Giant medial parameniscal cyst in an osteoarthritic knee.

Medial parameniscal cysts of the knee are typically 0.3 to 9 mm in diameter. Few cases of unusually large medial parameniscal cysts have been reported. We describe the treatment of a patient with osteoarthritis of the knee who presented with an extraordinarily large, ipsilateral, medial parameniscal cyst, 10 cm in diameter. We believe this to be the largest medial parameniscal cyst reported in the English literature. Based on the patient's severe tricompartmental arthritis and associated symptoms, total knee arthroplasty (TKA) was indicated with simultaneous excision of the parameniscal cyst. Perioperativley, the cyst was found to have penetrated beyond the menisco-capsular attachments that were adjacent to a degenerative posterior horn horizontal medial meniscal tear. Following excision of the cyst, the mass was measured with surgical tape to be 10x10x5 cm. After complete excision of the parameniscal cyst, TKA was performed. Histological examination confirmed a benign parameniscal synovial cyst. At 2-year follow-up, the patient was ambulating unassisted without difficulty. The wound had completely healed with no evidence of recurrence of the cyst. The patient's range of motion was 0 degrees to 110 degrees with no signs of instability. Our patient's pathology was most significant for the heretofore unseen large size of the paramensical cyst. Open excision of this giant parameniscal cyst followed by TKA was effective in treating the patient's degenerative joint disease and extraordinarily large, painful soft tissue mass.