Measuring arch height

YEAH ! I like your thinking ... assuming there is no down-side to cutouts.

rebecca

 

Hi Rebecca,

I would have thought that one of the major drawbacks would be that because we are extending the cut-out further proximally, there is an increased risk that a foot with a medially deviated STJ axis has more potential to pronate off the device? What do you think? Also, do you think this modification (probably in cases where we extend the cut-out back to the 1st met-cuneiform joint) could potentially affect the intended stiffness of the device i.e. making it more compliant when it should be stiffer?

Cheers,

Dan

 

I agree with you Dan. But in its simplest form, this is our aim - push up medial to the STJ axis and don't push up lateral to the STJ axis (as per pronated flatter foot type). Its just so simple its beautiful. Having said that, I don't even use cutouts. Plus I'm sure I'm not the worlds best at locating the STJ axis.

Regards

Rebecca

 

Hi Rebecca,

This is just a prediction Rebecca. I may not be correct. I haven't experimented with cut-outs either. I would definitely be keen to try them out but at the same time I'm a bit hesitant due to the reasons I previously mentioned.

Your not alone.

 

We performed reliability trials using the perspex plate and grid system back in 2002, it just never got written up for publication- shame. The results indicated that intra-observer error was fair (ICC= 0.78 for within-day, 0.76 for between day, inter-observer was poor (ICC = 0.66 within-day)

I don't know yet.

Trying to find it- I think it was by Bob Kidd or maybe Tony Redmond?? Anyone know the one, I know I didn't dream it!

{edit} Found it after much searching:
Orthop Sports Phys Ther. 2006 Aug ;36 (8):550-6 16915976
Weight-bearing passive dorsiflexion of the hallux in standing is not related to hallux dorsiflexion during walking.
Jill Halstead , Anthony C Redmond
STUDY DESIGN: Case control study. OBJECTIVE: To explore the validity of the assumptions underpinning the Hubscher maneuver of hallux dorsiflexion in relaxed standing, by comparing the relationship between static and dynamic first metatarsophalangeal (MTP) joint motions in groups differentiated by normal and abnormal clinical test findings. BACKGROUND: Limitation of motion at the first MTP joint during gait may be due to either structural or functional factors. Functional hallux limitus (FHL) has been proposed as a term to describe the situation in which the first MTP joint shows no limitation when non-weight bearing, but shows limited dorsiflexion during gait. One clinical test of first MTP joint limitation during standing (the Hubscher maneuver or Jack's test) has become widely used in physical therapy, orthopedic, and podiatric assessments, supposedly to assess for the presence of hallux limitations during gait. The utility of the test is based on an assumption that restriction during the static maneuver is predictive of functional limitation at this joint during gait. Despite a lack of evidence for the validity of such an assumption, the outcome of the static test is often used to infer risk of overuse injury or as an outcome for functional therapy. This paper examines the validity of the assumptions supporting this widely used static test. METHODS AND MEASURES: First-MTP-joint motion was assessed using an electromagnetic motion tracking system in cases (n = 15) demonstrating clinically limited passive hallux dorsiflexion in relaxed standing, and in 15 controls matched for age and gender and demonstrating a clinically normal Hubscher maneuver. Maximum hallux dorsiflexion was measured with the subject non-weight bearing (seated), during relaxed standing, and during normal walking. Results: Hallux dorsiflexion was similar in cases and controls when motions were measured non-weight bearing (cases mean +/- SD, 55.0 degrees +/- 11.0 degrees; controls mean + SD, 55.0 degrees +/- 10.7 degrees), confirming the absence of structural joint change. In relaxed standing, maximum dorsiflexion was 50% less in cases (mean +/- SD, 19.0 degrees +/- 8.9 degrees) than in the controls (mean +/- SD, 39.4 degrees +/- 6.1 degrees; P < .001), supporting the initial test outcome and confirming the visual test observation of static functional limitation in the case group. During gait, however, cases (mean +/- SD, 36.4 degrees +/- 9.1 degrees), and controls (mean +/- SD, 36.9 degrees +/- 7.9 degrees) demonstrated comparable maximum dorsiflexion (P = .902). There was no significant relationship between static and dynamic first MTP joint motions (r = 0.186, P = .325). CONCLUSION: The clinical test of limited passive hallux dorsiflexion in stance is a valid test only of hallux dorsiflexion available during relaxed standing. There is no association between maximum dorsiflexion observed during a static weight-bearing examination and that occurring at the same joint during walking.

