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Right then. Still a bit punchy so you'll have to forgive me if I'm a bit more "fighty" than usual.
I'm going to start from the top if I may, rather than try to piggyback in on Simon and Bob's debate.
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All, in another thread Robert wrote:
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Originally Posted by Robertisaacs
I don't know if anyone has ever done a repeatability test for nav drift / drop / planal dominance.
Here is a reliability study: http://www.japmaonline.org/cgi/conte...tract/91/5/262, regardless of their moderate reliability in the hands of these testers, what do the tests actually show? Hylton Menz assumed they were measures of foot pronation: http://www.japmaonline.org/cgi/conte...tract/88/3/119 Robert appears to be assuming that some inference can be made from these to planal dominance? Planal dominance, described by Don Green http://www.japmaonline.org/cgi/content/citation/74/2/98 relates to the relationship between a joints axial position and the amount of motion that should occur about that axis in the cardinal body planes. What does navicular drift / drop tell us about planal dominance, if anything, and if so, the planal dominance of which joint? Talonavicular joint? Navicular-cuboid joint? Navicular-medial cuneiform joint? Navicular-intermediate cuneiform joint? Navicular-lateral cuneiform joint? All of these? None of these? Given the relatively unconstrained nature of some of these joints, and Nesters contention that the axis is created by the motion, not vice versa, is the concept of planal dominance even valid and worthy of consideration for these joints?
First off. I think there is a dissonance here between biomechanics (as a theoretical study) and MSK Podiatry (as a clinical discipline). As I have ranted many a time, I think that they are profoundly different.
Can one derive, with any degree of accuracy, the position of the sub talar axis by observing the navicular drift / drop? I'd say no. Several reasons. Firstly, as Simon points out, this assumes that the sub talar axis is biplanar. The further the axis from the sagittal plane the more movement there will be in the frontal plane from that orientation.
Far more significantly, there is the issue of midfoot stiffness. If the midfoot is munted, and can dorsiflex freely and far, the navicular could be weight bearing with little or no sub talar motion at all! And just to really confuse matters, Sub talar pronation involves dorsiflexion so in the sagital plane, sub talar pronation could actually be raising the navicular in the sagittal plane, even as it lowers it in the other two..
And of course we are measuring the position of the navicular. The amount (if any) of navicular movement relative to the other tarsal bones may also be a confounding factor.
So no. In a biomechanical sense, I don't believe we can make any profound observations about the location of the sub talar axis based on nav drift / drop.
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Then, I'll spoil your fun and tell you that foot position doesn't necessarily predict pathology, and ask you why you are measuring this anyway? Yet, if I can apply a "braking" force in a direction directly opposite the translational vector of the navicular, via a foot orthosis, should this not be the most efficient point and direction to apply such a force... assuming deceleration of navicular motion is important?
And here is the MSK Podiatry bit. Why is the position of the axis important at the sharp end of the process? Lets keep in mind that from an MSK podiatry perspective the purpose of all our biomechanics is to make the right shaped bit of bent plastic (or, with a respectful nod to Bob, to know which bits to cut and which bits to leave alone and suchlike).
As Simon observed, knowing the direction of travel of the navicular, and its start and end point, offer a very useful piece of information when deriving prescription, both the shape of the device and potentially the covering material as well. From this perspective one might consider the movement of the gross morphology of the foot to be as important as the details of the axial componants of that movement. There are a limited number of places and ways we control the foot. Knowing what percentage of movement is originating from which of the tarsal joints is interesting from a theoretical, and perhaps diagnostic viewpoint. But if you were to know that the movement was 60% Talonavicular joint and 40% Navicular-medial cuneiform joint as opposes to 40% / 60% would it make a vast difference to the way you shaped the arch of your orthotic?
Here is another point to consider. I am a great fan of the "directly opposing force" concept Simon mentioned. If the navicular was a cricket ball and I want to control its fall, I want to hold my hand / orthotic perpendicular to its direction of travel right? But there is another element. As Simon and Kevin's recent paper illustrated, the effect of a force is not just about its location, but also its vector. The vector of the ORF, the equal and opposite reaction force, will, I suspect, depend significantly on the vector of the incoming force.
The idea of measuring navicular slide is a great one from a biomechanical point of view. But how much would that inform our treatment? We can't put anything in front of the navicular to stop it sliding forward, the cuniform gets in the way!
