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I was going over threads tagged with FnHL when another excellent question from Asher, which included this 'gem'

I have woken up in the midst of many a night worrying about same.
The width of the medial, posterior, and lateral heel flanges, and the "width" of the orthosis must determine where the orthoses fits in the shoe but surely the position of vamps and quarters, and breadth of footwear determines where the foot fits, maybe not necessarily where you want it in relation to the orthosis????
Maybe if the cast was made 'in-shoe' cast there would be no worries?, mark

 

Mark as ever, the threads that you start are thought provoking. As I've just spent most of my morning writing about the mechanics of foot orthoses my afternoons work was to attempt to put into writing something regarding the biomechanics of the foot-orthosis interface.

Here's a question that needs to be answered: When the foot is in a shoe with an orthosis in situ does the foot move on top of the orthosis?

 

One thing I like to do when I make my orthoses is to have a fairly significant curl upward along the lateral edge of the orthosis (done by altering the shape of the lateral expansion). The orthosis is an inclined plane and the foot will tend to slide down the slope unless there is something holding it up. Or within the shoe the orthotic will slide medialy unless there is something preventing it. So the lateral lip of the orthotic will either prevent the foot from sliding or become really uncomfortable as it digs into the undersurface of the 5th metatarsal.

In some shoes the sock liner curves upward at the lateral margin. This upward curve of the sock liner can push the orthotic toward the center of the shoe and this is a common cause of irritation of the lateral edge of the foot from orthotic. You can just cut part of the sock liner off so that the orthotic sits against the lateral edge of the shoe.

One of the things that John Weed used to teach was to look at the impression from the anterior edge of the orthotic in the shoe. That will tell you where the orthotic is sitting in that shoe. Once the hole is dug the orhtotic will tend to find its place in that hole. So placement is important the first few times an orthotic is put into a particular shoe.

Cheers,

Eric

 

I don't see how the foot can be prevented from moving while on top of an orthotic in a shoe. The hole in the back of the shoe studies confirm that the foot moves relative to the shoe.

regards,

Eric

 

Thank you Eric. So a given point on the plantar surface of the foot will effectively be being drawn up or down an inclined plane, during a given time period- right?

 

Oh, I see what you are saying. You are talking about medial to lateral movement of the foot relative to the orthotic. I was thinking about joint movement above the orthotic. You do have a valid question.

Eric

 

There also will be forward progression as well.

 

Do we have a valid answer? Any shear studies done on top of an orthosis?

 

Mark:

Excellent question. The exact fit of the orthosis inside the shoe is an important point that all clinicians that are making custom foot orthoses need to be aware of. I have seen even a 1 mm medial, lateral and/or anterior displacement of the heel cup of a foot orthosis inside the shoe cause significant discomfort for a patient. I have covered this topic of orthosis-shoe fit in my chapter on Troubleshooting Foot Orthoses in Ron Valmassy's book (Kirby KA: Troubleshooting functional foot orthoses. In Valmassy RL(editor), Clinical Biomechanics of the Lower Extremities, Mosby-Year Book, St. Louis, pp. 327-348, 1996). Ron's book is still in print and is available at amazon.com.

One common test, which I use frequently in my practice for these types of orthosis-shoe issues, I call the Barefoot Standing Orthosis Test (BSOT), which I first wrote about in my November 2000 Precision Intricast Newsletter (Kirby KA: Foot and Lower Extremity Biomechanics II: Precision Intricast Newsletters, 1997-2002. Precision Intricast, Inc., Payson, AZ, 2002, pp. 163-164). I may commonly assess the comfort of the orthosis outside the shoe with the BSOT if they are complaining of discomfort with the orthosis inside the shoe to try to rule out whether it is the actual shape of the orthosis that is causing the problem or whether it is the mechanical interaction of the orthosis with the shoe that is causing the problem. I use this test probably about 3 times a week to troubleshoot orthosis problems in my patient and it has saved many a good orthosis from being modified all for a poor shoe choice by the patient.

Hope this helps.

 

While I was looking for Van Langelaan EJ: A kinematical analysis of the tarsal joints. Acta Orthop. Scand., 54:Suppl. 204, 135-229, 1983

I put in "orthotic shear stress" and only came up with studies to do with silicone-porron top covers type questions and diabetic ulcers.

I did not find anything compairing EVA -POLYPROP or other types of orthotic materials.