Measuring arch height

If it doesn't pronate as far, but is initially pronating at the same velocity- what effect does this have on the rate of deceleration?

 

I would think that the more the STJ gets into a prontated position, the harder it is to stop the pronation motion. The subtalar joint tip-over sign comes to mind.

Rebecca

 

Daniel, Rebecca and Colleagues:

Let me try to make some sense of all of this for you. When a tendon or ligament has a tensile force placed on it, the stress (i.e. internal resistance force) within that tendon or ligament is increased. Stress, measured in force per unit cross-sectional area (e.g. N/mm^2), is simply a measure of how much internal force that material is offering to resist deformation. Now, a rubber band will never have a lot of stress within it since it can't offer enough internal resistance to deformation before it will break. However, a steel wire will be able to have much more stress within it, and be able to resist much greater forces, than will a rubber band, since it can offer much more internal resistance to deformation by external loads.

When a tendon or ligament has a tensile force placed on it, it will tend to elongate or stretch, which we measure by its strain. Strain is defined as the elongation of a material divided by its original length. For example, if the anterior talo-fibular ligament is measured to be 20 mm long and then, when a tensile force is applied to it, it stretches to 22 mm in length, the strain would be 0.1 (2 mm/20 mm = 0.1).

In material testing of ligaments or tendons, tensile loads placed on these structures will produce a characteristically shaped stress-strain curve which graphically represents the mechanical properties of that biological structure in response to loading force placed on it. The slope of the stress-strain curve at any point along the curve is the stiffness of the tendon or ligament.

For example, a ligament from a person with Ehler's-Danlos Syndrome (EDS) may show a less steeply sloped stress-strain curve than will a ligament from a person with normal ligaments. From viewing the two stress-strain curves from the normal person and the EDS person side by side, we would immediately note the different shapes to these curves and conclude that the EDS patient's ligaments are less stiff or more compliant. Stiffness is the inverse of compliance.

Since the word "hysteresis" has many meanings, depending on the scientific discipline it is used within, in our discussion, elastic hysteresis is a measure of the energy that a material loses when it returns to its original shape after having been stretched or elongated. The magnitude of elastic hysteresis may be determined by measuring the area between the two stress-strain curves when a material is first stretched and then relaxed. A larger hysteresis loop means that the material absorbed much of the energy of stretching and converted it into heat energy. A smaller or thinner "loop" means that the material is close to being nearly perfectly elastic and is a good "energy return" material.

The heat energy that a material produces when it absorbs tension energy and doesn't return all that tension energy back when the material returns to its original shape can be "felt" with a rubber band. Take a rubber band that is unstretched and hold it up to your upper lip to sense its temperature. Now, stretch it hard a few times and then touch the rubber band again to your lip to feel the increase in heat that has been produced by the elastic hysteresis within the rubber band. This is a nice little experiment for students if you ever want them to "feel" what hysteresis is all about.

Hope this helps.:drinks

 

Hi Simon,

You have gone missing on this thread. I wonder if my answer has fallen well short and left you disappointed with my effort - inspite of your efforts to enlighten me?

Regards

Rebecca

 

Sorry, I got side-tracked.
Lets try a motoring analogy: If you are in your car travelling at 60 MPH and a pedestrian walks out in front of you, the brakes of the car have to work a lot harder (provide more force) to stop the car within a distance of 50 feet than they would to stop it within a distance of 100 feet. In other words, given the same initial velocity, we can use less braking force to draw the car to a halt at 100 feet than at 50 feet, but it also takes longer to stop the car. So we are now talking about the impulse (impulse=force x time) here. We could generate the same impulse with a big force over small period of time as we could with a small force over a long time period. So our muscles can either provide a lot of force over a short period of time or less force over a longer period of time to get the same impulse required to decelerate the pronation to zero.

However, this is complicated by the fact that the more the STJ pronates, the further medially the STJ axis will be positioned, thereby reducing the lever arms of tib post and tib ant. So that the more pronated position requires more force to be generated by these muscles to develop the same moment. What it probably means is that we don't have a constant torque acting across the joint unless the muscles steadilly increase the force to exactly meet the decreasing lever arms. This is further complicated by the fact that as the joint continues to move it is also decelerating due to the ongoing action of these muscles.

