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Discussion in 'Biomechanics, Sports and Foot orthoses' started by markjohconley, Aug 21, 2009.

  1. markjohconley

    markjohconley Well-Known Member

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    Wouldn't an orthosis, fabricated using a 2D inshoe pressure image, off a shod mobile patient, be more relevant / accurate? regarding placement of the 'bumps' and 'angles', with the clinicians knowledge of bone anatomy to "join the dots", than off a static non(semi)-weightbearing foot. Surely the spatial locations of bony features, translated / reflected? to the transverse plane, differ.
    As for magnitude of the 'bumps' and 'angles' how relevant / accurate? can they be off that static non(semi)-weightbearing foot?
  2. Sammo

    Sammo Active Member

    I don't neccessarily think so. How can you know how high to make the arch? What about navicular drop/drift.. You can't get that information from a 2D pressure readings.

    What Bumps and angles are you talking about?
  3. markjohconley

    markjohconley Well-Known Member

    Goodaye Sam, i'd have to reply how can you know how high to make the arch from traditional casting methods, static, non(semi)weigthbearing? Drop/drift, i'll have to ponder that one. "bumps and angles" ~= contours of upper orthotic surface.
    Thanks for reply, it's okay, shoot me down, i'm married.
    Anyhow, have to take it back, as after you've fitted the orthoses, the spatial locations are going to change again.
    I'm taking up rocket science, i imagine it would be easier to apply the theories than biomechanics, mark
  4. Sammo

    Sammo Active Member

    why else do we post on here if is not to be shot down.. posting can be quite a masochistic past time in here!!!

    I'd say that with semi WB casts (i.e. foam boxes.. which personally I really like, but only use rarely in my current post) at least you have the shape of the foot how you have positioned it. Then when you have the cast it is a 3d shape on which to mould.

    With the 2d scan you have no idea of arch height at all... if your arch of the insole is lower than the lowest point to which the arch drops when pronating you will get no force applied from the orthoses against this movement = no reduction in pathological forces (if that is your aim)..

    Please don't shoot me down yet.. 2 months til my wedding :-o

  5. David Smith

    David Smith Well-Known Member

    I presume your talking about PoP cast or laser scanned methods V's orthoses made from pressure device data.

    Speaking for the PoP cast my polyprop orthoses are always a good fit visually, from my perspective, they are always comfortable to wear, from the customers perspective, and they almost always give good resolution of symptoms and improve signs in terms of gait and posture, without complications, from both points of view. How much more accurate or relevant does the cast need to be to get the prescription right or optimal?

    If you are speaking for the pressure device technique and you also equally achieve these positive measures of outcome / good results then that's great, who can argue with your experience.

    Do you make the orthoses yourself or do you have them made through the pressure device manufacturers service? Personally, in the latter example, I would not have enough input into the design to be confident about the effectiveness of this type of orthosis, but I know that many do use this option successfully, apparently.

    BTW I use Amfit about 70% of the time, which is a weight bearing 3D scanning system so it kind of bridges the two and I get the best of both worlds.

    Cheers Dave
  6. I love threads like this. They make you think.

    Firstly I think that there are several points to consider between an "non (semi) weightbearing foot and a fully WB functional foot.

    What Is the difference between a non wb foot, a semi wb foot and a WB foot? Research tells us that you get average 71% of the soft tissue spread and shape change based on the first 25% of loading. This is, of course, the average. Hypermobile feet spread more, rigid feet less. Given that we are talking about less than 10mm change (again, average), this moves semi WB casts to within a mm or two of the WB configuration which is close enough for me.

    Another big thing to consider I credit to Javier who chucked it in as a throwaway comment a few years ago and which has been lurking in my hindbrain ever since. If the foot is pathological, why to we want to capture its shape? Or to apply it to the present thread, if we accept that the foot changes shape on WB, which is going to allow us to apply force most efficiently, an orthotic made to the WB configuration or the NON WB configuration.

    Given that it is often residual moments which cause pathology and that we only start on those when a foot has reached end range of movement I would argue that the latter would be preferable.

