Orthoses

*Small Hole Orthosis-TM, copyright 2009

sounds a little kinky to me, Simon!

 

Were it truly extrapolative, even then it would be a questionable method to produce an orthosis. :deadhorse:

 

An interesting question... no one wants to have a crack, so here are my thoughts
I am guessing that with device 1 you are looking at the shape of the rigid material being defined by a relaxed stance posture, and we are looking at static pressure/ reaction forces.
Device 2 would probably have a broader spread of pressure as it is the flexibility of the material deforming which achieves the compliance.
In this situation (static) the pressure pattern on the plantar foot of the rigid device would be changes only minimally from the barefoot condition because the chape of the device is the shape of the foot- it should therefore not change pressure. If it did, then it would not be perfectly congruent...

Another hypothetical would be a comparison also of a compliant material which was the exact shape of the foot- and therefore would be the same shape as device 1- in this stuation it should be have exactly the same effect also as device 1 in the static position.

All this would change if the subject actually took a step of course... the effect on pressure and ORF would be specific to the individual.

 

In both examples above congruity is achieved, however we do not know if the shape of the foot-orthosis interface is the same in both cases. Since pressure = normal force / area the shape of the interface will determine the total pressure, even though in static stance the total load, i.e. ½ body weight per foot, should be roughly the same.

The physical factors that determine the stress distributions in both materials the devices are constructed of and at the foot-orthosis interface will be important. When force is applied through the interface, deformation of both the foot and orthosis will occur in both types of device. The strains in each of the two devices and the foot will be dependent upon their relative moduli and thickness, the geometry of the underlying bone and the loading applied. It is important to realize that the more compliant device 2 there will be greater deformation (strain) under load. Tension may be developed in the deformation of the surface and this may be transferred to the soft tissues of the foot (the relatively softer prefabricated devices and blistering I spoke about previously may be explained by this.

 

Or that the soft-tissue deformation is the same in both cases. Note that when we take a weightbearing cast the deformation of the soft-tissues that occurs during the casting process will be a function of the material properties of the casting medium employed. An "ideal" orthoses designed to reduce high areas of plantar pressure would be a device in which the plantar soft tissues would exhibit the same deformation as they would be in the unloaded state.

 

BTW, this is not true. In the barefoot condition the arch area of the foot would not be in contact with supporting surface but when standing upon the orthosis this area of the foot would be in contact with the supporting surface. Thus, the contact area of the foot would be increased with the orthosis. Viz. in barefoot standing the normal forces between the foot and the supporting surface would act over a smaller area and would all be vertical. In standing on the orthosis, a greater area of the foot would be in contact with the supporting surface and the discreet forces at the foot-orthosis interface would be angulated from vertical (mg x cos angle) both of these factors will reduce pressure since: Pressure = normal force / area.

Now, during dynamic function....

 

You know, all shoes used to be straight lasted until technology improved. Maybe, we are in the "straight last" stage of orthotic casting evolution. I cast several ways.
Full weight bearing for accommodation of arthritis with hyper pronated feet with a lot of bony prominences. Semi-weightbearing for accommodative soft supports for diabetic protection.
Non-weightbearing for correction.

To answer part of the original question, yes particular joints (esp in the fore-foot will often be in a different place once the patient in in the shoes and standing or walking. I often make the basic support and then mark the patient's foot with lipstick to determine the spots to add relief modification after they come back for their pick-up appointment.

 

I thought exactly the same thing, but...
In this hypothetical situation if there was contact enough to cause a change in area, then the congruency would not be 'perfect'- there must be increased pressure in the arch and decreased in the contact areas (heel and forefoot) and thereofore there must be a slight difference in the shape relative to the foot. I am viewing this as a completey rigid foot in a static position, with a no changes of force. If you 'spray painted' on a rigid shell while this subject was weight bearing, then you would have the congruency, and no change in plantar pressure- probabaly the same of you cast the foot directly in liquid plaster.
This was my point about the method of casting/ manufacture being significant.
This is obviously purely theoretical, and probably not what you were thinking with your original question. In any real sitaution you have forces acting on an orthosis even in a quiet stance position, and the foot is not completely rigid- for that reason alone you are correct that there would be a change in pressure due to contact area.

