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Subtalar Joint Axis Locator

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Kevin Kirby, Nov 16, 2008.


  1. Members do not see these Ads. Sign Up.
    Here is a video on the Subtalar Joint Axis Locator that Dr. Simon Spooner and I designed and developed together in 2005. We also had a radiographic study published on the use of the device in 2006 in JAPMA (Spooner SK, Kirby KA: The subtalar joint axis locator: A preliminary report. JAPMA, 96:212-219, 2006).

     
    Last edited by a moderator: Sep 22, 2016
  2. Admin2

    Admin2 Administrator Staff Member

  3. issy1

    issy1 Active Member

    Hi Kevin,
    I am becoming increasingly aware of the need to accurately locate STJA. I have never seen your STJ Locator before and from this side of the screen in U.K. it looks somewhat fiddley. Just wondered what your opinion is on the accuracy of foot scanner STJA locator?

    Regards
    Issy
     
  4. Issy:

    What does "fiddley" mean? I don't know what a "foot scanner STJ axis locator" is.
     
  5. Isabel, in another post which seems to have vanished, you asked about clinical methods of determining the STJ axis location. Perhaps the most commonly employed technique is that described by Prof. Kirby in this paper: http://www.japmaonline.org/cgi/content/citation/77/5/228

    I'm sure that if you ask nicely, Prof. Kirby will upload a copy for you.
     
  6. Fiddley = awkward

    Bum bag = fanny pack

    Weird unknown creatures = pigs

    Foot scanner STJ axis locator = ?
     
  7. Here's the deer I think I saw at Heythrop Park...sure looks like a pig to me!!:rolleyes:
     

    Attached Files:

  8. Atlas

    Atlas Well-Known Member

    I like it Kevin. Grid idea is easily understood and practical to implement.



    I (presume to) know what you have implied with ankle sprains...in that a laterally deviated STJ will give supinators maximal mechanical advantage (and accordingly less protective output of peroneals) etc and facilitate inversion etc....


    Any work or theories about the height of the STJ axis, and its connection between ankle instability?


    Ron
    Physiotherapist (Masters) & Podiatrist
     
  9. These papers describe the change in subtalar joint axis moments arms for ground reaction force acting on the plantar foot and for the extrinsic muscles of the foot as the spatial location of the subtalar joint axis is altered.

    Kirby KA: Methods for determination of positional variations in the subtalar joint axis. JAPMA, 77: 228-234, 1987.

    Kirby KA: Rotational equilibrium across the subtalar joint axis. JAPMA, 79: 1-14, 1989.

    Kirby KA, Green DR: Evaluation and Nonoperative Management of Pes Valgus, pp. 295-327, in DeValentine, S.(ed), Foot and Ankle Disorders in Children. Churchill-Livingstone, New York, 1992.

    Kirby KA: Biomechanics of the normal and abnormal foot. JAPMA, 90:30-34, 2000.

    Kirby KA: Subtalar joint axis location and rotational equilibrium theory of foot function. JAPMA, 91:465-488, 2001.


    The following paper by Bart Van Gheluwe, Friso Hagman and myself studied the effects of simulated genu valgum deformity on the kinetics around the subtalar joint and discussed the possible effects that the height of the STJ axis might have in the production of subtalar joint pronation/supination moments in genu valgum deformity. Hope this helps.

    Van Gheluwe B, Kirby KA, Hagman F: Effects of simulated genu valgum and genu varum on ground reaction forces and subtalar joint function during gait. JAPMA, 95:531-541, 2005.
     
  10. issy1

    issy1 Active Member

    Hi Kevin,

    Yes sorry, excuse my colloquial by 'fiddley' I mean awkward (Thank you simon.) I've no doubt you could show me how to use your STJA Locator but as I say from this side of the screen it looks abit 'fiddley';)
    I think my terminology 'foot scanner STJ axis locator' was perhaps misleading. I was referring to the line which the foot scanners here plot on your scanned foot representing STJ angle. IT looks at the medial heel & lateral heel zone - when these zones are equal the angle is said to be equal to the foot axis. For each 10% deviation in either direction the angle goes 1 degree in that specific direction. I know that this is not actual STJ axis location as demonstrated by your video but I'm wondering if the two are closely linked. In otherwords would the angle plotted by the foot scanner be similar to the true STJ axis location?? I would appreciate your thoughts on this.

    Regards
    Issy
     
  11. Is not that line that your talking about the Center of Pressure (COP) of the foot?
     
  12. Can you post some pictures of what you are talking about? When you use the term "scanner" are you referring to a system which captures a 3-dimensional image of the foot or a pressure mapping system?

