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Calculating orthotic prescription values

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Mark Russell, Nov 22, 2010.

  1. Members do not see these Ads. Sign Up.
    One thing for sure, Craig's excellent Boot Camps certainly get you thinking - the only problem is you leave with more questions than answers! Perhaps some patient, kind soul might assist with this one.

    When considering your orthotic prescription - for simplicity a Root device from a STJ neutral cast - what correlation (if any) exists between the mechanisms used - posts, skives, extensions etc., and the damaged tissue the device intends to heal? For example, can you calculate the forces generated from various inclination angles on rearfoot posts in the same subject - if so, how? If this has been discussed before - I'd appreciate a pointer in the right direction.

  2. Mark have you read this thread and got a copy of the paper ? I think the paper may help, written by 3 blokes you know. Helped me a lot when thinking about 3d geometry, vectors and the like.

    If you want a full copy PM with an email address and it will appear in your inbox.
  3. Thanks Mike - bedtime reading. Not sure it's what I'm after though. Let's make it simple.....

    Patient complaines of plantar faciitis and an examination reveals a medially deviated STJn axis with FnHL and delayed windlass. High supination resistance. She responds well to a course of inversion strapping and icing but there's a Hx of PF occurring over the last 20 years so we're considering bespoke orthoses.

    Ordinarily I would be looking at the differential between her relaxed and neutral stance positions and trying to get her everted calcaneus as close to the neutral position as possible. Let's say a 5 degrees medial wedge for argument's sake.

    Am I correct in thinking that if I increase the inclination angle - either directly or by adding a medial skive or removing the lateral heel cup - then I would be increasing the supination moments and facilitating the reduction of pronatory moments by a greater factor than if I were to post the devices to the old prescription? Most clinicians won't have in-shoe pressure measuring systems or the like - so calculating the GRF vectors will be difficult if not impossible for the average podiatrist. But if the foot pathology results in an abnormal force of say 40N and we try to counteract that force with medial wedging - what correlation is there (if any) between the inclination angle and the orthotic reaction force? Is it simply, higher angle = higher force?
  4. David Smith

    David Smith Well-Known Member


    You wrote
    The practical answer to Q2 is No. Answer to Q1 is: The more appropriate your design the better outcome you will have (deliberately ambiguous;))

    In theory it would be possible to calculate the change in forces applied to the foot pre and post orthotic intervention providing you knew parameters like initial conditions and biometrics and material properties etc. NOT likely tho is it.

    The better way to think about this is not by force change but by how the CoP vector will change relative to the axis of interest e.g. the STJ axis. So in simple 2D terms, lets say reductionist terms, the medial wedge moves the CoP medially and the higher the incline of the wedge the more medial the CoP becomes (up to a certain point of slippage in the shear plane, where the foot slides onto the lateral aspect of the foot.

    The medial skive works slightly differently, it is like a chock under a wheel, as the foot tries to roll over it the force vector from the skive becomes more vertical and higher in magnitude and therefore more medial, plus and at the same time, the force acting on the lateral aspect of the foot reduces in magnitude, and so the total CoP vector again moves more medially. Also the placement of the medial skive enables an initial condition of a more medial force to be applied to the foot than a normal wedge would, again the analogy with a chock under a wheel show this well.

    Determining exactly how much change in CoP position you might require will not be precise but consideration of the STJ axis position (for instance) thru the stance phase will help considerably in terms of the pathology of interest.

    The problem with using in shoe pressure insoles to make a reasonable estimate of change in CoP relative to the STJ axis for instance, is that the curved surface of the orthosis is characterised as a flat surface on the PC screen and so the force vector that appears vertical and medial (and causing supination moments) to the STJ axis may in fact be oblique and lateral (or rather as if lateral & causing pronation moments) to the STJ axis. You'll have to read the paper Mike quoted to get the full picture of this proposition.

    Does this help?

    Regards Dave
  5. David Smith

    David Smith Well-Known Member


    Reading between the lines of your last two posts: are you thinking, if you can't calculate forces then it might be reasonable to observe change in position.
    Change in position might indicate change in internal force applied but no change in position does not imply the converse i.e. no change in internal force.

    This is because the position can remain the same but the internal forces that held the foot in a certain position can be replaced by external forces e.g. orthotic reaction forces. So because the design of the orthosis means the CoP moves medially the stress on internal structures are reduced and the force on external structures are increased. In other words the same angular or rotational equilibrium is achieved but by utilising plastic materials instead of biological tissue.