So is it harder to stop the pronation motion, the more the STJ gets into a pronated position? It's a tricky one that probably needs/ deserves some more thought, but this is the best I can do right now. I guess the answer is dependent upon the initial velocity, initial position of the joint axis and muscular insertions, the length/ tension of the muscles and the distance over which the joint is decelerated. Anything else? Planal dominance of axis, angle of muscular insertions relative to joint axis.


Here's a question: for every degree of rotation of the STJ, how many degrees/ mm does the axis move?

 

I'd like to add one more thing and that is forefoot abduction.

The axis of the STJ is determined by the shape of the facets of the talus. So, as the talus moves relative to the rest of the foot (or vice versa) the projection of the STJ axis will move as well. As the STJ pronates there is an increase in range of motion of the midtarsal joint and there will be further abduction of the forefoot on the rearfoot with STJ pronation.

Here is an interesting motion analysis question. Are there some patients who have internal leg rotation with midtarsal joint motion and not subtalar joint motion? My bet is that there are. When there is abduction of the forefoot on the rearfoot then the projection of the STJ axis will become more medial to the plantar surface of the foot.

Regards,

Eric Fuller

 

In addition to Eric's points, the movement of the center of pressure (CoP) as the subtalar joint (STJ) pronates will greatly affect the tensile force necessary within the posterior tibial muscle-tendon unit in order to cause either deceleration of STJ pronation motion, stabilization of STJ rotation or acceleration of STJ supination motion. In general, the further supinated the STJ is, the more lateral the CoP and the greater will be the pronation moment arm for GRF to cause a STJ pronation moment, and the further pronated the STJ is, the more medial the CoP and the lesser will be the pronation moment arm for GRF to cause a STJ pronation moment. Of course, as Simon mentioned, the medial translation and internal rotation of the STJ axis as the STJ undergoes rotational pronation motion will also affect the posterior tibial tendon supination moment arm.

 

Are you still with us Rebecca?

 

Yep still here Simon.

I have enjoyed the last three posts of yours, Eric's and Kevin's. I think I understand the point you are making about deceleration of pronation. But surely a tibialis posterior muscle in a foot with a medially deviated STJ axis without orthoses has the longest possible time to pull up the pronation movement. A PTTD patient presents indicating that inspite of having a long time to pull up this pronation, it fails to, it cannot provide enough force. So the answer is not giving the tibialis posterior longer to provide a supinatory force (anyway, how do you give tib post longer, an orthosis would give it a shorter time to act?).

Do you agree or have I got this wrong? It makes more sense to me to reduce the amount of pronation thereby providing a shorter lever arm for the tibialis posterior to provide an adequate supinatory force.

I guess it depends on the where the STJ axis is in relation to the 3 body planes, particularly the sagittal plane.

Rebecca

 

It fails becase it has a short lever arm. An orthosis would only give it a shorter time to act if it influenced the kinematics. Specifically, by reducing the degree of pronation.

If we reduce the amount of pronation we should get a longer lever arm because the axis will not translate as far medially. What if the orthosis doesn't change the pronation excursion, how is it helping the muscle then?

Sorry to throw a question back at you, but its done with the best intentions: why does the STJ axis change it's position as the joint moves?

Good discussion.

 

So the deceleration of pronation is not a factor in treating posterior tibial tendon dysfunction. Decreasing overall pronation is the aim.

Where does this leave us with the issue of the (un)importance of deceleration of pronatory motion for other pronatory symptoms? Is it more a comfort issue?

To my way of thinking, its about doing the job for the muscle because it can't. The tib post muscle is unable to provide an adequate supinatory moment so we rely on an orthosis to provide that supinatory moment.

Ummm it just does - the angle that a joint axis makes to the three planes is variable at different positions throughout its range. I've read this and accepted it. I'm not sure why. Has it got something to do with the fact that the STJ has three articulations?

Rebecca

 

Rebecca and Simon: I would like to jump in here to make sure that everyone following along is not confused.