    One way I can think of to use a pressure mapping system to estimate the STJ axis location would be to have someone stand on a pressure mat and pronate/ supinate their foot. Provided we could isolate STJ movement, as the subject pronates their foot the pressure beneath the area of the foot that is medial to the axis should increase while the area lateral to it should decrease and vice versa as the subject supinates their foot. If you could identify the zone between pressure increase and decrease, this should correspond to the transverse plane projection of the STJ axis. However, as we know, the STJ axis changes its location as the joint moves so in fact, what we would see in a pronating foot is that the zone between pressure increase and pressure decrease would sweep from lateral to medial across the foot as the foot/ axis changed position. The reverse should be true in supination. Assuming that we can't isolate STJ motion, what this technique would actually show is the transverse plane projection of the net rotational axis of the plantar foot. Interesting, if a little 1-dimensional.

    I suspect this is something like what you trying to describe Issy.
     
  13. Issy:

    Are you talking about the RS Scan unit?
     
  14. Probably :drinks
     
  15. issy1

    issy1 Active Member

    Hi Kevin,


    Yes I am referring to a RsScan system - wasn't sure if I was allowed to use product names on this site.

    Regards
    Issy
     
  16. In which case, we would need to know how the software is calculating the "subtalar articulation angle". I've searched but could not find much on this. Using the protocol I described previously, the software could look at change in pressure in all sensors over time (i.e. between two adjacent "samples", this time period will be determined by the sampling frequency) and provide a transverse plane projection which might provide a reasonable estimation of axial projection. It would only really be valid when the loading onto the pressure mat was vertical though, so the calculation between consecutive samples would have to occur around midstance- obviously this only tells us the estimation of axial position between these two instances in time. However, from what I can glean from my reading, I don't think this is what it is doing.
     
  17. issy1

    issy1 Active Member

    To be honest I know no more than yourself about how this system calculates this angle. This is a system which I used 7-8 years ago in the Health Service and which I am now considering buying for my Private Practice. At the time I was not looking at this line as a prediction of what the axis of the STJ might be - since I was still prescribing orthotics with Root thinking ideas. It is only now when I am trying to grasp these new ideas re: STJA and heel skives etc. that I got thinking about this line which the RsScan plotted - from what I remember it often seemed to be medially deviated in a very pronated foot and laterally deviated in a very supinated foot, hence the thinking that the two might be a reflection of each other somehow. I guess I was hoping that this might be a sufficient indication as to whether the STJA was deviated and be adequate for deciding where ORF should be placed. That's as far as my physics goes on this thought I'm afraid.:confused:

    Issy
     
  18. There are easier and cheaper ways to estimate stj axial position that can be used to inform your orthoses prescription. Unless you want to use this pressure system to decide on your orthoses prescription for you and use their pre-fabs? In which case it doesn't really matter because if you buy into the concept you don't need to think at all. Not my cup of tea, but each to their own.
     
  19. There is a bit of a story about this "STJ axis line" on the RS Scan system. To make a long story short, RS Scan is based out of Belgium, if I remember correctly. Friso Hagman (a Belgian biomechanist who I have coauthored a paper with Bart VanGheluwe) did his PhD thesis on "Can Plantar Pressure Predict Foot Motion" in which he was trying to determine the amount of frontal plane foot motion from pressure readings. From this work, I believe RS Scan now has a STJ line on their readout. I told the developer of the system, when he visited my office in Sacramento a few years ago, that the name should be changed since it probably didn't reflect STJ axis location and may confuse clinicians trying to use the system. Maybe it has been changed...but I don't know for sure.

    As Simon stated, there are better ways to determine STJ axis location. Maybe, Issy, when you get out of this "fiddlley" stage you are in, you will learn the other ways that STJ axis spatial location may be more accurately assessed.:drinks
     
  20. In which case we are drawn to:
    http://www.japmaonline.org/cgi/content/abstract/96/4/305
     
  21. Kevin, any chance you could upload "methods of determination", please? For those not familiar with it;). This is the paper which fundamentally sparked my interest and accelerated my understanding of foot biomechanics- way back in the day. In my opinion, this is one of the most important papers that any student of foot biomechanics should read.
     
  22. I wrote this paper when many more hairs graced my head.......now the few strands remaining all have their own names....

    Kirby KA: Methods for determination of positional variations in the subtalar joint axis. JAPMA, 77: 228-234, 1987.
     

    Attached Files:

  23. issy1

    issy1 Active Member

    Thanks for this paper Kevin. I have been trying the palpation technique on several people and starting to get it - I think:D O.K. putting this to use then when prescribing my orthotics - am I right in saying that you use a medial heel skive when you have a medially deviated STJA and a lateral heel skive when you have a laterally deviated STJA from the so called 'norm.' At what point do you decide to use a heel skive rather than rearfoot posting on your orthotics - how deviated from the 'norm' before you switch to heel skive? Also do you tend to find that very pronated feet are always medially deviated and very supinated/cavoid feet are always laterally deviated? - This has probably been asked before but just haven't found it yet.

    Thanks
    Issy
     
  24. issy1

    issy1 Active Member

    Without trying to sound defensive - the pre-fab part of their software is not a part I have any interest in:boxing: Thanks for helping me to understand the importance of accurate STJA location - keen to change my old 'root' ways!

