Arthrocentesis, also known as joint aspiration, is removal of fluid from a joint utilizing a hollow needle and syringe. Arthrocentesis is performed for either diagnostic or therapeutic purposes. Used as a diagnostic tool, fluid is drawn and analyzed to determine the underlying cause of excess fluid. A small amount of fluid in joints is normal, but when fluid builds up (joint effusion), it is usually the result of arthritis or related condition, and pain and swelling will develop. Arthrocentesis will then be used therapeutically to remove the excess fluid thereby reducing pain and swelling in the affected joint. As an example, Arthrocentesis (diagnostic and therapeutic) is performed on humans for a condition commonly known as 'water on the knee', usually resulting from arthritis.