I don't mean to frustrate. Sometimes it helps to clarify your thought processes by arguing for something you don't necessarilly believe to be true.

 

You need to think about the visco-elasticity of the body tisssues.
Here are a couple of questions to lead you (not to frustrate you)- I could just tell you what I think, but that isn't learning is it?:
1. What does the area under the stress/ strain curve represent?
2. What happens to the stress/ strain curve when the tissues are loaded more rapidly and what effect does this have on the area under the curve?

It can, and is being done on orthoses. The question is does the increased compliance make it into a "shank-dependent" device? I still hate that terminology.

 

Internal oblique is an extrinsic rearfoot post (external stabilizer) that is extended on it's medial aspect- like a Thomas heel.

 

Who knows? Daniel, I'm from a "hard science" background dealing in quantification, so I tend to shy away from the qualitative "touchy feely" type of stuff- that doesn't mean it doesn't have a place. As Rebecca has pointed out, to a large extent I have been playing the devil's advocate with you both in order to ascertain what you and I think we know and to learn a little more about us. I guess if we quantified everything we could do some nice predictive modelling. That's not to say you can't do that with qualitative variables, just makes the analyses more complex. How do you measure orthotic success?

 

Rebecca and Daniel,

You raise some excellent points here. I'll try to address them all together:
I don't agree that there would be an increased risk that a foot with a medially deviated STJ axis has more potential to pronate off the device for the reason Rebecca outlined; you are increasing supination moment without increasing pronation moment. That is not to say feet don't move on top of orthoses!

One of the problems with cut-outs, or apertures in orthoses and simple insoles is that the pressure at the edge of the cut-out / aperture is higher. So while you may offload a lesion, the area around it has increased pressure acting on it, maybe problematic in diabetic patients?

The stiffness is easy to compensate for- either increase shell thickness or extend the rearfoot post or both.

One of the things I didn't mention regarding the reliability trials of the STJ axis assessment was that we also looked at improvement over time: everbody got more reliable the more they used the technique!

 

There is another way to create different stiffness areas on a shell without varying the shell thickness. Using different fibers (kevlar, carbon fiber, glass fiber, etc) and/or different intrinsic angulations (0º, 15º, 30º, 45º, etc) during a lamination process you can manufacture a device that it will respond more or less stiff/compliant in a certain point depending on the formulation when a load is applied.

You would have a less bulky device that it would fit a wide range of shoes. But, If someone is thinking to run to the patent office, I am afraid is currently patented (not from me )

Regards,

 

:good:

 

On the surface, I don’t’ technically “measure” orthotic success, I make my judgment based on whether there has been any improvement in pt symptoms. This is probably a very bias analysis though. In saying that, one of the challenges I find is that, how do we ascertain how the pt got better i.e. was it the devices, was it a placebo effect, or, they did they get better by themselves?

Regards,

Daniel

 

The clinician in me doesn't care as long as they get better. The researcher in me gets frustrated that we will never know.

I only wish everyone was as insightful as you are Daniel, I reviewed a paper today in which orthoses were used alongside anti-inflammatory drugs, the author's ignored the fact that the subjects were taking this medication and concluded that the orthoses used in their trial reduced pain. Hmmmmmmm:sinking: Pass me an orthosis my head hurts.

 

The removal of some of the medial portion of the anterior edge of the orthosis plate that occurs with a first ray cut-out would tend to decrease the ability of the orthosis to resist eversion moments from the foot, regardless of the stiffness of the device (unless a medial filler is added distally plantarly in the arch of the orthosis to make this portion of the orthosis shank dependent). For this reason, I don't use first ray cut-outs, but I do know many podiatrists that claim good results with foot orthoses that commonly use first ray cut-outs.

 

Firstly let me make it clear that I really don't know what I'm talking about inspite of reading what I'm sure is the relevant information :wacko: :

1. The area under the stress / strain curve shows deformation - there is an elastic area and a plastic area. How does this relate to accelleration/ deceleration?