Thats why I use the two dimensional test to derive a two dimensional vector.
The drift / drop test may not tell me, with any degree of accuracy, the planal dominance of the sub talar joint axis, the mid tarsal axis, or any other axis. But these axes are theoretical constructs designed to help us reach a goal, that being the shape of the device. So far as that is concerned, when I talk about planal dominance, I am happy enough to think in terms of the planal dominance of the movement of the arch as measured from its peak, the navicular. Its crude, but then so are orthoses. While biomechanics, as a pure science, has no room for pragmatism, I believe MSK podiatry does, provided it does not masquerade as other than it is. If I have, in the past, inferred such then I shan't do so again.
And if you made it to the bottom of that ramble you're a better man / woman than I!!
Re: Navicular drop + navicular drift = planal dominance?
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Originally Posted by Robertisaacs
Can one derive, with any degree of accuracy, the position of the sub talar axis by observing the navicular drift / drop? I'd say no. Several reasons. Firstly, as Simon points out, this assumes that the sub talar axis is biplanar. The further the axis from the sagittal plane the more movement there will be in the frontal plane from that orientation.
Far more significantly, there is the issue of midfoot stiffness. If the midfoot is munted, and can dorsiflex freely and far, the navicular could be weight bearing with little or no sub talar motion at all! And just to really confuse matters, Sub talar pronation involves dorsiflexion so in the sagital plane, sub talar pronation could actually be raising the navicular in the sagittal plane, even as it lowers it in the other two..
And of course we are measuring the position of the navicular. The amount (if any) of navicular movement relative to the other tarsal bones may also be a confounding factor.
So no. In a biomechanical sense, I don't believe we can make any profound observations about the location of the sub talar axis based on nav drift / drop.
I'm still not sure why the subtalar joint axis is even being discussed with regard to the navicular drop and drift, is the navicular part of the subtalar joint?
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Originally Posted by Robertisaacs
As Simon observed, knowing the direction of travel of the navicular, and its start and end point, offer a very useful piece of information when deriving prescription, both the shape of the device and potentially the covering material as well. From this perspective one might consider the movement of the gross morphology of the foot to be as important as the details of the axial componants of that movement. There are a limited number of places and ways we control the foot. Knowing what percentage of movement is originating from which of the tarsal joints is interesting from a theoretical, and perhaps diagnostic viewpoint. But if you were to know that the movement was 60% Talonavicular joint and 40% Navicular-medial cuneiform joint as opposes to 40% / 60% would it make a vast difference to the way you shaped the arch of your orthotic?
Here is another point to consider. I am a great fan of the "directly opposing force" concept Simon mentioned. If the navicular was a cricket ball and I want to control its fall, I want to hold my hand / orthotic perpendicular to its direction of travel right? But there is another element. As Simon and Kevin's recent paper illustrated, the effect of a force is not just about its location, but also its vector. The vector of the ORF, the equal and opposite reaction force, will, I suspect, depend significantly on the vector of the incoming force.
The idea of measuring navicular slide is a great one from a biomechanical point of view. But how much would that inform our treatment? We can't put anything in front of the navicular to stop it sliding forward, the cuniform gets in the way!
Thats why I use the two dimensional test to derive a two dimensional vector.
That's why you need to measure the slide: to work out the 3D movement vector. The slide measures the anterior/ posterior component and will ultimately influence the net vector of movement. So while you may not be able to directly stop the anterior / posterior component in isolation, it should influence the required opposing 3D surface angulation to decelerate the net movement.
Re: Navicular drop + navicular drift = planal dominance?
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I'm still not sure why the subtalar joint axis is even being discussed with regard to the navicular drop and drift, is the navicular part of the subtalar joint?
Its attatched to the talus. Not firmly, I'll grant you, but I think it gives you some inclination about what the talus is doing.
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That's why you need to measure the slide: to work out the 3D movement vector. The slide measures the anterior/ posterior component and will ultimately influence the net vector of movement. So while you may not be able to directly stop the anterior / posterior component in isolation, it should influence the required opposing 3D surface angulation to decelerate the net movement.
Certainly it will influence the net vector of movement. But not, I suspect (no evidence), so much as the position of the body over the foot.
