Will increasing the friction of the topcover increase the supination resistance of the foot? And if

...kidding about the lovvvvveeee making, last time, something to do with Y2k.......

 

Don't think so...
I make all my devices myself, so I guess i would say 'it depends' or enough to be 'effective'.
I should also say that for the majority of cases, I don't want the heel cup to be required to apply a great lateral to medial force. I view its role more to locate the orthosis in the shoe and the foot on the orthosis.

 

There seems to be some confusion here, Craig. Go back a step- why does a body slide down an inclined plane? Is it because of the moment applied to it? Or, is it due to the angle of the inclined plane and the co-efficient of friction between the contact surfaces? The supination moment may cause the body to roll down the plane, but it's friction which prevents sliding.

If we have a rigid body we can stop it sliding down an incline even when the angulation of the plane exceeds the angle of friction by putting a buttress (your lateral heel cup) on the inclined plane to prevent the rigid body sliding. But what if the body on the inclined plane isn't rigid, rather, it's deformable? The portion of the body which is against the buttress will be static while the rest of the deformable body slides towards it, if the angle of the inclined plane exceeds the angle of friction.

So do I, hence I gave my first lecture on this at the PFOLA meeting in San Diego 2007 where I first presented the finite element modelling I'd done of various orthotic prescription variables. How you getting on with yours?

I tried to upload the next in the series of lectures I gave at the ACFAOM meeting which dealt with load/ deformation, but it's not having it at the moment.

 

Craig:

I'm with you on this one. Simon and I had this discussion at the ACFAOM conference (at the pool after a few beers) and I believe I said that the contribution of frictional forces for most orthoses is likely minimal, at best especially when compared to the mechanical effects of orthosis geometry and orthosis stiffness. However, I did agree with Simon that paying attention to topcover mechanical characteristics would likely be important for treating plantar ulcers.

 

How does the orthosis geometry influence the ground reaction forces? It is the increase in the shear components of ground reaction force which differentiate walking on a flat insole from walking on a moulded orthosis.

 

That and of course, the reduction in pressure due to the increase in contact area...

 

You have got me thinking, and I do not disagree with you.

I am sure that shear forces have a role to play in how an orthosis affects the foot, but it is my opinion that the design of the geometry of the orthosis should minimise the influence of these.

In your example I think we should be comparing a flat insole to an angled flat insole (a simple wedge?), then comparing to a moulded orthosis and consider how changing the geometry (say via increased inversion?) and material characteristics of the moulded orthosis changes the force application...

When you think about this, it makes material characteristics VERY important-

Just thinking out aloud...

 

It should be flat then.

 

If we are talking dynamically then there are shear forces present on a flat insole too

Simon,
I am not denying the existence of shear forces or that they can be considerable with certain orthotic designs.... It is my opinion that in the majority of cases an appropraitely design orthosis should address shear forces via geometry and material characteristics- If shear forces are great enough that a high friction top cover DOES make a difference, then I am not likely to be happy with the design of the orthosis (as well as there also being the increased possibility of irritation)
Perhaps we should be talking about the sum of forces???

 

I'm obviously not understanding what it is that you are trying to say here, Craig. The geometry of the orthoses, to a large part determines the shear components of the reaction forces- ma x Sin theta, so the point to point surface angulation of the orthosis is key in determining the shear components. Do you want the foot to be coupled with the orthosis or uncoupled?

Material characteristics include co-efficient of friction, BTW.


Lets take two identical orthoses with two different top-cover materials (assume same load/ deformation characteristics, but different static co-efficient of friction), at no point on the surface of the orthoses at the foots interface does the point to point angulation exceed the angle of friction of either top-cover material. Will the kinetics be the same or different when standing on these two devices? Why?

Now, lets take two more orthoses with different top-cover materials but this time lets assume that some of the point to point surface angulations exceed the angle of friction for one of the top-cover materials, but not for the other. Will the kinetics be the same or different when standing on these two devices? Why?

Another way of thinking about this is to assume we have one orthosis with a top-cover that is frictionless and another with a top-cover which has an infinitely high coefficient of static friction. Would you expect the kinetics/ shear forces to be the same with both these orthoses in-situ?

 

Simon and Craig:

We know that friction is probably one of the factors affecting performance of a foot orthosis. However, the most important point here is whether the coefficient of friction of the dorsal surface of an orthosis is a significant factor in most types of orthoses where the barefoot is generally insulated from the orthosis by a sock and the geometry of the shoe and orthosis itself limits motion of the foot on the orthosis?

