How is fluid drained from the knee?

Needle. The fluid is usually aspirated by a needle and siring.
Via needle aspiratio. Check out material from aafp re knee joint aspiration: http://www.Aafp.Org/afp/2002/1015/p1497.Html. You can also watch youtube videos of knee joint aspiration: http://www.Youtube.Com/watch?V=clmfivdtope. Common point (pun intended): fluid drained from knee (and any other joint) using a needle & syringe after numbing medication & cleaning area off.