    Regards Dave
  6. efuller

    efuller MVP

    I don't calculate the moments when I make an orthosis. I assume that plantar fasciitis is pronation moment and high medial forefoot load problem. (assuming medial slip of fascia) Identify the source of pronation moment. (muscles or ground) Muscule caused pronation feet will tend to have laterally positioned STJ axes. Don't add a medial skive for these feet. Ground caused pronation will tend to have medially deviated STJ axes and these feet should get a medial heel skive.

    Use a forefoot extension to reduce load on the medial forefoot and increase load on the lateral forefoot.

    Then adjust as symptoms warrant. There's way too many factors to calculate it all out. You just have to go by the level of symptoms to see if you changed things enough.

  7. In order to calculate these thing you need a 3d force plate + in-shoe pressure + 3d kinematic analysis with multi-segment foot modelling. Can you imagine how long this would take on all patients and how much you'd have to charge them. Anyway the best evidence suggests a prefab will do the trick:rolleyes:

    Like Eric and Dave, I think more in terms of the position and direction of the forces, rather than their exact magnitude.
  8. This seems relevant here: http://matacq.free.fr/Publis/sheep.pdf :D

    And some of us realise that shamopody is weird, can you imagine this at an international conference with your mates and peers:
    "So what you working on at the moment?"
    "Sheep dragging kinetics"
    "You bet your ass".
    "No, really?"
    "Sheep dragging kinetics"
    "So, what are you benching these days?"
    "two-twenty"....... etc.
  9. Okay - firstly thanks for the replies - as always, much appreciated. Now by and by I think for the most part, I can understand exactly where your thinking leads and it makes a lot of sense. I've had any number of cases in the past who present with navicular/fascia/MTPj symptoms and yet my clinical examination didn't reveal immediately what pathology was at play to cause the symptoms in the first place. I guess it's only when you start thinking about the effectiveness of lever arms, resistance, elasticity etc., as well as alignment issues, you begin to see much more of the bigger picture! Oh happy days.

    So coming back to my original point - and thinking along what you said about position and direction of forces - we have a patient with a medially deviated STJn axis and we can see that there is adequate medial space on the calcaneus for a R/F post (lets forget about skives/MOSI/MASS devices for now) - should we post to the maximum amount we can - to where the patient finds it comfortable and doesn't slide off the device - and not worry about rearfoot alignment or inclination angle?
  10. Thought you'd made a typo until I opened the link..... Wonder how they organised the control group??
  11. My gut feeling is no, at least not always (supination resistance comes in to play). Remember we want the tissues to be within their zones of optimal stress (ZOOS). If we (for example, varus rearfoot) post to the maximal possible amount, the probability that we will create excessive supination moments in some people will be high. In other's you could stick 30 degrees under them and it still might not change reafoot position during function.
  12. Thanks Simon - makes a lot of sense. I can see my head's going to hurt over the next few weeks.... Has anyone written any protocols for determining prescription values yet, taking into account force vectors, loading, ZOOS, resistance etc?
  13. No, it can't be done to that kind of precision yet. The good news is, it doesn't have to be (hint: it's a zone of optimal stress not a single point)
  14. Mike - you're a wizard! That's the conversation I was looking for. Simon et al., I guess when we eventually manage to construct a protocol that takes into account all the variables in vivo/in vitro then we might come close to understanding exactly how our interventions actually work. Until then it's the ET technique and some informed reasoning.....

    ET = Eyeball Technique.
  15. efuller

    efuller MVP

    As a first estimate, I won't go to maximum unless there is an extremeley medially deviated STJ axis. When I first started using medial skives, I took my own orthotics and started adding more and more varus wedge under the rearfoot post of the orthotic. I reached a point where I could still stand comfortable on the orthotic, but in gait, I could feel fatigue beginning in my lateral leg muscles (peroneals) especially at heel contact. So, you can go too far.

    Just because there is "inadequate" medial space to cause supination doesn't mean that you should not add the varus wedge effect to your device. You have to remember this is a matter of leverage of the center of pressure relative to the STJ axis. Just because you cannot create a supination moment does not mean that you should not reduce the pronation moment as much as possible. (Increasing supination moment = decreasing pronation moment) The higher the pronation moment from the ground the greater the internal supination moment must be. The higher the internal supination moment the higher the stress in those tissues that are resisting the pronation moment from the ground.

