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Orthotic Reaction Forces (ORF)

Discussion in 'Biomechanics, Sports and Foot orthoses' started by markjohconley, Jul 27, 2010.

  1. markjohconley

    markjohconley Well-Known Member

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    Foot orthoses are used to alter the position?, direction, and magnitude of Ground Reaction Forces (GRF), at the plantar (& medial & lateral) surfaces of the feet, relative to those exerted by a flat (either ground or flat in-sole) surface. For medial and lateral I would figure the friction of the heel against surface transferring to the shoe upper/heel counters to the feet.
    How the devil do you INITIALLY estimate the modifications necessary to the orthoses to acquire those desired MAGNITUDES. Surely most podiatrists (and other orthotic manufacturers) wouldn't have a clue.
    And a good morning to all
  2. You must be a hoot at the breakfast table!

    Ok, first off, I don't consider my orthoses to be aiming at a target location and magnitude of orf. Rather I consider where it is now and how far I want it moved. For me this is the old dichotomy, are we trying to move toward normal or away from pathology.

    Besides that, the more bulk there is the desired side of the present cop (not the axis) the more moment you will affect. Vector wise, the higher the friction in the cover, the more vertical the vector (which is generally a good thing) although exactly how much this happens must depend on the vector of downforce as well.

    That's all I can manage this early. I think the answer to your question must, of necessity, be vague. Either that or have the words "it depends" a frustrating number of times!
  3. Mark,

    Great question. What is the relationship between the angulation of a surface and the vertical and shear components of a load applied to it?
  4. I think estimate is the best way to look at it.

    I also think one of the important things about a ORF that maybe sometime is forgotten is the Timing of the ORF as opposed to the GRF- ie it is also earlier.

    Things that I consider to help decide how much ORF to use

    - weight of person
    - speed of motion your attempting to alter
    - where the moment comes from - ie if it´s ground then your device must work harder than if it muscle based
    -distance from STJ axis the tissue your attempting to reduce load on - such as medial deviated axis and PTTD

    I read something the other day that said the Orthotics or ORF were just adjusting joint moments and that muscles were then better able to alter joint Kinemetics...

    thats all Ive got before coffee.
  5. Phil Wells

    Phil Wells Active Member


    I have had the same issues and simplified things by reducing my emphasis on GRF, looking at gross GRV and try to estimate the ORF on CoP (Centre of Pressure). (I have gone acronym MAD)
    Is it a bit more 'touchy/feely' but does work well with the tissues stress/ZOOS approached i.e. modify the pathological forces.

    Also while we are thinking about GRF from orthoses, how do you then incorporate the GRF from shoes - modified or off the shelf?

  6. RobinP

    RobinP Well-Known Member

    Head above the parapet here but simply, I would have thought that the greater the inclination of the surface, relative to the horizontal, the less axial load there will be on the supporting surface and the greater the shear force. In saying that, is there a point at which the inclination of the supporting surface has increased to the point that the shear force begins to reduce again?

    In the case of a conforming surface such as an orthotic, I would have thought that the load would rarely be a direct vertical component but rather an ever changing pattern of loads depending on the contour of the device and the direction of the ground reaction force. Thus changing the external moments by applying an external force can alter the relationship of compression/shear?

    I think I might be talking nonsense but I never answer these ones for fear of looking like a thickie(which admittedly might be justified)

    Looking forward to the whizzing bullets

  7. Mark:

    Good question. However, this question could also be the topic of discussion for a week-long seminar on foot and lower extremity biomechanics and foot orthosis therapy.

    Let me try to summarize my thoughts on this question. First of all, as clinicians, with each patient we see, we should keep in mind the goals of foot orthosis therapy using the Tissue Stress Approach (TSA).

    1. To reduce the pathological forces on the specific tissues that are injured.
    2. To optimize gait function.
    3. To not cause any other pathologies to occur.

    With these goals in mind, the clinician now has a framework about which they can use the TSA to initially estimate the orthosis modifications necessary to achieve the desired magnitudes of orthosis reaction forces that will allow the accomplishment of these goals.

    In using the TSA, the clinician must first accurately identify which anatomic structure is receiving the pathological stresses that has caused its injury. Second, the clinician must determine what are the most likely types of pathological forces (i.e. tension, compression, shearing) that are causing the injury. Third, the clinician must design a treatment plan that will best reduce the pathological forces on the injured structure, optimize gait function and not cause any other pathology to occur.

    In order to become proficient using the TSA, the clinician must be able to understand and utilize basic mechanical modelling techniques so they can better predict where to push on the plantar foot with the orthosis in order to accomplish the goals of the TSA. For example, the clinician must understand if they push in the medial arch of the foot medial to the subtalar joint (STJ) axis with an orthosis, then there will be created an increase in external STJ supination moment and that if there is medial or lateral deviation of the STJ axis, then this will have large mechanical effects on the orthosis pushing force.

    In addition, the clinician must also understand that every change in the temporal patterns, magnitudes and plantar location of ground reaction force (GRF) acting on the foot caused by their orthoses may produce both direct mechanical effects on the foot and also indirect neuromotor effects on the foot and lower extremity that are mediated by the central nervous system. In other words, the foot will not always respond by moving in the direction of the orthosis push since, ultimately, the kinematic patterns of our feet and lower extremities are mediated by the central nervous system.

    The clinicial approach that I have briefly outlined above will allow the astute clinician to quite effectively treat nearly any mechanically-based pathology of the foot and lower extremity with a success rate and confidence that will be unequaled by any other method.

    I have described this type of clinical approach to foot orthosis therapy in much greater detail in my three books.