 

Craig,

I think you are missing the point. In barefoot weightgbearing the arch is not in contact therefore has no pressure on it. On the orthosis, the arch is in contact therefore has pressure distributed to it. There does not have to be a change in position or total load for this to occur.

 

Think about it this way Craig: take a high heel shoe with a heel contact area of 2cm x 2cm measure the pressure between the shoe and the ground. Now take a knife and carve off the heel until it has a contact area of 1cm x 1cm. The foot in the shoe hasn't changed position, the person wearing the shoes hasn't lost weight, but the pressure between the shoe and the ground will be increased in the carved down heel. The reverse is true in the orthosis example, we increase the contact area at the interface between the foot and it's supporting surface with the addition of an orthosis.

 

Simon
You are actually missing my point- I understand all of this-

-that is not difficult at all- in fact very simple.
What about this...
If you had a flat bottomed container filled with liquid plaster and put this shoe and foot into it so it was fully weightbearing. You would have maximum pressure under the forefoot and and heel, and minimum pressure in the space between although full contact with the plaster. Would this change as the plaster hardens?
My point was more that the effect on pressure and congruency would be very dependent on how you defined congruency or how it was achieved. I still think you could have total contact with only a minimal change in plantar pressure - but perhaps that would then not be defined as congruency?
(I actually said no change in plantar pressure, whereas I should have said is that it would be possible to have congruency with minimal change in plantar pressure- not very likely, but, like I said, I was talking theoretically...)

 

Craig, congruency, as I stated in a previous thread doesn't tell us much about the interface pressure, the two surfaces could just be touching or could be pressing into each other with substantial force. Both would be congruent, but the interface forces would be very different. However, this is not related to the moot point, nor is your example above. The moot point being that you said the pressure at the interface between the foot and the floor in standing barefoot on the floor would change minimally from the pressure at the interface between the foot and a rigid congruent orthosis shell in standing. I do not believe this is true for the reasons I have outlined previously. Please forgive my simplistic explanation of this but it is a simple concept, I'm just trying my best to work out why you don't seem to get it?

BTW the pressure of the foot and shoe suspended in a fluid would be equally distributed, which is why divers don't implode.

 

Simon and Craig T.

I think that I see both of your points and can illustrate (I think) with the following example. I've made 2 pair of orthoses for my feet from an impression from a foam box. One impression was taken full weight bearing standing and the other was taken partial weigth bearing while sitting. No attempt was made to manipulate joint position when casting. When standing on them there is less arch contact pressure in the devices made from the fully weight bearing casts. There is contact in the arch in the full weight bearing devices, but much less pressure there and it feels as if there is more pressure on the heel and the ball than in the arch. This occurs even though the device is the same shape as the weight bearing foot.

I would explain the above by saying that in the full weight bearing cast the plantar tissues were fully deformed when casted and the shape achieved from that cast will need to have the soft tissue under the heel and ball fully loaded and compressed in order for the arch of the device to come into contact with arch of the foot.

Pardon, if I missed your points.

Regards,

Eric

 

Eric,
I have no problem with your explanation above as it agrees with my point re: deformation of the tissues and stress/ strain. However, I do not think this addresses the point which Craig and I seem to have got stuck on. Which is that Craig does not seem to agree that the pressure at the foot orthosis interface upon a congruent device would be different from the pressure of the same foot barefoot against it's interface with the floor. To be honest this has been a road block to the discussion I wished to develop. If you can help, please do. I don't know how to say that the surface area over which the load will act when the cavus foot is standing on the congruent orthosis will be greater than the area over which the load will act in the same "naked" weightbearing foot when standing on the ground in any other way. P = f /a, a will be greater standing on the orthosis due to the contact between the orthosis at the medial longitudinal arch than it will when standing barefoot on the ground. This seems too basic to be arguing over to me?????????

