Blister Prevention Office Hours - Breaking Down Posterior Heel Blisters

Blister Prevention Office Hours - Breaking Down Posterior Heel Blisters
➡️A monthly Q&A hosted by podiatrist Rebecca Rushton (the BlisterPod) for Podiatrists, Physios, Athletic Trainers, Wilderness Medics, Race Medics and Event Directors, Educators, Footwear Designers, Footwear Retailers, and anyone who deals with foot blisters and their prevention in a professional capacity 👣

[0:00] Intro
We are looking very closely at posterior heel blisters today.

[0:47] The common understanding of what causes posterior heel blisters
The common, and might I say incorrect, understanding of the cause of posterior heel blisters is that your heel rubs the shoe, or your shoe rubs your heel. It's actually impossible for your shoe of your heel, or for your heel to rub your shoe - because the socks in the way. So what is rubbing what?

[1:39] What is shear deformation?

[2:55] Relative motion (rubbing)
Rubbing (relative motion) is not the problem
Shear is the problem of blisters
And rubbing is often the solution
As long as it's a low friction rub

[9:41] Research
1. “Normal” relative motion
The first was that the heel moves down, relative to the back of the shoe at heel strike, by a whopping 1.5 centimetres.
2. Bidirectional or unidirectional?

So the moral of the story here is that keeping the skin, sock and shoe stuck together is not the primary aim in preventing posterior heel blisters. We've got to focus on the magnitude of shear deformation.

[14:40] My experience with posterior heel blisters
I had two conceptual challenges that I had to get my head around.
- The first, I eliminated the rubbing on my skin with tape, yet I'd still get a blister.
- And secondly, the thing that helped the most made rubbing easier.

Friction reduction strategies aim to allow the skin to move in sync with the bone at an earlier moment of time with every step. That's how they reduce the magnitude of shear deformation.

[17:13] HOW PREVENTIONS WORK
[18:14] COF REDUCTION AT THE SKIN-SOCK INTERFACE
Lubricants and moisture-wicking socks are coefficient of friction (COF) reduction strategies that work at the skin-sock interface.

[19:50] COF REDUCTION AT THE SOCK-SHOE INTERFACE
The Engo patch is a low friction surface that the sock is able to move up and down very easily over it.

[20:18] COF REDUCTION AT THE SOCK-SOCK INTERFACE
The idea with double-sock mechanisms, including Armaskin socks, is that the lowest frictional interface is the sock-sock interface.

[20:54] PRESSURE REDUCTION STRATEGIES
Basically, the harder two surfaces are pressed together, the higher the friction force.

[22:47] SHEAR ABSORPTION STRATEGIES
We're talking about the “give” in the material, or the shear deformation that happens within the material, external to the foot. Examples include padded socks, gel materials like Silipos sheeting and PelliTec pads.

[24:30] SPREADING SHEAR LOAD
The easiest way I can describe it is to compare it to how cushioning works with the perpendicular load.

[25:51] BONE MOVEMENT
Obviously if the calcaneus doesn't have to move up or down in the sagittal plane as far, then shear deformation magnitudes will be lower.

[26:22] OTHER FACTORS
Haglund's deformity - because there's more pressure.
Frontal plane motion of the calcaneus – perhaps this may be relevant. I can't say I've noticed a correlation.
Unidirectional or bidirectional shear events

[28:07] Recap
1) Blisters are caused by repetitive shear deformation, not rubbing.
2) Rubbing is not required for shear, or blister formation.
3) Low friction rubbing reduces shear magnitude because it frees the skin surface so it can move in sync with the bone at an earlier moment, with every step.
4) High friction rubbing sees higher shear magnitude plus risk of abrasion: Abrasion of the skin, Abrasion of sock, Abrasion of the shoe lining (I’m sure you have clients who wear holes in the back of their shoes).
5) Shear is normal.
6) Shear isn’t visible. At least, it’s not easily visually appreciated, because it’s happening under the skin, within the foot.
7) Rubbing isn‘t friction. Friction is the force that resists relative motion; rubbing is the relative motion.
8) Rubbing can happen at any material interface
9) COF reduction preventions promote earlier slip via reduced friction force.
10) So do pressure reduction preventions, because F = COF x N
11) Allowing rubbing doesn‘t mean the heel has to come out of the shoe
12) And finally, we can reduce shear magnitudes by:
Absorbing it
Spreading it
Reducing bone movement
Reducing friction force by reducing COF and/or pressure

[32:10] Questions
Q1: Is the management of blisters the same in diabetics as it is in non-diabetic patients?
Q2/3: What are the best treatments / how to manage blisters
Q4: Strategies to prevent blisters for clients with fragile skin?

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