In treating posterior tibial dysfunction (PTD), deceleration of pronation by a foot orthosis, or alternatively, increasing the external supination moment so that the PT muscle does not need to exert so much tensile force to decelerate subtalar joint (STJ) pronation, is a huge function of custom foot orthoses. Tendon injuries, in general, are nearly always due to eccentric muscular contractions not concentric muscular contractions.

Eccentric muscular contractions are where the muscle units are contracting but the distance from the origin to insertion of the muscle is lengthening. Concentric muscular contractions are where the muscle units are contracting but the distance from the origin to insertion of the muscle is shortening. In other words, the damage to the PT tendon seen in PT dysfunction almost certainly occurs from an eccentric contraction, not from a concentric contraction. The damage to the PT tendon likely occurs mostly during early stance phase, when the STJ is undergoing its largest magnitudes of angular velocity of pronation or during late midstance when the greatest magnitudes of STJ pronation moments occur during midstance in these feet while the ankle joint is also dorsiflexing.

Therefore, a properly designed foot orthosis will be able to not only increase the external STJ supination moments at early stance phase but will also be able to increase the external STJ supination moments during late midstance. In this way, the foot orthosis will have increased likelihood of reducing magnitudes of tensile stress within the PT tendon when the tendon is under the greatest tensile loads.

As far as Simon's other question, regarding the STJ axis and why it does move in space when the STJ pronates and supinates, you should read my paper on STJ axis location and rotational equilibrium (Kirby KA: Subtalar joint axis location and rotational equilibrium theory of foot function. JAPMA, 91:465-488, 2001). After you have read it, you should be able to answer Simon's question. I have attached my paper to this post for your (and your colleagues following along) education on the particulars of STJ axis spatial rotations and why they occur.

Hope this helps.

 

Hi Rebeca,

I think the key issue in PT dysfunction is load on the PT tendon. When structures are placed under more load than they can handle they will break.

In looking at deceleration of pronation you have to figure out the sources of moment acting on the joint. The moment from the ground is caused by the position of the center of pressure relative to the STJ axis. (That is why we were talking about the change in the position of the axis.) However, it is very difficult to change the position of the joint with an orthosis, especially in a foot with a medially positioned axis. Therefore, I don't see changing the position of the axis, by changing the position of the joint with an orthosis as part of the goal. On the other hand, putting a varus wedge under the heel of the foot will tend to change the center of pressure and hence the pronation moment acting on the foot.

Another source of moment acting on the subtalar joint is the posterior tibial tendon. The higher the force in the tendon the more supination moment applied by the tendon. The force in the tendon is dependent upon activation of the muscle which is essentially a Central Nervous System (CNS) mediated thing. Therefore, the CNS chooses how much force there will be in the tendon in response to the needs of the whole body that are required.

When there is a pronation moment from the ground the STJ will accelerate in the direction of pronation, unless there is a muscle creating a supination moment to decelerate the joint. (It's not the pronation that causes the injury, it's the stopping of pronation that causes the injury.) So, if the PT tendon does not stop decelerate pronation, then some other structure will and that structure may become injured. You often see sinus tarsi pain with PT dysfunction. So, the higher the pronation moment from the ground the higher the force in the tendon is required to decelerate the joint.

So, in my way of thinking, a varus wedge under the heel decreases the pronation moment, so less force is needed in the tendon to oppose the pronation moment from the ground. When there is less force in the tendon there is a smaller chance of injury, or if it is already injured, there is a better chance for healing.

Regards,

Eric Fuller

 

Thanks Kevin for helping me answer Simon's question "Why does the STJ axis change it's position as the joint moves".

The reason is because we know the STJ axis pierces the talus anteriorly at the superior aspect of the talar neck. So, as the STJ pronates (talus plantarflexes and adducts) and supinates (talus dorsiflexes and abducts), the STJ axis will be in a different position. Therefore, as the STJ pronates, the STJ axis will be more medially positioned.

Rebecca

 

Hi Kevin,

So the injury to tibialis posterior in PTTD occurs while it is contracting eccentrically to decelerate pronation. I understand this.

Kevin, from what you have written above, it seems to me that your orthosis aims at reducing overall pronation (by increasing external STJ supination moments).

Or do you infer that because STJ pronation still occurs, the orthosis is really decelerating pronation?

