When is a meniscal cyst not a meniscal cyst?

Two patients are presented where lesions adjacent to the joint line of the knee were diagnosed clinically and on magnetic resonance imaging (MRI) as meniscal cysts. No concomitant meniscal tears were seen, and the MRI signal was not completely homogenous. The diagnostic imaging influenced the surgical management in a manner inappropriate for the definitive diagnoses of fibromyxoid sarcoma and monophasic synovial sarcoma. Not all cysts produce a purely homogenous signal on MRI due to haemorrhage or high protein content fluid, and not all meniscal cysts are associated with a meniscal tear. In the absence of a meniscal tear and with mixed signal on MRI we advise caution in the diagnosis of a meniscal cyst and advocate shared management with either orthopaedic or radiological colleagues with a special interest in oncology to obtain a tissue diagnosis before definitive treatment.