    Kind regards
  7. This isn't true, think about it!
  8. Which bit, that it is often residual moments which cause pathology or that they happen when the foot has reached the end range of movement?
  9. I'm assuming you mean that we only start to effect these when a foot has reached is end of range.
  10. Graham

    Graham RIP

    I look at the orthoses shell as a frame, similar to eye glasses. It is meerly a mediam to deliver the prescription. If the foot deforms when in contact with a hard flat surface taking a cast in this position means your frame is already twisted.

    Non weight bearing casts, I believe, give me a consistent starting point. Cast modifications, such as arch fill etc will vary based on foot type. No science involved...yet!

    2D presure mapping can be usefull but unable to make that custom frame. In shoe mapping as a concept to acually make the orthoses would be worse as it would also be shoe specific. A little limiting.
  11. david3679

    david3679 Active Member

    I would come back to similar previous thread (cant think of the title)(Damn)
    does an insole need to be contoured or congruent to provide the changes of CoP CoM or Moments to help a pathology. No
    Does it provide more comfort to the patient, maybe?
    Could your orthotic be completly flat with additional wedges ETC, Yes

  12. And your point is?
  13. david3679

    david3679 Active Member

    is the assessment not more important than the medium of capture of the foot?
    If we can make an orthotic that works and bears no resemblance to the foot, I would say the understanding of the functional change is more important than a POP cast or cast box etc

  14. Dave, I agree that understanding the functional change is key. I just wrote this in the Genesis thread (they were never the same without Peter Gabriel):
    To this we could add, the method of obtaining a model of the foot should be employed to help dictate the morphology of our orthoses.
  15. You're right. Mark has made me question my assumptions perhaps more than anyone else who posts here. Your input has also been noted Robert. To re-iterate Kevin's recent post, it has been a delight to watch you grow.

    What needs to be understood is that neither weight-bearing nor non-weightbearing casts capture the shape of the foot during dynamic function unless you use a dynamic casting technique. If we assume many pathologies of the foot occur in response to the dynamic loading of the foot then it is easy to make an argument that neither static methods necessarilly capture the shape of the pathological foot. But realistically the static weightbearing cast is probably closer to it. Perhaps then the weightbearing cast is better because it shows us the shape we don't want the foot to be in? Better, perform dynamic weightbearing casts. Nice study there Robert, take static weightbearing and dynamic weightbearing and compare the two. The assumption here is that "foot shape" is important, is it? As Dave notes, we can alter forces without a model of the foot to work from.
  16. Graham

    Graham RIP


    I guess the apposing argument is why would you want to capture the foot as close to it's pathological position? Surly this is what we are trying to avoid and necessitates a device which can also deal with the foot as it changes position and shape over the course of a step.

    Of course that still doesn't answer why we should use a non weight bearing cast over a flat insole with wedges.
  17. Because, we then know that we have a cast of the foot in a pathological position. If we cast the foot in any other position we don't know whether or not this is another potential pathological position- right? At least there is a degree of certainty with the former:D. This is just a mental-gym for myself as much as anything Graham; don't get too hung up on it.

    If you wanted to edge your bets, you'd try and capture the foot close to mid-range as for the STJ at least, this should be near to the centre of the zone of optimal stress. Now there's a novel idea. ;-)
  18. Graham

    Graham RIP


    No worries there. I would have hung myself years ago if I wasn't able to say, with certainty, that certainty is what we don't have. (The Holy Grail). Perhaps Mr Creasote would have a thing to say on the matter of certainty:pigs:
  19. Arjen

    Arjen Active Member

    This very discussion happens here at Amfit all the time. Many have already weighed in on the 2D vs 3D dataset issue. I couldn't agree more that the 2D pressure data can be a useful diagnostic to detect gait anomolies, but it tells you little about the shape of the subject plantar surface.
    An interesting extension of that is the discussion of understanding the difference between the static measurement of the foot (casting method aside for a moment) and what happens in the gait cycle and our ability to accurately determine those dynamics. We spend a great deal of time trying to understand the relationship between these two (broadly speaking) states of the foot and how best to deal with them. We have ended up by using an inshoe pressure measurement system and evaluating pressure scans through time/gait cycle both before and after orthotic therapy. Does anyone else have a better technique?
  20. If you want to spend your time meaningfully at AMFIT develop your system such that each individual peg can be adjusted to a variable stiffness, then it will be very useful.
  21. Arjen

    Arjen Active Member

    I'd like to think that most of what we try to do here is meaningful.

