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Standing Subtalar Joint Axis Location Technique

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Petcu Daniel, Nov 2, 2018.

  1. Petcu Daniel

    Petcu Daniel Active Member


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    Hello,

    Just found Standing Subtalar Joint Axis Location Technique on Dr. Kirby's facebook page ( https://bit.ly/2P6wOKe ) but it is not very clear to me! How can be marked the anterior and posterior exit points of the STJ when those points are important to be first located? Why the lateral-superior quadrant of the posterior calacaneus is the exit point? Is this valid for both medially and laterally STJ deviated axes? And how can be identified the dorsal talar neck? Because in the Subtalar Joint Axis Locator ( https://bit.ly/2yLOXTq ) those points are identified in a non-weightbearing foot which is not the same situation with the weightbearing foot!
    A second question: which are the errors which can occur in the non-weightbearing method?
    Thanks in advance!
     
  2. Petcu Daniel

    Petcu Daniel Active Member

    Still unenlightened!
     
  3. Daniel:

    I just now saw your posting. I'm not following Podiatry Arena very much these days and am doing most of my comments and podiatric education on my Facebook page. I invite you and others following along to "like" my Facebook page to get the latest information on foot orthosis therapy and foot and lower extremity biomechanics at https://www.facebook.com/kevinakirbydpm/

    My July 2017 Precision Intricast newsletter I wrote on the "Standing Subtalar Joint Axis Location Technique", which is included in my fifth Precision Intricast Newsletter Book (Foot and Lower Extremity Biomechanics: Precision Intricast Newsletters, 2014-2018), was just posted up on my Facebook page. The book will be ready to be printed and shipped within the next two weeks. Preorders for the book can be made at
    http://precisionintricast.com/shop/PREORDER-Volume-5-p120696221

    Here is the newsletter on teh "Standing Subtalar Joint Axis Location Technique".
    https://www.facebook.com/kevinakirbydpm/posts/1891432124287355
     

    Attached Files:

  4. efuller

    efuller MVP

    This creates the question of what changes between weight bearing and not weight bearing. The shape of the bones does not change. The location of the axis, relative to the bones, won't change when the foot becomes weight bearing. (The position of the axis relative to the foot and ground might change.) The thing that could make a difference between weight bearing and not weight bearing is movement of skin markers.

    Possible errors in non weight bearing method. When you move the foot the point that does not move is, by definition, the location of the axis. Posteriorly, the marks are on a part that moves relative to the leg with STJ motion. (the calcaneus is on the distal side of the joint. Anteriorly, you have to choose a location on the talus. The problem is that the talus is on the proximal side of the joint, and it does not, (or should not) move relative to the rest of the leg when you move only the STJ. So to figure out a dorsal exit point you figure out a point in space that does not move relative the foot as it moves around the STJ. That creates some potential for error.
     
  5. Petcu Daniel

    Petcu Daniel Active Member

    Quote from the newsletter: " If the STJ is already maximally pronated during performance of the SAPT, when the examiner pushes lateral to the STJ axis, the STJ can pronate no further, thus making it difficult to
    determine the points of no rotation when performingthe SAPT."

    A question which I want to ask for a long time: When I've performed the SAPT [supposing I've done it correctly] I've felt in some patients the points of no-movement as points of unstable equilibrium [ http://wikieducator.org/Types_or_Concepts_of_Equilibrium ]- any points near to that point of unstable equilibrium will create movement. But in some patients having a STJ range of movement there was any point of unstable equilibrium bat a surface of no-movement. Could be this the sign of a STJ already maximally pronated?

    My problem with the position of the axis is that Eric is speaking about 'position relative to the bones' and Kevin about 'the posterior calcaneus and anterior talar neck'. Maybe I'm splitting hairs but I think in practice is used the anterior talar head midpoint as a reference [ which can be seen in a simulation of STJ movement. And probably this creates the opinion that the location of the axis relative to the bones won't change - which in my opinion is not true. If it could be possible to use the talar neck as reference [which is not possible!] then, I believe, this will modify the posterior exit point on calcaneus as this point will move as a function of the position of talar neck. The practical implication could be in defining the area where supinatory or pronatory moments can be created.
    So, regarding the SSALT technique my problem is with the definition of the exit point on calcaneus which I believe is not in a fixed position but depends by the position of the talar neck [which can be approximated by the position of the midpoint of the talar head]
    I don't know if I've succeeded to express enough clear my thoughts! Which is your opinion?
    ( simulation of STJ movement )
     
  6. The Standing Subtalar Joint Axis Location Technique, as the newsletter says, approximates the STJ axis spatial location while standing. Although it does have inaccuracies, since this technique is done in a weightbearing fashion, it eliminates the problem with the non-weightbearing bearing STJ palpation technique where there may be a shift in STJ axis spatial location when the patient's foot moves from a non-weightbearing to a weightbearing position. Both STJ axis location techniques have issues with them, but both techniques are much better than not doing it all and assuming that a vertical calcaneus with the STJ in neutral position represents a "normal foot".
     
