Meniscal cysts are commonly reported in MRI scans and it is important to have an understanding as to whether these can be attributed to the index accident or are incidental findings.
A meniscal cyst is a focal collection of synovial fluid within or adjacent to the meniscus1 typically at the meniscocapsular junction. Most meniscal cysts are parameniscular
A widely held view on the origin of parameniscal cysts is that they result from the extrusion of synovial fluid though an adjacent meniscal tear. The synovial fluid is forced though a tear and accumulates at the meniscocapsular junction forming a cyst. The fluid is then slowly resorbed leaving a thick jelly-like substance
- There is evidence that the prevalence of meniscal cysts is around 3.9% in asymptomatic knees; most of these subjects were presenting with derangements of the contralateral knee.
- There is some evidence from large patient series being investigated for internal derangement of the knee that the prevalence of meniscal cysts in this type of population is around 4 to 5%, similar to that in the asymptomatic knees.
- It seems likely that there is no trend to increased incidence with age; that meniscal cysts can occur in persons of any age and the mean age of those presenting with cysts in many case series was in the 28 to 40 year old range.
- MRI studies suggest that meniscal cysts are slightly more common in the medial compartment compared to the lateral. One larger study found a prevalence in the medial compartment of 2.8% compared to 1.4% laterally.
Traumatic or incidental
- There is a strong association of parameniscal cysts with meniscal tears which are of a degenerate nature, particularly horizontal and complex tears, as is repeatedly evidenced in many case series. Greater than 90% of cysts were associated with degenerate tears in some studies.
- In populations being investigated for internal derangement of the knee, of those with cysts detectable by MRI, case series data suggest that around 70% are asymptomatic.
- There is a suggestion that other pathology of the knee is also associated with the presence of parameniscal cysts however these associations are poorly investigated and reported.
- Of those with parameniscal cysts, 65% to 75% were found in men.
- Around 30 – 55% of those found to have a parameniscal cyst have a history of injury regardless of which compartment the cyst is located; this data relates to populations being investigated for derangements of the knee or presenting with swellings around the knee.