28 year old female with medial right knee pain; prior history of ACL reconstruction.
Parameniscal cysts are a helpful diagnostic clue: they only occur in the setting of a meniscal tear and are often easier to spot than the underlying tear. They're typically seen along the peripheral border of the menisci adjacent to a horizontal cleavage tear. The occur via a ball-valve mechism: joint fluid traverses the tear filling the cyst, but cannot escape the cyst. When surgically resecting the parameniscal cyst, care must be taken to repair the underlying tear, oherwise the cyst will recur.