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Effects of medially posted insoles in overpronating men - time to talk terminology?

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Trevor Prior, Mar 23, 2017.

  1. Trevor Prior

    Trevor Prior Active Member


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    Effects of medially posted insoles on foot and lower limb mechanics across walking and running in overpronating men

    Journal of Biomechanics 54 (2017) 58–63

    Anti-pronation orthoses, like medially posted insoles (MPI), have traditionally been used to treat various of lower limb problems. Yet, we know surprisingly little about their effects on overall foot motion and lower limb mechanics across walking and running, which represent highly different loading conditions. To address this issue, multi-segment foot and lower limb mechanics was examined among 11 overpronating men with normal (NORM) and MPI insoles during walking (self-selected speed 1.70 ± 0.19 m/s vs 1.72 ± 0.20 m/s, respectively) and running (4.04 ± 0.17 m/s vs 4.10 ± 0.13 m/s, respectively). The kinematic results showed that MPI reduced the peak forefoot eversion movement in respect to both hindfoot and tibia across walking and running when compared to NORM (p < 0.05–0.01). No differences were found in hindfoot eversion between conditions. The kinetic results showed no insole effects in walking, but during running MPI shifted center of pressure medially under the foot (p < 0.01) leading to an increase in frontal plane moments at the hip (p < 0.05) and knee (p < 0.05) joints and a reduction at the ankle joint (p < 0.05). These findings indicate that MPI primarily controlled the forefoot motion across walking and running. While kinetic response to MPI was more pronounced in running than walking, kinematic effects were essentially similar across both modes. This suggests that despite higher loads placed upon lower limb during running, there is no need to have a stiffer insoles to achieve similar reduction in the forefoot motion than in walking.
     
  2. Trevor Prior

    Trevor Prior Active Member

    Interesting article which looks at kinematics and kinetics - they use navicular drop to determine 'overpronation'.

    I know there has been plenty of discussion around terminology but I do wonder if we should try to get some form of working terminology. We all agree (I think) that there are structural alignments that can contribute to load across the foot rather than specific alignments causing specific kinematic function.

    Communication between colleagues, professions and patients needs commonly agreed terms that are reflective of what we are observing and feel relevant to the patient. If I wish to describe two patients and one demonstrates a foot with greater signs of the components of pronation than the other, how do we feel this should be achieved? Do we use a term that tries to describe the level of pronation or refer to the component parts (arch height / forefoot adduction, rearfoot eversion etc.)?

    I appreciate I may be opening up a can of worms here but, if as an example I took tibial varum and have two patients who are identical other than the range of motion available, the patient with the stiffer foot is likely to have a more lateral CoP than the one with a more mobile foot on static stance and the timing of the loading is likely to be different dynamically. One can then refer this to the likely loading on any specific structure / injury.

    The height of the arch, the degree of mobility etc. are also likely to influence the prescription of any orthosis (in other words there are some structural features that contribute to the prescription).

    I think one of the issues with colleagues struggling to move away from the STJ neutral theory is partly terminology. Many of the prescriptions used based on the Root theory are likely to increase moments one way or the other and thus work due to this rather than the STJ neutral theory. You see a foot in a pronated position, trying to move it to a less pronated position will increase supination moments yet unlikely change kinematics. Identifying that this foot type has features that result in increased pronation moments (i.e. tibial varum, dare I say forefoot inversion, calf inflexibility etc.), allows those features to be targetted to reduce the pronation moments - this may be directed at the forefoot, midfoot, rearfoot, include calf stretches, heel raises etc.

    I have some further thoughts around this but would be interested in others.

    Trevor
     
  3. it doesn´t need to be all that complicated unless you want it to be

    Workout what is broken

    workout why it is broken , maybe biomechanics maybe increased stress from say training etc etc

    begin treatment program to lessen loads on broken tissue, aid in healing

    but I can not find anything to disagree with your post Trevor, the issue is using over pronation and other similar terms like normal.

    As an aside I have always said that stiffer materials will just have a kinetic effect on the body faster than less stiff which need to compress/flex, and it is about the timing of the moment or Orthotic reaction force, so raised an eyebrow at the conclusion of the study
     
  4. Griff

    Griff Moderator

    I've only skimmed the full text once but initial thoughts on the paper:

    - Asymptomatic subjects with a navicular drop of greater than 10mm have been defined as "overpronators". No. Just no.

    - The authors offer no statistics on their reliability of navicular drop.