I would imagine that an orthosis made for a patient with a medially deviated STJ axis and with PTTD (lets say a 4.5mm polypropylene shell with a distally extended rearfoot post, a first ray wipe and a 15 degree medial heel skive - that's along the lines of what I would prescribe) would be a pretty stiff device in the proximal MLA area - it has to be to provide enough STJ supinatory moment. In regard to decelerating pronation, would you (or anyone else for that matter) consider any other prescription variable in an effort to provide deceleration of STJ pronation rather than pure reduction of STJ pronation.

Rebecca

 

The orthosis may or may not decelerate STJ pronation. However, the well-designed orthosis will nearly always decrease the tensile loading forces on the posterior tibial tendon since, if the foot orthosis is increasing the external STJ supination moments, then the internal STJ supination moments (e.g. from PT muscle contraction) will not need to be as high. This is why you can still see changes in symptoms of patients with foot orthoses without necessarily seeing signficant changes in their kinematics.

I would also invert the orthosis by 2-6 degrees, use minimal medial expansion plaster thickness and would use a 2-6 mm medial heel skive at 15 degrees. I originally described the medial heel skive as being standardly ordered by depth of skive, not by angular measurements (I originally described the medial heel skive as being a 15 degree varus angle of skive regardless of the skive depth). I now realize that since it has been over 15 years since the paper was published that orthosis labs around the world that offer the medial heel skive may vary the technique to some extent when compared to how I originally described it. I have attached my original paper on the medial heel skive for those of you following along that would like to read it (Kirby KA: The medial heel skive technique. Improving pronation control in foot orthoses. JAPMA, 82:177-188, 1992).

 

It also moves because the STJ is not a pure hinge joint. It is the translation between the bones that adds to the shift.

 

In addition to Kevin's suggestions, here, I use a deep heel cup (as deep as the shoe + patient can tolerate -almost like a UCBL), and an internal oblique rearfoot post design. This rearfoot post design seems to move the CoP more medially than a standard post design cut straight across the shell. (Joanne S. Paton and Simon K. Spooner: Effect of Extrinsic Rearfoot Post Design on the Lateral-to-Medial Position and Velocity of the Center of Pressure. J Am Podiatr Med Assoc 2006 96: 383-392).

The other advice I picked up from Kevin that he hasn't included here is to get them into walking boots.

I commonly use a 15 degree medial heel skive but in some cases I've used 20 degree medial heel skive in combination with these modifications + walking boots and obtained excellent results.

The papers that Kevin has attached in his last couple of postings are essential reading; if you haven't: do. I may even review them myself as it's been a while.

 

Thanks Simon and Kevin so much for your thoughts on orthosis design for PTTD, the pathomechanics of the condition and for attaching the papers, I really do appreciate it!

Getting back to the issue of deceleration of pronation, is this important or not? Was it considered important in the past and now not considered important? Or is it becoming an issue in more recent times (as per posts from orthosis manufacturers earlier in the thread). If it is important, how is it achieved - I appreciate that this has been discussed in part earlier in the thread but seemed to be a new thing?

Regards

Rebecca

 

Attempting to decelerate pronation with foot orthoses is an important goal of foot orthosis therapy for those patients with symptoms caused by excessive pronation moments, but deceleration of pronation isn't absolutely necessary to produce symptomatic relief for the patient.

 

In general, I agree and would add to this by suggesting that in decelerating pronation with foot orthoses the tissues will become less stiff and reduce their capacity to store and return elastic energy (this could be a good thing in pathology; perhaps not so good if trying to improve athletic performance- who knows?), hence orthoses should demonstrate a high degree of resiliency.

Found this wonderful analogy in Steven Vogel's book: Cats paws and catapults. The caveat being that he is not talking about visco-elastic materials:

"Consider stiffness. If you anchor a boat with a line made of unstretchy stuff, you'll get into trouble. If the boat moves about its anchorage, sooner or later the line will come taught and try to stop it. How much force will that take? According to the best authority, Sir Isaac Newton, the force will equal the large mass of the boat times the deceleration of the boat. If the line comes taught at a very specific length- that is, if it has a high stiffness- the boat will stop abruptly, with a lot of deceleration and thus a lot of force. That might break even a hefty anchor line, or worse it might rip off part of the boat or the dock. Much better to use a stretchier, less stiff rope, one that stops the boat less abrubtly and thus with lower force. Not only is damage less likely, but a weaker rope will do; using a material of lower stiffness allows using one of lower strength! Similarly a longer mooring line may be better than a shorter one. Extensibility may be the same for long and short, but the actual extension will increase with the length of the line."

