Supination resistance and orthoses prescription

I'm with you on this one, FTSE (whoever you are). The geometry of the orthosis largely determines the reaction forces on the plantar foot from the foot orthosis. In the case of the polypropylene orthosis with a large medial heel skive with either a polypropylene or high durometer EVA rearfoot post, the varus geometry will cause increased reaction forces medially since this part of the orthosis is more prominent (i.e. more thick) which increases the compression forces on the skin, adipose pad and bone of the medial-plantar heel.

Reaction forces such as that caused by a foot orthosis or even by flat ground are largely determined by physical contact with the contact surface and the geometry of the contact surface. In uneven surfaces with both high points and low points (imagine the feet walking on a road with many small pebbles or the buttocks sitting on some large uneven rocks), the high points of the contact surfaces will naturally receive more reaction force than the low points when the body part contacts that surface since these high points are not only physically contacting the body part first, upon initial contact, but will also receive the most reaction force under maximum loading since their physical prominence will cause more tissue strain and tissue stress than the less prominent low points of the contact surface.

I don't see anything confusing or difficult about this concept...it is one of the basic and fundamental physical properties of the world we physically interact with on a daily basis.:drinks

 

If only it were that simple Kevin. You've got to remember that where the heel cup of the orthosis is thicker, it has a lower spring constant (k). So per unit deformation, be those units inches or micro nano-inches, the reaction force per unit deformation is lower where the material is thicker. And in real terms if an area of the material is twice as thick as another area of the material it will have to deform twice as much as the other area to create an equal reaction force.

Lets take some objects for you to place your buttocks (fanny) upon. Start with a cube of soft sponge rubber 5cm cubed and place next to it a cube of concrete 1cm cubed, you sit on them, your buttocks contact the foam cube first followed by the concrete cube, which one will exert more tissue deformation from the time of initial contact to static equilibrium? Which will exert the higher reaction force?

Lets sit on two springs, one with a stiffness of K and a resting length of 1cm and one with a stiffness of 0.5K, and a resting length of 2cm, the buttock will contact the longer spring first, but which spring will cause the greater tissue deformation and which will exert the higher reaction force from initial contact to static equilibrium? Now add a third spring whose resting length is 3cm and spring constant is 0.1K, does this spring by virtue of it being longer create higher reaction forces from the time of initial contact to static equilibrium?

 

BTW, two identical springs both with the same spring constant (K), under loading one deforms one ten-thousandth of an inch the other spring deforms two ten thousandths of an inch, in percentage terms what is the difference in the reaction forces exerted by the two springs?

Two more identical springs both with the same spring constant (K), under loading one deforms one thousand inches, the other deforms two thousand inches, in percentage terms what is the difference in the reaction forces exerted by the two springs?

Get it? It isn't the gross magnitude of the deformation, it's the discreet variation in deformation per unit load (and variation in the spring constants (K) which come with variation in material thickness- i.e. the resting length of the springs) across the orthosis which will, in part, determine the distribution of reaction forces. So while the deformations in polyprop under loading might be small or even microscopic due to it's Young's modulus, this does not detract from the moot point. As I said this morning: "it's all relative".

How do dual density midsoles work in running shoes?

 

Another little thought experiment...

Look at the diagram which shows two reaction force vectors R1 and R2 sharing a common centre of pressure on a wedge of polypropylene. Both vectors were produced by an 10kg point mass impacting on the wedge with the same initial velocity: which impact force deformed the wedge more and why? Which reaction force has the higher magnitude and why?

Think I'll do an FEA model of this one for Manchester.

 

I really would like to see some evidence for this conjecture. Are there any studies which show initial contact is more medial with a varus wedged orthosis in situ?

 

Simon:

This problem, I believe, is relatively simple for a polypropylene orthosis with a polypropylene rearfoot post. All you need to know is what the final equilibrium geometry of the wedge is under each loading situation and forget about worrying about the "micro nano-inches" of deformation in the polypropylene material that basically undergoes no observable deformation under the loads of the foot.

For calculating the plantar reaction forces during the dynamic activity of gait with a more deformable orthosis material such as an softer EVA material, the problem becomes infinitely more complex. The only thing I can offer you is that I would approach the problem as a series of quasi-static equilibrium conditions where for each increasing plantar load on the dorsal surface of the orthosis, the deformation of the orthosis under static equilibrium conditions could be calculated. Therefore, for each set of finite equilibrium loading conditions of the orthosis during the time of loading and unloading of the orthosis, the deformation of the orthosis could be calculated so that the spring-like qualities of the orthosis could be derived from combining this series of quasi-static determinations into a dynamic model that should allow an approximation of the deformations that the orthosis undergoes during the dynamics of gait.

