Undermining Skin Edges

To undermine skin edges, you cut beneath the skin along the edge of a
wound to free the skin from its deep tissue attachments. The purpose
is to increase skin mobility, which is important for a tension-free
wound closure. It is also a necessary skill for performing local flaps.
Technique
Pinch the tissues around the edge of the wound with the forceps to
ensure that the local anesthetic is still working. Give additional anesthetic
as required. Lift the skin edge with the forceps, and with a knife
or a scissors cut into the deep subcutaneous tissue along the length of
the wound (try to stay at the same depth) until the skin has the required
mobility.
An alternative method involves separating the skin and subcutaneous
tissue from the underlying muscle. The plane of dissection is just above
the fascia, the thin layer of connective tissue that overlies the muscle.
By undermining the skin along this deeper plane, you may encounter
less bleeding than if you cut directly into the subcutaneous tissue layer.
Bibliography
Edgerton M: The Art of Surgical Technique. Baltimore, Williams & Wilkins, 1988.