Perhaps less stiff orthoses, with a large degree of deformation under loading may be more beneficial than "rigid" polypropelyne?

 

I can't think of many situations where a foot orthosis would have the capacity to significantly affect storage of elastic energy in the body, especially reducing the elastic energy storage enough to cause any decrease in metabolic efficiency of the individual. Can you give us examples of when orthoses may negatively affect elastic energy storage, Simon?

Also, are high magnitudes of resilience always a good thing in foot orthoses? I just don't see foot orthoses performing much of a "energy return" function, maybe a little in running and probably not at all during walking. Rather, I see foot orthoses as having more of a moment-altering function where the total metabolic energy is only changed in those individuals where the orthoses have altered their gait kinematics significantly enough to improve their metabolic efficiency.

 

Sorry Kevin I was editing my last post as I wasn't happy with what I'd written, when you posted this:

If an orthoses is capable of changing the rate of loading of tissues, it is also capable of changing the tissues stiffness under load. This must have an effect on the capacity of visco-elastic tissues to store elastic energy. So, if when wearing an orthosis the rate of loading of a given tissue was reduced, e.g. the plantar fascia or achilles tendon then its capacity to store elastic energy would also be decreased. As you know, McNeil Alexander's work on animal locomotion demonstrated the importance of storeage and return of elastic energy in running.

While not specifc to foot orthoses, the work of Mcmahon and others has demonstrated the effect of surface stiffness on metabolic efficiency in running. Since variation in orthosis stiffness will influence the net surface stiffness at the foot's interface with the supporting structures, I believe that a foot orthosis is similarly capable of influencing metabolic efficiency.

Here is where the two theories collide though, hence I have attempted where possible to play Devil's advocate and to sit on the fence a little in my discussions of the topic. McMahon's work showed that softer surfaces, i.e. surfaces which should reduce the rate of loading, tissue stiffness, and thus, energy storeage capacity, actualy improved metabolic efficiency in running. This creates a bit of a paradox (as McMahon himself noted). I think there is an optimal range though, and if the surface were too soft metabolic cost would rise- I believe that this has also been demonstrated experimentally. This also touches on Steve Robbins experimentation with cushioning in shoes and the Robbins-Gouwe hypothesis.

I think we don't yet have a strong enough grasp on the effects of manipulating the mechanical properties of orthoses to draw firm conclusions. Theoretically, if an orthosis can act as an external store of elastic energy and return it to the runner then this has potential to be a very good thing- see the blade runner thread. But probably of little benefit to the walker. Indeed, in other situations it may be beneficial if the energy is "lost" and not returned to the system. I know Bill Olsen believed resiliance was an important factor in orthosis design which he discussed in his chapter in Valmassy.

It may be helpful if you or I explain to the readers of this thread why we are differentiating walking (compound pendulum) from running (springs) so carefully here. I don't have time right now, but will come back to it if you wish (or if someone requests it), otherwise you go ahead if you think it is worthy of explanation, if not don't worry about it :dizzy::bash:

The thing about metabolic cost and foot orthoses is that we just don't have enough data yet. One thing that's drawn my attention is that a couple of EMG studies appear to support the theory that foot orthoses increase the activation levels of muscles such as tibialis anterior. Increased muscle work should increase energy cost- right?

Taking this post full circle- the effect of foot orthoses on muscle activation is also another potential mechanism whereby foot orthoses may influence elastic energy storeage capacity.

 

Many thanks for your insights Simon and Kevin.

I for one would appreciate if you would elaborate when you have time Simon.

Regards

Rebecca

 

Hi All

The STJ axis does not move.

Have a look atwww.musmed.com.au

"have a look at this" topic and start again.

There is lots more to come I can assure you all.

There has been too much stressed on little as fact.

