Meniscal Cysts

Meniscal cysts are seen in about 5%-10% of knee MRIs. Meniscal cysts are more commonly parameniscal (perimeniscal), but can also be intermeniscal or both. The majority, though not all, of meniscal cysts are associated with meniscal tears. Conversely, parameniscal cysts are seen in less than 10% of cases of meniscal tears.

Meniscal cysts are thought to arise from dissection of synovial fluid through meniscal tears, and entrapment through a one-way valve mechanism.

On MRI, they are more commonly seen in the medial compartment. Earlier studies, however, reported a ration of lateral to medial cysts of up to 7:1. The discrepancy is thought to be related to the higher sensitivity of MRI for the detection of meniscal cysts compared to physical examination (which is better at detecting lateral cysts due to the smaller amount of fatty soft tissue on the lateral aspect of the knee), conventional knee arthrography, and arthroscopy (due to the difficult surgical approach to the posterior horn of the medial meniscus, where most medial cysts are located).

Parameniscal cysts typically present as loculated fluid signal with a connection to the adjacent meniscus. Intrameniscal cysts present as increased signal within an enlarged meniscus with an expanded contour. The T2 signal of intrameniscal cysts may not be as high as that of parameniscal cysts.

When located medially, parameniscal cysts are usually adjacent to the posterior horn. The majority also extend anteriorly adjacent to the body. Cysts can also extend centrally and end up posterior to or surrounding the posterior cruciate ligament, simulating a ganglion.

When located laterally, parameniscal cysts are usually adjacent to the anterior horn, with half of the cysts extending posteriorly adjacent to the body. The image above shows a cyst adjacent to the body of the lateral meniscus in communication with a horizontal tear.

The majority of medial parameniscal cysts, as well as parameniscal cysts overlying the body or posterior horn of the lateral meniscus are associated with meniscal tears. By comparison, only about 2/3 of cysts adjacent to or extending to the anterior horn of the lateral meniscus are associated with meniscal tears. It is not clear why these cysts are less likely to have an underlying meniscal tear.

References

• Anderson JJ, Connor GF, Helms CA. New observations on meniscal cysts. Skeletal Radiol. 2010 Dec;39(12):1187-91.
• Bergin D, Hochberg H, Zoga AC, Qazi N, Parker L, Morrison WB. Indirect soft-tissue and osseous signs on knee MRI of surgically proven meniscal tears. AJR Am J Roentgenol. 2008 Jul;191(1):86-92.
• Campbell SE, Sanders TG, Morrison WB. MR imaging of meniscal cysts: incidence, location, and clinical significance. AJR Am J Roentgenol. 2001 Aug;177(2):409-13.
• De Smet AA, Graf BK, del Rio AM. Association of parameniscal cysts with underlying meniscal tears as identified on MRI and arthroscopy. AJR Am J Roentgenol. 2011 Feb;196(2):W180-6.