Arthritis and Other Joint Problems

- Popliteal Cyst (Baker's Cyst)

Introduction

A popliteal cyst (also called Baker's cyst) is a fluid-filled sac that appears on the back of the knee. Although the cyst raises a bump under the skin, it is benign, not cancerous.

Sometimes a cyst can burst open (rupture) and leak fluid into your lower leg. Although a ruptured cyst is not life-threatening, its symptoms mimic those of thrombophlebitis, a very serious condition in which a blood clot forms inside a swollen vein.

Anatomy

The Knee Joint , the lining of the knee (synovium) produces a clear liquid (synovial fluid) that lubricates the joint. Synovial fluid keeps the junction of the upper leg bone (femur) and the lower leg bone (tibia) "slippery," letting the bones glide smoothly inside the knee. If the joint lining makes too much fluid, some of it may seep out or push through the lining, forming a sac.

Causes

An injury, inflammation of the joint lining, or a disease such as arthritis or gout can damage the knee, causing the synovium to produce too much fluid.

Symptoms

You may notice a bulge on the back of your knee. The knee may also buckle or feel achy or tender. If you feel real pain, it may be caused by an underlying disease, injury, or inflammation.

If you feel pain and swelling in the back of your leg below your knee (calf), the cyst has ruptured. Although the fluid is reabsorbed by the body, the cyst will probably grow back in a few days.

Diagnosis

As discussed in the diagnosis section of The Knee Joint, your doctor will take a history and perform a physical exam. He will watch and feel your knee as you bend and straighten it, trying to detect the cyst. This examination is typically all that's needed to make the diagnosis. If your symptoms suggest a rupture, however, you'll need an ultrasound exam or Arthroscopy to confirm it.

Treatment

If your symptoms are mild and do not interfere with your daily activities, nonsurgical treatment is usually effective. Resting the knee and keeping it elevated for several days may help. Your doctor may also drain the fluid and inject cortisone into the area of the cyst to reduce swelling.

If these treatments fail, you will probably need surgery to remove the entire cyst and repair the hole in the joint lining. The surgery is usually done on an outpatient basis, so you will not have to stay overnight in the hospital. You will be given general anesthesia to put you to sleep or spinal anesthesia to numb the leg. The doctor will then make an incision in the skin over the cyst. After removing the sac, he may sew a synthetic patch over the hole in the joint lining.

After surgery, your knee will be bandaged with a dressing and splint for support. Most of the stitches used to hold the incision closed will be absorbed into your body. Any that remain will be removed by your doctor after 10 to 14 days. In the meantime, keep the incision dry for the first 24 hours. After that, you can shower, but dry the wound right away. Clean the incision 3 or 4 times a day with soap and water; or use a cotton swab dipped in a mixture of half water and half hydrogen peroxide. Once the stitches or staples are removed, your doctor may put small strips of tape across the wound. Leave them on as long as instructed.

Popliteal cysts form close to major nerve and blood vessels in the leg. Although it is unlikely, those vessels could be injured during surgery. Call your doctor if you feel numbness or weakness in your lower leg or foot. Other problems that should prompt a call to the doctor include:

Increased pain in your lower leg.
Tenderness or redness above or below your knee.
Increased swelling in your lower leg, ankle, and foot.
Trouble breathing.
Sudden chest pain.
High fever or chills.
Increased knee pain or trouble moving around.
Increased bleeding from the wound.

You should also be treated for the underlying cause of the cyst; otherwise it may grow back.

Illustration by: Calhoon, Borman, Mueller

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