What is the difference between undermining and tunneling in regards to stage 4 pressure ulcers?

Length of tunnel: Undermining is when a wound diameter is wider at its base (deep in the wound) than at the wound's skin edge. For example, the hole in the skin may be 5mm, but the base of the wound cavity is 10mm. Tunneling is when the wound tracks under the skin to another opening in the skin or to a deeper cavity. Tunnels are sometimes referred to as a "sinus" or "tract".

Pressure ulcer: These are characteristics of wounds; they do not need to be pressure ulcers in order to have this type of characteristics. A tunnel is an empty space that can extend in any direction from the wound; undermining is tissue destruction underlying intact skin along wound margins.

Complexity: These are terms that relate to the complexity of the wound. Other than that they are not terribly important. Undermining refers to the extent to which the skin is separated from the underlying tissues. Tunneling is more of a connection between two openings through such a separation.

They are the same: A stage 4 ulcer is the deepest type of pressure ulcer that forms over a pressure point due to diminished sensation and/or inability to move and relieve pressure. Examples are spinal cord injury patients or older stroke and debilitated bed-bound patients. Most vulnerable is the deepest (muscle) tissue, then the fat then the skin. This leads to the undermining seen at the base of the ulcer.