Orthoses

Eric, I agree on both points. I think the physical characteristics of the orthoses we produce must be important.

Further thoughts on “Time”, Michael:
We have a runner impacting the ground wearing our two types of orthosis: one stiff device that arrests the pronation (rotation) twice as fast as the other, a more compliant device. We can find the average force of the impact between the foot and the two orthoses:

Average force = rate of change of angular momentum,
Initial momentum = mass moment of inertia x initial velocity
Final moment = 0
Therefore change of momentum = mass moment of inertia x initially velocity – 0
Rate of change of angular momentum = change of angular momentum / time = average force of impact.

It's a no brainer to see that the compliant device results in a lower average impact force. The question then becomes, how important are impact forces on injury? Moreover, how does the body kinematically respond to the change in impact force?

Also, energy lost in the collision between the foot and the orthoses (measured by co-efficient of restitution) is significant in injury. The more momentum that is conserved the greater the potential for injury.

 

Alright heres what I´m Thinking..

1st we must think of newton ´to every action there must be an equal and opposite reaction´

When the foot comes incontact with the stiff orthotic there will be more soft tissue compression (intial reaction), as the soft tissue expands ( reaction) the less orthotic changes shape the soft tissue will begin to move bone position within the foot and therefore joint and axis location.

Therefore the stiffer the device the more effect on the foot ... as long as this device does not lose more energy in the initial collision. As the more energy loss the less soft tissue compression the less movement of bone.

 

I understand your thinking, but I don't think we can say "the stiffer the device the more effect on the foot". What we have is a number of effects that differ between the stiff and compliant devices. Your assumption regarding differences in the movement of the bones may only be true if the foot reaches equilibrium with the two orthoses in a different position, this may not always occur due to variation in the geometry of the orthosis. Moreover, it assumes that movement of the bone is needed for resolution of the patients symptoms- this may not always be required.

What about the higher average impact force with the stiff device?

 

The higher impact force could lead to pain from the patient and with the higher impact force there is more chance of energy to be lost in the collision between device and foot .

 

Why should there be more chance of energy loss with the stiff device?

 

I was thinking that with the higher impact it would lose more energy because things like heat and friction between the foot and device which would be greater with the stiff device.

Once the soft tissue has reached it´s full deformation and the device can not flex in the opposite direction because it´s stiff energy will be displaced.?

 

Energy transfer should be a problem related to work.
http://www.lightandmatter.com/html_books/2cl/ch03/ch03.html#fig:spring-pos-neg-work

 

So Ive just tried a little experiment.

Hand slap on a table v´s hand slap on a pt treatment couch. Ive felt much more impact from the table on my hand so therefore I lost less energy when comming to contact with the harder table.

So this should represent the two different devices so therefore I was very wrong
sorry for being dense.

 

I don't think you are being dense Michael. On the contrary, I thank you for working on this with me. It's certainly testing my brain power.

In your experiment above- which was louder?

 

The table.

Also there was some redness and heat producted on the hand.

 

I think one question here is change between shod, shod with rigid, and shod with compliant. How much change is there between the three situations and where on the foot is that change the greatest. A good place to start was Bobbert and Nigg paper on impact forces in running. The impact at contact was pretty much determined by the mass of the lower leg and foot with the "spring" of the knee and other parts slowing the impact of the entire body mass.

I agree that the compliant device will have a lower average impact force. However, this force in the arch will be higher than if there was no orthotic. Additionally, impact force will be lower on the normal weightbearing areas.

Eric

 

I was thining about this today. After the foot - device relationship we must also consider the device - shoe relationship especially when thinking about loss and return of energy.

 

Its definently been fun hope that It continues and I guess some of this discussion will play a part in the orthotic consensis

 

Nigg's work on preferred motion pathways has been inspirational to much of what I've been working on for the last several years. Indeed, in one of the papers they discuss the potential significance of material properties of foot orthoses to muscular work. I agree that the discussion thus far was focused on compliant versus stiff orthoses, and of course other factors such as the stiffness of the shoe mid-sole and supporting surface will also be significant. If we are going to prescribe a device for a patient, that is that the shod without orthosis option was no longer an option, how do we decide on the stiffness of the device to be employed? Traditionally there has been a flawed division of soft accommodative, semi-rigid, rigid etc., which frankly is a load of rubbish. Shouldn't we be aiming at being more precise than this?