    This is quite a ways off topic...We'd long considered doing what you are advocating but with 576 pins...the machine would quickly become unaffordable. It would be cool, but of little use if no one would buy it.
  22. The delight is entirely mine I promise you! :drinks

    Here is a thought to mull. We seek to change something within the foot by altering the way force is exerted on it. Generally speaking we do this by exerting a force across an axis to modify internal forces. One of the problems patients sometimes have with orthotics is "comfort" which is often a problem with pressure on certain parts foot.

    Kevin's "spikeorthotic" (which has become a useful abstraction) exerts all its force on the medial side of the STA, but is uncomfortable because its all in a spot.

    Try this for a model. Lets say that to be effective an insole must create sufficient supination moments across the STA to reduce the demand on a pathological internal tissue for supination moments from that tissue. Lets call that Volume of supination moments X.

    Lets call the surface area of the foot upon which that force is exerted Y.

    Lets call the pressure on the foot (lb / sqinch) Z

    Z*Y = X

    If Y is small (Spikeorthotic) then Z will be huge = ouch.

    The goal, to maximise effect and comfort, then becomes to increase Y. Obviously some parts of the foot are designed to take more pressure than others so its not exactly linear but in general....

    In which case the better the orthotic conforms to the pressure profile of the foot (which is not necessarily the same as the soft tissue profile) the more comfort per effectiveness...?

    Obviously how much the pressure needs spreading depends on the value of X and the compressive tissue stress threshold but is this what we're circling?

    Just thinking out loud.

  23. Pressure x area = moment

  24. Loose the resolution, come up with something more than a contact digitizer.
  25. Arjen

    Arjen Active Member


    Moment is a unit of torque...not pressure over area.

    In fact pressure is incorrectly used in this discussion.

    Pressure = force x area
  26. Arjen:

    Moment is torque, not a "unit of torque".

    In addition, pressure is force/area not "force x area".
  27. Supination moments are measured in N-m (Newton-meters) and is not a measurement of volume (cubic centimeters). It would be better to say "magnitude of supination moments".

    You are mixing terms incorrectly. The correct formula for pressure (P) is P = F/A, where F is force and A is area over which the force acts. Moment (M) is the mathematical product of force (F) and moment arm (L).

    You have left off an important part of the equation in your example above. You would need to know the moment arm for the supination force from your spikethotic in order to calculate the how much pressure the spikethotic would need to exert to produce the supination moment required to supinate the foot. A large STJ supination moment arm from your spikethotic will require decreased pressures and no STJ supination moment arm from your spikethotic will require an infinite magnitude of pressure to produce STJ supination. Therefore, without specifying STJ moment arm length, you can't calculate what the pressure would be.
  28. Arjen

    Arjen Active Member

    I stand corrected. Thank you.
  29. As kevin says, I've left out the whole bit about how far from the axis the force was exerted. Kind of on purpose for the sake of making a point but granted it's incorrect.

    If I modified to to assume that the cop of the orthotic was the same in all cases? I'm not really trying to come up with a formula for how an orthotic works, more to express the significance of the shape of the orthotic. Even if we included moment arms in one side of the equation and adjusted the terminology it would still be inadequate because it would not account for planal dominance, axial inclination, force vector of the orf, friction in the cover (which would inform friction) etc.

    Sorry. Bad form to mangle the maths to make a point. My bad.

  30. By the by, has anyone every made such an equation incorperating all of those points? Because fascinating intellectual exercise!
  31. Arjen,

    Robert's equation was way off, as is yours above. That is why I was asking if he was sure.
  32. OK, so go back a step. Rather than trying to express this mathematically say why shape is important. The shape of the orthosis will, in part, determine it's load-deformation characteristics; the discreet and net ground (orthosis) reaction force vectors; the comfort- right?

    Anything else you can think of?
  33. So Simon and others if I can paraphrase to see if I´m thinking correctly. If you take a foot which you want to reduce the medial deviation of the STJ axis after heel strike.

    1 you 1st need to determine where the axis of the subtalar joint is at the point of closed kinetic chain you want the orthotic to start working.

