Shank dependent vs shank independent foot orthotics

Well I would say its all about forces, nothing new there. We are changing forces in an attempt to reduce tissue stress, as you have stated. We are aiming to provide an orthotic reaction force to reduce damaging forces to bony and soft tissue structures such as ligaments, articular cartilage and muscles. Eg:

In orthoses for plantarfasciitis, we are providing an orthosis reaction force which reduces damaging forces to the plantarfascia such as a cluffy wedge which holds the hallux in a relatively dorsiflexed position (preloading the hallux) to promote earlier windlass mechanism establishment which reduces pathological stain on the ligament.

In orthoses for osteoarthritis of the dorsal medial midfoot, we provide an orthosis reaction force which reduces medial forefoot dorsiflexion moments, such as a FF valgus forefoot extension which promotes 1st ray plantarflexion, reducing dorsal compression forces at the medial midfoot and thereby reduces damage to the articular cartilage of these joints.

In orthoses for posterior tibial tendon dysfunction (PTTD), we are providing an orthosis reaction force which reduces the pathological load on the tibialis posterior musculotendinous unit with variables such as a medial heel skive or medial flange. So we are reducing muscle activity patterns.

I realise this is nothing you don't know and is probably not the answer you are after, but you have asked twice.

Rebecca

 

Hi Rebecca,

I agree that it is about forces. One way to figure out how we should try to alter the forces on the foot is through free body diagram analysis of the injured structures. Regarding plantar fasciitis and the cluffy wedge. In modeling the plantar fascia (Fuller, E.A. The Windlass Mechanism Of The Foot: A Mechanical Model To Explain Pathology J Am Podiatr Med Assoc 2000 Jan; 90(1) p 35-46) I would predict that decreasing pronation moment, and decreasing load on the first ray should decrease load on the plantar fascia. I really don't see how the cluffy wedge decreases load on the plantar fascia. Given a foot standing with and without a cluffy wedge, I could make the case that the wedge would increase tension in the plantar fascia. I don't think it would be a really good case, but I can't make a case for it decreasing tension in the fascia.


Regarding the dorsal midfoot arthritis. You are mixing motion and forces. I agree with the treatment, but I am quibbling with the explanation. The dorsiflexion moment on the medial forefoot is proportional to the load on the medial forefoot. A forefoot valgus wedge is added in attempt to reduce medial forefoot load and increase lateral forefoot load.

I think that this does address the question that Simon was asking. If an orthosis, or a wedge, or a wadded up facial tissue, alters the location of forces under the foot then we should expect some change in the internal loads. (Wadded up facial tissue would be shank dependent) We can use models to predict how altered loads will alter stresses so we can have a good idea of where to put the tissue.

Regards,

Eric

 

Hi Eric

Agreed

I understand what you're saying. (However, correct me if I'm wrong, but a cluffy wedge is not contraindicated in plantarfasciitis). A better example of an orthosis prescription variable would have been a medial heel skive or plantarfascial accomodation or FF valgus forefoot extension.

OK, if you say so, I mean it makes sense. But just like Kevin Kirby's Thought Experiment re: sinus tarsi compression forces, I figured that position could be the same but forces very different.

Thanks for your suggestions Eric.

Rebecca

 

Not really, so I'll have to ask again

How does it alter the location of forces under the foot?

 

How does an orthosis change the forces under the foot? Umm ... it just does. By pressing on the skin, subcutaneous fat, muscles, bones to not necessarily change position but to change .............................................................. (I want to say change forces but that's the question).:dizzy:

Well I don't know, but the net result is different stresses applied to ligaments, bones, tendons, different muscular activity etc.

I'm confused.

Rebecca

 

Simon,

I think you may have to elaborate further, on this one...

Is this related to the question above?

 

OK I’ll have another go.

The foot has to travel from heel strike to toe off – the stance phase of gait.

On its own (in a shoe, no orthosis) it gets there via a certain path. If that path causes pathology (pain, deformity , performance issues) we design an orthosis (or provide some other treatment).