  16. markjohconley

    markjohconley Well-Known Member

    ...Just interested in what exactly were you looking for and what keywords you used in your search to get this article listed?...only interested
  17. What happens when the STJn axis is so far out that a medial wedge actually increases the pronation moment? The only benefit I see in a R/F post in these cases would be to increase the arch stiffness - and in these cases, would it not be preferable to change the casting position - perhaps even to a MASS position? But then I'm probably wrong! Perhaps I'm still too entrenched in my comfort zone - if you have a pathological force - you should counter it with an equal or greater therapeutic force; be that rearfoot/forefoot posts, skives deflective cushioning - and probably my thinking is too simplistic insofar as if you were able to calculate the pathological forces, then there may be a method of doing the same with the therapeutic counterpart. As with Simon, Dave, Mike, Robert, Kevin etc etc - many thanks for your patience and forbearance, Eric. :drinks
  18. Yeah right - we're all too well aware of your idiosyncrasies in the wooly world matey. What it it they call an outback sheep farmer again.......... a pimp? ;) Don't worry - mum's the word!
  19. efuller

    efuller MVP

    Shifting the center of pressure more medially will decrease the pronation moment from the ground. The varus heel wedge effect will shift the weight more medially even if that location of force is still lateral to the axis.

    If the center of pressure without treatment is 6mm lateral to the STJ axis. Then with a varus wedge the center of pressure is 4 mm lateral to the STJ axis you have improved the situation. If you shifted the center of pressure to 8mm lateral to the axis then you would be making it worse. So, I don't see how the medial wedge could possibly increase the pronation moment.

  20. Mark, have you watched this Video Kevin has put up about Subtalar joint axis location?

    It helps with the image of the axis moving during gait, http://www.youtube.com/watch?v=Fn9Uujfo3iA

    Due to the fact that the STJ axis anterior exit point is stationary relative to the dorsal aspect of the talus neck , any Supination force created from medial to this point should cause a supination moment. ie I think it would be impossible for a medial skrive to be lateral to the stj axis and cause a pronation moment unless the talus was dislocated.

    hope that helps
  21. David Smith

    David Smith Well-Known Member

    Simon Solomon Spooner wrote in the thread 'Orthotic precrition Variables'

    This is well kept in mind as we might try to become more prescriptive as opposed to descriptive

    Regards Dave
  22. Griff

    Griff Moderator

    Haven't watched this video for ages! Kevin, is that your good lady you have standing on your kitchen table?

  23. Ian and Colleagues:

    This is actually my daughter-in-law, Keira, who is married to my oldest son, Keegan, and is the mother of my three grandchildren. Yes, this is our kitchen table she is standing on in our current home.

    She also was the subject for my midtarsal joint video up on YouTube which was also shot in our house on the couch with a black sheet backdrop.

    We're currently paying off her modelling fee by Pam and I babysitting her kids as much as possible.:rolleyes:
    Last edited by a moderator: Sep 22, 2016
  24. Actually it's Simon Kenneth Spooner, the Kenneth being after my late father.

    This was Solomon:

    "blessings are not just for the ones who kneel, luckily." U2- City of Blinding Lights

    "City Of Blinding Lights"
    The more you see the less you know
    The less you find out as you go
    I knew much more then than I do now

    Neon heart, day glow eyes
    A city lit by fireflies
    They're advertising in the skies
    For people like us

    And I miss you when you're not around
    I'm getting ready to leave the ground
    Oh you look so beautiful tonight
    In the city of blinding lights

    Don't look before you laugh
    Look ugly in a photograph
    Flash bulbs, purple irises
    The camera can't see

    I've seen you walk unafraid
    I've seen you in the clothes you made
    Can you see the beauty inside of me
    What happened to the beauty I had inside of me

    And I miss you when you're not around
    I'm getting ready to leave the ground
    Oh, you look so beautiful tonight
    In the city of blinding lights

    Time, time, time, time
    Time won't leave me as I am
    But time won't take the boy out of this man

    Oh, you look so beautiful tonight
    Oh, you look so beautiful tonight
    Oh, you look so beautiful tonight
    Yeah, the city of blinding lights

    The more you know the less you feel
    Some pray for, others steal
    Blessings are not just for the ones who kneel

    You know who you are and that this is for you. x

    For you Dave because I love you too: Yahweh
    Take these shoes
    Click clacking down some dead end street
    Take these shoes
    And make them fit
    Take this shirt
    Polyester white trash made in nowhere
    Take this shirt
    And make it clean, clean
    Take this soul
    Stranded in some skin and bones
    Take this soul
    And make it sing

    Yahweh, Yahweh
    Always pain before a child is born
    Yahweh, Yahweh
    Still I'm waiting for the dawn

    Take these hands
    Teach them what to carry
    Take these hands
    Don't make a fist no
    Take this mouth
    So quick to critisize
    Take this mouth
    Give it a kiss

    Yahweh, Yahweh
    Always pain before a child is born
    Yahweh, Yahweh
    Still I'm waiting for the dawn

    Still waiting for the dawn, the sun is coming up
    The sun is coming up on the ocean
    His love is like a drop in the ocean
    His love is like a drop in the ocean

    Yahweh, Yahweh
    Always pain before a child is born
    Yahweh, tell me now
    Why the dark before the dawn?