    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.
  8. I agree.

    However in the interests of fairness, I should point out that if anyone else said this about any other model, we (the thought police) would be down on them faster than you could say "evidence based medicine".
  9. Fair enough.
  10. Jeremy Long

    Jeremy Long Active Member

    All of these are interesting replies; however, there are other parameters that can mostly only be assumed. Since these devices are intended to be worn in shoes, the physical charateristics of the footwear would directly effect ground force with relation to the orthoses. Heel counter stiffness, rotational control of the shank, position and credibility of the forefoot flex position, amount and type of rockering, midsole compound bias all contribute or interfere with the function of any orthoses design. This doesn't even cover how the wearer will use the combination, be it in a chronically static, standing position or in more dynamic situations.

    Since there is no research beyond that done internally by various, large footwear brands (and as such kept mostly private), we are left to make design decisions based on experience and observation.
  11. Saab

    Saab Member


    Great point stating this what should be obvious in our profession. I believe it should be considered standard when allocating prescription variables in orthotic therapy.
    an orthotic in a men's dress shoe with a stiff heel will "feel" different in a casual shoe or in a sports shoe with a dual density midsole. a stiffer or bulkier ORF can be greated and cause too much pressure which can lead to more proximal symtoms like poor shock absorption and knee pain ( what ive found out clinically) and im sure leg stiffness has a part in this discusssion somewhere if anyone would like to jump in ( eric?) Still getting my head to appreciate this new ( to me ) thought process.
    Its tricky mechanics/physics.. i think i need an i phone app lol
  12. efuller

    efuller MVP

    You have to look at your treatment/ prescription variables. You can add a forefoot varus or valgus correction or a rearfoot varus or valgus correction. It is hard to know how much of those corrections you want initially. However, the tissue stress approach can tell you which one to try. Your question was how much to try. I base my rearfoot decision on STJ axis position. The more medially positioned the axis, the more varus heel wedge the patient gets.

    We don't need to know the exact magnitude of the change. We only need to know whether the change made the patient feel better. The first attempt is an estimate. An orthotic may need adjustments after dispensing. I've added quite a few varus wedges under rearfoot posts when I did not get the clinical results that the patient wanted.


  13. drsha

    drsha Banned

    Then why are you not doing that?

    Instead of treating me and mine like **** and his as Biblical?


    Dr Sha
  14. drsha

    drsha Banned

    We're getting defined:

    Tiisue Stress Approach (TPA) = The Previously Undefined Paradigm

    The Thought Police (TP's) = The Bullies of The Arena

    Kevin = Darth = The Prophet

    Dr's Sha, Glaser, Root, Dananberg = The Criminals

    Payne = The Law

    The Flock = ?????
  15. drsha

    drsha Banned

    So You Mean That SALRE is No Better Proven Than Foot Centering!!

    So Kevin:
    Is That What You Admit By Saying FAIR ENOUGH?

    Mr. Hyde:
  16. markjohconley

    markjohconley Well-Known Member

    I apologise for not, earlier, thanking those that gave advice. Everytime I start considering same I get 'side-tracked' by a previous thread, you know the ones! Thanks, Mark
  17. DaVinci

    DaVinci Well-Known Member

    We try to.

    There are differences in the way people present there work, promote their work and how they respond to those who criticise their work.

    There are also issues of how consistent an approach is with the available research evidence and how much intuitive sense they make.

    I do not profess to be any sort of expert, but read almost everything on Podiatry Arena and follow links to read the research reported, but I will make two observations:
    1. A lot what Kevin says does appear to be consistent with the evidence (at least the evidence that I am aware of), so it makes sense.
    2. The way people get treated by the 'general' community at Podiatry Arena is proportional to the way they treat people on Podiatry Arena. It goes both way.

    I do not necessarily see a ganging up on you, what I see is the community responding to the approach that you are taken. I am sure anyone else would be treated the same if they did the same.
  18. I'm not sure.

    I fully agree that how you address people has a big effect on how you are addressed. From a transactional analysis point of view dennis lives in the "child" ego state and that forces everyone into the "parent" ego state. Dennis has only himself to blame for this. The constant trolling is intensely annoying.

    But beyond the tone, we DO treat fft as s**t and the physics approach as, well of not biblical then certainly as pretty much accepted principle. We DON'T treat them the same.

    My theory is that we treat neoteric biomechanics as s**t because... Well it is, and physics as a reliable set of base principles because they are.
  19. DrPod

    DrPod Active Member

    I came across this saying recently and wrote it down:
  20. efuller

    efuller MVP

    At least SALRE is well defined. I'm still waiting for you to describe how centering works in terms of the anatomy. Perhaps that is why your pet theory gets no respect.

  21. I got a better one:

    "Talk sense to a fool and he calls you foolish."

    Euripides, Greek tragic dramatist (484 BC - 406 BC)
  22. drsha

    drsha Banned

    Can you please provide us with a paragraph or two, or the references that define SALRE so well.

    Dr Sha
  23. Dennis have you read Kevin´s 2 papers based on and around SALRE theory. He posted them up Here. You may have missed it.

    While we are doing to information exchange. Have you got that peer reviewed published info on your FFT that Robin asked for.

    or can you answer Eric question about the Vault ?
  24. markjohconley

    markjohconley Well-Known Member

    thanks eric, thnks to you guys, i attempt a similar path, except i haven't casted for ~2years, after i'm happy with the footwear i send a template of the insole shape to the orth lab with requests for addition/subtraction/modification's on a polyprop shell. i haven't a clue on the magnitude changes just attempting to alter the forces to ameliorate the pathology, all the best, mark

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