 

Hi Eric
You have exactly got my point! Thank you for your real world example.
Simon-

I never said that the surface area would not be greater, nor did I say that there would be no change.
You asked a hypothetical question, and I gave a response to try to demonstrate that simply saying...

...is not enough information as the effect of such a device on plantar pressure is so dependent on HOW this congruency was achieved.
In Erics example above, both devices would be congruent, but the plantar pressures would be very different.
I guess the problem is that I have tried to give an extreme model to demonstrate how different the effect on plantar pressure could be.
I am sorry that you feel this has been 'a block', but I think it is important.
So... for the record I agree that in the real world any rigid device that was manufactured to be congruent- weight bearing or non-weightbearing- would have a significant effect on redistributing pressure.
However you original question...

Cannot start to be answered with the information you have given.

...which is why I said weight bearing rather than suspended. Or is this comment just for general interest?

 

Craig, I disagree since you said:

Which sounds pretty much like you did not think the pressure would change from barefoot standing to standing on an orthosis to me! Which is why I've just spent too much time trying to go over very basic physics with you.

You then said:

Which suggests that at this point you believed that a device cannot be congruent if the plantar pressures are changed. Which is why I wrote: "Craig, congruency, as I stated in a previous thread doesn't tell us much about the interface pressure, the two surfaces could just be touching or could be pressing into each other with substantial force. Both would be congruent, but the interface forces would be very different. "

Yet now, after both Eric and I have posted further on this you are saying:

Which is right, but basically what I'd already said and not what you'd previously been saying.

The problem has been that you said:

Which is not true, as I pointed out, which then lead to you saying:

Which suggested to me that you did not understand the basic concepts, so now
I'm pleased for you that you now agree. I too think it is important, which is why I had spent the last few days going over very basic physics with you.

Yes it can, you've made a start at answering it here when you said:

Now consider what I wrote about stress and strain of the tissue and materials at the foot-orthosis interface and you have more than a "start". I'll come back to this discussion when I can find the time.

 

Hi Simon have you got the explanation It would be great to look closer at this.

 

Simon and Craig T:

I have been trying to follow your discussion here. :wacko: Maybe one of you can provide all of us struggling to follow along an illustration here to clarify things since I think this may also help each of you better understand each other.

 

Hi Kevin
Confusing yes, but I think we are on track now
I look back on the posts now and, knowing what I was trying to say, I think I know where the issues lie.
In simon's original hypothetical-

I have got caught up in the question of- 'congruent' vs 'matches exactly the contours of the plantar foot'.
While a device that matches the contours of the foot will be congruent, it can also be congruent without being an exact match to the contours- soft tissue compression causes congruency (in this example of a rigid device) if it is not a perfect match to shape of the foot.

In this hypothetical, how congruency was achieved was not what I should have been asking, but rather how you matched the plantar contour of the foot.
So when I said

I should have said it must not be a perfect match to the plantar surface of the foot. If it was a static weightbearing cast, then there would still be significant pressure (in this example) under the heel. 1st and 5th MTPJS.
Yes- there would be a change in pressure overall, but this would be minimal.
On the other hand there would be a significant change in pressure if the plantar surface was perfectly matched to the NWB foot before loading. Again it would be congruent, but vastly different effect.

So- in summary- congruence tells us very little (which is what Simon has been saying).

And yes Simon- you are right I did say 'no change in pressure' in my post- I should have said 'essentially no change'... I also said minimal change in the same post and subsequent posts, and corrected the fact that I said no change in a later post... you would make a good wife

 

Thank you Craig, I think we started going down a cull-de-sac, for no real reason because essentially we are in agreement, and essentially always have been. I can now see where your confusion stemmed from, in the original question I was trying to explain that the rigid device was moulded to the shape of the foot, while the flat device achieved congruence through deformation, I probably shouldn't have used the word "perfectly".

