Hi all,

Stanley I continue to refine and understand our respective positions.

Some cut from our previous post.
At 06:16 PM 1/9/2005 -0500, you wrote:

>>Stanley replied
>>
>>>I know you have this mental block about what I am trying to say. I think it
>>>is because you are thinking that the orthotic is controlling the foot, so
>>>the foot cannot pronate. Maybe this will help, as the foot slides laterally
>>>on the orthotic, the portion of the arch in contact with the foot is lower.
>>>so the arch does not provide support anymore.
>>
>>Part of my mental block comes from the fact that people should not slide
>>off of a properly made orthotic, so I don't see the pronation related to
>>sliding off of the device.  The other part of my block on what your are
>>saying is trying to explain how a modification to an orthosis that should
>>create a supination moment ends up causing pronation motion.  Less pressure
>>in the medial arch is plausible cause of a decrease in supination moment
>>from an orthosis, but I am having a hard time accepting it.  I still like
>>the muscular response explanation better.


Stanley in blue writes:


>Theoretically people should not slide off a properly made orthotic.
>However, it does occur. If there was no coefficient of friction, between
>the orthotic and the skin, everyone would slide off their orthotic. If the
>post was excessively high, people would slide off of it (you admitted that
>you do not post more than 6mm for this reason). What do you do in a case
>where a patient had a tibial fracture and the leg is 15 degrees in varus?
>You have to post maximally until the foot slides. In this patient it would
>be impossible to make a proper orthotic because of the limitations of the
>physical world, you just can't post that much.


The foot will slide until something stops it.  I thought we agreed that a
high lateral heel cup will stop sliding for up to some reasonable amount of
posting.  In addition, you can make a lip along the lateral edge of the
orthosis that will also prevent sliding.

>  Now let's talk about moments. As I said I was not much of a physics
> student, and I have forgotten most of it, just like Parasitology.
> However, you have forced me to jog my memory. If I make a mistake, I am
> confident you will find it. If I remember correctly, a wedge will push
> perpendicular to its plane. So if we take a vector of the varus wedge,
> the vector is vertical + lateral. So there is an abductory force. I guess
> this is why the foot slides laterally. This lateral force will when
> applied to the calcaneus will cause an eversion moment to occur. Due to
> the nature of the subtalar joint, this becomes pronation.  The steeper
> the slope, the more of an abductory force is applied, and therefore a
> greater pronatory moment.

Eric Replies.
Let's make sure we are understanding the force in the same way.  The varus
wedge will apply a force through the fat pad to the calcaneus in an
inferior to superior and a medial to lateral direction.  I don't see how a
medial to lateral force will cause abduction of the calcaneus. With an
orthosis with a relatively high lateral heel cup, this force can be
negated.  The heel will slide until the heel hits the lateral heel cup and
there will be a force lateral to medial at this location.  This lateral to
medial force from the lateral heel cup will cancel the medial to lateral
component of the force applied by the force perpendicular to the varus
wedge.  Once the heel sits fully in the heel cup there is no net horizontal
force.  My impression is that this would occur almost immediately.



>>Eric Wrote:
>>
>>
>>
>>>>If you believe in Newton's laws, you have to come up with a moment that
>>>>causes the motion.  If the varus wedge (or overposted varus othorsis) is so
>>>>high that there is little pressure under the lateral heel and higher
>>>>pressure under the medial heel, you know that the center of pressure has
>>>>been shifted to increase supination moment from the ground.
>
>As the heel everts on the wedge, the supinatory moment is negated.

Eric Replies
How often do you see eversion, in stance, or in gait with a varus heel
wedge?  Are we talking about the too big wedge?


>>>>The friction
>>>>from sliding down this slope will also cause a supination moment.
>
>You are assuming that there is no slop inside the fatpad.

In addition to the friction there is the lateral heel cup force described
above.   After the skin slides, relative to the calcaneus there will be a
point where the frictional force between the skin and orthotic will be
transmitted to the calcaneus.

Eric wrote:


>>>>So, if
>>>>you see pronation from a modification that should cause supination, then
>>>>the most logical source of moment that I can think of is the peroneal
>>>>muscles.

Stanley replied:

>You are saying that we have a maximally pronated foot. Now that we have
>changed the moment (not necessarily the position, the muscles will fire to
>pronate the foot. An analogous situation would be if we were on an
>unstable surface. Let us say that the surface moves so that there is a
>moment to make us fall over. We are starting to fall. Now all of a sudden
>the moment is reduced, but not to the point that we would be pushed back.
>You are saying that the muscles would react to this change of moment so we
>would fall.  I would like to think that after all these years of
>evolution, we have more sophistication of our nervous system.

Eric answers:
We agreed earlier, the maximally pronated STJ cannot evert more with
peroneal contraction.
I'm not sure I understand your analogy.  Are you equating falling down with
pronation of the foot?  This was my point earlier.  Evolutionarily it is
better to pronate than to sprain your ankle.
My point earlier was there are limitations even for a sophisticated nervous
system.  It takes time for nerve impulses to travel and then it takes time
for tension to develop in a muscle.  If you are walking along and your STJ
is balanced with range of motion available in either direction, an
unexpected bump in the terrain will cause immediate motion that may be too
much to recover from.  If you choose to be maximally pronated, you have to
worry less about spraining your ankle.