    2 determine the area and shape of the foot medial of the stj axis at that point

    3 the greater that area the greater the orthtoic contact with the foot should be. This will then

    a reduce the force required to cause a lateral deviation of the axis due to the greater area of contact with the foot.
    b be more comfortable due to the greater foot contact area.

    So the more medial deviated the axis the greater the force required from the orthotic due to the reduced area of foot medial to the stj axis to get lateral deviation of said stj axis. This may lead to less comfort.

    The less medial deviated the stj axis the greater the surface area of the foot medial to the stj axis therefore less force required due to thegreater surface area to lateral deviate the axis. Which may lead to more comfort.

    The above assumes that the orthotic is perfectly contoured to the foot.

    So if this is correct the problem is that at the moment we cannot take a 3d scan of the foot during gait, we cannot see the subtalar joint axis change during gait.

    We also be considering the relationship to all 3 body planes.( I finally got what Simon and Robert we discussing in the tread about high stj axis)

    Hope that it makes sense and I´m on the correct path. off to a wedding now.

    Michael Weber
  34. The pressure exerted by the orthosis on the plantar foot is, I believe, the key to patient comfort. If the plantar pressure is above a certain threshold of tolerance, the plantar pressure comfort threshold, then the orthosis will be uncomfortable and intolerable. If the plantar pressure is below this plantar pressure comfort threshold (PPCT), then the patient will feel that the foot orthosis is comfortable and tolerable.

    I also believe that the magnitude of PPCT will be different from one person to another and will be different from one area of the plantar foot to another. Individuals with more "sensitive" feet will perceive much lower magnitudes of plantar pressure as being uncomfortable than those individuals with "less sensitive" feet. Also, areas of the plantar foot that have thicker skin and are adapted to having large magnitudes of plantar pressure acting on them continually, such as on the plantar heel, metatarsal heads and lateral forefoot, will likely have a higher plantar pressure comfort threshold than will areas of the plantar foot which are typically not in contact with the ground, such as the medial longitudinal arch.

    Since the measure of pressure is in units of force per surface area of contact (P = F/A) then to make comfortable orthoses for our patients while still exerting sufficient orthosis reaction force (ORF) to alter the moments at the joint axes of the foot as are therapeutically desired, the goal of orthosis therapy should be to design orthoses that spread ORF over as large an area as possible to minimize the pressure and improve patient comfort. Trying to exert a large ORF medial to the subtalar joint (STJ) axis to supinate the foot in an area of contact that is too small will significantly increase the plantar pressures in this area of contact and will tend to create discomfort. My hypothetical example of an orthosis with a nail driven into the medial arch of the orthosis to "control pronation", the spikethotic, is an excellent example of how we must always be concerned about the magnitude of plantar pressure of our foot orthoses in order to optimize patient comfort and prevent patient injury.

    I believe that one of the reasons that the medial heel skive has been so successful as an orthosis modification to help increase ORF medial to the STJ axis is that the medial heel is a typical weightbearing area of the foot and can tolerate fairly large plantar pressures before the PPCT has been exceeded. However, I also have noted that if the medial heel skive orthosis can be designed to have increased medial longitudinal arch height (e.g. inverted balancing position, minimal medial expansion plaster thickness, stiffer orthosis plates), then the resultant increased plantar pressure in the medial arch from the orthosis will not only help shift ORF more medially in the midfoot to increase the magnitude of STJ supination moment, but will also allow higher depths of medial heel skive modification to be tolerated by the patient, presumably by decreasing the plantar pressure at the medial heel, than what would be present in an orthosis with a lower medial longitudinal arch.

    Therefore, the careful consideration of plantar pressure, the PPCT of each individual and each area of the plantar foot, and how it can affect not only the function but also the tolerabilty of foot orthoses is one of the keys to making foot orthoses that are both therapeutic and comfortable additions to our patient's shoes.
  35. Since pressure always acts perpendicular (normal) to the surface and pressure = force / area, is it the normal component of force that defines pressure at a given area on an orthosis surface or the total load on that area?

    The normal component- right?