  7. If you are familiar with using the technique for finding the anterior and posterior STJ axis exit points as described by Drs. Jack Morris and Lester Jones (Morris JL, Jones LJ: New techniques to establish the subtalar joint's functional axis. Clinics Pod Med Surg., 11(2):301-309, 1994), you will find that the anterior and posterior exit points I have chosen to use in my Standing Subtalar Joint Axis Location Technique correspond quite nicely to what you will find from one patient to another. There is some variation in exit position relative to the talar neck and posterior calcaneus, but not as much as you would think. Dr. Simon Spooner and I used the technique of Morris and Jones when we did our research on our STJ Axis Locator from 13 years ago (Spooner SK, Kirby KA: The subtalar joint axis locator: A preliminary report. JAPMA, 96:212-219, 2006).
     
  8. efuller

    efuller MVP

    I think you have it right. Here is an analogy. Place a book on top of a pencil on a table top. When the bottom of the book is parallel with the table you can push on either side of the pencil (axis) and the book will rotate. If one side of the book is resting on the table, and the pencil, you can push any where between the pencil and the side of the book touching the table and there will be no rotation because there is no range of motion. If you push on the other side of the pencil the book will rotate and it will rotate faster the farther away from the pencil that you push. You can still find the axis with STJ maximally pronated by starting laterally and gradually pushing more medially until you see motion and that is where the axis is.

    What determines the location of the STJ axis is the shape of the articular surfaces and the ligaments that keep the articular surfaces together. Body weight and ground reactive force will also tend to keep the articular surfaces together. So the location of the axis is quite constrained by the shape of the bones. The likely reason that different feet have different axis locations is that these different feet have different shapes of their articular surfaces. This is why I believe that the position of the axis will not move relative to the talus and calcaneus. The talar neck reference point is just a good guess at where the axis exits. The motion determines the axis, not where the talar neck is. The axis is defined as the point that remains stationary relative to all points of the bone as the bone moves. So, if you drew several points on the posterior surface of the calcaneus and moved the STJ, the point that moved the least would be closest to the location of the axis. The motion that occurs is what determines the calcaneal exit point, and not where the talar neck is. Another analogy. Draw several points on the surface of a disc sander. Spin the sander, the point that moves the least is the point closest to axis of rotation of the disc. The posterior surface of the calcaneus is simlar to the surface of the disc in that parts of the bone are on one side of the axis and parts of the bone are on the other.

    Eric
     
  9. Petcu Daniel

    Petcu Daniel Active Member

    I'm strongly convinced that both techniques for determining the STJ axis position are mandatory in teaching foot biomechanics. But is the same thing for clinical practice? Once you know from theory the importance of STJ axis and that 'there is some variation in exit position relative to the talar neck and posterior calcaneus', but not as much as we would think, you can imagine that a flat foot with a medial deviated talar neck will have a small plantar surface where to apply the supinatory moments because of a medial deviated STJA. What could be different? Which are the cases where foot morphology is not correlated with the position of STJ in the transverse plane?
     
  10. The bottom line, Daniel, is that both the standing STJ location technique and palpation techniques can be useful clinical techniques in understanding STJ axis location in each foot of each patient . However, more importantly, using and understanding these techniques and the mechanical consequences of medial and lateral STJ axis deviation will inform the biomechanically-knowledgeable clinician as to the abnormal internal forces and moments that have resulted in the patient's pathology, and how best to mechanically treat these pathologies.
     
  11. Petcu Daniel

    Petcu Daniel Active Member

    One more question: how can we differentiate through these techniques a high [close to vertical] or low STJ axis? Because, if there isn't any movement [or if there is little movement] it could be a maximally pronated STJ axis or a high STJ axis. Am I right?
    Thanks,
     
  12. efuller

    efuller MVP

    True, a force parallel to the axis will not create a moment. You can break down an applied force into component vectors. The component that is parallel to the axis will not create a moment about that axis and the component perpendicular to the axis will create a moment in proportion to its lever arm. The closer to parallel the force is to axis the smaller the perpendicular component will be. As long as there is some perpendicular component a moment will be created and you will see movement. The smaller the moment the lower the acceleration. That would be one way to tell if the axis is closer to vertical: notice that you have to push harder to get motion.

    I guess the time when you care if the axis is high or low is when you are deciding to alter STJ moments through some other method than shifting the location of force under the foot. For example, using a lace up ankle brace to resist talar head adduction. Most of the time we are altering STJ moments by shifting the location of center of pressure under the foot. If the axis is truly vertical there will be small moments from ground reaction force and it will be less likely those moments will be high enough to cause pronation related pathology.
     
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