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Persistent Knee Swelling

Fluid Behind the Knee (Called a Baker's Cyst)

A Baker's cyst (named by an English surgeon, Dr. William Baker, in 1885) is a collection of fluid behind the knee. It generally collects there because when enough fluid is present in the actual knee joint, it has nowhere else to go. Because of the knee's anatomy, once fluid forms within a Baker's cyst, it cannot return to the joint and the cyst may continue to enlarge or even rupture, causing fluid to extend into the lower leg and calf. Pain and pressure behind the knee are common. An ultrasound is a good way to diagnose this condition if the cyst has not ruptured; MRI or an arthrogram (in which dye is injected into the knee) are alternative tests that may be useful.

Sometimes, despite a thorough review of the circumstances surrounding your knee swelling, a full examination, and, perhaps routine x-rays, the cause of your swelling may not be clear. If it persists and is associated with significant symptoms (pain, instability, or locking, for example), your doctor may recommend one or more of the following tests:

  • tapping the joint for synovial fluid analysis - after numbing the skin, a needle is inserted into the knee and joint fluid is removed; the fluid is sent to the laboratory for analysis, including the number and type of white cells (measures of inflammation), tests for infection (called gram stain and culture) and inspection under a special microscopic looking for crystals (as seen in gout or pseudogout)

  • MRI - a large magnet is used to create pictures of the inside of the knee, including bones, cartilage, ligaments and tendons

  • CT scan - a specialized series of x-rays that create a three-dimensional picture of the knee

  • arthroscopy - a tube with a light and camera on the end, roughly the size of a pencil, is inserted into the knee to directly view the interior of the joint; certain abnormalities, such as damaged cartilage or a loose bone fragment, can be repaired with this instrument.

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