Parameniscal Cyst

cystis parameniscalis

A parameniscal cyst (meniscal cyst) is a collection of mucoid fluid adjacent to a meniscus, communicating with a meniscal tear via a check-valve mechanism. Lateral meniscal cysts are three times more common than medial cysts and appear as palpable masses at the lateral joint line in the middle third. They are almost always associated with a horizontal cleavage or oblique meniscal tear that acts as the fluid-producing source and check valve.

Region: Knee

Clinical Relevance

Clinical Notes

Parameniscal cysts present as firm lateral (or less commonly medial) joint line masses that fluctuate with knee position: they are largest in slight flexion (5-20 degrees) and smaller or impalpable in full flexion (fluid forced back into the joint) and full extension. MRI demonstrates the cyst and its communicating meniscal tear simultaneously, guiding treatment. Arthroscopic treatment addresses the meniscal tear (the source) rather than the cyst directly; the cyst usually resolves when the tear is treated, but persistent cysts can be decompressed through needle aspiration or arthroscopic cyst fenestration.

Pathology

Common Injuries & Conditions

Lateral Meniscal Cyst from Horizontal Tear Check Valve

A horizontal cleavage tear of the lateral meniscus produces a check-valve mechanism that forces synovial fluid from the joint into the parameniscal cyst space on knee flexion, creating a palpable lateral joint line mass that varies with knee position; arthroscopic meniscal repair or partial meniscectomy addressing the tear source resolves the cyst in most cases without direct cyst excision.