The MOSI

[twirly]

Hi Dave,

I understood this bit --->

[glow=red,2,300]Hold your right palm up so that it faces your face. Make a fist. [/glow]

Then this bit lost me ---->

[glow=red,2,300]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.[/glow]

<sigh>



I just knew not joining the convent for bemused middle aged women was a BIG mistake.



twirls in a wimple....

[blinda]

if you`re looking for me, you`ll find me cowering in the blue corner................

[robertisaacs]

Oky dokey. Lets see if we can't get twirly out of her wimple (as fetching as i'm sure it looks on you) and Blinda out of the corner.

Don't be afraid, its only directions

Davids Biometrics right hand rule style kung fu position looks like this



Now, remember that x is generally the way you are traveling not pointing towards you as above. If you are bendy enough you can twist your hand around so your thumb is facing away from you. You will then see that your Z (ie middle) finger is pointing to your right.

However i had an easier time understanding it using my LEFT hand. The Right hand rule using your left hand style kung fu position looks like this



Make a gun with your left hand (thumb up and first finger pointing forward) then deploy your middle finger pointing to the right.

So, X finger points forward therefore x + is forward, x - backward
The y thumb points up therefore y + is up, y - is down
The z finger points to your right therefore z + is right z - is left

this


is eagle style kung fu. Irrelevant to the present discussion. But a nice picture i'm sure you'll agree.

Is that clearer?

Regards

Sempai Robert

[dtt]

Hope thats all very clear

Luv you all Dave

SMIFFY,DONT YOU EVER CALL ME FLAMBOYANT AGAIN AFTER WRITING A POST LIKE THAT. EVER, EVER ;D ;D ;D

Great thread ( but Ive got SUCH a headache )

EVER

Cheers
Derek

[blinda]

Thanks Robert, i now get the forces/left hand (you`re right, it makes more sense visually than using the right, tho hubby wants to know why I`m shooting my PC) rule and that they can be measured pretty effectively in stance with a MOSI.

I would be interested to hear what results Paul had using an F-Scan in shoe device with the MOSI.

Cheers,
(Homey) Bel

[robertisaacs]

Thought i'd update y'all on the patient i put a mosi in.

He came back after 10 days for a review. He said he had found the wedge to be somewhat aggressive and uncofortable and that he had not had much releif in his ankle pain.

So i had a rethink and tried a variation As you can see it involved putting material lateral to the STA (counterintuative)





Reviewed this one week on and he has some modest improvement (which is more than anyone had acheived so far!!).

Which made me think. How can a modification which is Lateral to the axis cause a supination moment? Does'nt make sense.

So here is my Theory

[glow=red,2,300]WARNING, COMPLEX / VERY SHAKY BIMECHANICS AHEAD[/glow]



By sticking something under the medial aspect of this foot the foot inverts. So given that the axis is lateral to this point why might that be? Well the only reason i could think of is that because the foot is maximally pronated and very nearly fixed in that position there is a considerable amount of gravitational force being exerted through the sinus tarsi. The top diagram shows the weight bearing state of this foot in rotational and translatory equilibrium (yes Dave, i know, its ALWAYS in equilibrium, but for the sake of simplicity let it slide!)


The second diagram shows what i imagine happens when i put a block under a point between the STA and the Sinus tarsi (as the foot is loading). In this state there is a new axis, the block. The rotational equation is now the STA gravitational force * the lever arm between the STA and the BLock and the Sinus tarsi gravitational force * the lever arm between the ST and the block. Because the latter is obviously larger the foot inverts.

So now we have inversion from an object lateral to the STA. Weird but true, and indeed observable. However its not supination. Because If the STA supinated at all the Sinus tarsi gravitation force would vanish and the equation would change again. So how has it inverted?

I contend that in fact the WHOLE LEG has changed position so its angle of incidence is now more lat - > Medial (ie the knee is now more laterally positioned in space.

Which, I hear you cry, will not be helping matters in the ankle. If the STJ is not supinated it will not releive any of the residual moments which are causing the ouchness.

However consider this. If the whole unit is inverted the axis has shifted more laterally. In which case the distance between the STA and the GRF points (D1 and D2 on the diagram) are smaller.



And if GR1*D1 + GR2*D3 is smaller then G2 * D2, the Sinus tarsi compression, along with any other structures not on the diagram which are contributing to the supination moments will be reduced. In short, we have reduced the residual STJ pronatory moments even in we have not acheived movement in the STJ.

And if the axis moves sufficiently laterally to cross the position of the block before the sinus tarsi opens up then it may even be possible to acheive a supination MOVEMENT. Keep in mind that D1 is as short a distance as i can make it!

What we have here is a double axial model.

Does this make any kind of sense to anyone or do i need to up my meds again.

Regards
Robert

[lawrencebevan]

I tink that in some cases large moments applied to the STJ axis can lead to some perception by the CNS of lateral instability in late midstance and the peroneii are fired leading to late pronation.

A small valgus wedge provides a counter-moment switching off this reflex.

[robertisaacs]

I tink that in some cases large moments applied to the STJ axis can lead to some perception by the CNS of lateral instability in late midstance and the peroneii are fired leading to late pronation.

A small valgus wedge provides a counter-moment switching off this reflex.

Sorry Lawrence. My fault for putting on that post before i put up pictures.

Its still a varus wedge. But it is lateral to the STA.

Does it make more sense now?

