Why Shear Forces Wreck Diabetic Feet

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Notice the skin moving underneath the ball of the big toe, this is called shear.

When we walk, there is always a little bit of shear with every step. Think of how we rub our hands together to create warmth. If we rub the hands enough, calluses will develop, like on the hands of a carpenter or weight lifter. They can wear gloves to prevent some of this callus buildup. However, if they keep going, calluses can rip off or become blisters, sometimes even blood blisters. In feet that don’t feel pain, the same thing happens, only they are more likely to go beyond the point of blistering, which is an ulcer. They also look different from what we see on our hands because the soles of the feet are so much thicker. Let’s talk about what exactly are blisters, blood blisters, and ulcers.

Progression of calluses

Blister diagram
Blood in the blister tells us there is an ulcer.

Feet with diabetes and neuropathy don’t experience pain, which means no limping, continued walking, and continued repetitive shear forces. This creates a hot spot, so the body will try to compensate by growing thicker skin, called callus. The skin basically has two layers and the callus is in the outer layer, called epidermis. When the callus gets thick enough it can get rubbed away from the layer underneath, called dermis. A clear liquid will seep between these two layers forming a blister. Continued walking at this point means more rubbing to the dermis. At some point, it wont be able to hold itself together anymore and will break apart. A small hole in the dermis will form, and at this level it will start to bleed into the blister, forming a blood blister. This small hole, called an ulcer, can get bigger from continued walking. At this point, looking from the outside, it can look like a bruised callus. The ulcer is hidden underneath all that thick skin. The callus on the outside is acting more like a bandaid. If the blister didn’t pop, it is best to leave it alone. If it pops, this becomes dangerous because now the ulcer is open and exposed to germs.

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Foot shearing against a shoe
Ulcer formation
Animation showing how a callus can turn into a blood blister, creating an ulcer.

It may take awhile to fully understand this animation because there’s a lot going on, but it basically summarizes how a callus progresses into a foot ulcer, and this can happen in a day if you walk enough. This is normally extremely painful, but unfortunately neuropathic feet don’t feel pain. Notice in the very end of the animation how the callus on the outside hides the ulcer. You can’t see the ulcer unless the callus is removed. This is why the best way to prevent foot ulcers is to check your feet daily for calluses. If you have a foot ulcer, there are ways we can treat them.

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