The MOSI

[robertisaacs]

I like it...but not the way its placed on a bin

There is NO pleasing some podiatrists

Hows this then?
Its a new range of MOSI. Not mini nor midi nor even maxi

I present (drumroll please)

The DANCE mosi.

tinypic.com/flek.php?f=2lm5rb4&s=2

What do you think. Better? worse?

Regards
Robert

[dtt]

;D ;D ;D

Nice one !!
D

[ianl]

OK Robert, since when have you been spying on me at Salsa lessons!!!

Ian

[podiathing]

Robert, how did the fitting go?

Paul

[robertisaacs]

Robert, how did the fitting go?

Tolerably well.

Comfort wise all seemed fine, the patient reported no discomfort. I do have a mild concern with footwear, because (on our advice) he has purchased shoes which are both wide and soft i'm not 100% convinced that the wedge won't push his foot into the lateral part of the heel of the shoe.

I have booked him for a review in 2 weeks so we'll see. If that IS the case i shall have to pad the inside of the heel.

I'll keep you all informed.

Regards
Robert

[podiathing]

i'm not 100% convinced that the wedge won't push his foot into the lateral part of the heel of the shoe.

This could be very interesting, as by incerasing the vertical force (Fy) by improving the alignment of the post it decreases the horizontal force (Fx). Theoretically there should therefore be less shear laterally off the orthosis.....

Forgot to reply to....

There is NO pleasing some podiatrists

I'm actually really honoured at how you have had a go with this with one of your own patients and been happy to discuss it in such an open forum. So, thank you.

Anybody else had a try?

Regards

Paul Harradine

[davidsmith]

Paul

I'm actually really honoured at how you have had a go with this with one of your own patients and been happy to discuss it in such an open forum. So, thank you.

Anybody else had a try?

I have ben using the Amfit CAD CAM system for many years and usually skew the rearfoot post to suit the foot biomechanics. When using Talar Made to make an orthosis I have also requested a kirby skive to be set at a certain skew that more closely approximates the STJ axis angle, which is difficult since it changes thru gait. But you obviously get the picture. Never done a MOSI exactly as you describe tho. I usually extend towards the navicular with my own designs.

My main concern is that the force vector in a dynamic situation, ie in gait, will be +Fy +Fz and - Fx when considering a right hand rule protocol. In other words the force vector will be in an upward, lateral and posterior 3D direction.
The STJ axis is also vertically inclined as well as internally rotated (in the type of foot of interest in this discussion) If the CoP progression is from lateral posterior heel at heel strike and moves toward the centre of the heel at early weight bearing then forward roughly thru the centre of the foot in midstance. For our purposes the effective time of operation for the orthosis ends at the propulsive stage, assuming there is no propulsive posting etc.
Therefore during the braking stage the force vector and its progression thru the foot are opposite to that which one might imagine would be optimal for the MOSI to be effective.
More simply put the foot is moving away from the post and not on to it, which intuitively would seem more effective.
In stance the MOSI may work very well since the force vector is only +Fy ie vertical. For the MOSI to work during gait I would expect to see a significant change in CoP progression toward the medial heel but I'm not sure this is possible. There's a nice little research project right there. Except for the fact that one would probably require to use an insole system which in my opinion are pretty unreliable for CoP progression when used in the foot - orthosis interface.

Paul Have you done any static 3D model analysis to see how the MOSI might perform. May not be too valid sice you probably wouldn't have a data set using a MOSI. Doh!

Nice work and you might be onto a winner.

Cheers Dave

[podiathing]

Hi Dave

Before i get into any real detail, do you agree that in using an orthosis to influence the COP in a foot with a medial STA, you will need to apply moments to the medial side of the axis? Or just to the medial aspect of the COP?

Regards

Paul Harradine

[robertisaacs]

We interupt your enjoyment of this thread for this important announcement.

Dave, being the obscenly clever bloke that he is, has dropped the "right hand rule" into the debate. I do not think i speak out of turn if i say that most of us sub MSc types have not come across this as part of the degree. You forget, Dave, that you, like enron, are probably the cleverest man in the room!!!

So for the benifit of those like me who would like to understand what the hell you are talking about!:-

THE RIGHT HAND RULE



This is the terminology which clever folk like dave and paul use for discussing biomechanics. Its just a quick and easy(?) way of discussing directions.

X is the direction of travel (forward and back to you and i)
X + is forward, X - is backward

Z is the side to side direction
Z + is medial to lateral Z - is lateral to medial

Y is up and down.
Y + is up, Y - is down

F means force

So for example Fx+ is more force forwards, Fx- is more force backwards

I think thats right. If not somebody please correct me.

We now return you to your regularly scheduled programming

Regards
Robert

[robertisaacs]

My main concern is that the force vector in a dynamic situation, ie in gait, will be +Fy +Fz and - Fx when considering a right hand rule protocol. In other words the force vector will be in an upward, lateral and posterior 3D direction.