2. This is hysterisis is it?

Hysterisis is when a viscoelastic material is loaded and unloaded, the unloading curve will not follow the loading curve. The difference between the two curves represents the amount of energy that is dissipated or lost during loading (this is a quote from http://www.engin.umich.edu/class/bme456/ligten/ligten.htm )

Does that mean that each time the tissue is loaded, it doesn't return to its original state. I'm not sure how (if) the concept of hysterisis relates directly to acceleration / deceleration of loading. Although, the stress / strain characteristics of viscoelastic materials have a time element involved, unlike other material (non-viscoelastic). So maybe the quicker the load is applied, the longer the tissue takes to return to its original state?

Do you know to what capacity? Is it by a few clever podiatrists in isolation or certain labs? Is is a matter of grinding an orthosis material thinner once its pressed, or laminating thin layers to certain areas for more thickness? ... Just read your post Javier. What about polypropylene, or can it only be done with layering / laminating, as far as you know. Do you mean different fibres in the one orthosis shell?

Rebecca

 

I agree with you Kevin from what I have seen of 1st ray cutouts and from what I assume will happen.

But the first ray area of an orthosis fro a foot with a medially deviated STJ doesn't need to provide resist pronation / eversion moments. Pushing up here will either increase pronatory moments or as you say compress the underlying soft tissue.

Narrow devices however (which a 1st met / ray cutout is) slide medially and stuff up one's 'precision' orthosis prescription, which annoys me. Maybe it would make a difference if a top cover was added to make the orthosis stay put on the floor of the shoe?

Rebecca

 

This would be true if you look at that area of the device in isolation, however as Kevin mentioned...

This is because with a shell device particularly (or non shank dependent) part of its characteristic is that you have support characteristics from parts of the device that are not under direct pressure. Imagine if you continued a first ray cut out all the way to the 3rd or 4th MT... you would definately decrease the inversion/eversion 'control'. Imagine if you chopped it in half and only had a rearfoot stabiliser- it would have very different characteristics.
It is for this reason that if I do a first ray cut out, I always apply it intrinsically before the shell is pressed to ensure full width of the device.
With this in mind... Simon said...

It would make sense as this point would be the most anterior point where you could apply a supinatory force with the orthosis medial to the STJ axis, and the most posterior point where you could encourage first ray plantar flexion without detrimental effect on this supinatory force.
I routinely do a large 1st ray cut out using an Amfit, but then mill a model and press a thermoplastic device. The size of this cut out follows discussion with Bruce Williams. Could this be the linking of sagittal plane theory and rotational equilibrium???
Hope I haven't blurred terminology and caused confusion.
Good discussion this.

 

Ah, I see your (and Kevin's) point. Thanks Craig T

What do you mean, you make a depression in the medial fill (plaster depression) so there is still shell material under the area but it is on a lower level. Is that what you mean Craig T? So you still have a distal medial edge as per normal.


Rebecca

 

Yep- spot on.

 

Javier

We currently do something similar with our CAD orthoses -possibly Simon S has had a go as well as he uses his own CAD system - which involves making the orthoses thicker at key points and in a specific way.
We routinely make the orthoses 1mm thicker at the distal medial edge of the rearfoot post as this used to be a common place for fracture.
We then will add 'cross members' to the underside of the shell. The direction and thickness of these allows the us to engineer the flex of the shell.
This really works as you don't make the orthoses any thicker.
It is also a lot cheaper than the carbon fibre method and infinatley more variable.

Phil

 

Rebecca and Colleagues:

When one looks at the action of the dorsal surface of a foot orthosis on the plantar foot as a whole, the goal of treating a patient with symptoms related to increased magnitudes of subtalar joint (STJ) pronation moments (e.g. posterior tibial dysfunction) with foot orthoses is to increase ground reaction force (GRF) medially and decrease GRF laterally on the plantar foot versus the non-orthosis condition. This is how a foot orthosis mechanically acts to increase the external STJ supination moments on the foot to help relieve the symptoms and gait abnormalities of pathologies such as PT dysfunction.

However, the orthosis does not magically start working only medial to the STJ axis and not working lateral to the STJ axis to perform this STJ supination moment production function. Even pushing harder on the foot lateral to the STJ axis with the foot orthosis will produce a STJ supination effect as long as this push lateral to the STJ axis by the orthosis simultaneously occurs with a concurrent decrease in orthosis pushing force farther lateral to the STJ axis so that, overall, the magnitude of STJ pronation moment is decreased.

Said another way, , there are three ways to get a STJ supination effect from a foot orthosis:

1) Increase the magnitude of STJ supination moment.