Help me out. Give me an example. Foot 1 has no slide and a 45 degree movement medial/plantar of, say, 1cm. Foot 2 has a 0.5 cm anterior slide at the same medial plantar vector. So we have 2 parts drift, 2 parts drop and 1 part anterior slide. How would I make my insole differently to take this into account?
I suppose higher friction cover would make a difference, but I tend to use those anyway....
Re: Navicular drop + navicular drift = planal dominance?
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Originally Posted by Robertisaacs
Certainly it will influence the net vector of movement. But not, I suspect (no evidence), so much as the position of the body over the foot.
we are talking about the movement of the navicular so the slide component is part of that movement. What drives the motion is something completely different.
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Originally Posted by Robertisaacs
Help me out. Give me an example. Foot 1 has no slide and a 45 degree movement medial/plantar of, say, 1cm. Foot 2 has a 0.5 cm anterior slide at the same medial plantar vector. So we have 2 parts drift, 2 parts drop and 1 part anterior slide. How would I make my insole differently to take this into account?
Draw it out as a vector diagram, that will give you the resultant translational axis, your orthotic shell would be best able to decelerate translation along that axis if it were normal to it.
A quick analogy, you drive your car along a horizontal surface into a concrete wall (orthotic) which is vertical and perpendicular to the direction of motion; you then drive you car into a concrete wall which is a wedge shaped inclined plane and therefore at an angle <90 degrees to the direction of motion. Which wall will decelerate the car more effectively?
Missed the bit about the talus. But you are not measuring the movement of the talar head, you are measuring the navicular displacement, and while the navicular articulates with the talar head, it also articulates with a host of other bones. Lets say I fused the navicular at all of its articulations other than the talonavicular joint and then moved the subtalar joint in isolation, what motion of the navicular might we observe?
Re: Navicular drop + navicular drift = planal dominance?
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Draw it out as a vector diagram, that will give you the resultant translational axis, your orthotic shell would be best able to decelerate translation along that axis if it were normal to it.
A quick analogy, you drive your car along a horizontal surface into a concrete wall (orthotic) which is vertical and perpendicular to the direction of motion; you then drive you car into a concrete wall which is a wedge shaped inclined plane and therefore at an angle <90 degrees to the direction of motion. Which wall will decelerate the car more effectively?
This I understand. Much as the cricket ball analogy (which I had to include because its seasonal). This is how I apply the drift / drop. But I can't see how one can make the orthotic surface normal to the X+ element because to do so would require the orthotic to occupy space which is occupied by the medial cuniform. To take a hypothetical, if we made our foot 1:1:1 movement, forward, down and sideways, it would require an orthotic surface at 45 degrees to the ground in all planes. But that space is occupied.
I'm not explaining this well. I feel a video with a skeleton and some cardboard coming on...
Re: Navicular drop + navicular drift = planal dominance?
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Originally Posted by Robertisaacs
This I understand. Much as the cricket ball analogy (which I had to include because its seasonal). This is how I apply the drift / drop. But I can't see how one can make the orthotic surface normal to the X+ element because to do so would require the orthotic to occupy space which is occupied by the medial cuniform. To take a hypothetical, if we made our foot 1:1:1 movement, forward, down and sideways, it would require an orthotic surface at 45 degrees to the ground in all planes. But that space is occupied.
I'm not explaining this well. I feel a video with a skeleton and some cardboard coming on...
I think you are looking at it as individual components rather than a single resultant motion. When I put an orthotic under a pes pancake foot, with the navicular touching the ground, I'm introducing the orthotic into a space which is "occupied" too. Can I suggest that rather than cardboard you start with a cube of foam, from this create a triplane wedge angled 45 degrees to each plane, then slip this under the arch of your foot.
Re: Navicular drop + navicular drift = planal dominance?
The cardboard video is uploading as we speak but I tried it with foam and that illustrated the point rather better. If I get time I'll do that one this PM.
Re: Navicular drop + navicular drift = planal dominance?
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Originally Posted by Robertisaacs
The cardboard video is uploading as we speak but I tried it with foam and that illustrated the point rather better. If I get time I'll do that one this PM.
Good thread BTW. Thanks for starting it.
Here's an even easier illustration, palpate and mark the navicular tuberosity, as you would if you were about to measure navicular drop, drift and slide. Then take a tack nail and place the flat head of the nail against the navicular tuberosity, let this be a portion of the orthotic shell and the spike part of the nail be the net translational axis, change the angulation of the nail relative to the cardinal planes such that it simulates an axis roughly 45 degrees to each plane along which the marked point of the navicular should move plantarly, medially and anteriorly, you should be able to do this and keep the head of the nail pretty much in contact with the foot. Now try varying the angulation of the nail..
