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 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 ~4% of cases, meniscal cysts may involve both menisci within the same knee 5.
- discoid meniscus 11
- 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
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 radiographs may show soft tissue swelling at expected locations.
High resolution musculoskeletal ultrasound can be highly sensitive and usually shows anechoic or more commonly 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 horn) is limited. For definite diagnosis of para/meniscal cyst one should confirm not only the presence of the cyst but also meniscal tear and communication between them; otherwise it should be reported as 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 communicating meniscal tear present it can be treated arthroscopicly
- if noncommunicating meniscal tear present open surgery 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
The knee is a complex synovial joint that can be affected by a range of pathologies:
- bone and cartilage
- distal femoral condyle fracture
- tibial plateau fracture (classification)
- patella fracture
avulsion fractures of the knee
- Segond fracture
- reverse Segond fracture
- anterior cruciate ligament avulsion fracture
- posterior cruciate ligament avulsion fracture
- arcuate complex avulsion fracture (arcuate sign)
- biceps femoris avulsion fracture
- iliotibial band avulsion fracture
- semimembranosus tendon avulsion fracture
- Stieda fracture (MCL avulsion fracture)
- patella fracture
- chronic avulsion injuries
- chondromalacia patellae
- osteoarthritis of the knee
- osteochondral defects
- osteochondritis dissecans of the knee
- pattern of bone contusion in knee injuries
- knee fractures
- meniscal lesions
- synovial lesions
- fat pad
- popliteal fossa
- 1. Campbell SE, Sanders TG, Morrison WB. MR imaging of meniscal cysts: incidence, location, and clinical significance. AJR Am J Roentgenol. 2001;177 (2): 409-13. AJR Am J Roentgenol (full text) - Pubmed citation
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- 3.Tschirch FT, Schmid MR, Pfirrmann CW et-al. Prevalence and size of meniscal cysts, ganglionic cysts, synovial cysts of the popliteal space, fluid-filled bursae, and other fluid collections in asymptomatic knees on MR imaging. AJR Am J Roentgenol. 2003;180 (5): 1431-6. AJR Am J Roentgenol (full text) - Pubmed citation
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- 9. 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;196 (2): W180-6. doi:10.2214/AJR.10.4754 - Pubmed citation
- 10. Wu CC, Hsu YC, Chiu YC et-al. Parameniscal cyst formation in the knee is associated with meniscal tear size: an MRI study. Knee. 2013;20 (6): 556-61. doi:10.1016/j.knee.2013.03.008 - Pubmed citation
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