The orthosis causes the foot to get from heel strike to toe off via a different path. We use prescription variables to cause this change of path. It does this by altering forces (kinetics). The orthosis changes internal and external moments to achieve this. It may or may not change position and movements (kinematics).

Having said that, I can’t discount the fact that an orthosis does change position to some degree. Maybe not of bones (hence we can’t rely on measuring kinematics to explain why orthoses produce good clinical outcomes). But the orthosis does change the position of the soft tissues (skin, subcutaneous fat, fatty pads, muscles). Think of the soft tissue of the heel sitting on a medial heel skive, it gets compressed. And of the soft tissues of the midfoot with an orthosis contouring the medial or lateral longitudinal arch. Does this explain why orthoses work – I don’t know. But one of my colleagues thinks its important – me I don’t understand it – but I have an open mind.

Simon, do you have an answer and you’re waiting for someone to say it? If so, what is it? If not, what’s your best guess?

Rebecca

 

Hi Asher,

I dont see any differences between your two answers. This doesn't mean that I think you have answered the question incorrectly though. I think on both occassions you have appropriately addressed what Simon has asked.

Simon, I really dont understand the "how do they do this" part of what you are asking? How has Asher not answered the question? Are you suggesting or trying to figure out if there is something else happening other than a reduction in tissue stress i.e. forces?

I will be looking forward to you elaborating further...

Cheers,

Daniel

 

Hi DSP

Agreed, I have just repeated what I said initially, just differently. But its all I've got.

Me too.

Rebecca

 

Rebecca:

If you size your font much smaller..... I will need to get out my glasses to read your posting!

I think you are doing very well in answering Simon's question, Rebecca. If we compare a foot orthosis to walking on a flat surface with no contour, then certainly the mechanical effects of foot orthoses are at least partially caused directly by the change in reaction forces acting on the plantar foot. Standardly, this reaction force is known as ground reaction force (GRF). In the special circumstance of the reaction force acting from a foot orthosis on the plantar foot, I have coined the term orthosis reaction force (Kirby KA, Green DR: Evaluation and Nonoperative Management of Pes Valgus, pp. 295-327, in DeValentine, S.(ed), Foot and Ankle Disorders in Children. Churchill-Livingstone, New York, 1992).

In another thread, I put the mechanical effects of foot orthosis into two main categories, passive mechanical factors and active mechanical factors:

In your example of the medial heel skive, the increased varus plane of the heel cup in a medial heel skive orthosis will increase the orthosis reaction force (ORF) on the medial calcaneus and decrease the ORF on the lateral calcaneus when compared to an orthosis without a medial heel skive. This redirection in ORF toward the medial heel and away from the lateral heel will increase the subtalar joint (STJ) supination moment and/or decrease the STJ pronation moments that are caused by ORF when comparing a medial heel skive orthosis to a standardly designed foot orthosis. In addition, a medial heel skive orthosis will exert greater STJ supination moment versus no orthosis (i.e. flat surface) due to this same shift in reaction forces toward the medial plantar heel and away from the lateral plantar heel. This shift in ORF or GRF medially on the heel that causes increased STJ supination moment would be considered a passive mechanical effect of the foot orthosis.

However, there are also effects of orthoses that are mediated by the central nervous system (CNS) that we commonly may see in our practices, but are less commonly understood. These active mechanical factors may involve the CNS in causing paradoxical motions of the foot in response to the passive mechanical effects of the foot orthosis. For example, if a foot orthosis is over-inverted, so that individual's CNS senses that the dramatic increase in external STJ supination moments from the foot orthosis may cause an inversion ankle sprain, we may see our patients undergo significant late midstance pronation which is not directly caused by the passive mechanical actions of the orthosis. Rather, this late midstance pronation is caused by the indirect CNS effects that will actively increase late midstance peroneal contractile activity that will increase the internal STJ pronation moment and prevent the individual from suffering an inversion ankle sprain from the over-inverted foot orthosis.

The effects of foot orthoses on the human locomotor apparatus are not simple, and we don't know all there is to know about them, however, our understanding of how they work is much better now than it was just 10 years ago.