Again, for clarity dave, .

It took me a while to figure out what you were talking about, i suspect i'm not the only one struggling.

Is what you mean the following?

We are looking at the direction (vector) of ground reaction force (the way the ground presses into the foot).

So when the heel lands the foot is traveling forward (Fx+) which means the ground pushes backward against the foot (Fx-).

The foot is traveling down into the ground (Fy-) which means the ground pushed UP against the foot (Fy+)

And the foot loads from lateral to medial (Fz-) which means the ground is pushing medial to lateral (Fz+)

Is this right?

Cheers
Robert

[davidsmith]

Paul

Hi Dave
Before i get into any real detail, do you agree that in using an orthosis to influence the COP in a foot with a medial STA, you will need to apply moments to the medial side of the axis? Or just to the medial aspect of the COP?

To change the moments about an axis there needs to be a change in the sum, position or direction of forces.

In gait the integral of forces or force impulse are finite, we can however change the magnitude, timing, direction and position of the forces by an intervention eg and orthosis or shoe type.

Considering CoP in a flat plane, as is often done, is erroneous.
The CoP is a ficticious position where ALL the forces (not just vertical Fy) acting on a surface, if summed, have a combined single resultant vector acting on a single point. (As I'm sure you are aware Paul but I'm just keeping this clear for everyone). The key word is VECTOR the force has a 3D direction and that direction is seldom entirely vertical, ie not in the global reference frame. Even when it is mostly vertical the moment arm available to a large vertical force is often small and conversely the moment arm available to a horizontal force is often relatively large in terms of the STJ. Therefore resultant moments of Fx, Fz, or Fy can be equally important to the axis of interest. (Where Fx = forward and back, Fz = side to side Fy = vertical.

So to get back to your question about CoP.

The CoP could be in the same place but the resultant force vector could change and so change the moments about the axis of interest.
Therefore to change moments about the STJ axis we need not necessarily change the CoP although it is likely that we would.
We do need to change the magnitude or timing or direction of forces acting about the STJ axis at a certain time of interest.

Therefore to reduce pronation moments we need to increase supination relatively.

Not an entirely simple answer but then it was not an entirely simple question as it might seem at first.

This was why I asked in my first post if you had considered a 3D analysis of forces in terms of the MOSI addition. Intuitive consideration of the CoP alone (which is usually in a single plane ie transverse) might be misleading to any conclusions about the MOSI action potential.

All the best Dave

[podiathing]

Hi Dave

Not an entirely simple answer but then it was not an entirely simple question as it might seem at first.

I know, and thank you for a great reply. My quesiton was left a bit purposefully open, i wanted to see where you were coming from! I think too often when CoP and STA get discussed together people get confused between the two.

Therefore to reduce pronation moments we need to increase supination relatively

We do need to change the magnitude or timing or direction of forces acting about the STJ axis at a certain time of interest.

Yes, and as the axis moves medially a post more inline with this will apply forces more optimally. It may have greater effect in stance, and when the foot is most pronated in gait (and so the axis most medial), but this is still a theoretically greater effect than a medial skive or rearfoot post. Seems ideal?

To answer some of your first points, i don't often see a 'normal' CoP in the type of feet i'm prescribing a MOSI for. I see CoPs that start more medially and stay medial with a short excursion, i see CoPs that start lateral and move lateral and I see CoPs that start lateral and move medially early.

Now, being honest, i've not done an F-Scan in-shoe on this type of patient since i've been using the MOSI, but i will now! Hopefully i'll get a couple in today. However, i have been seeing changes in pronation angles using the MOSI on a semi-rigid shell. Also improved symptom relief. This is why we published our findings. Dave, you also state you have been using a similar idea on your rearfoot mods, and so must have been seeing some benefit too? Do you feel our explanation in Podiatry Now was in the right direction, or is there another way it could work? This is very interesting, and why it's great to be able to use this maibase to discuss such ideas openly.

This was why I asked in my first post if you had considered a 3D analysis of forces in terms of the MOSI addition. Intuitive consideration of the CoP alone (which is usually in a single plane ie transverse) might be misleading to any conclusions about the MOSI action potential

No, i haven't done this. I don't really have the resources. It would be very interesting to do though.

And Robert, thanks for the great explanations as well.

respectfully

Paul Harradine

[davidsmith]

Paul

Dave, you also state you have been using a similar idea on your rearfoot mods, and so must have been seeing some benefit too?

I appear to have relatively good succuess with my prescriptions and most people get better. I make my prescription according to how I judge the optimum design to be. Of course I don't make 2 different pairs and compare the outcome. I just give the patient the design I think will work best for them. So without research its difficult to give an objective answer to your question. By my experience I sem to be doing the right thing which might indicate that modifications that I use such as skewing the angle of the posts may be more effective than orthoganal posting.

