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The frequency of these cysts is also a matter of controversy. In general, they are thought to present in ~5% of knee MR studies 4.
Clinically the patient with meniscal cysts may present with a palpable soft tissue swelling, with or without knee pain.
There are multiple theories regarding the etiology of meniscal cysts. Surgical and arthroscopic series report lateral meniscal cysts as a more frequent occurrence as compared to their medial counterparts. A systematic review of MR literature says that the frequency of medial and lateral meniscal cysts is almost equal 1. In ~4% of cases, meniscal cysts may involve both menisci within the same knee 5.
- meniscal tears are very highly associated with the vast majority (>95%) of cysts except at the anterior horn lateral meniscus (~65%) 9, this reduced rate may be partly explained by cysts at this location possibly arising from anterior cruciate ligament fibers 14
- some authors suggest a critical size of a meniscal tear at 12 mm along the circumferential axis as identified on MRI as a threshold value for parameniscal cyst formation 10
- discoid meniscus 11
The cysts of the medial meniscus are usually located along the posterior aspect of the posterior horn, whereas those of the lateral meniscus are located along the anterior/anterolateral aspect of the anterior horn or the body.
Plain radiographs may show soft tissue swelling at the expected locations.
High-resolution musculoskeletal ultrasound can be highly sensitive and usually shows an anechoic or, more commonly, a hypoechoic lesion, in keeping with the cystic nature of the lesion. It may also demonstrate the associated meniscal tear.
MRI is the investigation of choice for comprehensive imaging of the knee. Many of the cysts are non-palpable and these patients present with knee pain hence MRI can demonstrate the meniscal tear as well as the cyst.
Intracystic signal characteristics include:
- T2: high signal (but often not as high as synovial fluid 4)
- T1 C+ (Gd): no enhancement
MRI may be more sensitive than arthroscopy in detecting cysts since arthroscopic evaluation of the posterior region of the knee joint (posterior to the posterior meniscal horn) is limited. For a definite diagnosis of meniscal/parameniscal cysts, one should confirm not only the presence of the cyst but also meniscal tear and communication between them; otherwise, it should be reported as a possible meniscal cyst.
Treatment and prognosis
Surgical excision of the cyst can be performed along with repair of the underlying meniscal tear.
Based on imaging findings treatment of a cyst differs:
- if a communicating meniscal tear present, it can be treated arthroscopically
- if a noncommunicating meniscal tear present, open surgery is required
Longstanding large cysts may cause erosions or scalloping of the adjacent bone.
As a broad differential, consider other cyst-like lesions around the knee.