    So, the angulation of the orthosis surface is key in determining the pressure exerted by the orthosis at any point on the plantar surface of the foot. The greater the angulation, the lower the pressure (given a constant total load). So a 15 degree medial heel skive should actually result in lower pressure at the area on the plantar surface of the foot that it is in contact with, when compared to the same area of the foot contacting a horizontal surface. This begs the question then, if plantar pressure is key in determining comfort, how could any foot orthosis with it's multiple inclined planes be less comfortable than the relatively more horizontal surface of the inside of a shoe? Obviously Kevin's point regarding areas of the foot which are used to load bearing is valid, but with increasing angulation we also get increasing shear force components. Too high shear forces are likely to cause blistering = discomfort. What is the most common sign of orthosis intolerance? Blisters- right? Where do they occur? Medial longitudinal arch- right? This area of the foot is not used to load bearing and the orthosis shell is highly curved (inclined) in this area = increased shear forces.

    So why do I see more blistering resulting from prefabricated devices than I do from custom foot orthoses?
  36. Even though angulation of the surface of the orthosis is likely to be a determinant of the amount of pressure that an orthosis may exert on the plantar foot, it is also likely that it is the congruity of the orthosis to the plantar foot which is a much more important determinant of plantar pressure than the absolute angle of the orthosis surfaces relative to the ground.

    For example, if I were to use a 45 degree medial heel skive that narrowed the heel cup of the device by 10 mm, the highly angled surface of the medial skive would certainly have much more pressure on it than would a flat and horizontal medial heel cup. The reason for this is that both the 45 degree and flat medial heel cup of the orthosis are no longer a three-dimensional match to the plantar foot and so the poor congruity more significantly affects the pressure than does the absolute angle of the orthosis. Of course, the reason for this is that the load on the orthosis has changed in this area which Simon has mentioned above.

    In another example, if I were to make an orthosis that had a flat, horizontal medial arch for a patient that had a pes cavus deformity, then there would be no plantar pressure in the medial arch of this foot since it wouldn't even contact the medial arch (no load in medial arch) even though the orthosis was horizontal to the ground. Again the plantar contour of the foot relative to the orthosis plate during loading conditions is likely a much more important determinant of the pressure on the plantar foot than the absolute angle of the orthosis surface to the ground.

    However, as Simon mentioned, given a constant load, the horizontal orthosis surface should have increased pressure relative to the angled orthosis surface due to pressure being measured normal (i.e. perpendicular) to the orthosis surface. I just thought I should expand on these ideas since they are critical to understanding how orthoses work and don't work for our patients.
  37. Usually with a prefab there is less arch height and therefore the MLA height will reduce during pronation, when this happens the foot gets longer so when the skin comes incontact with the device you have a shearing stress in mutiple directions which I believe leads to blistering.

    Michael Weber
  38. So orthosis stiffness is also important. If we have a very stiff device it must be shaped to fit the contours of the foot to be congruent. However, a more compliant device will allow the foot to sink into it to become congruent. Indeed, with a compliant enough material we could take a flat horizontal insole and still achieve total contact with the plantar surface of the foot; if it were thick enough this could even be achieved in a cavus foot.

    The biomechanical properties of the soft-tissues and relationship of the underlying bone at the orthosis interface will also be significant.

    If we have a foot, lets say it's a cavus foot. We make two orthoses for this foot: device 1 is a stiff devices which is congruent since it matches exactly the contours of the plantar foot; device 2 is a flat horizontal sheet of material but is complaint enough such that when loaded by the foot the congruence between the foot and device is equal to that achieved by device 1. Question: what, if any, will be the differences in pressure and reaction forces at the interface between the foot and the two devices?
  39. I read a nice explanation of this at work today. I don't have the book with me. I'll post it up tomorrow.
  40. So why do we concern ourselves with posting angulation? And why do we still try to rationalise this in a scientific manner?;)

    Indeed, why not start with a weightbearing impression of the foot, form an orthosis to this model and then remove sections of the orthosis to manipulate the congruency at discreet areas of the foot? I'm calling this Spooner's orthoses subtraction technique aka the SOS technique* (acknowledging Markjohconley for talking about removing sections of an orthosis a while ago, but he spoke about removing sections that weren't important, the SOS technique realises the importance of the removed sections in manipulating plantar pressures).

    *SOS technique- TM, copyright 2009

    In all seriousness, rather than a track of pins AKA the spikethotic, why not a track of holes, we know that an aperture in an insole will result in increased pressure at the periphery of the aperture, so if we run a track of small holes along the surface of the orthosis...

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