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Musculoskeletal Imaging

Synovial cyst

a fluid-filled para-articular mass lined by a synovial membrane, with or without communication with the neighbouring joint. Those that communicate commonly become distended with fluid and show an elevation in intra-articular pressure. Typical processes in which synovial cysts occur include rheumatoid arthritis, the seronegative spondyloarthropathy, osteoarthrosis and crystal deposition disease. Synovial cysts are encountered most commonly about the knee and hip, with the shoulder, elbow, wrist, foot, ankle and hand being involved to a lesser extent. Synovial cysts are of clinical significance because they may appear as a periarticular mass, cause pain or limitation of joint mobility, or compress adjacent neurovascular structures; they may also rupture acutely or dissect or become infected secondarily.

In the knee, any inflammatory, degenerative, traumatic or neoplastic condition that produces a knee effusion can lead to synovial cyst formation. The most characteristic location is the posterior aspect of the knee related to distension of the gastrocnemius  semimembranosus bursa in response to a knee effusion. These cysts are designated Bakers cysts or popliteal cysts and may dissect between the muscles of the leg or rupture with extravasation of fluid, producing clinical manifestations resembling those of thrombophlebitis. Ultrasonography, radionuclide arthrography, standard arthrography, CT, computed arthrotomography and MR imaging may be useful in the diagnosis of a Baker's cyst. MR imaging may provide the most detailed information regarding the distribution and extent of the process and the degree of synovial inflammation.

Rupture of a Baker's cyst is associated with soft tissue extravasation of fluid contents. Ruptures occurring posteriorly can simulate a compartment syndrome or thrombophlebitis. MR imaging is effective in demonstrating ruptured popliteal cysts. Typically a popliteal cyst is a well-defined mass of variable size with signal intensity characteristics of fluid. Changes in signal intensity characteristics may indicate haemorrhage or intrabursal osteocartilaginous bodies.

The arthrographic appearance of an abnormal synovial cyst varies. In most instances, a well-defined, lobulated structure filled with air and radiopaque contrast material will be revealed. It may have an irregular surface related to hypertrophy of its synovial lining. Alternatively, the entire cyst or a portion of it may rupture, with extravasation of contrast material.

In the hip, any process that leads to elevation of intra-articular pressure results in escape of fluid from the joint through a number of anatomic pathways and serves to decompress the joint. Typically, the fluid passes into a surrounding synovial sac which, with distension, can be seen or palpated. These synovial cysts can also be assessed with imaging methods, including ultrasonography, CT, arthrography, computed arthrotomography and MR imaging.

Synovial cysts are a well-known manifestation of rheumatoid arthritis. The cysts may arise as rupture of the joint capsule with extravasation of fluid and secondary encapsulation or as herniation of the synovial membrane, but usually they represent abnormal distension of various bursae that communicate with the adjacent joint. Rheumatoid synovial cysts have also been described at other sites, including the calf, the ankle, the plantar aspect of the foot, the hip, the hand and wrist, the elbow and the shoulder.

Synovial cysts may also develop in a degenerated apophyseal joint, most frequently at the L4 - L5 spinal level. Cysts that are medial to the ligamentum flavum appear as nearly round structures that displace epidural fat and may indent the dural sac. The capsule of the cyst is usually noticeably denser than the fluid contents and may be calcified. In some cases the fluid in the cyst is replaced with gas.

Intraspinal synovial cysts are more common in women, with 90% of lesions occurring in the lumbar region, predominantly at the L4 – L5 level. Clinical manifestations include motor and sensory deficits and reflex changes. The vast majority of such cysts arise adjacent to apophyseal joints with which they commonly communicate, and these joints are frequently involved with osteoarthritis. Routine radiography in cases of intraspinal synovial cysts shows nonspecific findings. Myelography may also reveal nonspecific findings. CT typically shows a soft tissue mass adjacent to a degenerative apophyseal joint; the mass may contain gas or possess a partially or completely calcified rim. The MR imaging findings of intraspinal synovial cysts include a mass lesion with variable signal intensity, signal void related to the presence of gas, and degenerative changes with fluid or gas collections in the apophyseal joints.
 
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