Re: Navicular drop + navicular drift = planal dominance?
Here Ya go.
Its not just for you simon. In fact its mainly for those folk following along (I hope there are a few) who are wondering what the hell we are talking about.
For some reason, try as I might, I cannot get these things to embed. Craig, would you mind? Thanks.
Then take a tack nail and place the flat head of the nail against the navicular tuberosity, let this be a portion of the orthotic shell and the spike part of the nail be the net translational axis
Well I suppose so if you have a really sticky out navicular prominance. But how much surface area are you going to catch that way? The more surface area, the bigger the lump in the side of the arch and the more navicular drift, the more likely that is to cause irritation!
Re: Navicular drop + navicular drift = planal dominance?
Your orthotic surface is too big, end of story. Take the card off of your pencil and use something about the diameter of your pencil. Then do it on a foot. In fact push with your finger with a 45 degree vector, the medial cuneiform still in the way? Of course not.
You're wrong on this Robert, and making video's doesn't make you right. We are talking here only about the orthotic surface which is in contact with the medial portion of the navicular, when you measure navicular drift and drop what's the surface area of the point you mark to measure this? Indeed what is the average area of the navicular on the medial aspect of the foot? 1.5 cm squared? Try with a smaller piece of card.
Re: Navicular drop + navicular drift = planal dominance?
And in your second video once again the wedge is way too big and out of proportion. Funnily enough I did exactly the same thing today, drilling a cast and offering up a much smaller wedge, and guess what, there was no issue with it "occupying the space of the medial cuneiform". It's all a matter of scale Robert. Next....
Maybe I'll make some video's if I find time tomorrow and post them up here.
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Originally Posted by Robertisaacs
Well I suppose so if you have a really sticky out navicular prominance. But how much surface area are you going to catch that way? The more surface area, the bigger the lump in the side of the arch and the more navicular drift, the more likely that is to cause irritation!
Yeah, that's why it's called the tuberosity of the navicular, because it's the sticky outy bulging bit
Your contention regarding orthotic irritation and "more surface area" is not well made and indeed by orientating the geometry of the orthotic shell perpendicular to the relatively small navicular area and perpendicular to it's plane of translation, shear forces should be reduced, reducing the friction and irritation, not increasing them as you maintain.
P.S. when you stuck your cocktail stick through your foam cast with the triplane 45 degree vector, you translated the position of the navicular anteriorly too, the stick should have exited from the same point as the first hole, only angled anteriorly.
"Lets see what response that gets". Yeah, lets.
Oh and I just spotted the dent in the heel section of the foam cast caused by your biplane wedge based on navicular drop and drift alone. Does this mean we should perform a heel amputation when you are prescribing devices based on navicular drop and drift, Robert? Maybe the next time we get together I can explain the concept of bias to you? Like I said, just because you made a video, it doesn't make you right. Although, as i'm sure you understand the power of marketing, TV ads cost more than paper ads for a reason.
Re: Navicular drop + navicular drift = planal dominance?
Hello men.
Forgive me if I'm being a bit stupid here, but how do you hope to apply your equal and opposite force to the navicular to prevent the drift or drop using orthotics? I can't see a patient wearing an insole that does not generally conform to the shape of their medial longitudinal arch without complaining of severe discomfort. I'm guessing that this thread is more of a 'thinking exercise' about how your orthotic is effecting the particular point of interest rather than it being an immediately clinically useful discussion (please do not take that as being a criticism of the thread).
Lets say you can work out relative three dimensional rotation of all of the bones of the foot, or even 3d displacement, vectors, etc... - if you were to attempt to apply this to designing an orthotic that provides an equal and opposite force to each one of these bones at whatever point in their motion you like you would produce a very uncomfortable insole. We all know that there is more in a foot than just bones. I could be way off the mark here, but the idea reminds me of Rx lab's polysectional triaxial casting thing that Ray Anthony talked about some time ago. You probably recall that they would take the positive cast, cut it at key points where they sought to correct perceived problems, move the sections of the cast into the corrected position and plaster it back together. Then they would smooth it off and in-fill plaster here and there so if would not be too uncomfortable to wear. What they ended up with was an orthotic that was not significantly different to the standard thing you could have made via far simpler methods.
