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Hi all,

I have noticed that individuals/ companies that sell high density foams or bladders full of some type of material for taking weight-bearing impressions of feet are elevated at the heel. I am assuming this is because if their impression material were flat that the heel would sink in further than the ball of the foot (dorsiflexing to a degree the ankle) therefore not producing an ideal impression. Similar to if you stand in a box full of sand, your heels sink in further than the forefoot.

My question is, is there an accepted degree of heel elevation in relation to the forefoot when taking a weight-bearing impression, so to equalise the forefoot and heel sinkage into the impression material? Are there any other considerations when taking a weight-bearing impression using a soft mouldable material to maximise the benefit of the orthoses that are then made from these moulds?

Thanks in advance.

 

I can't directly answer your question, but had an interesting and productive meeting with the product development manager of ASICS last week. I hope I do not breech commerical sensitivities here, but he pointed out that the industry standard for running shoes (not just ASICS) is a 10mm heel raise - and that it was a purely comfort thing --- anything less had been found to be uncomfortable.

As part of the development of a shoe for another sport, they made the heel height differential 8mm and the feedback on the shoe was not as good as liked. This feedback changed when they went to 10mm.

So maybe there is something magic about a generic 10mm, though I have no doubts that there will and should be subject specific differences .... it just how do you tell them.

 

Thanks Craig,
I will try and take an impression of my foot so that the heel sinks into the material 10mm less than the forefoot and see how it feels. The orthotic arch would be more comfortable in a device made this way compared to if were in level with the forefoot.

 

tarik et al

To the best of my knowledge in the manufacture of shoes there are several measurements (according to Rossi W A (editor) 2000 The Complete Footwear Dictionary (2nd edition) Kreiger Publishing Co.)

Heel height is a measurement from the floor to the shank taken at the heel breast (anterior aspect). Heel heights are tradionally measured in increments of 8th inches.

A common mistake made by lay persons is to use the heel elevation which is measured from the extreme rear (neck of the heel) as the height of the shoe. This captures a greater distance from the ground because the heel seat (the cupped section on which the foot's heel rests) lies on a slant (or heel pitch). The heel pitch describes the verticle slant or angle of the heel at the rear from heel seat and this is called the heel angle or angle of the heel seat.

This value probably corresponds closest to the foot heel elevation in a block cast (eg Oasis brick) that tarik referred too.

Toe spring describes the elevation of the under surface of the sole at the toe and gives the shoe a slight rocker effect (across the metatarsal heads). Toe spring and heel height values must correspond and these dimensions are built into the last to allow the forepart of the shoe to bend with the metatarsal phalangeal joints. To do otherwise would cause the foot to bend in an ungiving shoe. Shoe makers make their shoes with this in mind but elevating feet during casting (weightbearing or otherwise) would have no significant benefits to foot orthoses.

Raising the heel during casting may result in unlocking the mid tarsal joint, causing the forefoot to become unstable (and splay).

A common casting exercise used in many biomechanics classes was to compare anthropmetric measurements between foot casts taken in various ways. Statistical analysis demonstrated a significant constant which supported referenced casts captured the shortest linear distance between the heel bisection and centres of the 1st and 5th Met heads; and
the angle between these points (breadth of the foot) were the narrowest. The values for prone and supine casting in the referenced position were consistent indicating either practised method was acceptable but weightbearing or semi weightbearing casting was likely to capture a pronated foot.

>My question is, is there an accepted degree of heel elevation in relation to the forefoot when taking a weight-bearing impression, so to equalise the forefoot and heel sinkage into the impression material?

I have never measured creep (shrinkage) in Oasis blocks, but plaster of paris bandage will creep and the sooner the full cast is made the better. How significant the creep rate is again to the best of my knowledge remains unknown.

>Are there any other considerations when taking a weight-bearing impression using a soft mouldable material to maximise the benefit of the orthoses that are then made from these moulds?

If you have an infirm client or want a pronated cast then blocks are fine (but a very expensive alternative). I once did a cost benefit analysis and was staggered.

If you have a mind to simulate the conditions within a shoe (ie using a heel lift) this adds potential risk to the client and the process of trying to get a reference position (in a weightbearing client) becomes a complicated procedure and I would doubt whether the effort is merited in the final product.

toeslayer

 

Good post.

 

Thanks for the excellent reply.

I did more impression testing last night with people of different weights. As you mentioned all had pronated impressions with first ray dorsiflexed. Also attempted elevating the heel to various degrees with mixed results.

Thanks again