Why is a medial arch needed in an orthosis?

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Apart from its indication in orthoses to be used in a wound clinic for decreasing plantar metatarsal head ORF, why does it have to be included?
In 'medially deviated STJ' patients the arch would either be increasing STJ pronatory moments, be 'beneath' the axis or providing such a short lever arm to be insignificant?
Obviously not wanted in a 'laterally deviated STJ' patient's orthoses.
To increase rearfoot dorsiflexion moments / increase forefoot plantarflexion moments, if contains a 1st ray cutout, fair enough, any other indications?

 

To influence the MTJ's.

 

You also need to consider if you add a Medial skive the STJ axis position will be more deviated laterally and of course the STJ axial position is always moving and then there is the timing of ORF to consider in relation to the STJ axis

You also need to consider the MTJ or Talo-Navicular Calcaneo-cuboid joints and how the medial arch will affect those joints ( as Simon Said)

Plus there is Plantar tension ie Plantar ligaments, fascia Muscle to consider

 

Thanks Mike for replying

How would this relate to mla arch v no mla arch?

Have been busily (still had dinner though) through the PA threads tagged with 'midtarsal joint' and so far haven't come across any mention of how the 'medial arch' in an orthoses affects the midtarsal joint, will continue tomorrow ....

Haven't come across this also, how the medial midfoot shape of the orthosis affects MTJ kinetics, but will keep at it, thanks again to you and Simon, mark

 

Mark:

There are numerous functions of the medial longitudinal arch (MLA) of a foot orthosis:

1. Increases the ground reaction force (GRF) in the MLA of the foot.
2. Increases the external subtalar joint (STJ) supination moment acting on the foot versus no MLA in orthosis.
3. Increases the external rearfoot dorsiflexion moment and decreases the external forefoot plantarflexion moment (i.e. decreases the flattening moment on the MLA of the foot).
4. Will decrease tension load on plantar tension-load bearing elements of the MLA of the foot (i.e.plantar ligaments of MLA, plantar fascia, medial plantar intrinsic muscles, posterior tibial muscle, etc).
5. May function to decrease GRF on lateral column of foot by shifting more GRF to medial plantar midfoot.
6. Will promote more normal 1st ray-1st metatarsophalangeal biomechanics during gait in foot with functional hallux limitus by reducing tension in medial slips of central component of plantar aponeurosis.
7. Works along with medial heel skive modification to increase external STJ supination moment and increase comfort and "anti-pronation-controlling" ability of foot orthosis in patients suffering from pathologies due to excessive external STJ pronation moments.
8. Reduces dorsal interosseous compression forces in medial midtarsal and midfoot joints.

Hope this helps.:drinks

 

Generally agreed although with number 6 I'm sure one of Geza Kogler's papers showed an arch support increased tension in the medial fascia (within the limitations of a static cadaver testing set up).

The paper is on my other laptop right now so I stand to be corrected but is it reasonable to assume a more convex MLA will may increase tension in the medial fascia, where a concave arch profile may not?

 

Two big problems with Kogler's papers on cadaver feet: 1. The Achilles tendon was not loaded in any way and 2) the study was static, not dynamic.

Any paper studying plantar fascial dynamics without loading the Achilles tendon, which has been shown to have a direct action on plantar fascial tension, will not answer the questions we need to have answered.

That being said, I am certain in some instances, a foot orthosis with an exceptional high, stiff MLA could increase the plantar fascial tension. In patients where I want to specifically decrease the tension within the medial band of the plantar fascia, I put a plantar fascial accommodation into the MLA of the orthosis to "stress-shield" the plantar fascia. Clinically , this seems to work very well. However, until dynamic studies are done in live subjects, we simply don't have adequate data to either support or refute these hypotheses.

 

Thanks Kevin, what has me 'worried' is when a prescribed design variable has both desired and unwanted applications of force to different structures. I recall your diagram of the medially deviated (transverse plane component?) STJ axis every time I consider the 'arch' of the orthosis. Thanks for the post, have re-read several times, not difficult to comprehend just so as I 'retain' it, mark

 

I thought that the ORF from an 'angled' region of the orthosis, say the medial arch area, was indeed 'angled' having a vertical and a horizontal component, thus wouldn't it be applying a STJ pronation moment for the most part as the STJ axis passes superiorly to the midfoot. This is where the transverse plane STJ axis has its shortfall as a predictor of STJ moments surely, mark

 

... fully realising the horizontal component would have both a medial-lateral and a, depending on which part of the orthosis arch, distal-proximal or proximal-distal; and that the ORF's would be subject, activity, surface (as in stiffness and slope) specific; and most importantly, i suppose, the part of stance phase the foot is at.

 

To me it will be interesting to define first what the "orthosis medial arch" is !

In the case of an "arch support" an interesting article (also from PA) is: EFFECTS OF FOOT ARCH IMPEDANCE ON NAVICULAR DROP AND CALCANEAL EVERSION DURING WALKING: http://www.asbweb.org/conferences/2004/pdf/277.pdf
"SUMMARY
These preliminary findings suggested a greater calcaneal eversion might occur when the foot arch is impeded during the midstance of walking. As orthoses are often prescribed to correct excessive rearfoot motion, constraint on midfoot navicular drop
needs to be considered."

