Category 4 (FIG. 1)
This is now an extremely serious, life threatening condition. Bone will be exposed, leaving the bone to dry out and die and for osteomyelitis (bone infection) to set in.
The wound will fill with dead tissue and anaerobes (bacteria that like an oxygen free environment) will inhabit the dead tissue. At the same time, Putrescine and cadaverine (foul-smelling compounds produced when amino acids decompose) will also inhabit the dead tissue in order to break it down. These two molecules are produced during the decomposition of proteins and largely cause the terrible odour in dead animals.
Some of the odour of dead tissue is produced by the actions of the bacteria, which break down the tissues in the body into gases and salts. Along with the putrescine and cadaverine, the odour of a pressure ulcer is terrible and unforgettable and memory of the odour will live with relatives of the sufferer, perhaps for a lifetime.
The bacteria will produce toxins (poison) which will enter the body, causing injury to the body organs. Someone is likely to die of kidney failure which is secondary to pressure ulcers making the pressure ulcer the primary cause of death.
If the wound bed cannot be seen it is ungradable.
If bone is not exposed or felt but muscle can be seen, then it would be a Category 3.
If bone is exposed or felt, it would be a Category 4.