Bedsores evolve through stages:
Stage 1 bedsores involve superficial tissue only. The skin is not broken, but a red mark may be warm or cool to the touch.
Stage II bedsores are open wounds. The outer layer of the skin is broken and the wound enters the dermal layer of the skin. At this point, the body is more prone to infection.
Stage III bedsores run even deeper and the amount of skin loss is severe. Fatty yellow tissue is often visible. The skin inside the sore is often necrotic and may appear grey or yellow.
Stage IV ulcers are the most serious and involve muscle and tendon damage. The bone might be apparent through the sore in the skin.
Bedsores can be very serious because once they graduate past stage I patients are at increased risk for serious and life-threatening complications. These include infection, cellulitis, osteomyelitis, sepsis, and more.