|With walking, the clam-like split may allow fluid to enter, but a valve-action hinders it from leaving. Fluid pressure builds up inside the cavity.||Eventually the fluid pushes out the outer rim of the meniscus.|
How to diagnose a meniscal cyst
Meniscal cysts are more frequent on the lateral side of the knee, often in association with a horizontal tear of the meniscus. External rotation of the tibia may cause the lateral mass to become more prominent, while internal rotation of the tibia may cause the mass to completely disappear. Ultrasound may be useful in establishing the presence of fluid in the lump, and the diagnosis can be confirmed with MRI.
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MRI characteristics of cysts and “cyst-like” lesions in and around the knee: what the radiologist needs to know. Perdikakis E and Skiadas V. Insights Imaging. 2013 Jun; 4(3): 257–272.