You may design an orthotic that perfectly resists your abnormal vectors, but then you're going to have to smooth it off so the patient can tolerate it. How much this will deviate your device from your ideal I do not know.
Edit - sorry Robert, just watched your second video and I agree with the last bit. You guys make more insoles than me and see more of these patients - how well tolerated are your devices and have you actually tried something along the lines of Simon's suggestion?
Re: Navicular drop + navicular drift = planal dominance?
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Originally Posted by bob
What they ended up with was an orthotic that was not significantly different to the standard thing you could have made via far simpler methods
that probably tells you more about the lab than the prescription concept.
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Originally Posted by bob
You may design an orthotic that perfectly resists your abnormal vectors, but then you're going to have to smooth it off so the patient can tolerate it. How much this will deviate your device from your ideal I do not know.
This is a theoretical discussion as you rightly point out. The point is we could theoretically manipulate the geometry and the stiffness of the shell, in discreet areas, so it doesn't necessarily = "smoothed out device", at least, the smoothed out device could have higher stiffness sections which will act in a similar way, while still providing a smoother interface surface.
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Originally Posted by bob
Edit - sorry Robert, just watched your second video and I agree with the last bit. You guys make more insoles than me and see more of these patients - how well tolerated are your devices and have you actually tried something along the lines of Simon's suggestion?
Since I only actually thought of it last night.... and I don't think the navicular slide test is described in the literature, then I'm also fairly certain nobody has actually tried to create an orthotic surface normal to the navicular translation axis which interfaces with that area of the navicular.... I guess you could say its theoretical.... Like I said, reference this post in your paper.
Re: Navicular drop + navicular drift = planal dominance?
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Originally Posted by Simon Spooner
that probably tells you more about the lab, than the prescription concept.
This is a theoretical discussion as you rightly point out. The point is we could theoretically manipulate the geometry and the stiffness of the shell, in discreet areas, so it doesn't necessarily = smoothed out device, that is, the smoothed out device could have higher stiffness sections which will act in a similar way, while still providing a smooth surface.
Since I only actually thought of it last night....
That is all fair comment Simon. As far as material properties of your devices go I think it could be a great concept, but my materials science knowledge in and around orthotics is severely limited so I'll politely STFU regarding that . Do you think you still might have issues regarding comfort if you could provide such a device? I am thinking about the extremes of density in EVA orthotic materials placed next to each other. It must be tremendously difficult to provide a device that provides the right force in the right direction to a dynamic structure such as the foot whilst remaining comfortable. I wonder about the longevity of these devices?
Re: Navicular drop + navicular drift = planal dominance?
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Originally Posted by bob
I am thinking about the extremes of density in EVA orthotic materials placed next to each other. It must be tremendously difficult to provide a device that provides the right force in the right direction to a dynamic structure such as the foot whilst remaining comfortable. I wonder about the longevity of these devices?
Not all orthoses are made of EVA, Bob. And there is more than one way to skin a cat
Re: Navicular drop + navicular drift = planal dominance?
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Yeah, that's why it's called the tuberosity of the navicular, because it's the sticky outy bulging bit
Really? I thought that was because it was something to do with root vegetables!
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Your contention regarding orthotic irritation and "more surface area" is not well made and indeed by orientating the geometry of the orthotic shell perpendicular to the relatively small navicular area and perpendicular to it's plane of translation, shear forces should be reduced, reducing the friction and irritation, not increasing them as you maintain.
Meh. I'm not sure. If what I'm thinking your thinking is what your thinking then what I'm thinking is that there has to be a protruberance in the medial flange of the orthotic. Or "bloody great lump" as patients like to call them.
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P.S. when you stuck your cocktail stick through your foam cast with the triplane 45 degree vector, you translated the position of the navicular anteriorly too, the stick should have exited from the same point as the first hole, only angled anteriorly.
Duly noted.
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"Lets see what response that gets". Yeah, lets.
Oh and I just spotted the dent in the heel section of the foam cast caused by your biplane wedge based on navicular drop and drift alone. Does this mean we should perform a heel amputation when you are prescribing devices based on navicular drop and drift, Robert?
Could try it I suppose. Hard to get volunteers for a study I guess. Ethics would be a B**** as well.