    - I don't think what they have defined and issued as "medially posted insoles" is comparable to how most of us would define and issue medially posted insoles. (Difficult to be certain as no pic available)

    - Whilst kinematics and kinetics are often discussed as separate entities for teaching reasons (and general simplicity), we know they are inseparable in reality. I fear the conclusion is misleading in the way it has been written in this regard.

    It's great to see orthoses research looking at and quantifying kinetic measures though.
     
  5. When you find someone who listens, takes on board what you are saying and then expresses that in their own writing, you know you are lucky to have got the right guy on board in the first place. If someone could ante-up the full-text I should be delighted to have a read and pass opinion. Looks interesting.
     
  6. efuller

    efuller MVP

    Anti-pronation orthoses, like medially posted insoles (MPI), ​

    I agree that definitions are important. Medially posted could mean forefoot or rearfoot. It could mean an external rearfoot post only on the medial side of a frontal plane symmetrical (heel balanced vertical) orthotic shell. Or it could mean a shell made from a cast with a medial heel skive. etc. Hopefully they described their medially posted orthosis.


    The kinematic results showed that MPI reduced the peak forefoot eversion movement in respect to both hindfoot and tibia across walking and running when compared to NORM (p < 0.05–0.01). No differences were found in hindfoot eversion between conditions. ​

    From the above it seems that the medially posted orthosis was under the forefoot. I can't be sure without reading the paper.

    Communication between colleagues, professions and patients needs commonly agreed terms that are reflective of what we are observing and feel relevant to the patient. If I wish to describe two patients and one demonstrates a foot with greater signs of the components of pronation than the other, how do we feel this should be achieved? Do we use a term that tries to describe the level of pronation or refer to the component parts (arch height / forefoot adduction, rearfoot eversion etc.)?
    Which brings us back to why is pronation/pronated bad. As alluded to in another thread, many of the foot posture index items that make the foot more "pronated" are associated with a medially deviated STJ axis. The problem is not that the foot is "pronated" the problem is that there is a high pronation moment from the ground. Perhaps we should just be talking about transverse plane location of the STJ axis.

    I appreciate I may be opening up a can of worms here but, if as an example I took tibial varum and have two patients who are identical other than the range of motion available, the patient with the stiffer foot is likely to have a more lateral CoP than the one with a more mobile foot on static stance and the timing of the loading is likely to be different dynamically. One can then refer this to the likely loading on any specific structure / injury.​

    I like that we are moving toward using terms that have an engineering definition like stiffness. However, we still need to understand the limitations of this term. Absolute range of motion is still an important concept. In the quoted paragraph above the important concept is where within the range of motion the joint becomes stiff. Those feet may be equally stiff within there range of motion. They may be equally stiff (resistant to further bending) at their end of range of motion. The difference between the feet is where the stiffness increases. As the STJ is moved from a more supinated position, to a more pronated position, one of those feet will have its stiffness increase at a more inverted position. This is not necessarily a stiffer foot.

    Trevor, your last paragraph was also very interesting. Unfortunately, I don't have time to comment on it now.
    Eric
     
  7. The best reason to move away from subtalar joint (STJ) neutral theory is that it, as a theory, simply does not take into account the prevailing abnormal joint moments that can cause pathology. I was taught by the professors of biomechanics at the California College of Podiatric Medicine (CCPM) that the foot should function close to the STJ neutral position and if it didn't the orthosis balanced with the heel vertical (this was used about 95% of the time) would "prevent abnormal compensations" and help the patient's pain. There was no discussion of "moments" nor "rotational equilibrium" but rather a focus on "foot and lower extremity deformities" and how they will "compensate" to cause pathology. The concept of modifying the foot orthoses specifically with the intent to reduce tissue stress at CCPM during the late 1970s to mid 1980s would have been as foreign to us at CCPM during that period as hearing of someone talking about a "smartphone".

    STJ neutral theory was based on the false premise that "calcaneal bisections", "neutral calcaneal stance position", "forefoot to rearfoot relationship", "rearfoot to tibial relationship", etc could not only predict pathology but also predict gait kinematics. Obviously, we know now that they don't. STJ neutral theory was also based on the false premise that these measurements advocated by Root et al could be reliably be done from one clinician to another. Again, we know now that these measurements have very significant inter-examanier errors which make their use suspect.

    And Trevor, STJ neutral theory had a big problem with the concept that one may need to add pronation moments to the foot to treat pathologies. They did allow valgus forefoot intrinsic orthosis posting if the patient had a measured everted forefoot to rearfoot deformity (i.e. forefoot valgus), but not if the patient had peroneal tendinopathy and had just a "rearfoot varus deformity" without a "forefoot valgus deformity" also. You were told that "thou shalt not pronate the foot"...maybe not in biblical language but that was the meaning of what we were taught. Dr. Spooner has a great story about his experiences with that idea.

