OA of the knees sometimes causes fluid to fill the joint, which causes swelling and pain. While this condition is commonly called “water on the knee,” doctors may refer to the increased fluid more formally as a joint effusion. All of the movable (diarthrodial) joints in the body are surrounded by a layer of cells called the synovial membrane.
This layer of cells normally produces a small amount of fluid, whose purpose is to lubricate the joints. OA can lead to irritation of the synovium and cause it to produce larger than normal amounts of joint fluid. This extra fluid may create visible swelling of the knee as well as increased pain and stiffness. If you develop swelling and pain in your knee, your doctor may suggest that you have the fluid removed in a procedure called an arthrocentesis.
Arthrocentesis
The removal of fluid from a joint; also called joint aspiration. In this procedure, a sterile needle and syringe are used to drain fluid from a joint that is inflamed or infected.
Your physician may remove this fluid for two reasons. First, the procedure is diagnostic and can help your doctor determine what is causing the swelling. Second, the procedure itself may be therapeutic and help relieve the pain and stiffness of the knee.
The symptoms of swelling, pain, and stiffness in the knee can also be caused by processes other than OA, the most dangerous of which is an infection of the joint. If your doctor is not sure what is causing your knee pain and swelling, removing the joint fluid from the knee can help him or her to make a diagnosis of OA and exclude more serious problems such as joint infection (septic arthritis).
Examination of the physical properties of the joint fluid, such as its color and clarity, and its white blood cell count can assist the physician in making these important determinations.
The removal of excess joint fluid can relieve pain, swelling, and stiffness, and improve the knee's range of motion and flexibility. Additionally, chronic knee effusions sometimes contribute to leg weakness. Removing extra joint fluid can be helpful in rebuilding quadriceps strength as well.
The doctor may also inject corticosteroids (“cortisone”) into the knee in an attempt to relieve arthritis symptoms. Sometimes this type of injection helps to prevent the joint fluid from reaccumulating. Corticosteroids can be injected approximately every three months, but if necessary the knee joint fluid can be drained more frequently.
Before performing an arthrocentesis, your doctor will clean your knee and the surrounding area with an antiseptic soap.
Next, the doctor will inject a medication, such as lidocaine, into the skin to numb the area. Finally, he or she will use a hypodermic needle to draw off the fluid. In experienced hands, this procedure does not cause much discomfort and usually takes five to ten minutes. You do not need to do any special preparation before this procedure, and you will usually feel better immediately.