The big surface was as you say, big. A smaller one (your 10p size) brings its own problems. That has to be a lump to get in front of the navicular tuber.
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Maybe the next time we get together I can explain the concept of bias to you?
Yours or mine? Or both? I'm pretty sure I read a book about it once.
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You're wrong on this Robert, and making video's doesn't make you right.
It surely doesn't. And saying I'm wrong on this doesn't make you right either . Quid pro quo clarice. Since, as you say, the idea of making a surface perpendicular to the triplanar vactor, using navicular slide as an indicator, is a shiny new idea and has never to our knowledge been used on an insole, we neither of us really know do we?
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Like I said, just because you made a video, it doesn't make you right. Although, as i'm sure you understand the power of marketing, TV ads cost more than paper ads for a reason.
Yes indeed. Although advertising implies I have something to sell, and I don't. I just happen to disagree with you. Assuming, as I say, that I've understood you right. I just don't see how an orthotic shell can be modified to place a surface normal to the vector of the navicular, or even the navicular tuberosity, without creating intolerable pressure on the medial side of the medial cuniform. But I am open to be proved wrong, and I would love to see a picture (no need to do a video) of such a surface. Whittle yourself a foot, or an insole shape, and show me the sort of angled surface you have in mind!
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sorry Robert, just watched your second video and I agree with the last bit. You guys make more insoles than me and see more of these patients - how well tolerated are your devices and have you actually tried something along the lines of Simon's suggestion?
Nope. This concept is new to me today. And I don't make a precise (hahaha) angled measurement in my orthoses. I use it as a gross indicator. If its loads of drift, I'm thinking high flanges in strong footwear. Loads of drop I'm thinking shank dependant in a softer material and not getting stressed over footwear. Points in between make points in between.
By the by, this is not a private barny. Everyone else is invited . This must be the longest run that Michael has not posted in for months!
Re: Navicular drop + navicular drift = planal dominance?
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Originally Posted by Simon Spooner
Not sure I understand your question. Are you asking if material "strength" is related to material thickness? Or something else?
I'm wondering if there is a material out there that can fulfil your needs to create a shell that provides your equal and opposite force whilst still remaining comfortable, but does not fail (or wear out) at different rates so that it has a reasonable shelf life? Your earlier post that mentions "..higher stiffness sections.." got me thinking that the lower stiffness sections may wear out sooner and cause some discomfort?
Re: Navicular drop + navicular drift = planal dominance?
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Robert, you're beginning to irritate me, and I don't want to be irritated by you. So I'll leave this for now.
Like Kevin, I feel it may be time for a break, I'm sure now that the Arena has a new bread of biomechanics experts, life will go on.
Oh don't be like that! You know that its not my intention to irritate and certainly not to piss you off. You've had people disagree with you before. I just want to understand why I'm wrong, not least because I have a sneaking suspicion I have the wrong end of the stick entirely!
We both know I can't do what you do. I'm not ripe yet so don't you dare bugger off.
I just don't geddit is all! Someone, Kevin, Craig, Eric, Ian, Mike, someone pitch in here on this.
Re: Navicular drop + navicular drift = planal dominance?
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Originally Posted by Robertisaacs
Oh don't be like that! You know that its not my intention to irritate and certainly not to piss you off. You've had people disagree with you before. I just want to understand why I'm wrong, not least because I have a sneaking suspicion I have the wrong end of the stick entirely!
We both know I can't do what you do. I'm not ripe yet so don't you dare bugger off.
I just don't geddit is all! Someone, Kevin, Craig, Eric, Ian, Mike, someone pitch in here on this.
I need a break Robert, nothing personal. I've presented some new ideas, make of them what you will. I'm tired and I can't be arsed to fight.
Re: Navicular drop + navicular drift = planal dominance?
AND this is the last one for now: "here is your stage Robert. Stand upon it. And if anyone should ask or question, be sure to tell them that I sent you..."
Now, I am ****ing off for a bit, so stop ****ing replying - give me a ****ing break,...K? K.
Re: Navicular drop + navicular drift = planal dominance?
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. Do you think you still might have issues regarding comfort if you could provide such a device? I am thinking about the extremes of density in EVA orthotic materials placed next to each other. It must be tremendously difficult to provide a device that provides the right force in the right direction to a dynamic structure such as the foot whilst remaining comfortable. I wonder about the longevity of these devices?
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