This is one of the reasons I mostly prefer polypropylene orthoses with polypropylene rearfoot posts for my orthoses. I don't need to care if the medial aspect of the varus wedged heel cup of my polypropylene orthosis with a polypropylene rearfoot post deforms .006 mm and the lateral side of the same orthosis deforms .001 mm since it isn't mechanically signficant. The bottom line is the geometry of the orthosis material at each stage of the loading and unloading process and, for my polypropylene orthosis, the geometry doesn't change enough for me to worry about. I worry more about the sole of the shoe that the orthosis is being used in and I worry more about the geometry of the heel cup of the orthosis, of the numerous things I must concern myself with in producing quality foot orthoses for my patients. I really don't need to concern myself with the deformation of polypropylene material itself since, it is so miniscule, it is a non-factor in my deciscion making process regarding the changes in plantar forces that are necessary to produce the desired therapeutic results of the orthosis.

Simon, it is time to move on to a more practical and useful discussion. We'll definitely need to have a few beers in Manchester to figure this one out. I'm greatly looking forward to it.:drinks

 

Dr. Spooner:

I meant, in general, higher profile contact surfaces will contact an object first rather than their lower profile contact surface counterparts, all other factors being equal. Inside the shoe, all bets are off due to shoe fit, activity being performed, and orthosis geometry to name a few factors. If the shoe is laced onto the foot in the normal fashion, the orthosis will be in physical contact with the plantar heel even during the swing phase of gait so initial contact is a non-factor in these circumstances. In a looser fitting shoe, the heel will not be in contact with the orthosis during swing phase. If you were so interested in performing this sort of study, you would probably need to establish a contact force threshold between the orthosis and the foot in order to generate meaningful data.

 

Dave Smith did this with angular displacement of the wedge and uniform loading... the rate of change of velocity was identical across the wedge. i.e., the reaction forces were the same at any instant in time across the surface of the wedge...

With the greatest respect, you appear to be missing the point, Kevin. It's not the gross magnitude of deformation of the heel cup of the orthosis which counts, whether its six feet or 0.001mm, this matters little to this concept, it's the fact that variation in these deformations exist across the heel cup. Variation in the distribution of the magnitudes of the discreet reaction forces, and ultimately the position of the centre of pressure exist due to variations in load/ deformation characteristics across the heel cup- agreed? Think about how centre of pressure is calculated, Kevin. Moreover, think about how reaction force are generated. Ultimately, how does the geometry of the orthosis generate the reaction force? What is the science behind "object's pushing back"? Why don't I fall through the floor or hover above it?

The distribution and magnitude of the discreet forces beneath the foot are all about the rate of change of momentum.... Whether the orthosis is made of polypropylene or gold..... it's still about rate of change of momentum. How does the rearfoot wedge alter the rate of change of momentum?

Kevin, you've known me for many years now, do you really think that I would be pursuing this line of reasoning if I didn't believe there were practical and clinically useful ideas within it?

 

I don't think momentum is that important and can't agree with you on this one. It is the orthosis reaction forces that are most important. I guess we will need to agree to disagree. It's been fun Simon, but I can't afford any more time, of what little time I have, to discuss this topic further.

 

We are assuming that we contact the high side of the wedge when comparing the wedge to no wedge situations. If the relationship of the foot to the ground remains the same at heel contact in wedge versus no wedge situation then you have to hit the high side first. In walking, touchdown angle, the angle of various body parts at the instant foot contact with the ground, will vary with surface and interventions. However, this is a CNS mediated change and not a purely mechanically mediated change. So, I think it is a good assumption to consider the mechanical effect of the wedge to have more pressure on the high side.

The classic assumption was that the calcaneus had a flat bottom and that flat bottom would force the calcaneus to become congruent with any wedge placed under it. If memory serves, that was one of the things that led Kevin to do the anterior axial projection study. I've placed enough feet on wedges to be able to say that you will not get 5 degrees of inversion with a 5 degree varus wedge in the vast majority of feet.

The force will be higher because of the wedge effect. So, if you choose a material that is firm enough (I would bet most materials used in orthotics are firm enough), then the material will not deform enough on its thick side to load the thin side as much. Some force is required to deform the wedge. You could make a wedge out of spenco and see if the pressure felt higher on the high side of the wedge.

What observations from that paper were you referring to? In that study the subjects preformed to practice jumps before they were tested at each height. In dancers, I would bet that the CNS would be responsible for many changes seen across the various angle of floor.

So, if the heel changes orientation, the mechanical effects will be hard to measure and be different. One of the reasons that the mechanical effects will be hard do measure is changes in CNS activiation. I have seen the somewhat rare foot that responds to a varus heel wedge with more pronation in gait. They are usually feet with laterally positioned STJ axes and I sure wich I could measure peroneal muscle activity when I see this happen.