In my opinion, the design and construction of the shoe that the orthosis is being placed in is much more important in most cases than the coefficient of friction of the dorsal surface of the orthosis.

 

Of course it is significant; think about the two extremes of zero friction and infinitely high friction. But you do raise another important point here, Kevin. Where would we prefer sliding/ shearing to occur? At the sock-orthosis interface or the sock-skin interface? Subcutaneous tissues? Or, would we prefer no sliding at all?

In my opinion, the construction of the shoe is a completely different argument and just another factor which requires consideration. But lets run with it and take our thought experiment from above with one orthosis with a top-cover that is frictionless and another with a top-cover which has an infinitely high coefficient of static friction and place them into the same shoe. Would you expect the kinetics to be the same with both these orthoses + shoes, Kevin?

 

Kevin, out of interest how would you rank these three orthotic characteristics in terms of their "importance"?

Geometry at foot-orthosis interface
Load/deformation characteristics at foot-orthosis interface
Frictional characteristics at foot-orthosis interface

 

Missed this on first read Kevin. This statement assumes the foot is not a deformable body. It is.

 

1. Geometry at foot-orthosis interface
2. Load/deformation characteristics at foot-orthosis interface
3. Frictional characteristics at foot-orthosis interface

 

Why do you think that anything I said here makes you assumes the foot that I think the foot is not a deformable body?

How do you know that the frictional characteristics of the dorsal surface of the orthosis is not translated into a shearing force that only moves the sock relative to the skin and the skin and subcutaneous fat relative to the plantar osseous structures of the foot, resulting in negligible shearing forces on the osseous structures of the plantar foot themselves?

 

Sorry, wrote this between patients...

Should read:

 

Simon
Perhaps we have quite different orthotic designs in our minds when we are looking at this?

I have attached a couple pics-

• The first is a cross section (at the heel) of an orthosis I designed with a typical (for this design) heel cup.
• The second is the same orthosis with the heel cup removed.

Surely the net effect if shear forces will be different? (NO denying that shear forces will be acting)- won't they will be acting on the lateral heel also ??

Going back to your presentation the orthosis you have placed your biometer on is quite different in profile to these...

 

Kevin, you have placed geometry at the top of the list as being most important. As you know, the geometry of the device will influence it's load deformation characteristics so the these two of obviously interr-related. Lets make two device with identical geometry but manufacture one from a low density plastazoate (a bit like the "sham" devices used in the Landorf study) and one from polypropylene. Would these device have the same kinetic effect on the foot? Obviously not, so despite having the same geometry clearly the most important factor in differentiating the mechanical effect of these devices is not their geometry as you suggested, but rather their load deformation characteristics.

Lets take two identical devices, same geometry, same load deformation characteristics and make one frictionless and one with infinitely high friction. Would they have the same mechanical effect at the foots interface? What factor would differentiate their function?

 

No your orthotics are not too different from mine. Regardless, the same laws of physics apply. Here you have cut a cross section through a medial heel skive, so if we look at the right hand side of the orthotic in both images, the skive section if you like, you have an inclined plane there of about 25 degrees. So, if we took someone wearing cotton socks and had these devices made out of polypropylene with no top cover this section of the orthosis would be angled greater than the angle of static friction for cotton sock on polypropylene, hence the area of the sock in contact with this section of the device should be trying to slide down it from right to left.

If however, we used EVA as a top-cover material and again stood on it in cotton socks this time the angle of 25 degrees does not exceed the angle of friction for cotton on EVA, so the sock would not slide against the orthosis surface in this area of the interface.

These two situations will result in different kinetics at this area of the foot (sock) -orthosis interface.

Now, the central section of both of the heel cup images is pretty much horizontal in both the orthotics. So the angle of friction at this part of the sock-orthotic interface is not going to be exceeded even with the "bare" polypropylene device. Hence the sock at this section of the interface will be stationary with the section of sock to the right hand side of it (that area on the skive) sliding down onto it on the polypropylene device (remember the angle of friction was exceeded in this area so sliding will occur). The sock will deform and "crease" in response. This will not occur on the EVA covered device.

The left hand orthotic has a lateral heel cup while the right hand one doesn't. The surface angulation of the lateral heel cup varies between about 30 and 70 degrees, so depending on which point we are talking about and depending on the co-efficient of friction between the sock and orthosis, the sock may well be sliding down this slope too, from left to right this time even with an EVA topcover. Again, the section of sock in the middle is static because the angle of friction in the central part of the heel-cup is not exceeded, which will result in bunching of the sock on that side too.