The point is (and I think this is what you were trying to say) if we take two devices which are congruent (i.e. the entire dorsal surface of both orthoses are in total contact with the plantar surface of the foot), if no deformation takes place at the foot-orthosis interface it would not matter what material the two devices were made from, the redistribution of pressure would be the same for the two devices so long as the contact interfaces had the same area. (My point was that the area of the interface would be increased from the barefoot state so pressure must be different between the barefoot to orthosis conditions:hammer. However, we could have two devices made of the same material which are both congruent: one which causes x amount of tissue deformation and one which causes y amount of tissue deformation, both devices are congruent, but the pressure at the foot-orthosis interface will be different between the two. In the example I gave the two devices achieved congruence in different ways, via differences in their deformation. So the manner in which congruence is achieved is significant. In other words the strain in both the foot orthosis and the foot are significant in orthoses pressure characterisation. This was my point.

 

Now, during dynamic function; lets take our two devices again: 1 very stiff, 2, very compliant. As the foot strikes the ground and "collides" with the orthoses the compliant device will deform to a greater extent and over a longer duration than the stiff device- right? What difference will this make to the stress /strain in the tissues of the foot? (remember- the tissues are visco-elastic). Viz. what is the relationship between the load-deformation of the orthosis and the load-deformation of the foot?

 

Simon and Craig:

Thanks for explaining yourselves better now....I'm finally now able to follow along. :drinks

One of the problems lie in what we consider to be the definition of congruence. The foot orthosis that is congruent to the plantar foot (i.e. three-dimensional (3D) contours of plantar foot match the 3D contours of the dorsal orthosis surface) in a non-weightbearing setting, such as in supine neutral suspension casting position, may still have contact to the plantar foot when that individual stands down on top of the orthosis, but once they are weightbearing on the orthosis, should we still use the term "congruence" to describe the close fit of the medial longitudinal arch of the orthosis to the plantar foot?

This is a complex issue with movement of joints, compression of soft tissues which all will change in static vs. dynamic function of the foot. I suggest we first work out a concrete definition of what congruence is, or many will not be able to follow along with the discussion due to the confusion that may be created.

 

Wow, so many variables. Is the bottom line the amount of force applied to the foot in the shape the foot is in when it is on the device? I could still see that you could theoretically arrive at the same forces applied to the foot in the same position with a compliant device that started with a higher arch and deformed to the shape of the stiff device. However, it would be pretty hard to calibrate the rigidity of the high arched device so that it ended up at the height that you wanted or that the patient needed.

I had a teenage girl in the other day who wanted more arch in her orthotics. Very flat feet. Very medially deviated STJ axis. I added more arch by a rearfoot varus wedge under the post and temporary felt in the arch. I'm very curious as to whether she will be able to tolerate the pressure. I know some people could, by I don't think that I would. The ability to tolerate pressure in the arch is another variable in the equation.

If we knew both the optimal position of the foot and how much force should be applied in that position then we could save a lot of effort in prescribing orthotics. However, there still is a lot of guesswork in making orthotics. It is important to realize that you can tweak the rigidity of a device by adding or removing rearfoot post length.

Cheers,

Eric

 

I agree the term "congruent" is tricky. The concise OED gives:

congruent adj 1. agreeing 2. Geom. (of figures) coinciding exactly when superimposed.

So:
coincide adj 1. occur at or during the same time 2. occupy the same portion of space 3. be in agreement, have the same view.

superimpose lay (a thing) on something else.

So to my mind a congruent orthosis is one in which the entire dorsal surface of the orthosis is simultaneously in contact with the plantar surface of the foot during weightbearing. Congruence may be phase of gait dependent and the degree of tissue and / or orthosis deformation that has occurred in order to achieve total contact at the foot-orthosis interface is unknown.