Regards
Robert

[davidsmith]

Robert

I'm not sure if I quite understand your diagram free explanation but it appears that you moved the GRF application point lateral to the STJ axis nearer to the STJ axis. Uhhh!

;D No what I mean is the summation of discreet force lateral to the STJ may have a locus at point P. P is 2cm lateral to the STJ axis. Add a varus or block post to the lateral side of the STJ axis and this may reduce P to 1.5cm for instance. GRF * 1.5 is less than GRF *2 so there are less pronation moments ergo relatively increased supination.

Is this what you are saying?

Dave

[davidsmith]

Robert

Nice diagram! Have you considered becoming an anatomical illustrator. NOT!

So now I have seen your (crap, sshhh! who said that - rude boy, stand outside with hands on head) diagram, I can see that you are indeed moving the lateral CoP/CoF closer to the STJ axis. Allegedly!

LoL Dave

[robertisaacs]

No what I mean is the summation of discreet force lateral to the STJ may have a locus at point P. P is 2cm lateral to the STJ axis. Add a varus or block post to the lateral side of the STJ axis and this may reduce P to 1.5cm for instance. GRF * 1.5 is less than GRF *2 so there are less pronation moments ergo relatively increased supination.

Is this what you are saying?

Yes. Therefore (alledgedly) applying a force lateral to the STA may produce a supination effect. Which i thought was a bit wierd.

So now I have seen your (crap, sshhh! who said that - rude boy, stand outside with hands on head) diagram, I can see that you are indeed moving the lateral CoP/CoF closer to the STJ axis. Allegedly!

You rude git .

Look, i sketched it on a bit of scrap paper whilst having lunch. You should be grateful there are no coffee rings and sausage roll crumbs on it.

mutter mutter ungracious sod mutter.

Regards
Robert
Can't draw, sing, or speak french.

[davidsmith]

Robert

Can't draw, sing, or speak french

.

Je peux seulement concorder monsieur

This diagram took about 2 mins to draw and about and hour to scan and find out how to upload it in the correct format. (I had to phone a friend)


Hooray it worked

So if you add a post with a medial wedge effect it will move the CoP medially and if you add a post with a lateral wedge effect it will move the CoP laterally. and the moments about the axis of interest will change accordingly.

However your modification appears to be moving the post more towards the midfoot. (its a bit difficult to tell for sure by the photo's) If it is the this is bringing the post more inline with the normal GRF vector. (where normal means perpendicular to). Is this interpretation correct?

I'm not sure that there can be any force applied to the Sinus tarsi . It would cause pain ie sinus tarsi syndrome.

I have always thought that the sustentaculum tali may bear weight and tend to resist supination. However some years ago when I wrote this on the Jiscmail forum (I think it was) I didn't get a very good recption to the idea.

Certainly there appears to be a theory that the instantaneous STJ axis translates medially as well as rotates medially thru the stance phase when the STJ pronates. Whether there is impingement on the floor of the sinus tarsi I'm not so sure but the STA resistance seems to make sense.

Cheers Dave

(And I'm quite good at Karaoke at least it sounds that way after a few JD's)

[davidsmith]

Blinda

Thanks Robert, i now get the forces/left hand (you`re right, it makes more sense visually than using the right,

True

Left hand rule, rules OK. Nice one Robert, it even works better in the plane of the paper that one does the diagrams on.
Oh yeah! diagrams- Now regarding diagrams can anyone take Robert in hand. (Oh he couldn't let it lie!) ;D ;D

LoL Dave

[robertisaacs]

Ok. I conceed. That is a MUCH prettier prettier picture.

However as per usual i have questions

So if you add a post with a medial wedge effect it will move the CoP medially and if you add a post with a lateral wedge effect it will move the CoP laterally. and the moments about the axis of interest will change accordingly.

Normally true. The question i am bandying is how does a medial wedge move the COP medially if all for the wedge is lateral to the STA

However your modification appears to be moving the post more towards the midfoot. (its a bit difficult to tell for sure by the photo's) If it is the this is bringing the post more inline with the normal GRF vector. (where normal means perpendicular to). Is this interpretation correct?

Sorry, confused. Do you mean moving more towards the midfoot as in moving more anterior?

I'm not sure that there can be any force applied to the Sinus tarsi . It would cause pain ie sinus tarsi syndrome.

Yep, he's got lots and lots of that!

I have always thought that the sustentaculum tali may bear weight and tend to resist supination. However some years ago when I wrote this on the Jiscmail forum (I think it was) I didn't get a very good recption to the idea.

Seems a reasonable idea to me! I wonder how it would go down on pod arena these days.

Certainly there appears to be a theory that the instantaneous STJ axis translates medially as well as rotates medially thru the stance phase when the STJ pronates. Whether there is impingement on the floor of the sinus tarsi I'm not so sure but the STA resistance seems to make sense

Great. Just when i thought i had a model . Translating axis does make sense considering the articular surfaces of the joint.

More later.

Cheers
Robert

[robertisaacs]

On the translation of the STA.

Kevin has uploaded a paper by Benick on Pod arena called "the constraint mechanism of the human tarsus. Its 120 pages worth of x ray / cadaver / in vivo studies of the way the tarsals work.

On page 69 there is diagram showing translation and rotation of the axis.

Run, don't walk, to read it! Its a 120 page white knuckle rollercoaster of a page turner.

Actually thats a fib, its like wading through treacle, but it IS relevant and it IS interesting.

Anyone has trouble downloading the paper drop me an email and i'll forward it.

Regards
Robert