The back of the knee is a hard area to heal because of constant moving. Rinse the cut with warm water, use a mild anti bacterial soap if you have it, nothing with extra fragrance or colors. Use an antibiotic ointment such as Neosporin to reduce the chance of infection, and use it daily. Don't use anything with lidocaine in it. Keep an eye on it, check it everyday to be safe. If it starts to seep white or yellowish liquid, it's infected. Clear liquid oozing is ok, but it's better if it doesn't ooz at all. If it is a really bad cut it may need to be sewn up by a vet.
-Beth