However, as an after thought, a device may be congruent with some areas of the foot at some time during gait and congruent with another area of the foot at another time during gait. So perhaps we don't need the entire surface to be in contact; perhaps we can specify a certain part of the foot and a certain part of the orthosis which come together to be congruent at a certain time. That's about as clear as mud: blame the OED, not me.

 

I agree, this is complex and I'm not going to suggest I have the answers yet. One of the things I was thinking about was the rate of loading of the tissues and deceleration of motion. With the stiff device the loading rate and deceleration should be faster than the compliant device. The loading rate will influence tissue stiffness and their ability to store energy.

Obviously if the two devices started out with the same surface geometry, the compliant device should allow more motion to occur before equilibrium is achieved. Thus, as you infer, the foot may be in a different position at this point when the two devices are compared. Also as you said, we could start with two different shaped devices that have the same shape at their point of equilibrium with the foot. But the loading rate and decelerations of the foots joints and tissues would have been different between the two devices. Therefore, the stress/ strain curves for the visco-elastic tissues may be different also.

I'm thinking out-loud here, so forgive me if it gets a bit difficult to follow. It's clear(ish) in my head, but this thread has taught me that I should not presume that what I write is always clear to the reader.

 

I wanted to edit this but my 120 minutes were up (which goes to show that I spend too much time thinking about this ****). Anyway here's what I was going to add:

Given the above, if we need to define the amount of deformation that has occurred at the foot-orthosis interface in order to achieve congruity, then I suspect we need another term. Rambling now, I'll see what y'all think in the morning.
i

 

So, we need to consider impulsive forces. According to Newton’s second law of motion, rate of change of momentum is proportional to the force that produces it. Thus, if momentum changes rapidly, the acting force must be comparatively great, if it changes slowly the force must be comparatively small.

If during gait the foot impacts upon the compliant orthoses, the foots momentum is arrested less slowly; in other words the foot delivers up its momentum slowly, and the force of the impact is small. If the foot impacts with the stiff orthoses, its momentum is delivered up rapidly, the force of impact is consequently greater. The force of the impact is therefore measured by rate of change of momentum.

 

And, if the foot is "harder" than the orthosis, the orthosis will deform more than the foot = less strain on foot. But if the orthosis is "harder" than the foot, the foot will deform more than the orthosis = more strain on foot.

 

Hi Simon,

I've been following this along (or at least trying to but truth be told alot of it is a bit over my head) but this statement has really made me think hard about some stuff. I have to be honest I had never thought in this way before, coming more from the background of 'it has to be rigid enough to be worth being there'. Just to be clear is the suggestion here that a more rigid device will result in more strain/deformation of the foot?

Ian

 

I think so, relatively speaking. Basically as the foot and orthosis come together due to impact with the ground they will both deform until equilibrium is reached and they start to move apart from each other (restitution). Think about bouncing balls made of different materials off floors made of different materials. Or try this: make a foot out of play-doh and impact it with an orthosis made out of poly-prop, then make a foot out of polyprop and impact it with an orthosis made out of play-doh, note the differences in deformations that occur- get the idea? In real-life situation the rate of loading will be important too because of the visco-elastic nature of the tissues, as will some of the factors Eric and I talked about last night. It's really comes down to forces, motion and foot position and which of these are important in injury- stiff devices will do some things well. Compliant devices will do other things well. Somewhere in between should give us a broad sweep. But it might be necessary to use either a very stiff or very compliant device in certain individuals.

 

I agree Ian intersting.

If we take a cross section of the foot we have various tissue with a degree of elastic properties. We also have bone which does not have the same properties. If the harder/stiffer device causes more strain deformation of the foot wont we also have a deformation from another direction which will create a sandwitch effect of the soft tissue. If the bone is moving if we say opposite from the orthotic push we will shearing stress both from the orthotic foot and bone soft tissue.

