Teaching Case

6 month history of pain and intermittent swelling of the left knee. On examination there was swelling and tenderness over the medial aspect of the knee joint.

The patient presented with a 6 month history of pain and intermittent swelling of his left knee. On examination there was swelling and tenderness over the medial aspect of the knee joint. A radiograph showed soft tissue swelling over the medial aspect of the knee and a large, associated erosion of the proximal tibia. MR imaging confirmed a large, lobualted cystic lesion which was displacing the medial collateral ligament and causing erosion of the adjacent tibia. A horizontal tear of the posterior horn of the medial meniscus was also shown. Arthroscopy confirmed the imaging findings and a partial meniscectomy and cyst decompression were performed.

Meniscal cysts(also known as ganglion cysts or juxta-articular myxomas) are multiloculated collections of mucinous material occuring at the periphery of a meniscus[1]. They usually affect young adults but are being increasingly diagnosed on MR imaging in other age groups. They are commoner on the lateral side where they often present with swelling and pain at the joint line. When they occur on the medial side they are often large and commonly asymptomatic. Although their cause is unknown the majority are associated with horizontal tears of the adjacent meniscus and it has ben postulated that there is an influx of synovial fluid through the tear into the parameniscal region forming a cystic lesion. Fluid within the cyst is identical to synovial fluid[2]. However, in the absence of a tear, meniscal cysts may develop from a compression injury to the periphery of an already degenerate meniscus. Complications may arise due to the size and location of the cyst. Meniscal cysts usually extend laterally and large cysts may dissect through the joint capsule and collateral ligaments to form mobile masses. They may dissect for considerable distances in the soft tissues. Cysts arising from posterior meniscal horn tears may extend centrally to lie in close proximity to the posterior cruciate ligament(PCL), mimicking a PCL ganglion cyst. Postero-medial extension from the lateral meniscus causing pressure effects on the popliteal artery has also been reported[3]. Diagnosis is usually made clinically and imaging performed to further evaluate the lesion. Occasionally gas formation and bone erosion may be seen on radiographs. MR imaging demonstrates the mucinous contents, the lobulated nature and the precise anatomical location of the cyst to periarticular and intra-articular structures.