Teaching Case

A 22-year-old female presented with pain and swelling on the lateral aspect of the left knee since two weeks, associated with restriction of movements. She had a history of fall and trauma to the left knee three years back. She had no history of fever. On examination, a palpable swelling with localized tenderness was present. An ultrasound examination performed previously in another institution had revealed a cystic lesion in the lateral aspect of the knee joint which was thought to be a ganglion cyst.

A non- contrast MRI (Magnetic Resonance Imaging) of the knee was performed. Anterior cruciate ligament (ACL) was thickened with diffuse high signal intensity on all the sequences, suggesting mucoid degeneration. Medial meniscus, posterior cruciate ligament, medial and lateral collateral ligaments appeared normal. The lateral meniscus showed a bucket handle tear with a medially flipped fragment. A well-defined, cystic lesion measuring 36x29x19mm was seen arising from the anterior horn of the lateral meniscus. It showed thin internal septae and smooth margins. It was seen to bulge into the Hoffa’s fat pad anteriorly with mild scalloping of the underlying anterior tibial cortex. A small cyst was seen adjacent to the posterior body of the lateral meniscus.

All bones showed normal marrow signal intensity. There was no joint effusion. The patient was referred to a higher institution for surgical management of the large parameniscal cyst.

Knowledge of anatomy is vital to classify the cystic and ‘cyst-like’ lesions when imaging a post-traumatic knee. These lesions include- popliteal cysts, ganglion cysts, synovial cysts, meniscal cysts and intraosseous cysts while the cyst-like lesions include fluid-filled bursae and recesses which may mimic cysts. Causes other than trauma include osteoarthritis, rheumatoid arthritis, gout and systemic lupus erythematosus. A parameniscal cyst is a well-defined fluid collection seen adjacent to a meniscus. They may show lobulations, internal septations and rarely bony erosions. Intrameniscal cysts are seen within the meniscus. [1] Ultrasonography, CT, arthrography and MRI are the various imaging techniques used in their evaluation. MRI is most preferred modality as it is non-invasive, does not use ionizing radiation and has superior soft-tissue contrast resolution. [2]

Ganglion cysts may be extra-articular, intraarticular, periosteal or intraosseous but are not associated with meniscal tears. The first type is commonest and presents as a cystic mass surrounded by dense connective tissue without a synovial lining. Meniscal cysts are associated with horizontal meniscal tears. It is believed that they are formed due to extrusion of synovial fluid through an adjacent meniscal tear. [3] Synovial cysts can be distinguished from other juxta-articular lesions by their synovial lining. [4] Most common examples are Baker’s cyst and proximal tibiofibular joint synovial cyst. Cysts in the extremities are usually asymptomatic but pain or discomfort may arise from inflammatory changes, rupture, haemorrhage or infection. Malignancy needs to be ruled out in complex cysts with associated soft tissue mass.
Surgical excision, aspiration and steroid injections are treatment options, however, recurrence rates are high when managed conservatively. [5]