So we also need to considered reducing the amount of bone change because this will also provide deformation of soft tissue internally.

I´ll put this out there I might be about to be shot down.

 

Absolutely. Remember the stress/ strain curves for the soft-tissues will not be linear, so with enough load and enough deformation these will become stiffer, reducing the effects you outline.

We can also think of the foot's stiffness as a "whole" which should take into account the tension/ compression in the soft tissue, the position of the bones etc. It's the "net" hardness of the foot I was talking about.

As I said in my last post, other factors will be important. Take for example a stiff device and a compliant device exhibiting the same surface geometry in their unloaded states. Lets say that during gait with the stiff device in situ the subtalar joint pronates to 8 degrees, walking on the compliant device the subtalar joint pronates to 12 degrees. The deformation in the tissues that limit subtalar joint pronation will be different between the two devices, with greater deformation of these tissues in association with the compliant device. As I said- its relative!

If we started out with two devices, one stiff, one compliant of different surface geometry in their unloaded state, but achieved equilibrium with the foot such that the foot's position was the same on both orthoses, what differences might we predict?

 

here goes.

The device with the greatest congruecy will need to exert less presure on the foot because
Pressure = normal force / area. Taking into consideration what Kevin stated about different areas of the foot being able to take different pressure.

Time as it may take the compliant device longer to achieve equilibrium due to less deformation of soft tissue.

I´m sure there is more...

 

Michael, I don't understand what you are trying to say here. If you are trying to answer the question I posed at the end of my last post, I was talking about variation in the stiffness of the device. Lets ignore compliance, if we can, for a moment although they are obviously linked. Now I'm confused!

Ahh, when you say "Time" you mean there will be a difference in the loading rate! Right. So the tissues will be less stiff in the compliant device and therefore able to store less elastic energy. Anything else? What will this do to the force impulse?

 

yep was so here goes again. The complient device got the same result with less deformation of soft tissue than the stiff device.

 

Im going to have dinner and think back later I´m a bit confused for a minute and the wife a bit upset the dinners cold

 

Me too, I edited my last post maybe after you had responded. I've got jerk chicken, what you got?

 

Swedish meatballs in tomato sauce with creamy mash spuds.

 

Ok to start again.

at the minute we forget about congruecy (we will be back)

Loading rates (thanks simon for translating to a better phrase from time) compliant will take longer time to get desired equilibrium.

I´m guessing here that the stiff device will have a force impulse spike where as the compliant will be less on a spike more like an inverted U

The complient device got the same result with less deformation of soft tissue than the stiff device.


Hope I´m less confusing now

 

Classic as sold in IKEA. Plank steak is my favourite Swedish dish!

Stockholm, Stockholm! Happy memories.

http://www.youtube.com/watch?v=OxrFvy3lyg8

Stockholm- New Fast Automatic Daffodils
Lately, lately, I find I rush.
Can't piece together the sun in the sky or the spots on my face.

And I must be stuck, my feet already are, I must be lost, my feet already are,
And I must be fast, my feet already are, you'll soon be dust your deeds already are,

Chorus:

You'll soon be dust your deeds already are,
You saw no orb no fiery bushes either,
I must be drunk I feel unsteady,
No monster me, sadly no saviour either...
Walk back into Stockholm - Stockholm -

Our travels take us further north,
where there's fish in the sea and food on the shelves.
And I must be cold, my feet already are,
You must be old, so take it easier,
And I must be cold, my feet already are,
You must be old, so take it easier...

[chorus]

Walk back into, walk back into, wicked, wicked, wicked...
Belly, belly, and slap
I love to feel the food in my guts and the fat in my veins.

[chorus]

At the moment "I can't piece together the sun in the sky or the spots on my face" I'm worn out -I'll come back to this tomorrow.