 

By altering the contact surface geometry. If you took two flat metal plates and then pressed them together there would be even pressure throughout the contact surface. Now, throw a grain of sand inbetween those two plates and at the location of the grain of sand there will be increased pressure.

Translating this to feet and orthoses is a little more difficult. There are two somewhat deformable objects that will be pressed together between ground reaction force and body weight. However, if you start with one orthosis and one foot, you can alter the forces at the foot orthosis interface by adding grains of sands, or something else.

You could also alter the location of forces by altering the stiffness of the orthotic material. Changing the stiffness of the orthosis will alter the contact surface geomtry of the loaded device.

It is very interesting to think about how changing the contact surface geometry will change forces. Simon, are you asking us to think about the molecular level. Surely, you must have thought about this with the modeling that you presneted at PFOLA a couple of months ago.

Regards,

Eric

 

Sorry about that, I had lost the whole post and did it again on a word document. Will up the font size next time!

Wow, I have never considered "active mechanical factors". I wonder, are these predictable from one person to the next? That is, will everyone react similarly via the "active mechanical factors" to an intervention. Maybe that's why (from Craig's Bootcamp lectures), some people pronate more compared to without an orthosis and some pronate less compared to no orthosis.

Rebecca

 

This concept goes hand in hand with Benno Nigg and colleagues' theory of Preferred Motion Pathway that the body will try to keep its motion pattern relatively constant even when subjected to external forces, thus causing a change in muscle tuning with different interventions. When an inverted orthosis is used, some people use their peroneals more than others, which is probably related quite a bit to the subtalar joint axis spatial location (i.e. a person with a STJ which is more medially deviated will have less peroneal activity than a person with a more lateral axis when walking on an inverted orthosis).

 

Indeed Eric!

We talked about surface geometry here: http://www.podiatry-arena.com/podia...p?t=2345&highlight=biomechanics foot orthoses from post no. 51 onwards, so I won't repeat myself here at the risk of Mark Russell's disapproval- nothing new under the sun Mark and indeed my first posts to this forum demonstrated my frustration with the repetition of subjects that we had previously discussed on the JISCMAIL forum.

In addition to what has previously been said on surface geometry, here's where my head is currently at:

The fundamental purpose of a foot orthosis is to transmit loads from the point of application to the point of support and, ultimately, through the shoe to the ground. During activities of daily living the orthosis may be subjected to a wide variety of loading patterns. In order to produce equilibrium, it must provide an equal and opposite reaction. When loads are applied to an orthosis, the orthosis tries to absorb its effects by developing internal forces that, in general, vary from one point to another. The intensity of these internal forces is called stress. When subjected to external loads, orthoses change their shape; though sometimes imperceptibly. This change in shape is termed deformation and is measured by the mechanical strain. Thus, the necessary orthosis reaction is generated by the stress caused by the action of the loads within the orthosis material, and by the ensuing strain in the elements of the orthosis structure. Stress and strain within the orthosis are the direct outcome of the action of forces and the deformations they produce.

The stress/ strain ratio defines the stiffness of the orthosis. The inverse of stiffness is compliance. The stiffness of an orthosis is dependent upon the mechanical properties of the construction materials and the geometry of the orthosis. It has not, to date, been determined exactly what the clinical relevance of stiffness is in a foot orthosis. As such the optimal degree of stiffness is not readily definable. However, it has been demonstrated that by manipulating the stiffness of the supporting surface to be within an optimal range, a runner’s performance may be enhanced by 2-3% and their risk of injury may be reduced by 50% 1. Additionally, Kerdock et al have shown that 12.5-fold decrease in surface stiffness resulted in a 12% decrease in metabolic cost with the runner’s support mechanics remaining “essentially unchanged”2. This appears to add support to the preferred movement pathway and muscle tuning model proposed by Nigg3 and to the contention that changes in the mechanical properties of foot orthoses might produce adjustments in the muscular response of the locomotor system4,5,6.