    Issy
     
  25. Depends on a number of factors, supination resistance being one......
     
  26. issy1

    issy1 Active Member

    Can you elaborate on the number of factors?

    Issy
     
  27. efuller

    efuller MVP

    Basically, that is what I do. When you do the palpation test be aware of reactive peroneal muscles. Sometimes you will see the peroneals contract in response to pressure medial to the STJ axis. This usually happens in feet with laterally positioned axes. In addition, I will often add more skive in either direction dependent on symptoms. For example, hallux limitus with medially positioned axis will get more of a medial skive or peroneal tendonitis with a laterally positioned axis will get a little more lateral skive. Both of these situations will get a flat rearfoot post to which it will be easy to add rearfoot wedging if needed.

    Generally this is true. However, you will see flat flexible feet with laterally positioned axes and high cavus feet with medially positioned axes. So, you do have to check the axis and can't just look at the arch height.

    Cheers,

    Eric
     
  28. Lets go back a step and see if we can get a deeper understanding. Issy, what does a medial heel skive do? BTW, lets assume that a medial heel skive can have any angle you want in the frontal plane.
     
  29. issy1

    issy1 Active Member

    From what I have read so far about this I am a assuming that for example a medial heel skive increases ORF medial to the STJ axis since the ORF is now further away from the 'hinge' if you like and will therefore give a greater rotational force with less compression force on the soft tissues thus giving a more powerful invertion moment. I assume that this sudden, powerful invertion moment at heel contact will make what's going on in the rest of your orthotic more effective and the greater the angle of the skive or the further away from the axis the force is applied the greater the invertory force will be?

    Issy
     
  30. Issy, go back a step further, lets say we have a positive cast and we put a 3mm deep 15degree heel skive onto it and then press a shell onto this cast; lets take an identical cast but rather than adding a medial heel skive we simply press a shell over this and then add 15 degrees of extrinsic rearfoot post. What differences will we see in the topography of the foot-orthoses interfaces in the two devices?

    How do foot orthoses change the position of the net ground reaction force vector?
     
  31. issy1

    issy1 Active Member

    Eric, What way do you post/skive the rearfoot of your orthotics for patients with flat flexible feet with laterally positioned axes or patients with cavus feet and medially positioned axes?

    Issy
     
  32. Depends on the presenting problem; which tissue you need to off-load.
     
  33. issy1

    issy1 Active Member

    The device with the plaster addition will change the shape of the heel cup of thee orthosis and therefore change the calcaneal angle in the frontal plane. The orthosis without plaster addition and rearfoot extrinsic posting will change the angle of the orthosis. Is this what your asking me??

    Issy
     
  34. efuller

    efuller MVP

    I agree with Simon. It depends entirely on which structure(s) you are trying to reduce stress in.

    regards,
    Eric
     
  35. efuller

    efuller MVP

    Both changes will essentially put a varus wedge under the calcaneus. A 15 degree varus wedge under a flat rearfoot post will lift the medial arch and the anterior edge of a device when the plantar surface of the wedge sits on a level surface. What happens in the shoe is a little different. I've never tried a wedge that large, but when you put a 5 degree wedge under a flat rearfoot post and the patient stands on it, the orthosis will flex until the medial anterior edge hits the supporting surface. As the foot applies force to the orthotic to make it flex there is an equal and opposite force from the orthotic acting on the foot.

    So, the device made over the cast without the skive and a varus wedge under the rearfoot post will push upward more in the arch than a device with a medial heel skive. This may, or may not, be a good thing.

    Regards,

    Eric
     
  36. Thanks, Eric. While both devices will result in a varus wedge under the calcaneus, the shape of the heel cup will be different in the two devices. This geometrical difference should result in differences in the the stress / strain profiles for this section of the orthoses and additionally will influence the surface angulation at a given point on the two shells. Variation in surface angulation and load / deformation characteristics of the orthoses will impact upon the pressure distribution and net GRF vector at the foot- orthoses interface.

    Also, changes in arch profile will impact on the stiffness of the arch section of the device.
     
  37. issy1

    issy1 Active Member

    O.K. - From what I have learned above can you give me a few clincial examples of when you would use medial heel skive rather than extrinsic rearfoot posting and why to help me work through the difference in my head:confused: Thanks

    Issy
     
  38. Can you venture any ideas of your own, based on what you have learned?
     
  39. issy1

    issy1 Active Member

    If I used a medial heel skive on a patient with excessive pronation and HAV formation I assume it will give a greater supinatoy action at heel contact and therefore less stress will be placed on the medial column of the othotic and likewise on the soft tissue structures in the arch area than if I used an extrinsic rearfoot post with a less powerful supinatory action?

    Don't think I'm answering this as you would like. Maybe you could suggest some reading related to orthosis and their effect on pressure distribution and net GRF.
     
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