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Professor Kirby wrote in an earlier post ("Persistent Heel Pain"; 2006) ... "try a custom foot orthosis first to see if the improved congruity to the plantar contours of the foot from a custom foot orthosis helps relieve her pain...."
My query is why would more congruous contours necessarily alter forces for the better? Is this only true for the heel or can be applicable to other sites of pain / pathology? Thanks, mark

 

pressure = force / contact area. Increase "congruity" = increase contact area = decrease pressure per unit area.

 

Goodaye Simon, what i'm thinking is a congruous shaped orthoses wouldn't result in equal ORF? to different, but equivalent in SA, plantar regions of the foot due to the different deep structures, bone is nearer the surface in some areas rather than others where there would be much greater depth of soft tissue. And aren't increases in forces at certain regions desirable in relieving pain?

 

No.

I don't know what SA means?

 

Mark:

For plantar heel pain, where the medial calcaneal tubercle is the area of maximum tenderness, using a custom foot orthosis that exactly contours the shape of the patient's plantar heel will decrease the ground reaction force (GRF) in the central aspect of the plantar heel (i.e. where the pain is located) and will increase the GRF in the medial, posterior and lateral aspects of the plantar heel (i.e. where there is no pain). My idea is that most plantar heel pain is caused by increased duration and magnitude of compression forces from GRF acting on the medial calcaneal tubercle, the weightbearing surface of the calcaneus (Kirby KA, Loendorf AJ, Gregorio R: Anterior axial projection of the foot. JAPMA, 78: 159-170, 1988), along with increased duration and magnitude of tensile forces from the plantar fascia pulling on the medial calcaneal tubercle

In order to increase this mechanical effect of reducing GRF on the medial calcaneal tubercle, I may also have the lab use one or more of the following modifications:

1. Order for thinner lateral expansion plaster on the lateral heel of the positive cast (i.e. only 1 mm thick) in order to decrease the radius of curvature of the heel cup of the orthosis.

2. Order for deeper heel cups (16-18 mm) in order to increase the contact area between the orthosis and the medial, posterior and lateral aspects of the plantar heel.

3. Order a plantar "bubble" into the plantar heel of the orthosis and fill it with a soft material such as poron or neoprene.

4. Order a plastazote #3 orthoses made with the heel contact point of the orthosis 3-4 mm thick and then grind the plantar heel contact point of the rearfoot post area into a concavity (i.e. thinner centrally, thicker on periphery) so that the plantar heel has less GRF on it during weightbearing activities.

Over the past quarter century, I have used all of these above orthosis modifications in my foot orthosis treatment of thousands of these patients with plantar heel pain with generally good success.

Hope this helps.

 

Thanks gents,
Simon, sorry, SA = surface area, so does "no" you're wrong still apply?
Kevin, thanks, makes sense (of course), have printed it out and put in my study folder.
mark

 

No, a congruous shaped orthoses will not create equal orthoses reaction forces across the bottom of the foot.

 

So congruity important when decreasing pressure to a region(s) is desirable eg. plantar ulcer, whereas not for biomechanical interventions where increasing pressure to a region(s) is desirable.
And would it be more efficacious that a medial skive be flat directly beneath the medial process of the calcaneal tubercle?
Am I getting annoying?, cheers, marcus

 

This isn't what I said, you asked if a congruous device would result in equal orthoses reaction force distribution, it will not. Congruent devices should decrease the overall pressure (or stress) by increasing the contact surface area of the foot when compared to incongruent devices. However, we can still have congruency, but increase pressure to an area of the foot. Think of a foot with a navicular height of x mm from the supporting surface, we make two "congruent" orthoses that have a height beneath the navicular of x mm and 2x mm respectively, both devices will be congruent, but the pressure beneath the navicular in the second device will be higher.

It would be most efficacious to have a heel skive that is positioned and angled such that the orthosis reaction force vector is positioned as medially as possible relative to the STJ axis (assuming one wishes to provide increase in supination moment) during the desired period of the contact phase.

You're rarely annoying; often entertaining.

 

"Of course!" he says smashing the palm of his hand into his forehead!

 

A better question is congruent: where and when?

I think I missed your point re: heel pad thickness under the medial calcaneal tubercle. This becomes an issue of force attenuation.