Answer by Steve Harris, physician and medical researcher:
Scab is a technical anti-shibboleth. The preferred medical term is crust or eschar, the latter being Greek for scab.
The edge of an eschar can keep a wound from closing by secondary intention. And an eschar is a sign that a wound is too dry to heal as fast as possible. Having noted that, a crust is the best nature can do in many situations. It is protective.
In the best of all possible worlds, it is possible to do better. Yes, the first time that you remove an eschar, you do traumatize the wound, so it is not good to keep doing it. But once it's done, if you never let another eschar form, you can accelerate healing.
How? The fibrocytes in a wound are trying to lay down collagen and heal the wound by drawing it together and forming the tissue that turns into skin and healed scar. You want to encourage them to divide and grow. They can't do this if they dry out. It happens well only if they are moist but not too moist. They are like amoebas—you want (and they want) existence of a gooey wound soup where they can crawl around and place themselves, but not so moist that the fluid becomes a seroma that is easily turned into pus. You don't want large collections of fluid far away from capillaries, as that provides a place for bacteria to grow, but too far from the white cells that kill them.
So, you ideally need a thin layer of tissue moisture in a wound, but no more. And you need oxygen, because some of these cells need it but don't have enough if you cover them. And you want to kill anaerobic bacteria to make sure you don't get gangrene.
Trying to get these conditions is the idea behind bio-occlusive dressings. Such a dressing is just a membrane that lets in oxygen and keeps a wound from drying out but also allows excess fluids to get out and be soaked up by a next absorbent layer.
These things are expensive even now, and doctors of old did a reasonable job just by slathering a wound with an antibacterial ointment like silvadene, putting down a layer of petroleum-covered gauze so that the delicate fibroblasts are not ripped off when the dressing is changed, and then adding a layer of something absorbent (can be an alginate sponge or simple dry gauze) so that excess fluids don't leak through into bedding but have a way to get away from the wound.
Finally, on top of all that goes any dressing that protects all the rest—an example is the self-sticking latex stuff from 3M called Coban, or silk surgical sticky tape, or even an ACE bandage (though that's expensive if you have to keep throwing it away at each dressing change).
You change all this every 48 to 72 hours, depending on amount of exudate and the stage of the wound. The idea is that once you remove eschar, you never let it form again, due to the wound damage involved in removing it. All of these are helpful in treating a large wound bed (like a large burn, scrape, or something else where skin is gone).
A last thing: I have seen the order "wet-to-dry dressings" too many times. Why too many? Because too many doctors have no idea what this is! Wet-to-dry was a bad old system where you put wet dressings on wounds, let them dry to eschar, and ripped the whole thing off. Clearly that is not what you want. There are better ways. Wet-to-dry is not a substitute for intelligent "debridement," where a doctor or wound-management nurse carefully removes moistened eschar and then treats the remaining wound bed as described in the steps above.
That was longer than intended. Think of it as the A-B-C of large-wound care in one answer.
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