Do you feel our explanation in Podiatry Now was in the right direction, or is there another way it could work?

Your theory appears sound but once published it begs to be proved. Empirical clinical evaluation is one thing robust scientific evidence is quite another. Ooh! I'm begining to sound like Robert. (hey! not a bad thing - - mostly ;D)

Perhaps you could do a kinematic study of rearfoot angular displacement with and without the MOSI and compared to a regular posted orthosis. If you were sponsored for the cost of hiring a motion analysis lab, the time of data collection and processing would be quite small.

Cheers Dave

[podiathing]

I appear to have relatively good succuess with my prescriptions and most people get better. I make my prescription according to how I judge the optimum design to be. Of course I don't make 2 different pairs and compare the outcome. I just give the patient the design I think will work best for them. So without research its difficult to give an objective answer to your question. By my experience I sem to be doing the right thing which might indicate that modifications that I use such as skewing the angle of the posts may be more effective than orthoganal posting.

Thats the joy of the materials / methods the authors to this paper use. i can take the post off, remold and put the post back on in about 8-10mins(Which we did in the small sample research i discussed previously). Hopefully, when i've had time to do the reliability data, a larger sample will be forthcoming. We are using Dartfish at the moment, and it should supply enough data for a preliminary study.

Your theory appears sound but once published it begs to be proved. Empirical clinical evaluation is one thing robust scientific evidence is quite another

Totally agree with this. Bit we need to remember that not many orthoses modifications have this research either.........i'm not saying this is a good thing obviously!

Regards

Paul

[davidsmith]

Robert

Yes thanks for those excellent explanations of axis reference terminology. (you really ought to stop giving me more credit than I deserve and get on with a higher degree yourself you have an exceptional way of thinking and the motivation necessary to follow up your thoughts with research)

Did you draw the hand diagram yourself? If you do it again it is better if the 3rd finger (the one next to the index finger) point towards the observer.

Hang on let me explain; (Tell me how to paste a diagram in this forum PLEASE)

Hold your right palm up so that it faces your face. Make a fist. Extend your thumb so that it points to your right. Extend your index finger so that it points vertically, extend your 3rd finger (next to youur index) so that it poit directly at you face. These fingers indicate direction and sign of the axes X, Y, Z. F denotes it is a force in that axis, the sign (+/-) denotes the direction. So imagine that someone walks in the direction of your thumb, as it is now. That is forward and +Fx, your index finger points upward in + Fy and your 3rd finger points to the right (relative to the forward direction of your thumb) = +Fz. Negative signs are obviously in the opposite directions.

Now here comes the really good bit. Direction signs of moments.

With your hand in the same position and considering the finger representing +Fz (third finger) Turn it as though you were doing up a right hand screw thread, This is +Mz. Do the same with the other fingers the direction of rotation is +Mz. Another way is to look along the finger (axis) of interest in the positive direction and the +M is a clockwise direction. +My is a positive moment on the y axis.

Not confused yet ---

Now in the global reference frame (the orientation of the room your in for instance) the directions are absolute and not related to which side of the body you view the subject from.

Therefore +Fy is always vertically upward and +Fx is always forward (of course) but +Fz is always to the right and not defined by medio-lateral or lateral-medial. So on the left foot the +Fz (left to right) direction is lateral to medial and on the right foot the +Fz is medial to lateral.

This is only one popular convention however. Sometimes Fx is vertical and Fy forward Fz is almost always left to right.

Now if we move ito the local axis set they are relative to the limb of interest and are not fixed in the global axis frame.

Therefore if we imagine that the tibia of a person standing is vertical and assigned the axis reference y the longitudinal axis of the subjects foot is assined axis x and the flexion extension axes of the ankle and knee (assume they are congruent and pin type) are assigned axis z. If the subjext now flexes his knee 90dgs, the local axis set moves thru 90dgs in the saggital plane. The z axis is uchanged but relative to the global axis the local y and x axes are changed. Now +y is forward and +x is downward, relative to the global axis, but actually in the same position relative to each other. Add some forces and moments into this and now you could start to feel your head spin. You can see why a robust system or protocoal for defining kinematic action is required to stop any ambiguity.

Robert your explanation of applied forces was spot on.
Usually when speaking of forces we talk in terms of force applied to the limb and assign the relevant sign +/-F,y,x,z. These usually start in the global axis and are roted by trigonometrical functions into the local axis set if required.

The local axis set is considered when we want to evaluate the moments about a joint in its true anatomical (local) reference.

So if we look at the moments about a knee in terms of the global reference the moments about the kne joint are expressed in terms of z axis orthoganal to the global axis, However the knee may be abducted so that the anatomical joint is 30dgs oblique to the global axis. Therefore the moment about the anatomical knee joint is changed by the trigonometrical function of 30dgs.

Hope thats all very clear

Luv you all Dave