Resilience is the property of an orthosis to store energy when it is deformed elastically and then, upon unloading to have this energy recovered. In other words, it is the maximum energy per volume that can be elastically stored. It is represented by the area under the curve in the elastic region in the stress-strain diagram. Modulus of Resilience, Ur, can be calculated using the following formula: Ur = ?2 / 2E = 0.5 ? ? = 0.5 ? (?/E), where ? is yield stress, E is Young's modulus, and ? is strain. In general, orthoses that exhibit high values of stiffness and low mass will exhibit greater resilience7. The significance of resilience in foot orthoses is also unclear. However, the ability of a foot orthoses to serve as an external store of elastic energy that could then be returned to the body to assist in locomotion would appear to be advantageous 8,9,10,11,12


So I guess the answer to the question I posed is: by developing mechanical stress and strain. This is really part of a first draft of the writing up of my PFOLA presentation- I hope this is helpful to those I've confused with my questions and should welcome your comments.

Sorry- cutting and pasting ruined the equations by inserting lots of "?", I'll update when I get time.

 

Hi Simon,

I am enjoying digesting your last post. One thing though, you have numbers 1-12 as if referencing but no reference list. Do have that handy?

Rebecca

 

Hi Simon, Asher, Kevin - I have really enjoyed this thread

What I glean; understanding how an orthotic works is fundamental to understand our universe.

then i read this http://www.theonion.com/content/node/29554 so I am just going to knock my degree course on the head and buy a white coat. :wacko:

 

Firstly, thank you very much to all the senior posters out there. You have no idea how much your knowledge and experience helps me on a daily basis.

I have been using the techniques described by Ray Anthony in his book " the Manufacture and Use of the Functional Foot Orthosis" to make shank independant orthotics but am now trying out some EVA shank dependant devices. Up to now I have adapted some of the positve cast modifying and manufacturing techniques described by Ray to suit working with EVA. Is there a similar book or manual out there that is evidence based and specifically deals with making shank dependant devices?

Thanking you

Chris Delpierre
Podiatrist
Sports Science Institute of South Africa
Newlands, Cape Town

 

Off the top my head, I can't think of anything that I do differently in my positive cast prep for EVA and polyprop devices. If I think of something I'll come back to you. So why do you need another book?

 

At the moment there is a bit of trail and error going on with selecting which shore hardness to use for which application. Obviously the weight of the patient, which shoes they will be used in, what activity etc all play a role. I was hoping others that use these devices could give me some guidelines.

Thanks,
Chris

Chris Delpierre
Podiatrist
Sports Science Institute of South Africa
Newlands, cape town

 

Chris:

When using a shank dependent material, such as ethylene vinyl acetate (EVA) or Plastazote, to make custom foot orthoses, the bottom line is that the material will never be as durable as a good shank-independent orthosis (i.e. polypropylene) but does have distinct advantages over shank-independent materials.

Here are the advantages of shank-dependent orthoses:

1. Orthosis may easily be modified on its dorsal or plantar surface to make adjustments for contour, orthosis stiffness and shoe fit.

2. Patients often perceive the orthoses as being "softer" and therefore more comfortable.

However, here are the disadvantages of shank dependent orthoses:

1. Large change in orthosis stiffness and/or function may occur when orthosis is moved from one shoe to another due to change in shoe fit and changes in shoe shank contour.

2. Durability is much less than that of polypropylene and other shank-independent materials.

3. Orthoses will change shape more rapidly over time.

Remember, it is not the material with which the orthosis is constructed that necessarily is the determining factor in optimizing orthosis therapeutic efficacy. Rather, it is the shape of the orthosis, the response of the orthosis to weightbearing loads, the fit/function of the shoe-orthosis combination and the clinical skill of the practitioner which most determines how well orthoses will work to relieve a patient's symptoms and to improve their gait function.

 

shank dependent vs non-shank dependent

Hi,

I've recently heard some refer to shank dependent and non-shank dependent orthitics - would someone be able to explain what this means please?

Many thanks!

 

Re: shank dependent vs non-shank dependent

l am sure Admin will be along shortly to put up threads.
But in short .........
Shank dependent; EVA type orthosis that require the support of the insole/shank of the shoe to retain their shape.

Shank independent; Polyprop type orthosis that require no additional support.

By the By, l disagree with the "shank" term been used as most of the shoes in the market dont feature actual shanks these days :wacko: