Hi, I'm new to the nursing forum ... saw your post. It's been a while, so you may already have clarified your problem. Basically, sinus tracts & tunneling is the same thing, just different terms. Undermining is a wider area of wounding that lies beneath the wound opening. Think of undermining like this: the wound is larger than the "hole" or entrance of the wound. Tunneling is like a cave, undermining is like a cavern or cavity. Hope I've helped. Have a great day.