    The biggest problem with the ideas of Root et al, for all the good that Root and coworkers did for podiatry and our profession (and for myself), is that they basically ignored talar head and neck position relative to the plantar foot. They were focused so much on "calcaneal bisections", which have very little to do with STJ pronation/supination moments, that they couldn't, I believe, see the forest for the trees. It is my understanding that on the east coast, Dr. Root's contemporary, Richard Schuster, DPM, who taught at the New York College of Podiatric Medicine, was a big advocate of paying attention to talar head and neck position in his clinical teachings. Since talar head and neck position determines STJ axis location, and calcaneal bisection does not, maybe if Dr. Root and colleagues had paid more attention to talar head and neck position and STJ axis spatial location, things would have very different for him and his theories which are now coming under attack.

    Very interesting to consider this. I believe Dr. Spooner has some good thoughts on this also. Maybe he can take the time to lay out his considerations for Root's theories relative to the British podiatry profession.
     
  8. efuller

    efuller MVP


    I agree with what Kevin discussed regarding STJ neutral terminology and treatments. To add to that. We still need to be able to describe positions and motions. Root et al, used and may have increased their importance and usage. However, the neutral position disciples still have a hard time describing how an orthosis works especially in terms of moments. Kevin's description of peroneal tendon injuries is a good example of this.

    I would like to question the notion that tibial varum increases pronation moments. This is related to my earlier comments on stiffness versus range of motion. The subtalar joint has a range of motion. Within that range of motion the stiffness is very low. When the STJ reaches it's end of range of motion the joint stiffness increases. The location, in space, where the stiffness changes is important for understanding the moments acting around the STJ. The concepts taught by Root et al are still very important for understanding this. Lets draw some lines and play along with the assumption that we can measure them accurately to examine this concept. Say you have two feet that both have 5 degrees of eversion relative leg. Now one of those feet has a leg with 0 tibial varum and one has 3 degrees of tibial varum. Both sit with their heel bisections at vertical. (This is using the relatively poor assumption that the forefoot is perpendicular to the heel bisection in all positions of the STJ. The motion [or change in location of stiffness] of the MTJ is a big problem in making STJ calculations like this relevant.) There would not necessarily be an increase in pronation moment with foot with more tibial varum. The feet will probably reach an equilibrium where there still is range of motion (low stiffness) in the direction of pronation. Now, if the same foot has a leg with 7 degrees of tibial varum, the range of motion will be used up and stiffness of the joint will increase and there will be a tendency to increase lateral forefoot load because the STJ cannot evert to bring the heel to vertical. The increased lateral forefoot load will tend to increase pronation moments acting on the STJ. So, tibial varum does not necessarily increase pronation moments, but it does increase the relative everted position of the STJ. Position is still important and we cannot just look at stiffness.

    Eric
     
  9. Trevor Prior

    Trevor Prior Active Member

    Eric

    Thanks for that - I would agree with your comments entirely although would add that this applies to the person in static stance. In the example you have given, when the foot contacts the ground, the tibial varum will cause a pronation moment on the foot until it reaches equilibrium and the relative position of the structures and in all likelihood the STJ axis will be different if everything else is equal - their effective start and finish points are different even if they end up in equilibrium their journey to that position has been different.

    Trevor
     
  10. efuller

    efuller MVP

    Classic Root disciples would assume that the foot starts in neutral position and then compensation happens from there. I've always had a problem with this. The foot contacts the ground in gait and the moments from ground reaction force and the muscles will determine what motion occurs. In static stance, you don't start in neutral position. You just look at where the foot is. Tibial varum will not cause a pronation moment in static stance. All other things being equal, the foot with more tibial varum will tend to rest in a more everted position of the STJ. However, tibial varum will not create more pronation moment.
    Eric
     
  11. I concur, I read Trevor's post a few days ago re: tibial varum " causing moments", but wanted to let Eric respond as it was their discussion. Let's go back to basics define a "moment" in the physics sense, then tell me how a bony position can "cause a moment"? This may appear pedantic, but check the title of the thread- if we are talking terminology we need to be pedantic. Still haven't had time to read the paper yet, but will soon.
     
