... it's a deroofed blister! That's a blister with it's top (roof) rubbed off. First the skin tears, then it deroofs.
Why do some blisters remain intact while others deroof?
Firstly, it depends on the characteristics of the skin. The epidermis varies in thickness and stiffness at different areas of the body. For example, the skin on the eyelids is very thin and bendy. And it's very thick and stiff on the sole of the foot. Simply, thinner skin is less resistant so the blister roof is more likely to deroof. Thicker skin is more resistant so the blister roof is more likely to remain intact.
Secondly, frictional forces. When I say frictional forces, I mean:
shear: internal rubbing - of the bone relative to the skin
rubbing: external rubbing - of the skin relative to the sock
Relevant factors here are pressure and the coefficient of friction(COF). Note: Rubbing: will deroof a blister - sometimes. Encouraging movement can actually keep it intact. Turning a rub into a slide will make all the difference! And that all depends on the COF. If the COF is high, the blister will deroof. If the COF is low, the blister roof will remain intact. The skin 'slides' rather than 'rubs' against the low friction surface.
4 Ways to prevent a deroofed blister
Here are the 4 best ways to stop a blister from deroofing (the last two with video demonstrations).
(1) No pressure / Nonweightbearing
The best way to prevent deroofing is by eliminating all pressure. By wearing shoes that don't touch the blister (eg: thongs or scuffs for blisters on the toes or back of the heel) or total nonweightbearing (eg: crutches for blisters on the ball of the foot). This will allow the blister to resolve in its own time. In reality, this is not always possible, like if you are in a race situation or miles from support.
This will serve as protection. I favour non-adherent dressings. Adherent dressings like Compeed are meant for abrasions and deroofed blisters. The hydrocolloid material is the best replacement for the blister roof. But when used on intact blisters, not only are they unnecessary, when you come to remove it, it may tear the blister roof off with it.
(3) Donut Pads - Reduce pressure
By cutting a hole in a piece of thick orthopedic felt / moleskin and adhering it over the area so the blister is in the cavity, pressure and movement against the skin can be avoided or at least minimised. This will keep the blister roof intact. Podiatrist Emily Smith shows you how in this video.
(4) ENGO Patches - Keep a low coefficient of friction
Reduce the COF between the shoe and sock. This is blister prevention (and deroofing prevention) GOLD. Why? Because you allow the sock and skin to move as a single unit so the sock actually protects the skin. Watch this video to see what I mean. Or you can reduce the COF low at the skin-sock interface (lubricants, powders, moisture-wicking socks, double socks). It's not quite as effective but better than leaving the COF high.
If you have a blister, you've missed the boat for pain-free blister prevention. But you'll be wanting to keep the roof intact so it's less sore! It will also keep the area sterile so it doesn't get infected - pretty important! In fact it's your priority at this point. The best ways to achieve this are by reducing friction levels and reducing pressure.