    Take this city
    A city should be shining on a hill
    Take this city
    If it be your will
    What no man can own, no man can take
    Take this heart
    Take this heart
    Take this heart
    And make it brave
  25. Mark:

    You have started a very good thread here since I'm sure a lot of others following along here have similar questions but are not willing to ask them on this forum.

    As the others have stated so far, it would be very difficult to actually calculate the external moments acting from ground reaction force acting across the subtalar joint (STJ) without a force plate and without a very good idea of the spatial location of the STJ axis.

    The good news is that when using Tissue Stress Theory to prescribe foot orthoses, you don't need to know the absolute magnitudes of moments acting across the STJ. All you need to know is which direction one should change the STJ moments in order to produce the reduction in stress acting within the injured tissue in order to produce the desired therapeutic results.

    For example, let's take the example of the runner with medial ankle pain and see how one would use Tissue Stress Theory to design an orthosis that would make the runner less symptomatic.

    First of all, after taking the history of the runner, one should determine which of the structural components of the medial ankle area of the runner is symptomatic. Palpation reveals that the runner is most tender at the posterior tibial (PT) tendon. If the runner relates no history of direct trauma (i.e. an external compression force) to the PT tendon, then one must assume that the most likely cause of PT tendon pain is excessive magnitudes of tensile stress within the PT tendon.

    Now, since we know that the function of the PT tendon is to increase internal STJ supination moment, then the most reasonable way to reduce PT tendon tension is to decrease the contractile activity within the PT muscle. It makes sense if we can increase the external STJ supination moment with a properly designed foot orthosis, then the central nervous system (CNS) will recognize the increase in external STJ supination moment from the orthosis from its afferent joint/tissue receptors and reduce the efferent contractile activity to the PT muscle. This reduction in PT muscle contractile activity will also likely reduce the pain in the PT tendon and allow it to heal uneventfully.

    How do we then increase external STJ supination moment with a foot orthosis? There are many ways to accomplish this task from a technical aspect. Orthosis modifications which may increase external STJ supination moment include adding a medial heel skive, inverting the orthosis, increasing the medial longitudinal arch height, stiffening the medial longitudinal arch of the orthosis, adding a rearfoot post, adding a varus forefoot extension or lowering the lateral arch of the orthosis. Basically, any orthosis modification which reduces the ground reaction force (GRF) on the more lateral aspect of the plantar foot and/or increases the GRF on the more medial aspect of the plantar foot will increase the external STJ supination moment and will likely produce a therapeutic effect for the runner with PT tendinitis.

    Mark, I would have to say that the most difficult task in prescribing orthoses is not in the decision of which direction to "push the foot" with the orthosis. Rather, the most difficult task is knowing how much "correction" and which orthosis modification one should use so that our example of the runner with PT tendinitis is given the proper amount of external STJ supination moment so that 1) the abnormal magnitudes of excessive PT tensile stress are reduced so that proper healing may occur, 2) the gait function of the runner is optimized, and 3) the increase in external STJ supination moment is not too little to produce insufficient reduction in PT tensile stress but also not too great that it may cause pathologies from excessive STJ supination moments such as iliotibial band syndrome, peroneal tendinitis, lateral ankle instability, or plantar fascial compression irritation.

    For most clinicians who are highly skilled with foot orthoses, it takes years upon years of trial and error experimentation to find which combination of foot orthosis modifications work best for each patient. Unfortunately, due to a lack of proper training, or being taught improper orthosis theory, many clinicians are unable to achieve the full potential of custom foot orthosis therapy.

    However, for the clinician that has worked to obtain a better understanding of how the foot works by studying and using mechanically coherent theories, and has worked to better understand how foot orthoses work by the alteration of GRF into orthosis reaction force (ORF), such a clinician can become one of the most valuable foot-health clinicians in their community and provide their patients with a medical service that can not be obtained from any other medical professional.

    I suggest, Mark, that if you spend time reading my first book and then, if you so desire, spend time reading my second and third books as time allows, that you will gain a much better understanding of the process I have outlined above. My first book has now been reprinted is now available again at the Precision Intricast Website.