Look at thedata from 40+ years ago.

Remember I sated about 4 years ago that the biceps femoris muscle was a foot muscle and was boo-hooed by the biggest blogger.

Well I was right.

Data on request.

Hope to hear from a tleast one person in the last 500to have a look!!!!!!!!!!

 

Paul:

Instead of continually directing us to your website for the "latest information", why don't you offer us some published peer-reviewed research on Podiatry Arena that shows "the STJ axis" does not move. Just because it is on your website doesn't mean that it is fact.:drinks

 

Simon:

I don't think that Tom McMahon's research on the effects of the stiffness of running surfaces can be directly applied to the stiffness of foot orthoses. In McMahon's research, the surfaces with less stiffness deformed more which caused a lowering of the whole foot and distal lower extremity more with ground impact. This change in downward movement of the whole foot and distal lower extremity at contact is a much different thing than having the medial and lateral arches of the foot being supported by an in-shoe appliance and having their vertical motion patterns altered, and not the vertical motion patterns of the proximal calcaneus, distal forefoot and distal lower extremity being significantly altered. That is not to say that there is no effect, but I think the "surface stiffness effect of foot orthoses" on the energetics of human locomotion is quite minimal, and can probably be considered insignificant. This is especially true considering the known negative effects that the added mass of foot orthoses to the distal lower extremity have on the metablic efficiency of human locomotion.

 

The vertical compliance is an issue- one of the reasons I was asking about rearfoot post design- need to get up to a max of around 10mm vertical travel- space in the shoe is the biggest problem. In his running shoe patent, McMahon didn't appear too concerend with the forefoot. I'll remain open minded until the research has been done- IF I had a research lab at my disposal and/ or an interested backer, it might get done a lot sooner! However, the limited research on this topic that I am aware of suggests that it is possible to improve metabolic efficiency with foot orthoses: Otman S, Basgoze O, Gokce-Kutsal Y. Energy cost of walking with flat feet. Prosthets Orthots Inter 1988;12:7376.

 

Paul, to me your postings frequently lack substance and appear as simple subterfuge to advertise your site here. You could always just pay for the advertising space like the others do.

I look forward to reading your data when it is published in a quality rated journal. If I'm lucky I might even get to review it prior to publication.

 

Read the paper I attached to the biomechaical modelling thread here: http://www.podiatry-arena.com/podiatry-forum/showthread.php?t=7672

And also the one I've attached here.

 

Simon:

I agree. If the orthosis could modify the vertical displacement of the whole foot, then I could see the foot orthosis being able to optimize running gait as does McMahon's tuned track at Harvard. In the study by Otman et al, which is a walking study, not a running study, the increased metabolic efficiency seen in the flat-footed subjects with orthoses is most likely due to increasing the mechanical efficiency of the gastrocnemius-soleus complex at causing a rapid heel rise during propulsion. Without the orthoses, these muscles likely cause more of an arch-flattening effect, thereby delaying heel-lift and shortening the propulsive period of gait. In other words, these orthoses likely increased the stiffness of the longitudinal arches of the flat-footed individuals so that the gastrocnemius-soleus complex was now acting on much longer distal propulsive lever arms than without the orthoses. Mechanical efficiency is thus improved for these walking patients with flat foot deformity, but not by the mechanism proposed by McMahon for the increases in mechanical efficiency seen in running on his tuned track.

Excellent discussion on my 51st birthday....and seeing 30 patients at my office today.....

 

Hi Paul,

I hope you have been well, you haven't posted for a while. You've been very busy with your research no doubt.

Anyway ...

I'm not sure what you are saying. I don't understand what the angles are that you have measured. Aren't these nonweightbearing images? I am a bit dim - would you mind explaining it to me regarding STJ axis?

What data?

I don't understand what you are saying?

With the force plate stuff you've done, do you plan to apply statistical analyses to your results? To me, the results look a bit variable.

Respectfully

Rebecca

 

Rebecca

These people although supine are applying a force onto their feet at about 90-95% body weight.

There are another 13 sets of data to come.

These show a huge difference to the dorsal views A1and A2 but very little changes to the posterior views A3 A4 A5 and A6. These reflect the position of the Subtalar joint.