  12. Trevor Prior

    Trevor Prior Active Member

    I must be missing something here - probably why I failed physics A level - actually, I failed all my A levels so perhaps there was another reason. Anyway, more seriously. We know that the foot generally contacts the ground with the rearfoot inverted (in relation to the ground rather than around 'neutral' position) and then everts thus it is fair to say that there has been a pronation moment on the foot. Of course, this moment will be present even if the foot does not move for all the reasons discussed previously.

    However, if there is something that is structural that changes the loading point of the foot such that the CoP may be more lateral, will this not increase the moment or, where motion occurs, the velocity of that motion?

    Is it not possible that someone with greater tibial varum has greater lateral loading or maintains that load for longer? Thus structure has the potential to effect function

    Trevor
     
  13. Moment = force x perpendicular distance to point of interest. Centre of pressure is interesting, but it is the sense of the vector when it is perpendicular to the point of interest that is key. Thus, you can have a centre of pressure (the point of application of the force) which is on one side of a joint axis, while the vector actually passes on the other side of the joint axis. I discussed this here: https://www.ncbi.nlm.nih.gov/pubmed/21084541 Ground reaction force is a 3D vector (actually it's a 4D vector quantity). Shear forces, both anterior-posterior and medial-lateral are a pain in the ass, but we can't ignore them, which is why the current in-shoe pressure systems ultimately fail- end of story.

    BTW, no-one is saying structure is not important in function. It's just not as simple as some would have us believe. I spent a day in Belgium recently lecturing on the inseparability of kinematics and kinetics. Like I said- inseparable, but not simple.

    Here I am demonstrating that concept with a bit of fun and some laryngitis.

    So, if we look at 3D kinematics in isolation, that is a very blunt instrument; if we look a 3D kinetics in isolation, that too is a very blunt instrument. We need both.
     
    Last edited: Mar 28, 2017
  14. Trevor Prior

    Trevor Prior Active Member

    Thanks for that and actually, raises the point well about terminology. So some thoughts on this current discussion under this thread:

    Tibial varum has the potential to alter the point of application (CoP) of the force (I believe you term this the normal force component). This is likely to be lateral to the STJ axis and thus potentially cause an increased pronation moment dependant upon the degree and direction of the shear component.

    Taking this a stage further, if the position of the CoP in relation to the STJ axis cannot determine the overall vector and thus the relative pronation or supination moment, the only value in knowing this in relation to the plantar aspect of the foot is to focus the point of application of any force we wish to apply to the foot i.e. medial rearfoot if medially deviated for example.
     
  15. Nope, we term this the centre of pressure or point of application. The "normal force" is the component of the ground reaction force which is "normal"- viz. perpendicular, or at 90 degrees, or at a right angle to the supporting surface. Not the same thing. Moreover, you assume that the degree of tibial varum alters the point of application of the ground reaction force- does it?



    Or potentialy increases the supination moment, dependant upon the magnitude of the shear components- you see how your own inherent bias is influencing the way you are writing here? Life ain't so simple when you look at it from the skeptical position is it, Trev? We got a 3D vector, vertical component in isolation doesn't do it; normal component in isolation doesn't do it.. we need the 3D- nobody doing that at the foot interface in-shoe.

    Or, to focus our vertical component of GRF. The question becomes: does the palpated position of the STJ axis predict pathology?
     
  16. Trevor Prior

    Trevor Prior Active Member

    Point of application because it is where the vertical and shear forces both act – ok, see that.

    Surely tibial varum potentially alters the point of application or the magnitude or the rate of loading. That would need researching clearly. However, we have all seen many patients over the years with one form of alignment or the other, I am using tibial varum as an example, and will use this within our assessment approach and subsequent management. The leg and foot that has more mobility will respond to ground reaction forces differently to the one with less motion – or at least, we often see the point of application and the direction of loading vary.

    Agreed on your last comment and I accept some bias there although I am more than prepared to accept your point. Taking tibial varum as the example and referring to range as above, one can be fairly predictive of the way in which the foot will load dependent on the relative motion available. If then, the injury in particular follows that loading pattern, can we not infer something from that?

    It is interesting having looked at however many inshoe analysis over the years. There are many we can be predictive on, but some we cannot. What we are usually seeing there is the influence of the rest of the body on function and, in all likelihood the ones that have greater shear influence as a result.

    Is it the vertical component alone? If we used an angled wedge or contoured shell etc., by nature, are we not affecting shear as well? After all, was that not your point with using pressure insoles to assess orthoses, the effect of angles and the lack of the shear component?
     
