The edges of the wound should pull together easily; if not, undermining needs to be done. Undermining means cutting the fibrous septae that connect the skin to the underlying fascia, and generally this is accomplished by using the scalpel to cut the septi just beneath the dermis as shown here. Undermining has the advantage of freeing the wound edges and decreasing tension on the wound which allows for faster wound healing and lessens the chance of scar spreading and keloid formation. The disadvantage of undermining is that it cuts blood vessels and nerves, and thereby slows wound healing and creates an area of anesthesia that generally is temporary. One rule of thumb is that one should undermine about the same radius as the maximum width of the wound. 
John L. Bezzant,M.D.