The right hand orthotic has no lateral heel cup so slippage between the sock and orthosis here will not occur.

Will the kinetics at the sock-orthosis interface be the same in these two devices? No.

Will the kinetics at the sock-orthosis interface be the same for these devices regardless of the static co-efficient of friction of their top-covers? Potentially not.

I've been talking here about slippage occurring at the sock-orthosis interface, but it could occur at the sock to skin interface. It all depends on the co-efficient of friction at these two interfaces. Zang and Mak said the angle of friction of sock on palmer skin was about 30 degrees, so lets assume the same for a plantar foot. If we say the angle of friction for cotton sock against polypropylene is 22 degrees, sock on poron 26, sock on vinyl is 24 and sock on EVA is 30 degrees, its pretty obvious that the angle of friction for the polypropylene, poron and vinyl is less than the angle of friction between sock and skin, so slippage will occur at the sock to orthosis interface before it occurs at the sock to skin interface. With the EVA it could occur at both simultaneously. If for some reason you wanted to engineer slippage to occur at the skin sock interface rather than the sock to skin interface, you'd need to use a top-cover with an angle of friction against sock of greater than 30 degrees (and obviously have areas of the orthotic surface angled at greater than 30 degrees). Designing an orthotic so that slippage occurs at the sock-orthosis interface, rather than at the skin-sock interface may be clinically important in reducing the transmission of shearing force to the skin and subcutaneous fat relative to the plantar osseous structures of the foot, Kevin. In order to do this we need to know a few things: the point to point surface angulations of the orthosis, viz. the 3-dimensional geometry of the foot-sock interface, the static coefficient of friction between the sock and orthosis top-cover, viz. the angle of friction and the static coefficient of friction between the skin and sock.

Just for fun Craig, which section of the orthotic heel cups will have the highest load/deformation characteristics: the central horizontal portion or the right hand portion midway along the skive area?

 

Here you go Craig, here is an analysis of the variation in normal force components on the surface of a foot orthosis that I did for our paper on in-shoe pressure measurement. In the analysis each point on the orthosis surface was loaded vertically with the same force, say 10N. The magnitude of the normal component of the force at each point on the orthosis is proportional to the length of the green arrow, so too will be the magnitude of the shear components (parallel to the orthosis surface at each point). So, the shorter the normal component, the bigger the shear component and vice versa. You could do the same thing with your heel cups.

 

Hi Simon
I will have a closer look at this in a couple of weeks... packing for a weeks holiday at the moment...:drinks

 

Opposite direction to what? To the shear components on the medial heel cup, yes. Where did I say they wouldn't be? Enjoy your holiday.

 

There's a nice little JAVA applet here: http://www.mhhe.com/physsci/physical/giambattista/iplane/iplane.html

 

Because you appeared to be suggesting that shoes could prevent slippage of the section of the foot in contact with say, point A on the orthosis surface such that it wouldn't tend to slide toward the section of the foot which interfaced with point B on the orthosis surface even when the surface angulation at point A exceeded the angle of friction between the foot and orthosis at this point and when the surface angulation at point B did not exceed the angle of friction between the foot and the orthosis. This might be the case if the foot were a rigid body, but not in reality because the foot is a deformable body. How do shoes prevent this occurring in the presence of a "deformable" foot?

Here's what happens when you have a deformable object (let the car = foot) sliding into an immoveable buttress (lets say the wall here is the lateral heel-cup of an orthosis + the shoe):
http://www.youtube.com/watch?v=gPsRbIAvDDs&feature=related See how the wall (lateral heel cup) prevents the car (foot) deforming with it's wheels not sliding toward the buttress and instead arrests the motion dead without deformation. Not. Indeed, does the wall/ buttress/ heel cup/ lateral shoe improve this problem or actually make it worse? That'll be worse then. The solution might be to increase the co-efficient of friction between the wheels (foot) and the road (orthosis) so the car (foot) doesn't slide into the wall (lateral heel -cup + shoe) in the first place. But if sliding (shearing) doesn't occur at the orthosis-sock interface nor at the sock-foot interface it will occur within the subcutaneous tissues and the underlying osseous tissues. This could be good if you want to change the forces acting upon the underlying osseous tissues, but not so good if the shearing is so excessive that it creates pathology. Yet, that's only if you believe that the frictional characteristics at the foot orthosis interface are important.