Daniel

 

And even this will not answer to "why is medial arch needed...?" at least will try to answer to the question "how the arch height is determined ?" in the case of Blake Inverted Orthotics
http://www.drblakeshealingsole.com/2010/09/inverted-orthotic-technique-determining.html

Daniel

 

Goodaye Petcu, couldn't think of a better short descriptor, I suppose I could have described it as, "you know the bit of an orthosis that's roughly in the middle front to back sloping uppity then down and extends from a little bit in from the outside, sloping uppity all the way to the inside". Am certainly open to an apt title.

Ah you're talking about the inside one, aren't you

Thanks Daniel will definitely peruse that tomorrow, it's 10pm here, should have, "Now I lay me down to sleep ....." 'ed a while ago, my CPAP awaits, and thanks for the reply and reference, mark

 

I do not know to be another one ! Am I wrong ?
Daniel

 

Indeed, the net GRF vector on top of an orthosis probably has an increase in the shear components of the vector when compared to shoe-only conditions due to the increase in non-horizontal surface areas beneath the foot introduced by the foot orthosis. These shear components are comprised of both medial-lateral shear and anterior-posterior shear. Without knowing both the 3 dimensional position of the STJ axis during dynamic function and the GRF vector at each instant in time, we simply cannot judge the sense of the external moment- I tried to explain this here: http://www.ncbi.nlm.nih.gov/pubmed/21084541 . The caveat being that if we observe motion at subtalar joint, we must conclude that the net moment (internal + external) is in the direction of the motion.

 

I suspect Mark was referring to the lateral longitudinal arch. So when we talk of "an arch support" it could refer to either the medial longitudinal arch, the lateral longitudinal arch, the transverse tarsal arch or all of the above.

 

The angles of the arch are not very high, so most of the force is vertical. If there is horizontal force, from the medial arch that would be pushing the foot from medial to lateral. If the foot is not accelerating from medial to lateral then there must be some other force from lateral to medial that brings the net force to zero. (F = Ma. a is zero so net F must be zero) This can either be friction of from lateral heel cup. It could be possible that the medial to lateral and lateral to medial components have some affect on STJ axis moments. However, the largest contributor to STJ moment is going to be where the vertical force applied at center of pressure is located relative to the transverse plane projection of the axis.

Eric

 

What would be considered as a "high" angle?

Which is dependent upon the top-cover materials co-efficient of friction. So, in a study I conducted the angle of friction for various top-cover materials were as follows- against cotton hosiery: EVA= 30 degrees, vinyl= 24 degrees, Poron= 26 degrees, polypropylene= 22 degrees. Against nylon hosiery: EVA= 29 degrees, vinyl= 22 degrees, Poron= 23 degrees, polypropylene= 20 degrees.

For those following: tan α =μ
where:α = Angle of friction; μ = Coefficient of friction

Out of interest here is a slide from one of my lectures where I measured a couple of angles from the superior surface of an orthosis shell, as you will see, we got angles here exceeding the angle of friction for certain top-cover to hosiery interactions.

 

Agreed.:drinks

 

Disagree for the reasons outlined above. I'll repeat the question: what would be considered a "high angle"? Moreover, how do foot orthoses alter the frontal plane component of the external knee joint moment if not by modifying the magnitude of the "vertical" component of the net GRF by increasing the shear components? Is it that the angle of the net GRF vector is altered by the orthosis or simply that the magnitude is altered? Yes "most force is vertical" but we could make strong argument that foot orthoses work by increasing the non-vertical components to the net GRF. After all, when we break-down a foot orthosis, it is just a series of inclined planes... and that calls for a basic physics approach to their analysis- don't ya' think?

 

That sure was a frustrating explanation. He described the shape, but did not mention the height.

 

I don't see how the orthortic is going to reduce the vertical component. The vertical component will vary with the accelerations of the center of mass. At rest, the vertical component of the force on the foot has to equal body weight. Orthoses work by altering the location of application of force, rather than the magnitude.

The horizontal forces must cancel when there is no acceleration of the foot relative to the orthotic. Now, there might be some configuration where the lateral heel cup or lateral flange forces could create some net moment at the STJ. But the fact that there is not net lateral to medial forces makes me thing we can get away with just looking at center of pressure and transverse plane projection of the axis.

Eric

 

Why it was a frustrating explanation ? Because if you have the shape (which in my opinion give more information than height !) you know where the highest part of the arch and also it's value ! On the other hand, seems that the height is dependent by the inverting degree ( standard 25 degree in that example ). How it will help you to know only the value of arch height ?

Daniel

 

It was frustrating because he did not answer the question: how high do you make the arch? With the long answer that he gave he didn't say how he decided how high to make the arch. At least with MASS there is an attempt at explanation of how high the medial arch should be. Rich Blake did say that he puts high point of the arch proximal to the first ray and I agree with that. You can have a 25, 30 & 35 mm high arch with the high point proximal to the first ray.

I find that some people like extra pressure in the arch and others can't wear the device that has too high of an arch. Since I've been doing standing medial arch height measurment, I haven't been pushing the limits, nor have I been experimenting with very high arch heights. I would bet that those that can't tolerate high arches are those that have high supination resistance and medially deviated STJ axes. High arches would push up on the foot in a location where there is little leverage to cause supination.

Eric