversion moments at the proximal phalanx, medial cuneiform to create equilibrium, but this will cause a flow effect which is amplifyed during gait from change in ground shape, change in COP and COM etc.

    The other thing to consider is making a change going to reduce the effectiveness on the muscle. By inverting the met and therefore a eversion moment at the medial cuneform does that effect the ability of the TA to do its job ?
  29. does anyone have a copy of that ?
  30. I found the discussion about bending moment in the article intersting and can across this, I think it will be at least a 2-3 read for me to get my head around it all but some might find intersting

    Attached Files:

  31. Griff

    Griff Moderator


    Bear with me on this one... it's taking a while for the penny to drop and a good nights sleep doesn't seem to have helped matters sadly. I extrapolated the STJ axis and foot outline in the pic to help me (attached in green).

    Why will this tend to supinate the foot and not pronate it? (Assuming there was a kinematic effect/motion observed). The majority of the wedge is lateral to the STJ axis, with only a tiny portion of it (less than 5% at a guess) being medial to the axis and having any kind of supinatory lever arm. Yes its a varus wedge, but it is still predominantly lateral to the STJ axis. Is it purely due to the shift of CoM?

    Attached Files:

  32. Lawrence Bevan

    Lawrence Bevan Active Member

    To my mind what is more relevent is what force this will exert about the ankle joint axis. This is particularly relevent in the foot with a medial STJ axis and equinus deformity.

    My experience is that if you place this wedge on an orthotic and observe the patient statically you will get increased STJ supination moment. In walking gait, late-midstance there will often be a pronation moment especially with equinus deformity. For me its not a "walking" orthotic modification except only very rarely. If I have a non-reducible dorsiflexed 1st ray (not uncommon) then I tend to use a Morton's extension.

    I use it occasionally in runners - if they adequate ankle joint RoM, the effect on the 1st MTP is far less marked. 2-3 degrees is often enough and I fond it can work well when made with soft materials like poron.
  33. Lets say we ignore all the other weightbearing areas of the foot and focus on the section of the foot beneath the MPJ's. What the wedge will do is effectively bring the loading closer, still lateral to the axis, but closer to it (less lateral) than without the wedge. Thus the net pronation moment (force x distance) will be reduced because the wedge has basically altered the moment coming from each met head.

    For example: Lets imagine that without the wedge each of the 4 lateral met heads has a GRF of 10N acting on it and that the lever arm distances from the STJ axis to each of the met heads is as follows:
    2nd met= 1cm
    3rd met = 2cm
    4th met = 3cm
    5th met = 5cm

    This gives us pronation moments of:
    2nd met= 1cm x 10N =10
    3rd met = 2cm x 10N = 20
    4th met = 3cm x 10N = 30
    5th met = 4cm x 10N =40
    = 100Ncm

    Now lets say that when we add in the varus wedge more load is proportionally born by the 2nd met with decreasing load on each successively lateral met (no met can have zero load). Will still have a total load of 40N lateral to the STJ axis that we have to share between the 4 lesser metatarsals.

    2nd met = 1cm x 30N = 30
    3rd met = 2cm x 5N = 10
    4th met = 3cm x 3N = 9
    5th met = 4cm x 2N = 8

    This gives a total pronation moment = 57 Ncm

    So by moving the loading more medially the net pronation moment has been nearly halved.

    However, when we stand on a flat horizontal surface the ground reaction forces (GRF's) are pretty much vertical, when we insert an inclined plane beneath the foot the vertical component of the GRF's decreases (mg cos theta) and the medial-lateral shear force component increases (mg sin theta). Thus, in the presence of the wedge the centre of pressure (CoP) may well shift more medially, but the direction and potentially the magnitude of the vectors will also change. So while we may see a medial shift of the CoP in association with the varus forefoot wedge, it does not necessarily follow that the net STJ supination moment has been increased (STJ pronation moment decrease), unless of course we see an increase in supination motion / supinated position of the foot.

    Hope this helps, Ian.
  34. That was my thinking. being lateral to the axis it cannot, grf wise, create a supination moment, but it can affect / reduce the pronation moment.

    To return to our see saw analogy, it is the equivilent of sliding one person up the bar, nearer to the middle. That end of the see saw will then rise, in spite of there being no new up or downforce introduced and in spite of the fact that we are only changing the position of the downforce.

    It's a bit of a mental wrench init.

    Leave alone the practical and dynamic aspects and considered as a purely theoretical puzzle it should be pretty east to test. There's plenty of medially deviated axes out there.


    Ps. I'm glad this thread has attracted comment. I hope those who were at the day are all Reading along!
  35. Simon.

    Ref the force vectors and where the would cross relative to the sta.

    How much of an impact would the friction co efficient of the wedge have in practical terms?

    How much, and what impact will the inclunation of the axis in the saggital plane have? Intuitivly I'm thinking if the axis is higher when it passes over the forefoot the vector should matter less?

  36. Griff

    Griff Moderator

    Ker-ching. Penny dropped. I'm on board. Thanks for the example and the numbers - massive help when visualising this - more so than the horrendous scribblings that I was attempting which have now found their way into my bin. Pythagoras is turning in his grave.
  37. efuller

    efuller MVP

    The force of gravity on the metatarsal is very small. Some of the force of gravity acting on the rest of the body above the metatarsal will find its way through the cuneiform and be applied to the metatarsal. So the diagram should have a force from the cuneiform acting on the metatarsal. The net vertical force from the floor acting on the metatarsal, in equilibrium, would be equal to force of gravity acting on the metatarsal plus the vertical component of force from the cuneiform acting on the metatarsal.

    Haven't read through everything yet, but need to clarify COM versus CoP.

    The Center of mass is an imaginary point where the average point of mass is located. It can also be considered to be where the force of gravity can be considered to act. If you are standing and you abduct your right arm out to the side, so that is horizontal with the ground your CoM will be moved sligthly to your right.

    The center of pressure is the average location of several forces acting on the bottom of the foot. So there is force on the heel and met heads etc and these forces can be average to a single point and ground reaction force can be considered to be acting at this point.

    Gravity pulls the body downward and is acting at the center of mass. The body is not accelerating downward in static stance so there must be a force countering gravity. Ground reaction force is acting upward at the center of pressure (average of both feet if standing on two feet and at the center of pressure of one foot if standing on one foot.) Ground reaction force is different than gravity.

    All I got time for now.

  38. Friction is significant in so much as it determines whether the foot is sliding on top of the device or not- this will influence vector position. The angulation of the force vector in relation to the transverse plane shift in the CoP position needs to be considered. If the orthoses moves the CoP medially by 1mm but changes the angle by 5 degrees from vertical toward lateral this may result in a significant increase in pronation moment even though the CoP moved medially. So we need to know the spatial orientation of the axis and the 3D force vector. The higher the axis the greater the chance a smaller change in angle has of influencing the net moment see figure. - time for your Pythagoras, Ian.

    Attached Files:

    • stj.jpg
      File size:
      33.3 KB
  39. Donnchadhjh

    Donnchadhjh Active Member

    Yup - still reading along, also glad to see its an active debate.

    I'll be totally honest and say a lot of the finer points of what Simon is saying is going over my head - but I am following the general flow of the conversation and the basic principles behind it.

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