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Cyst

A cyst is an abnormal discrete collection of fluid in a body space.

Several types of cyst may make their appearance in the knee:

• Baker’s cyst or popliteal cyst
• meniscal cyst
• ganglion cyst
• bone cyst

A Baker’s or popliteal cyst occurs in the back of the knee, the area known as the popliteal fossa. It is really a symptom of other problems inside the knee which are causing excessive joint fluid to form. Once the joint fluid is sufficient to cause pressure, the fluid pushes out at the back of the knee where the capsule is thinnest.

Treatment of a Baker’s cyst must include treatment of the underlying condition, when the cyst will often spontaneously resolve. A very large cyst may be problematic in its own right, causing pain and pressing on structures in the back of the knee. In this case the cyst may be cut out, but attention still needs to be paid to the underlying condition which precipitated cyst formation.

A very tense Baker’s cyst may rupture, and the fluid may be forced into the calf when it may mimic a deep vein thrombosis (DVT).

A meniscal cyst is quite different, and forms within a damaged meniscus, forcing its way eventually to the joint line on the side of the knee.

A ganglion cyst is associated with ligaments or tendons, and is a kind of degeneration of the mucoid matter within the sheath. In the knee ganglion cysts are generally associated with the cruciate ligaments.

Bone cysts are areas of bony degeneration, and there may be a number of causes.