2) Decrease the magnitude of STJ pronation moment.

3) Do a combination of #1 and #2.

Therefore, regardless of the spatial location of the STJ axis, as long as the foot orthosis is shifting GRF medially on the plantar foot, the foot orthosis will have a STJ supination effect on the foot, even though the orthosis may only be contacting the foot lateral (or on the pronation side) to the STJ axis.

 

Hello Phil,

It is interesting. Can you develop further your 'cross members' process?

Regards,

 

Simon

Do you not think there may be some merit in adding a ppt D pad.

Assuming a linear coefficient to the Limit of Proportionality (LoP),If one were to require variable coefficient of stiffness then a thinner shell would not do this, it would only reduce the coefficient of stiffness (CoS) and give a more flexible shell linearly across all applied loads. The ppt pad may give a certain 'low' CoS and as this compresses would become more stiff per unit of cross sectional area. Then the stiffer orthosis shell would deform more under the greater stress so that you would have a high deflection to force ratio building up to a low deflection to force ratio. This may suit the needs of some prescritions.
Isn't it the deflection to force ratio over time, IE the power ratio, that we would like to control (if we need to) in terms of controling arch height thru the stance phase of gait. So with a ppt pad we would start off with a low power device which becomes a high power device as the applied force increases.

Cheers Dave

 

I can see you are getting a little bit muddled here. I'll try to explain:

1. The area under the stress/ strain curve represents the energy stored. This is the same as the resiliance that I talked about here: http://www.podiatry-arena.com/podiatry-forum/showpost.php?p=30042&postcount=54 only this time we are talking about energy storeage in the body tissues rather than in the orthosis.

2. When the tissues are loaded more rapidly the gradiant of the stress/ strain curve is increased, i.e. the tissue becomes stiffer and the area under the curve is increased, therefore resiliance is increased and more elastic energy can be stored in the tissue and returned to the system.

So what happens to the stiffness of the supinatory tissues when we decelerate the rate of pronation? What effect does this have on their capacity to store energy?

I know of a couple of labs who are thinking this way. Remember- we don't need to vacuum form material if we are using CAD/CAM, instead we can mill the device directly from a block of material, e.g. polypropylene, so you don't need to layer up material, you just mill the shell out at different thicknesses across its surface. Craig T talked about "lowering" the area of the device under the first ray, traditionallly if you were vacuum forming you would build this area up on the positive cast. Using CAD/CAM you can design this directly into the orthosis.

 

Yeh, been playing with ribs. With regard to the increased thickness along the distal edge of the post- wouldn't a fillet work much the same?

 

Dave, good to have your brain here. I take your point re: laminates and foams. The addition of the ppt pad will tend to load the tissues over a longer time period (decelerate motion more gradually)- providing it's not bottomed out before we start. So is this a good or bad thing? I don't think we know.

Also, a ppt pad will result in an increase in horizontal compliance. Energy will be stored and returned in this horizontal deformation. How does the direction of deformation and elastic return influence it's usefulness to the body above?

If laminating the materials in this manner is beneficial, why just add an arch pad as oppose to covering the whole superior surface of the device?

Interestingly, the research that has demonstrated increased performance and reduced injury rate by manipulating surface stiffness has not, to the best of my knowledge, employed surfaces with the mechanical properties you describe above. Nice project.

 

Simon

A more organic shape seems to create less stress points and is easier to mill without moving into finer tooling - I use 12mm ball nose tools for very fast swarf removal and overall speed. We mill a lot of devices and use multiple tooling which although managed through an auto tool changer does add to the millingtime.

Phil

 

Javier

CAD orthoses can be designed using a top and bottom shell. This way the top shell is designed to contour to the foot and the bottom shell hold the posts.
When designing the shells, the gap in between them is what is left once milled. This alloows variable thickness's to be designed in.
In addition you can leave extra bars of material - or any shape you want - to change the thickness of the materials and it stiffness in any direction.

Does this help?

Phil

 

Dave

We have been playing about with a new CAM material that offers this intrinsically.
First impression is that the low shore value of 20 is too soft to control loading but due to the materials 'sponge like' qualities- and leaving it shank dependant when finishing - as the material loads it seems to offer more resilience and ORF.
patient compliance is good and the material does not seem to bottom out like EVA.
I keep you posted once I have finished testing on patients using F-Scan.

Phil

 

:good: Thank you Kevin!