  17. efuller

    efuller MVP

    The increase in tibial varum will only tend to shift the CoP laterally if you run out of range of motion of the STJ. (with the bad assumption of forefoot perpendicular to rearfoot.) Say you have 3 degrees of eversion of the heel bisection to the leg. Compare this foot with 0, 3 and 5 degrees of tibial varum. Between 0 and 3 there might not be any change in location of CoP as the STJ could equilibrate with CoP in the same position. Now you compare the 3 to 5 degree varum. The STJ runs out of range of motion before the heel can get to vertical and the forefoot would be two degrees inverted to the ground (this neglects change in ROM of the MTJ, but we will continue for the academic point assuming the forefoot stays perpendicular to the heel bisection through this excercise) There will be a lateral shift in the cop with 5 degrees of tibial varum, in this example, because the STJ cannot evert far enough to fully load the medial forefoot. So, to answer the question could a foot with more tibial varum have an increased pronation moment in stance the answer is it depeneds on available range of eversion range of motion.


    This is what we do clinically already. We don't need to calculate the exact moment from ground reaction force. We only need to figure out how to change the moment from ground reactive force to reduce stress on the injured anatomical structure.

    Simon is correct in that we do need to know the horizontal components of force to precisely calculate the moment from ground reaction force acting on the STJ. However, for most activities, the vertical component of force will easily have the largest magnitude. Moment is magnitude of the force times distance from the axis. So unless the distances of the horizontal components from the axis are relatively large we can concentrate mostly on the vertical component. As Simon pointed out the question is whether the palpation of the location of the axis, in the transverse plane predictive of pathology. If the answer is yes, we can ignore the horizontal components of force for most activities. We do have to be aware that we are making the assumption that moments from horizontal components are small.
    Eric
     
  18. For the discussion of theoretical effects of various structural alignments of the tibia within the frontal plane, I believe it is helpful to consider Trevor's assumption that an increase in tibial varum deformity will alter the subtalar joint (STJ) moment, all other factors being considered equal.

    For example, if during relaxed calcaneal stance position (RCSP) in an individual with a vertical tibia, the STJ is maximally pronated and the calcaneus is vertical to the ground with the forefoot being only able to evert 3 degrees from the ground in this position there may not be excessive STJ pronation moments that will cause pronation-induced pathology to occur.

    However, if we now change that same individual to having a 5 degree tibial varum (and nothing else changes), the STJ will still be maximally pronated in RCSP but there will be a greatly increased lateral column pressure from the ground during RCSP due to the lack of ability of the plantar forefoot to now evert sufficiently to have even medial-lateral weightbearing forces on the plantar forefoot. In this instance, the magnitude of STJ pronation moments have increased "due to the increased tibial varum deformity" due to the increase in ground reaction force (GRF) acting lateral to the STJ axis.

    If, on the other hand, we start with a foot and lower extremity which has a calcaneus which has a vertical tibia with a RCSP being 6 degrees everted and maximally pronated at the STJ and a plantar forefoot which can evert 10 degrees while the foot is in RCSP, adding tibial varum will likely cause a different effect. In this foot, adding 5 degrees of tibial varum deformity will bring the calcaneus to being only 1 degree everted while it is maximally pronated which will, in effect, alter the spatial location of the STJ axis to a more lateral position relative to the plantar calcaneus. This medial shift in the plantar aspect of the medial calcaneal tubercle relative to the STJ axis (which occurs as a result of the added 5 degrees tibial varum) will increase the external STJ supination moment from GRF acting on the plantar calcaneus which would, overall, increase the STJ supination moment due to the tibial varum (as long as the plantar forefoot medial-lateral distribution of GRF is unchanged from the vertical tibia example).

    In other words, changes in frontal plane tibial alignment don't always cause an increase in STJ pronation moments nor cause an increase in STJ supination moments. The relative pronation-supination moments from the plantar forefoot and plantar rearfoot must be considered before the STJ moments effects of a an increased tibial varum deformity may be fully appreciated.
     
  19. Trevor Prior

    Trevor Prior Active Member

    Eric / Kevin

    We are in complete agreement over this and ti was what I was trying to get at - the underlying structural alignment can influence the loading on the foot but the net result is due to a range of factors (i.e. not just the tibial varum alone) and how they interact, importantly, the range of motion available is a key factor.

    I guess I was trying to make the point that we do need to consider the structure as it gives us an indication as to how much range we have to 'play' with and how we may wish to modify load.

