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Meniscal cyst

Meniscal cysts occurs when synovial fluid becomes encysted secondary to a meniscal tear. When they extend beyond the margins of the meninscus they are termed parameniscal cysts.

Epidemiology

The frequency of these cysts is also a matter of controversy. In general they are thought to present in ~4-6% of knee MR studies 4

Pathology

There are multiple theories regarding the aetiology of meniscal cysts. 

Surgical and arthroscopic series report lateral meniscal cysts as a more frequent occurrence as compared to their medial counterparts. Systematic review of MR literature says that frequency of medial and lateral meniscal cysts is almost the same 1. In approximately 4% of cases, meniscal cysts may involve both menisci within the same knee 5.

Associations
  • while meniscal tears are associated with the vast majority of cysts, anteriorly located lateral parameniscal cysts are less likely to have underlying meniscal tears 9.
  • some authors suggest a critical size of a meniscal tear at 12mm along the circumferential axis as identified using MRI, as a discrimination cut off for parameniscal cyst formation 10.

Clinical presentation

Clinically the patient with meniscal cysts may present with a palpable soft tissue swelling with or without knee pain.  

Radiographic features

The cysts of medial meniscus usually are along the posterior aspect of posterior horn whereas those of lateral meniscus are along the anterior/anterolateral aspect of the anterior horn or body.

Plain film

Plain radiographs may show soft tissue swelling at expected locations.

Ultrasound

High resolution musculoskeletal ultrasound can be highly sensitive and usually shows the cystic nature of the lesion. It may also demonstrate the associated meniscal tear.

MRI

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 horn) is limited.

Complications

Longstanding large cysts may cause erosions or scalloping of the adjacent bone. 

Treatment

Surgical excision of the cyst can be performed along with repair of the underlying meniscal tear.

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