>>Stanley replied
>>
>>>I see what you are trying to say. You are saying that there are 2
>>>supination moments. 1. From the wedge and 2. From the friction of the skin.
>>>As the calcaneus slides within the fat pad (remember that the skin has
>>>friction to the ground) and abduction occurs at the midtarsal joint, the
>>>pronation moment for the subtalar joint comes from body weight via the
>>>midtarsal and subtalar joints.
>>
>>Body weight and ground reactive force both effect the joints of the
>>foot.  Their effect is determined by the location of the center of pressure
>>of ground reaction force relative to the STJ axis.  You cannot look at the
>>motion of one joint causing motion in another joint.  When you do this you
>>get circular reasoning such as Supination of the LMTJ causes pronation of
>>the STJ.
>
>Eric, are you suggesting that there is a longitudinal axis of the MTJ?
>Didn't we have a very long discussion prior to Kevin inviting me to
>joining this list serve, in which I said there were 2 axes, and you and
>Kevin said that this was an antiquated theory. I guess you have finally
>seen the light. Congratulations!! So if there is supination of the LMTJ,
>then gravity will pull the foot to the ground which can be done through
>gravity (If you believe in Newton's laws). I understand how supination of
>the Subtalar joint causes pronation at the LMTJ, but the converse can also
>be true.

That's not what I'm suggesting.  You may have missed the part where I said
"You cannot look..."  Motion does not cause motion.  Moments cause
rotations and force causes linear motion.


>>No, pronation of the STJ causes supination of the LMTJ.  A better
>>way to look at this is that a moment causes motion.  The force that moves
>>one joint may move other joints  It is also important to define the
>>starting position carefully.
>>
>>So a foot is placed on top of an orthosis with too much varus heel wedge
>>effect and a minimal lateral heel cup.  Initially as the foot is placed on
>>the device there will be a supination moment from medial position of force
>>and there will be a medial to lateral force on the heel causing a slide in
>>that direction.  Your contention is 1 that this slide will cause abduction
>>of the forefoot on the rearfoot and 2 this abduction causes foot, not
>>necessarily STJ, pronation.  In this example there may not be enough
>>friction on the forefoot to prevent pivoting between the forefoot and
>>ground.
>
>When I stand the patients on the orthoses and they slide, it tends to be
>the calcaneus. The sides of the shoe are up against the lateral forefoot.
>Just put your hand inside your shoe and see where things really are. This
>will also aid in stopping the forefoot from sliding. Also if there was the
>rotation you described the end result would be pronation, as the foot is
>adducting.

Eric replies
I'm saying that as the calcaneus slides approximately 5mm from medial to
lateral the whole foot will move as a unit.  There is not necessarily
motion between the calcaneus and forefoot.  You may understand this as
"foot" pronation, because there is a slight internal rotation of the foot
relative to the leg.  However, I still maintain that the pivot point is the
forefoot relative to the ground and not at the MTJ.  I think part of our
problem in communication is the difference in between foot pronation and
STJ pronation.


>>  It is my sense that this is where the motion is most  likely to
>>occur, unless there is a pronation moment from somewhere, for example the
>>peroneal muscles.  If this were gait, heel contact and the sliding would
>>occur long before forefoot loading.
>
>
>We are not talking about gait.

I guess we will have to disagree on where this motion occurs.  The point
about gait was that your position certainly does not apply to gait and it
still may not apply to stance.


>>So I don't really see how abduction of
>>the forefoot on the rearfoot can occur.  If it does occur then there would
>>be "foot" pronation, but not necessarily STJ pronation.
>
>Stanley wrote:
>As you define foot pronation later in your post, I agree. But STJ is
>necessarily a component of the motion.

Eric replies:
No its not.  It is possible to have forefoot abduction on the rearfoot
without STJ pronation.   It is also possible to have internal leg rotation
without STJ pronation.  The foot is made of many independent joints that
don not have to all move in the same direction all the time.  Why do you
think STJ pronation must occur with foot pronation?

>>Eric wrote



>>The peroneals can fire when the foot is sliding or not sliding. However, I
>>am saying that too much supination moment from the ground, will cause the
>>peroneals to fire.
>
>
>As I said above this makes no sense unless there is movement.

When you try and lift a 50lb barbell with 25 pounds of force there is no
movement, but there is a change in force.  I don't understand why the above
does not make sense unless there is no movement.  When an individual stands
on a varus wedge that changes moment enough to cause supination, that
individual will learn to contract their peroneal muscles before there is
motion.