    Kirby KA: Foot and Lower Extremity Biomechanics: A Ten Year Collection of Precision Intricast Newsletters. Precision Intricast, Inc., Payson, Arizona, 1997.

    Kirby KA: Foot and Lower Extremity Biomechanics II: Precision Intricast Newsletters, 1997-2002. Precision Intricast, Inc., Payson, AZ, 2002.

    Kirby KA: Foot and Lower Extremity Biomechanics III: Precision Intricast Newsletters, 2002-2008. Precision Intricast, Inc., Payson, AZ, 2009.

    Hope this helps.:drinks
  26. A geat help, Kevin, thank you. :drinks I guess this was the nub of what I was after - perhaps I am just jumping the gun a little. It will be interesting if, one day, we are able to conduct clinical tests to determine accurately, the various forces and stress that underpin foot pathology and aleviate these factors with a foot orthosis calibrated precisely - be that by extrinsic/intrinsic modifications, material characteristics or casting technique. The elusive holy grain of podiatry. Until then - informed guesswork and trial and error!

    All the best
  27. Unfortunately, since we know so little about the central nervous system (CNS) control of gait, and the response of the CNS to foot orthosis modifications, then the goal of even being able to accurately pin down the "optimum orthosis formula" for every patient will likely always be subject to some trial and error experimentation. This is why gait examination is so important when foot orthoses are being evaluated in a patient.

    No matter how much we are convinced that a certain orthosis prescription should be the "optimum orthosis formula" for a patient, if their gait function deteriorates with application of our "optimum orthosis formula", then the orthosis must be adjusted to optimize their gait function. This is why it is so important to develop a better understanding of how the CNS controls gait function since, without the CNS, there would be no gait function.

    As an aside to this great discussion, we are driving this morning up to Lake Tahoe to a cabin on the lake for our family Thanksgiving celebration. I'll take some photos and send them along to make everyone jealous. Lake Tahoe just got about 5 feet of snow in the last few days!

    Happy Thanksgiving everyone!
  28. David Smith

    David Smith Well-Known Member

    Last edited: Nov 26, 2010
  29. markjohconley

    markjohconley Well-Known Member

    Even when you have the "3d force plate + in-shoe pressure + 3d kinematic analysis with multi-segment foot modelling" that's only the forces at the orthoses/skin interface. What happens to those forces between skin and bone through the soft tissue, are those forces transmitted in straight lines? Bone surfaces, and that's where we're interested in with the forces, isn't smooth, even and equidistant from the skin surface.
    Have we covered this? or isn't this relevant?
    And goodaye to you all, it's been a beautiful day.
  30. Should be the same thing as the orthotic but even more complex with shape of bone, compression force, there should be internal shear forces going in every direction be a nightmare.

    Sometimes I think we over think things a little, while interesting discussion do come from them there must be a point where clinically we say here is the sum of a forces being too much on this tissue, add a device to change the COP to move the force from the stress tissue to another tissues , and if it doesn´t work make adjustments.
  31. Great discussion!

    Greetings from a snowy but clear Lake Tahoe. Today is the annual Kirby Christmas Photo Shoot and sledding with the grandchildren/children. Tomorrow, we expect about 4-6" of new snow. Fun!

    Attached Files:

  32. Looks rubbish :rolleyes: When I'm a famous podiatrist, I'm going to have a place by Spooner Lake (not too many miles from where you are right now Kevin), just so I can come over and bug you :D.
  33. Griff

    Griff Moderator

    Ah man - you get a lake named after you and all I get is a pond in Massachusetts... says it all really...
  34. Must try harder. But don't think of it as work, the whole point is just to enjoy yourself.
  35. Griff

    Griff Moderator

    National Lampoons and golf... you spoil me.
  36. I just want a pair of strides like ya' man there. I might even get interested in golf if I had a pair of them.
  37. Griff

    Griff Moderator

  38. markjohconley

    markjohconley Well-Known Member

    Yep, fair enough Michael, just think of it when I'm using a rearfoot valgus wedge, that, would it be creating equivalent 'modification of ground reaction force' MAGNITUDES to a similarly dimensioned rearfoot varus wedge as the plantar surface of the calcaneus is irregular with medial process of the tuberosity more prominent? than the lateral. (I realise the moments about the axes would be very different in magnitude)

    Beautiful day here in Aus mate!
  39. And here's the results of the 32nd annual Kirby Christmas Card Photo Shoot.....expecting 4-8 inches of snow tomorrow, wind gusts to 25 mph....starting to feel like winter up here! (Do you know how hard it is to get 5 adults, 3 children and 3 dogs to all be looking toward a camera at any one time?!)

    Attached Files:


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