    Kevin,

    You noted:

    "If, on the other hand, we start with a foot and lower extremity which has a calcaneus which has a vertical tibia with a RCSP being 6 degrees everted and maximally pronated at the STJ and a plantar forefoot which can evert 10 degrees while the foot is in RCSP, adding tibial varum will likely cause a different effect. In this foot, adding 5 degrees of tibial varum deformity will bring the calcaneus to being only 1 degree everted while it is maximally pronated which will, in effect, alter the spatial location of the STJ axis to a more lateral position relative to the plantar calcaneus. This medial shift in the plantar aspect of the medial calcaneal tubercle relative to the STJ axis (which occurs as a result of the added 5 degrees tibial varum) will increase the external STJ supination moment from GRF acting on the plantar calcaneus which would, overall, increase the STJ supination moment due to the tibial varum (as long as the plantar forefoot medial-lateral distribution of GRF is unchanged from the vertical tibia example)."

    Do you think it would be fair to say that, in this example, that there could still be increased lateral pressure, just not as much as the first case.

    One final question from me this evening:

    When we contact the ground dynamically, the foot is generally relatively inverted and move towards eversion with the degree dependent on the range of motion available (assuming no other factors altering this function). This means that there is a pronation moment on the foot at this stage. What factors could be influenced with a higher degree of tibial varum assuming all else is equal:

    The size of the pronation moment
    The velocity of motion
    The duration of motion
    None or all of these?

    Thanks for all the input, an interesting discussion

    Trevor
     
  20. efuller

    efuller MVP

    It is interesting to look at the center of pressure path over time. If you have the whole foot print, you can draw a line representing the location of the STJ axis. At heel contact the COP will tend to be very close to the STJ axis. At forefoot contact, the Cop will be "pulled" distally and laterally, assuming the foot hits inverted and the distal force is on the fifth met head. This will increase the pronation moment at the STJ. If you look at slow motion videos, you can see that STJ pronation does not really start at heel contact, but at the beginning of forefoot loading.

    At the beginning of forefoot loading, with the forefoot contact at the fifth met head, the moment from ground reaction force at this time, is completely independent of tibial varum. The pronation moment from the ground will cause STJ pronation until something stops STJ pronation. Tibial varum may change the anatomical structure(s) that stop pronation. When the medial forefoot hits the ground the CoP will be "pulled" medially. It is possible that the CoP could be pulled to a point medial to the STJ axis and this would cause a supination moment that would stop STJ pronation. The increased medial force could dorsiflex the first ray increasing tension in the plantar fascia and this could create a STJ supination moment that would stop STJ pronation. For the above two scenarios the motion could be stopped while there was still range of motion available of the STJ. (The lateral process of the talus has not yet reached the floor of the sinus tarsi.) Same points for muscle and end of RoM of STJ.

    Since the proportionate size of the pronation moment will be determined by the distance of the CoP to the axis. the velocity of pronation will be unaffected by tibial varum. EMG studies show that the posterior tibial muscle is usually active at this time so the velocity of pronation will be affected by the sum of moments from ground reaction force and the posterior tibial muscle.

    Duration of motion will be dependent on velocity and number of degrees of pronation. The number of degrees of pronation will be dependent on what stops the pronation.

    Eric
     
  21. Kevin wrote:

    "If, on the other hand, we start with a foot and lower extremity which has a calcaneus which has a vertical tibia with a RCSP being 6 degrees everted and maximally pronated at the STJ and a plantar forefoot which can evert 10 degrees while the foot is in RCSP, adding tibial varum will likely cause a different effect. In this foot, adding 5 degrees of tibial varum deformity will bring the calcaneus to being only 1 degree everted while it is maximally pronated which will, in effect, alter the spatial location of the STJ axis to a more lateral position relative to the plantar calcaneus. This medial shift in the plantar aspect of the medial calcaneal tubercle relative to the STJ axis (which occurs as a result of the added 5 degrees tibial varum) will increase the external STJ supination moment from GRF acting on the plantar calcaneus which would, overall, increase the STJ supination moment due to the tibial varum (as long as the plantar forefoot medial-lateral distribution of GRF is unchanged from the vertical tibia example)."

    Trevor asked: "Do you think it would be fair to say that, in this example, that there could still be increased lateral pressure, just not as much as the first case."

    Kevin replies: Yes, there is a possibility there could be increased lateral forefoot pressure, all other things being equal. However, this is the difficult thing about frontal plane tibial alignments and predicting their biomechanical effect. The effect of different frontal plane tibial alignments will be dependent on range of motion of the subtalar joint and relative frontal plane position of the forefoot relative to the rearfoot and ground, among other factors.