I am also mindful that thus far we have been talking about the stiffness of the orthosis as if it was uniform across the whole of the device, which of course it is not. What we need to know then is where within the dorsal surface of the orthosis do we want more or less stiffness? Lets take a foot with PT dysfunction, and the area of the orthosis corresponding roughly to it's interface with the navicular, would it be more beneficial for this area of the orthosis to be relatively more or less compliant? Moreover, if we divide the dorsal surface of the orthosis shell into nine sections by projecting a noughts and crosses (tic-tac-toe) grid on it, which sections should we stiffen and which sections should we make more compliant in PT dysfunction? In the past I have attempted to stiffen the device on the medial side of the STJ axis in such cases, but is this right?



Michael, I don't know if this will have anything to do with the consensus project as apparently this is being discussed at the PFOLA meeting and I won't be there. I have been asked to speak at a biomechanics conference here in the UK next summer, where I hope to talk about some of the issues we have been discussing here, along with other stuff I have been working on.

 

I haven't latched on to the preferred motion pathway. One thing that I always found interestng is that an orthotic can change some movement pathways. The classic one for me is late stance phase pronation with internal leg rotation. How do you relate material properties of orthotics to preferred motion pathways? The material properties of orthotics is an interesting area.

Agreed on the accomodative/ functional garbage.

I really like accomodating (putting a depression under) the orthotic in those patients that have a weight bearing callus under their navicular. Perhaps if the device was really flexible in that location.

Regards,

Eric

 

Nigg seems to relate the material properties of orthoses to impact forces and the frequency of the shock-wave transmitted up the lower limb.

What about if the navicular is not weight-bearing, would you still go more compliant in that region?

Perhaps Kevin will comment on this as (if memory serves correctly) he advocates (among other things) inverting the cast in PT dysfunction, which should result in a higher arch profile, and thus an increased stiffness in this area of the orthosis (all other factors being equal). Are we just skinning cats here? Viz. do the other modifications such as a medial heel skive counter-act the increased stiffness due to inversion, or is increased compliance the way forward for this condition in this area of the orthosis?

BTW, for those following along, there are at least three things you should have realised by now:
1. If I place identical orthoses under the feet of two or more different people the deformation of the orthoses that will occur during function will be different in these people.
2. If I place different shaped orthoses under the feet of several people, the orthoses will deform differently in each of these people.
3. A large proportion of the published research into foot orthoses is fundamentally flawed, due to 1 and 2 above.

 

Is it not also important to consider energy return in the materials as well. Say a poly-rubber mix which will deform more when under load but also return more energy to the foot which may help alter the navicular position during non weight bearing ?

 

Simon,

In your opinion is this why we tend to see subject specific kinematic responses to orthoses?

Ian

 

And kinetic and other subject specific responses. We are not comparing like with like. It's like a drug trial where everyone gets a different dose of the same drug, or the same dose of different drugs. Not good science.

 

Think about the relationship between navicular position and stress strain in the PT tendon, is the energy return to the foot significant in the stress in the PT tendon?

 

no because the stress has already occurred, so stop it in the first place as you indicated in the post before.

 

Hi Guys
Sorry to come back to this so late- pretty crazy week!

The concept of impact forces has popped up a couple of times in the discussion (such as above).
I have always held the view that we do not so much impact an orthosis, but rather impact with an orthosis.
It is always in contact with the foot, so I the foot/ orthotic combination (and shoe) impact the ground arther than the foot impacting the orthosis. Therfore the orthosis changes how the GRF is applied to foot by creating a different ORF.
There will changes in how forces are applied to the foot according to design and materials but is impact the best description?

 

How do you know the entire surface of the foot that has the potential to be in contact with an orthosis is always in contact with the orthosis? Think about swing and centripetal force, is this likely to draw the foot and orthosis together or apart? (why does a loose boot comically fly away from the foot when kicking a ball?), What do in-shoe pressure studies show? Are the same number of force transducers in the area of the orthosis-foot interface firing in open-kinetic chain as in closed kinetic-chain?

I'm also thinking this: the foot and the orthosis could be travelling along nicely together at the same velocity, however due to differences in their masses on striking the ground, the impact force (f= m x a) will result in differences in their relative accelerations since the mass of the orthosis is different to the mass of the foot and lower-limb. Therefore at some stage, they impact against each other.

Edit: Thinking out loud, it's probably that the orthosis and foot are moving at different velocities away from each other during swing due to differences in their respective masses and at strike the orthosis motion is arrested first, its direction reversed and it accelerates toward the foot which is still accelerating downwards towards the ground. We are not talking big time periods here, we're talking nano-seconds difference. Then we get differences differences in their relative accelerations due to ground reaction impact force since the mass of the orthosis is different to the mass of the foot and lower-limb. Therefore at some stage, they impact against each other.

 

Stop it slowly, or stop it fast? Remember everything we've discussed, especially visco-elasticity.

