It is not generally painful to remove water from or aspirate the knee because a doctor gives the patient a local anesthetic such as lidocaine to deaden the nerves when inserting the needle, explains Dr. Thomas Zuber for American Family Physician. Draining the fluid helps to relieve the discomfort and pain associated with water on the knee or knee effusion.
Water on the knee occurs when too much fluid collects in the bursa, a fluid-filled sac that acts as a buffer between the moving parts in a joint, according to Medical News Today. Although the bursa reduces friction and eases the joint's movement, it can become inflamed and filled with more than the normal amount of fluid. This puts pressure on the nerves and knee, causing pain and discomfort. Knee effusion has many different causes with just as many treatments. While removing the fluid can temporarily remove pain, treating the effusion usually requires treatment of the underlying problem.
Knee effusion can be caused by rheumatoid arthritis, osteoarthritis, traumatic injury to the knee, cysts and gout, states Mayo Clinic. While draining the fluid through aspiration or arthrocentesis can relieve pain, joint replacement can be necessary for a badly damaged knee. An individual can also take corticosteroid injections and anti-inflammatory medications to manage the swelling.