that 11 of the other cases had been misdiagnosed as ganglion cysts before surgery. Ganglion cysts are not limited to the hands and feet. They may occur near the knee, commonly near the cruciate ligaments, but they may occur at the origins of the gastrocnemius tendon, and anteriorly on Hoffa's infrapatellar fat pad. At the shoulder, they typically occur at the acromioclavicular joint or along the biceps tendon. Other From their common origin at a joint or tendon, ganglion cysts may form in a wide range of locations. Rarely, intraosseous ganglion cysts occur, sometimes in combination with a cyst in the overlying soft tissue. Rare cases of intramuscular ganglion cysts in the gastrocnemius muscle of the calf have been reported. It is possible for a cyst to be considerably displaced from the joint. In one extreme case, a ganglion cyst was observed to propagate extensively via the conduit of the common peroneal nerve sheath to a location in the thigh; in such cases surgery to the proximal joint to remove the articular connection may remove the need for a riskier, more extensive surgery in the neural tissue of the thigh. The cysts may intrude into the spine, which may cause pain and dysesthesia in distant extremities. Cystic adventitial disease, in which a cyst occurs within the popliteal artery near the knee, has been proposed recently to occur by an articular mechanism, with a conduit leading from the joint, similar to the development of ganglion cysts, that spreads within the peroneal nerve. Cysts that were compressing one or more nerves and causing bone erosions have been reported to occur near the shoulder joint. Causes The most commonly accepted probable cause of ganglion cysts is the herniation hypothesis, by which they are thought to occur as an out-pouching or distention of a weakened portion of a joint capsule or tendon sheath. This description is based on the observations that the cysts occur close to tendons and joints. The microscopic anatomy of the cyst resembles that of tenosynovial tissue. The fluid is similar in composition to synovial fluid. Dye injected into the joint capsule frequently ends up in the cyst, which may become enlarged after activity. Dye injected into the cyst rarely enters the joint, however, which has been attributed to the apparent formation of an effective and one-way "check valve", allowing fluid out of the joint, but not back in. In synovials,