 

I feel a bit like the punk in Dirty harry " is it 5 bullets or 6 punk, do you feel lucky well do yah "

So with the quotes from the discussion we want to stop it fast because the PT will become stiffer faster with the stiff device and store more elastic energy which will deal with the position of the navicular during non- weight bearing.

And then to come back to mark original question if the device follows the exact shape of the foot with its lumps and bumps the greater surface contact the less pressure applied to the foot therefore less chance of blisters and irratation.

so is is 5 or 6 bullets.

 

I think I look at this from a different angle - if load/deformation occurs faster (a more steep linear portion of the curve in the elastic region) and this results in increased tissue stiffness, would this not potentially increase risk of tissue injury?

Ian.

 

Need to realise the orthosis should be applying force in the opposite direction to the direction of loading causing strain in the tendon.

 

So to stop the stress 'fast' will decrease the tissue stiffness?

This thread makes my head hurt...

 

My thoughts regarding the PT tendon are that we want to arrest the motion of the subtalar joint to limit the stress and strain within the tendon. We could use either a rigid or a soft device that arrests the STJ pronation such that the elongation in the PT tendon is the same in both devices (obviously the two devices would need to have different surface geometry in their unloaded states). The question is really how much influence the rate of deceleration of the elongation of the PT tendon would have on it's stiffness, that is as it decelerates and load is applied less rapidly does it become less stiff and less able to store energy enough to make any real clinical difference? We also need to consider that the orthosis does not just influence the target tissue.

 

Worth a read if you are not familiar with it already:
http://journals.lww.com/cjsportsmed...2001&issue=01000&article=00002&type=fulltext#
http://www.orthlab.com/pdf/running_pdf,_clinical_Biomechanics.pdf

 

Yes, I do usually invert the positive cast in treating posterior tibial tendon dysfunction (PTTD) along with using a medial heel skive and other anti-pronation measures in custom foot orthoses.

As far as the rest of this thread goes, the stiffness/compliance in each region of the foot orthosis is critically important in addition to other factors such as the three-dimensional contours of the orthosis, shoe morphology, etc when treating mechanically-based pathologies of the foot and lower extremity.

The way forward here is to realize that each change in the shape and load-deformation characteristics of foot orthoses will have both external and internal effects on the forces and moments of the foot and lower extremity. Unfortunately, we are just now starting to understand what these changes are at this stage in our accumulated research on foot orthoses. Much more research needs to be done.

Like Simon, I also like the preferred motion pathway model of Nigg to explain some of what we see with foot orthoses, especially in regard to the neuromotor effects of foot orthoses (i.e. those effects caused by central nervous system control of foot and lower extremity muscle activity as a result of the mechanical interactions with a foot orthosis). However, I believe the preferred motion pathway theory is somewhat incomplete and will need to be further modified in the future in order to match up with what I see clinically with foot orthoses on a daily basis.

Good discussion.:drinks

 

Unfortunately, designing research protocols to effectively test the influence of stiffness is not so easy because as Kevin notes the three-dimensional contours of the orthosis and stiffness are inter-related. We can make two devices with the same surface geometry out of two different materials of differing stiffness. But during function the deformation in the orthoses will be different, altering their surface geometry. Ideally I guess we would have two orthoses that exhibit identical surface geometry when deformed during function. Need to use FEA to design such devices- not easy.

I suppose the 1st protocol here would be a good start.:drinks

 

Rebecca, Ian, Michael,

Thanks for your thanks, now you've read the Nigg paper, read the paper on ground stiffness and metabolism I linked to here:
http://www.podiatry-arena.com/podiatry-forum/showpost.php?p=23522&postcount=5

Edit: this gives a nice overview: http://www.udel.edu/PT/davis/stiffness_update.pdf you should probably read this first to get your head around the concepts.

 

This is interesting: http://www.journals.elsevierhealth.com/periodicals/jclb/article/PIIS0268003305000823/abstract
as is this:
http://www.bertec.com/publications/research/papers_products/Williams-Gait and Posture-2004.pdf

 

please delete:bash:

 

Here's a copy of the paper "Plantar feedback contributes to the regulation of leg stiffness" that Simon gave a link to that supports the theory that afferent input into the central nervous system from the plantar foot may affect the efferent output from the central nervous system to the muscles of the lower extremity during weightbearing activities.

 

Ive now read all the articles twice and your statement to mine below now makes much more sense to me within this discussion.

Also the Nigg articles also help explain the Question way some patients who have there devices in the wrong shoes get better.

I´m not sure if the thread is done or a new should be started on Preferred motion pathway model its very intersting lots to think about