    Trevor asked: "One final question from me this evening:

    When we contact the ground dynamically, the foot is generally relatively inverted and move towards eversion with the degree dependent on the range of motion available (assuming no other factors altering this function). This means that there is a pronation moment on the foot at this stage. What factors could be influenced with a higher degree of tibial varum assuming all else is equal:

    The size of the pronation moment
    The velocity of motion
    The duration of motion
    None or all of these?"

    Kevin replies:

    Increasing the tibial varum alignment will most influence the magnitude of external STJ pronation or supination moment (i.e. that coming from GRF). The velocity of STJ motion and duration of that motion would be dependent on how much internal STJ pronation and/or supination moment the muscles of the foot and lower generate to resist the external STJ moments from GRF which, in turn, is controlled by the central nervous system (CNS).

    Taking this all into consideration, especially with the Preferred Movement Pathway Theory proposed by Benno Nigg and the recent research on plantar intrinsic muscle EMG activity by Luke Kelly and coworkers, it becomes very evident that we can no longer assume a certain foot and lower extremity "deformity" (as suggested by Root et al) will cause a certain kinematic function of the individual. The CNS has control over motions of our feet and lower extremity. Foot and lower extremity structure are not the sole determinant of foot and lower extremity kinematics during gait.

    Therefore, the research recently published by Hannah Jarvis et al, that measures only kinematics relative to Root's "foot deformity classification system", really doesn't tell us whether the kinetics of the foot changes with certain "foot deformities" nor whether these "foot deformities" are predictive of pathology. The authors of this paper overstated the conclusions that one can derive from their research which only looked at kinematics, and did not look at all into kinetics.

    To summarize: Alterations in foot and lower extremity structure will always change foot and lower extremity kinetics, but does not always change foot and lower extremity kinematics.
     
  22. Trevor Prior

    Trevor Prior Active Member

    Eric said “At the beginning of forefoot loading, with the forefoot contact at the fifth met head, the moment from ground reaction force at this time, is completely independent of tibial varum. “

    Kevin said “Increasing the tibial varum alignment will most influence the magnitude of external STJ pronation or supination moment (i.e. that coming from GRF).”

    These two would appear contrary and I point this out simply so we can clarify. Eric, accepting that how the foot responds will be dependent upon the range of motion available, and assuming the set conditions we have been discussing, tibial varum can influence the pressure under the lateral forefoot and thus the relative pronation moment?

    Eric said"It is interesting to look at the center of pressure path over time. If you have the whole foot print, you can draw a line representing the location of the STJ axis. At heel contact the COP will tend to be very close to the STJ axis. At forefoot contact, the Cop will be "pulled" distally and laterally, assuming the foot hits inverted and the distal force is on the fifth met head. This will increase the pronation moment at the STJ. If you look at slow motion videos, you can see that STJ pronation does not really start at heel contact, but at the beginning of forefoot loading."

    Mapping the axis to the plantar aspect of the foot is fine if we are static. However, as the foot starts to load, to have an axis, the joint must be in motion and it will generally move medially with motion of the talus, thus the relative amount of the foot lateral to the axis increases. Indeed, it is this medial position that most logically explains why the foot stays pronated through midstance until the chain of events that allows things to reverse. However, this chain of events and reversal will be greatly influenced by factors proximal to the foot.
     
  23. While being individual it would be a interesting discussion in how much, I would say the position of the STJ will be an easy place to start the more lateral the more a role the GRF and foot will play in late stance supination
     
  24. These two seemingly opposing views are really just two very different scenarios. Eric is talking about the fact that the effects of GRF acting on the forefoot of any foot will be independent of the frontal plane tibial alignment of that individual relative to the STJ moments that GRF is causing. What I was talking about was a scenario of a certain foot structure now having, for example, a 5 degree varus high-tibial osteotomy which will result in a change in STJ moments due to the increase in tibial varus alignment.

    I don't see this as being theoretical ideas that are necessarily in disagreement with each other. I agree with Eric's analysis.
     
  25. Petcu Daniel

    Petcu Daniel Well-Known Member

    How a line drawn on the whole foot print (2D) can represent the spatial location (3D) of the subtalar joint in dynamics (partial foot-prints: heel, forefoot, ...) ?
    Daniel
     
  26. Griff

    Griff Moderator

    Because it's a model...
     
