Meniscal cyst

Illustration to show how a horizontal cleavage tear can suck joint fluid into it, to create a cyst. — 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.

Illustration showing fluid being forced into a horizontal cleavage tear to form a cyst. — Eventually the fluid pushes out the outer rim of the meniscus.

How does a meniscal cyst form?

A meniscal cyst starts with a horizontal cleavage tear, perhaps from a twisting injury.

The sliding motion of the upper and lower part of the meniscus can trap fluid in the cleavage, and then the lip can close like a clam so that pressure of the fluid inside the cleavage forces a passage through to the outside of the meniscus.

"....Meniscal cysts are relatively rare, but may be present in patients with complaints of knee pain. The majority of cysts are associated with meniscal tears and are often identified incidentally during MRI examination for a suspected internal derangement...."

Michael S Crowell et al

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.

"....Meniscal cysts will typically present with focal knee pain, swelling, and, on occasion, a palpable mass at the joint line....Confirmation of the diagnosis of a meniscal cyst will usually require magnetic resonance imaging (MRI) due to its ability to delineate the cyst and evaluate any associated meniscal pathology...."

Hamilton Chen

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Recurrent meniscal cyst

Even after surgery a meniscal cyst may reoccur.

"Various theories for the recurrence of meniscal cysts even after arthroscopic surgery have been provided including inadequate debridement, poor disruption of the “check-valve mechanism”, as well as a concomitant large meniscal tear that may hinder a surgeon’s ability to adequately debride the cyst and the connecting tract"

Aryan Haratian et al