I agree that center of pressure is a blunt instrument. It's really hard to know if the changes you see are purely mechanical or CNS mediated.

Eric

 

We need only to look at in-shoe pressure measurement data with and without orthosis in-situ. My recollection of playing with these systems showed zero contact force in swing and an initial sensor cell "lighting-up" upon strike. Is the sensor cell which first lights-up more medial as the varus inclination of the rearfoot post increases? I'll be more than happy for anyone to post some data. Obviously sampling rate and sensor density will be important, but any data is better than pure conjecture.

Anyone?

 

Kevin, force = rate of change of momentum. This is Newton's second law of motion. So how can rate of change of momentum not be important here? Rather, it is quintessential here: the rate of change of momentum at the foot-orthosis interface determines the magnitude of the impact and reaction forces- end of story. Unless Newton was wrong? If he was, the world is waiting.....

 

No, you get contact between the foot and the orthosis first where the foot hits the ground. The highest point of the orthosis is probably in the medial longitudinal arch- is this were initial compression forces are seen between the foot and the orthosis?

Don't know what this has to do with the CNS, but lets assume we are talking about direct mechanical effects for now. If i placed a water filled ballon on the heel cup of the orthosis would the pressure within the balloon be higher on the high side?

No argument from me, but how is this relevant? It's about moments about the axis.

Eric, can you explain this wedge effect to me please?

So, then we have the situation I was talking about previously in the orthotic stability thread in which the line of loading lies outside of the central third of the foot-orthosis interface and we get a tendency for separation of the foot from the orthosis surface at the lateral side, and there is then a tendency for the foot to evert relative to the orthosis surface at an axis of rotation at the foot-orthosis interface.

The observation that force increased medially with lateral angulation and laterally with medial angulation.

Like I said, I'm not sure of the CNS thing here. For now I'm just trying to get my head around direct mechanical effects.

Thanks Eric.

 

Not necessarily. In gait, at heel contact, the force will be on the heel. It's not possible to get significant force on the meidal arch of the orthosis until the anterior edge of the orthosis hits the ground. So, a varus heel wedge will still have the highest point of contact, on the heel, medially, so there should still be more force medial and less lateral with a varus heel wedge compared to no wedge.

Pressure in a balloon is a lot different than pressure from a wedge. I believe it is Poiseuille's law that says pressure is related to the radius of the ballon wall. The bigger the radius the higher the pressure (When you squeeze a ballon, the part of the ballon that bulges out has a bigger radius and has increased pressure.)

Practically, I don't see how it's possible to get the center of pressure outside the central 3rd of a heel cup. I don't see how it will shift that much. In an unposted device with a varus wedge effect heel cup (medial heel skive) you see deformation of the heel cup. I'm assuming there is a slight medial shift in the center of pressure of force between the heel cup and the foot. The force from the floor vs bottom of orthotic is lateral to the center of pressure at the orthosis foot interface and this creates a force couple causing the cup to flex. I don't see how there is a tendency for separation laterally. I'll have to go back and look at your explanation of where the tendency of the foot to evert relative to the orthosis comes from. I don't think I understood it the first time I looked at it.

Eric

 

hi simon how r u i have just joined this my god u r a bright lad what do u make of the use of contactand stretch techniques , my wife (no20) had a forefoot valgus and fhl which i reduced , cured using the above is it ok to practice on your wife have i stumbled on some great secret and more importanrtly r u a happy chap
tim

 

For those that don't know, Timmy Harmy and I go way back to student days and drunken parties at Pevensey Road. Tim didn't live with us, but he used to sleep there. If you didn't watch it, he'd try and nick your bed for the night and anyone that might have been in there at the time. I had to remove him from mine on several occasions.:drinks:dizzy:

Tim,
Generally I'm good, although my mother is currently in hospital so this is playing on my mind right now. If you stopped stretching your wives you may not go through them so quickly. No, you don't a have permission to practice on my wife, its not OK- right?

Good to have you contributing here Tim, enjoy yourself. I trust you are on good form?

"That bog is ****ing disgusting, there's no ****ing toilet paper"- Tim Harmy emerging from the toilet at Pevensey Road one Saturday morning.

"You don't go in the toilet, so what do you care?"- Me

Take care.

 

Yes sorry to invade your forum .It is wife no 2 and i am reformed and tamed which is good u cant be an idiot all yor life but it was fun.sorry to hear about your mum , i wiil try to investigate Stretch and contract techniques in an appropriate forum ,by the way kim my wife has a doube 1st in maths maybe u need some high end matematics applied applied to your problem.I did once get her to look at some biomechanical problems and her conclusion was it is very complcated and some of the equations etc being applied were not appropriate or matematical correct all the best