  27. efuller

    efuller MVP

    Yes, the axis moves relative to footprint with motion of the STJ. However, the vast majority of the time, pressure on the fifth met head will be lateral to the STJ axis and cause a pronation moment. This is why, the vast majority of the time, you see pronation of the STJ rapidly accelerate when there is lateral forefoot contact in heel to toe gait. As Griff said it is a rough model that can explain observations.

    Eric
     
  28. Petcu Daniel

    Petcu Daniel Well-Known Member

    I understand this is a model but all the time I've seen it only in a representation related with the position of the transversal projection of the STJ relative to the whole footprint. I'm trying to imagine how it looks when there is only heel or forefoot contact. Do you have such a representation?
    Daniel
     
  29. efuller

    efuller MVP

    Have you seen the "movie" of a pressure map over the course of a step. What you often see is the maximum pressure plot, which is the maximum pressure achieved at each sensor. This will give you the whole foot print. You will also often see the center of pressure line on top of the maximum pressure plot. However, you can also look at the pressure map at each instant that was sampled. Often with those pictures you will get a dot for where the center of pressure was at that point in time. Before forefoot loading the pressure map will only have pressures for the heel. You just draw the same line for the whole footprint as you do for when the pressures are just on the heel. I'll see if I can find some pictures.

    Eric
     
  30. Petcu Daniel

    Petcu Daniel Well-Known Member

    This will be as in the attached image no.1. And I'm sure you'll agree that the real situation will be different - probably something like in image no.2 or no. 3.
    And in my head the question is: can be the second or the third image, if correct, of any help in understanding the moments around STJA or is just an unuseful complication of a rough model?
    Maybe a sagital view will add more useful information!
    Daniel
     

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  31. Interesting. Some years ago I was trying to work out how so called "high and low gear propulsion" influenced the moments about the STJ axis and realised I needed to know the spatial orientation of the axis during propulsion, when the heel is off the ground. During discussion with my mentor we came up with an in-vivo solution...

    If you look at figures 6, 7 and 8, hopefully you can see the results of our endeavours at this time.
     

    Attached Files:

  32. Daniel:

    You must understand that unless we know where the three-dimensional (3D) location of the subtalar joint (STJ) axis is relative the ground reaction force (GRF) vector at any instant in gait, we can not know for certain what the external STJ moments are at any instant in gait. In other words, we are guessing but certainly with a more educated guess than others who don't have a clue where the STJ axis is. If the STJ pronates, the STJ axis will become more medially translated and more internally rotated and if the STJ supinates, the STJ axis will become more laterally translated and externally rotated relative to the ground. If you use those ideas, or have a STJ axis locator, as Dr. Spooner and I published about a few years ago, then you should have a much better idea of where the STJ axis is and what the external STJ moments are, as long as the foot is walking over a force plate to determine GRF vector spatial location.
     
  33. efuller

    efuller MVP

    It comes down the reasoning you use in choosing the position of the axis in diagrams 2 and 3. Yes, there will be movement of the STJ axis relative to the foot print with both ankle and STJ motion. There won't be that much movement with ankle motion. The more STJ motion, the more the axis will move. If the talus only internally rotates a couple of degrees the axis will internally rotate a few degrees. Since the vast majority of feet pronate after heel contact we will know that the vast majority of the time the axis will be more externally rotated at heel contact and then rotate internally when the STJ pronates. I thnk it is a pretty good assumption that the axis location in static stance will be close to the location of the axis in the midstance portion of gait. (Assuming the talus is in about the same postion that it is in stance) Additionally, feet that have medially deviated axis in stance will tend to have more medial position at heel contact when compared to the feet that has an average STJ axis location. This assumes that the talus internally rotates the same amount. So, if you know where the talus is relative to the foot compared to where the where the talus was relative to the foot when you palpated the location of the axis, you can know how you should change the axis to get a better calculation of the moment.

    The model is still good. It is still true that more lateral the center of pressure, the greater the pronation moment there will be. It will still tend to be greater in the foot with the medially deviated STJ axis and treatment directed at shifting the location of center of pressure will still alter the moments about the axis.

    To calculate a moment you need to know the distance from the line of action of the force to the axis of rotation. In the transverse plane the force is mostly vertical so that it can be represented as a point (the center of pressure). Then by inspection, in the transverse plane, you can easily see the distance from the force to the axis. In the saggittal plane neither the force vector, nor the STJ axis are perpendicular to the field of view so you can't look to see the distance from the force to the axis. Of course, the best way is to have equations for the lines representing the axis and line of action of force so that you can calculate the distance between them 3 dimensionally. It's a lot easier with the graphic representation where the force is directly pointed at you.

    Eric
     
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