Teaching Case

A 12-year-old girl presents with pain and swelling around the lateral side of her left knee. She underwent a MRI scan following a normal X ray of her knee.

A 12-year-old girl presents with a 2 year history of swelling and intermittent pain around the lateral aspect of her left knee. The pain was exacerbated with physical activity. There was no history of trauma locking or giving way. Examination showed a 2x2cm swelling with minimal tenderness around the head of her fibula. The overlying skin was normal and there was no effusion around the knee joint. She had full range of movement and the joint was stable. Anterior-posterior and lateral views of her knee were unremarkable.

The MRI of her knee was performed using the following protocol:

Sagittal T1W TSE, T2W FFE, PD SPAIR.

Coronal T1W TSE, T2W FFE, PD SPAIR, T1W TSE SPIR, T1W TSE SPIR + GADOLINIUM.

There is a cleft of abnormal signal within the lateral meniscus extending beyond the lateral margin, measuring 2x3x2 cm. This extends into the adjacent soft tissues as shown in figures 1-4. The signal characteristics are consistent with a tear within the lateral meniscus and a cystic fluid collection laterally, suggestive of myxoid degeneration.

Meniscal injury due to overuse results in myxoid degeneration with formation of a hollow space filled with mucopolysaccharide. This leads to an appearance of a cyst. The connective tissue will undergo an abnormal reaction following local mechanical stress. This defect tends to weaken the structure of the fibrocartilage tissue, precipitating a tear in the meniscus. The material within the cyst is either secreted by the meniscus cells or will be derived from the synovial fluid [1]. Cellular secretion is implicit in the theory of myxoid degeneration.

They are commonly identified with a MRI scan of the knee [2, 3], which also aids in defining the origin and size of these lesions [4]. Tyson et al reports that most of the meniscal cysts (91%) in his study of 62 MRI scans occurred immediately adjacent to the meniscal tear (98% of which were horizontal cleavage tears) with the tear leading directly into the cysts. It has been reported that meniscal cysts of the knee are a complication of meniscal tears that extend to the capsule and are three times more common on the lateral than the medial side [5, 6]. They are also most commonly found in men aged between 20 and 40 years [6]. Conventional spin-echo T1 weighted, proton density-weighted or gradient-echo sequences are necessary for identifying tears in the meniscus with cystic lesions. [7]. Myxoid degeneration is rarely found in children. It has been reported that a meniscal tear associated with degenerative changes might be regarded as the first sign of osteoarthritis of the joint. [8]. The presence of a tear is an important feature for the orthopaedic surgeon as it may influence their surgical approach [9]. If the patient was asymptomatic treatment would be conservative. In symptomatic cases, arthroscopic repair would be reccomended.