>>>>>some cut
>>>>
>>>>Stanley wrote when describing the mechanism of pronating within the fat pad
>>>>and STJ pronation:
>>>>
>>>>>Abduction of the midtarsal joint is a component of pronation which occurs
>>>>>with the abduction. If you pronate the midtarsal joint, you also get
>>>>>dorsiflexion. The talus must adduct and plantarflex, for the forefoot not
>>>>>to move. This is subtalar joint supination.Again, stand someone on an
>>>>>orthotic with too much rearfoot post and see what happens.
>>>>
>>>>Eric Replies
>>>>
>>>>Although this is slightly off topic....
>>>>By STJ pronation I'm talking about motion between the talus and
>>>>calcaneus.  Are you talking about foot pronation in which you also include
>>>>the MTJ and tibial rotation?
>>>
>>>Yes
>>>
>>>>Most of the time the STJ is maximally
>>>>pronated in stance.  When you ask someone, who is maximally pronated, to
>>>>evert their foot you will often see additional internal leg rotation and
>>>>forefoot abduction on the rearfoot without  calcaneal eversion.  If you ask
>>>>this person to invert their foot you will see calcaneal inversion and then
>>>>as they go back to resting position you will see eversion up until they are
>>>>maximally pronated.  It is possible to have abduction of the forefoot on
>>>>the rearfoot without calcaneal motion.  If the arch lowers there will be
>>>>ankle joint plantar flexion, but not additional calcaneal eversion.
>>>
>>>Eric, you are telling me that when you make a pair of functional orthotics,
>>>the patient is maximally pronated in the subtalar joint.
>>
>>
>>They are maximally pronated before I make the orthotics.  The orthoses that
>>I make for them may not move them from maximally pronated, but will relieve
>>the patients symptoms.
>
>So you agree that the foot that you are treating can still be maximally
>pronated. This is why they can't evert any more.
>
>
>>>  I was thinking
>>>today why we are having this long discussion, and it seems that we are not
>>>resolving one simple point. It occurred to me that we are talking about 2
>>>different things, like an Abbott and Costello comedy. I am talking about an
>>>orthotic that changes a foot's position and you are talking about an
>>>orthotic that changes a moment.
>>
>>Changing position and moment are not necessarily two different things.  You
>>can change moment without changing position, but you cannot change position
>>without changing moment.
>
>I agree So you do not disagree with what I said.

You and I are looking at things from different perspectives.  You are
looking from the perspective of position biomechanics (kinematics) and I am
looking from force and moment biomechanics (kinetics).  From your
perspective there is a dramatic difference between no motion and
motion.  From my perspective there is not.  Say you take a foot in which
there is 10 Nm pronation moment from ground reaction force.  If you make an
orthosis that changes the moment from the ground in that position to 1 Nm
supination moment the foot will change position to where the moment from
ground reaction force readjusts to zero net moment.  However, if I made an
orthosis that changed the moment from 10 Nm pronation moment to 1 Nm
pronation moment, the Foot will not change position and there is a dramatic
difference in stress acting on the foot pre and post orthosis.  So, from my
perspective we are describing essentially the same thing.

Some cut
Stanley asks:

>Eric, If you were to put an abductory force on the calcaneus in a non
>maximally pronated foot, with the forefoot not able to slide, what would
>happen at the STJ and MTJ?

There will be slop between the fat pad, and skin under the
metatarsals.  The forefoot will be able to slide.  How far will the
calcaneus slide if it is pushed?  Here's an experiment.  Take a standing
person and push on the medial side of their calcaneus. When I do this
gently I don't see STJ nor MTJ motion.

>Also do you notice if the higher heel cup decreases the pronation?
>What happens in an orthotic that has a 4mm heel skive in a patient with no
>structural abnormalities?

Above 20mm I don't see much additional decrease in pronation.
It may supinate, it may not depending on the position of COP relative to
the STJ axis.  If the peroneal muscles contract, it will pronate.  A 4mm
skive in a narrow heel creates more of an effect than in a wide heel.

>So at this point we have a lot of agreements.
>1. We both acknowledge that a varus wedge can cause a foot to pronate.
>2. Eric's orthotics do not necessarily change a person's maximally
>pronated position and therefore it is impossible for the foot to pronate
>further.
>3. Eric agrees that abduction of the foot can occur with "Foot pronation"
>
>And some differences
>
>1. We have a difference as to explanation.
>2. Eric feels that if there is a lateral positioned STJ axis, the wedge
>causes too great a supination moment which is compensated for by peroneal
>activity. The brain cannot respond quick enough, so it over causes greater
>than necessary peroneal activity for safety reasons.
>3. Stanley feels that the heel slides down the wedge, and this causes
>pronation of the midtarsal and then the subtalar joint.


Good discussion.


Cheers,

Eric Fuller

-----------------------------------------------------------------
This message was distributed by the Podiatry JISCmail list server

All opinions and assertions contained in this message are those of
the original author. The listowner(s) and the JISCmail service take
no responsibility for the content.

to leave the Podiatry email list send a message containing the text
leave podiatry
to [log in to unmask]

Please visit http://www.jiscmail.ac.uk for any further information
-----------------------------------------------------------------