When I publish the views of over pronated feet, you will see that there are changes only in the dorsal images and almost nothing in the subtalar joint.

Anatatomists of 50 to150 years ago have shown that the subtalar joint does move between 1-3 degrees.

If you add up the Angles pre and post of A3 A4A5 and A6 the angles combined are within 1.6degrees (within their anatomy discussions).

Bojan-Muller he was convinced that the joint only moved within a few degrees of
zero.

As regards the Biceps femoris. Embryologically the dorsal foot muscles , the lateral compartment of the lower limb are all derived from the Dorsal muscle mass.

This from Human Embryology,by William J Larsen page 291.

This in treating the foot dysfunction, one must treat the biceps femoris. In the past about 4 years ago I was always put down for suggesting treatment in this area.

Hope this helps
Regards from sunny Sydney

Paul

 

Paul:

Numerous recent studies show that the subtalar does can move more than 3 degrees and that the subtalar joint axis moves within space relative to the talus and calcaneus during subtalar joint rotational motions. The subtalar joint axis also rotates and translates within space with normal walking. The subtalar joint axis and can be tracked now using fast-MRI scanning of the foot (this is research I was recently involved in which should be published within the year in J. Biomechanics with the biomechanists from Penn State Biomechanics Lab and NIH).

Can you please me more specific and less cryptic regarding what you mean by "the joint only moved within a few degrees of zero." I don't have a clue of what you mean by this statement.

 

Kevin

Interesting, but how do they do fast-mri scanning and is it performed under load? as my work has been.

What I meant by a few degrees of zero was that the changes between pre and post mobilisation when each individual result was added before averaging the angles in degrees was within a few degree of each other.

Glad you made it to 51.

Paul C

 

Paul:

Thanks for the reply. We designed a special apparatus of plastic (so it can be used in the powerful magnets of the MRI scanner) that attaches to and loads the forefoot of the subject. Then the loaded forefoot is moved by the apparatus in such a way to maximize subtalar joint motion and minimize tibio-talar motion. Retroreflective markers in live subjects on only their calcaneus and tibia then can be used to closely estimate subtalar joint axis spatial location, without drilling bone pins into the talus. Very cool research...even if I do say so myself. (Lewis GS, Cohen TL, Seisler AR, Kirby KA, Sheehan FT, Piazza SJ: In Vivo Tests of an Improved Method for Functional Location of the Subtalar Joint. Submitted to J. Biomechanics November 2007.)

 

Kevin
Sounds fascinating. Marking is such a problem with MRIs not only because of the metal/magnetic thing but also because of very limited space.

Will look out for it
Regards

Paul

 

Paul:

There are now MRI scanners that allow weightbearing examinations. It would be very cool to do fast-MRI images with weightbearing images of the foot during closed kinetic chain motions. The future appears very bright for learning more about the intricate kinematics and kinetics of the osseous segments of the foot with these new technologies.

 

Kevin

From what i have read about their imaging there is very little to see compared to the supine/prone MRI imaging. I am certain with loading there will be some changes.

I have compared three people to MRI and CT underloading and there is very little difference between the two methods. When having the MRI's they almost dislocated their shoudlers for pulling for 20+ minutes versu 1-2 minutes with the CT's we have done.

I have almost 120 sets of CT images under load including bare feet, with orthotics soft hard and shoes like Berkenstoks etc. ie those with built in raises.

Most results despite the quality of the podiatrist is poor after the angles of Paulex are compared to 'normal feet' although visually they look good.

'normal feet' are sets of feet that at least three podiatrists have called within normal range.

A slow study

Regards

Paul

 

Simon:

I agree with you here. It is absolutely critical that if we are to discuss whether foot orthoses have the potential to increase the metabolic efficiency of walking and/or running that we specifically address whether we are referring to walking or running in our discussions. Walking and running are very different activities. Unfortunately, most clinicians simply view running as an accelerated form of walking, of which it certainly is not.

Here is my favorite review article on the differences between running and walking from a biomechanics and an energetics standpoint (Novacheck, Tom F.: The biomechanics of running. Gait and Posture, 7:77-95, 1998).

All of you following along with our discussion must read this paper!!!