 

Thanks so much for your explanation Simon!

In the 'Shank-dependent' thread that you linked to, you explain that:

From what you have said, it seems that deceleration of pronation is not important. In fact, it would be an advantage to have a higher pronatory acceleration so that eg: tibialis posterior could return the energy is has stored from the rapid pronation and use it to provide a supinatory force.

Is this right? It sounds counterintuitive to me.

Thanks again for your help Simon.

Rebecca

 

Hi Asher,

I’m currently still busy trying to comprehend all this information. However, I have to query your last post:

I would have thought deceleration was important (to reduce force within soft tissue), especially if there is a suspected pathology occurring in the post tib. tendon (For e.g. post tib. dysfunction). From how I understand it so far, the modulus of resilience is the quantity of energy a material can absorb without suffering damage i.e. the amount of stress applied to the material is within the threshold which allows it to return to its original shape. However, if we had higher pronatory acceleration then wouldn’t it be more likely for the strain to exceed the yield point, which would mean the material is deformed irreversibly? Am I missing the point here?

This is all still foreign to me at the moment and I am probably wrong. Looks like I will probably have to wait for Simon’s input.

Cheers,

Daniel

 

Daniel

I agree, it doesn't seem right. I would have thought deceleration would be a good thing. However, I'm sure science / physics will tell us the correct answer (via Simon).

Rebecca

 

Subjecting a ligament or tendon or bone to higher energies is not always a good thing. When a structural component of the body is subjected to too high of an energy, tissue damage will more likely occur. However, tissues that may absorb and then release more elastic energy, are able to return more energy back to the body. The problem is knowing how much external energy may be applied to the foot, with each individual foot geometry, without exceeding the elastic limit on the stress-strain curve for each individual structural component of the foot. The knowledge is not there yet, but the knowledge may be there in the future with better modelling techniques.

 

It is somewhat counterintuitive, but you have arrived at the point I wanted you to be at. Is deceleration important? Probably, because there is a limit to how much energy a tissue can deal with, too much =injury, but so is tissue stiffness, stiffer= greater capacity to store energy. So it's all a question of balance between tissue stiffness and loading rate.

 

Hello Phil,

Yes, I understand. It is usually done in different kind of orthoses. Thanks for the explanation.

Regards,

 

Agreed. 3) Do a combination of #1 and #2. should be the most effective.

We discussed something similar before when I suggested that a centre of pressure (CoP) that was medial to the STJ axis could result in pronation moment because of the direction of the vector. You noted at this time the effects of frictional forces that would tend to keep the vector angled toward the vertical; giving rationale for the medial heel skive techniques effectiveness, despite it's apparent potential to place the vector "on the worong side of the axis". As you pointed out at this time, given that the orthosis reaction force (ORF) vector will not be normal to the orthosis surface angle due to frictional effects, if we made an orthosis that only contacted the foot lateral to the STJ axis, do you think it would be possible to generate a net ORF vector that passed medially to the STJ axis? Would this be from lateral heel-cup?

Sorry to switch betweeen force and pressure. Pressure = force /area, if the force remained constant but we reduced the contact area, pressure would increase. Could we get a medial shift in centre of pressure simply by having a flat insole that on the medial side of the STJ axis had holes driled into such that the contact area on this side of the foot was reduced?

 

So the next question is this: if we have a tissue,i.e. the tib. post. tendon that is exhibiting symptoms of excess mechanical loading, can we provide mechanical relief to this tissue without decelerating the rate of loading?

 

1. Decrease the total range of (pronatory) motion
2. Increase the strength of the muscle

Rebecca

 

Simon and Colleagues:

My point was that as long as a foot orthosis is decreasing the STJ pronation moment acting on the plantar foot from GRF, the concept of rotational equilibrium tells us that the mechanical effect of this decrease in STJ pronation moment is the same as an increase in STJ supination moment. Therefore, using this logic, a foot orthosis does not necessarily need to push medial to the STJ axis in order to produce a "STJ supination effect". A foot orthosis may also produce this "STJ supination effect" by acting only on the lateral side of the STJ axis, as long as the orthosis is able to shift GRF from a more lateral to more medial position on the plantar foot. This reduction in magnitude of STJ pronation moment from the orthosis, even though it does not necessarily cause any STJ supination moment, may cause the STJ to supinate, due to the concept of rotational equilibrium.