MRI imaging. Sagittal SPIR sequence.
A 48-year-old man with no previous clinical records complained about having non-traumatic posterior knee pain. No palpable masses were noticed by an orthopedic surgeon, who referred the patient for a magnetic resonance (MRI) of the right knee, in order to rule out meniscus tear.
MRI of the right knee showed a well-defined cystic soft tissue lesion located just above the popliteal fossa on the posterior side of the knee, deep to the popliteal vessels. The lesion had low signal intensity high signal intensity on T2-weighted images (WI) sequences (Fig. 1, 2 and 4) and low to intermediate signal on T1-WI (Fig. 3) and a lobulated contour, with well-circumscribed and thin-walled margin. On its superior part, the lesion abutted upon the posterior cortical bone of the femoral diaphysis and had a partially subperiosteal position (Fig. 2 and 4). No haemosiderin nor nodular components were found within the lesion (Fig. 2). After intravenous contrast administration the lesion had no significant enhancement (Fig. 5).
Posterior knee pain could be caused by a wide variety of diseases, the most frequent ones being Baker cyst, meniscus tear and soft tissue or bone tumours. Neurological or vascular causes are less common .
Ganglion cysts are very frequent soft tissue lesions around the joints, mainly in the extremities when degenerative or inflammatory joint disease occurs. Most of them are asymptomatic, but sometimes they can be related to pain caused by compressive effect in adjacent structures, or inflammation, infection, rupture or haemorrhage .
Periosteal of subperiosteal ganglion cyst is a very rare condition, presumably produced by mucoid degeneration of the periostium of long bones, usually found in the lower extremities typically located at the outer cortex without intramedullary component. Some of the cases reported show extension to the adjacent soft tissue .
Though definitive diagnosis is reached by histopathology (myxoid cyst surrounded by fibrous tissue with an inner layer or pseudosynovial cells), MRI imaging may be diagnostic under specific clinical circumstances  as it shows cystic lesions with high signal intensity on T2WI sequences and no enhancement or complex characteristics, only thin internal septa, sometimes. Therefore, MRI is considered the imaging technique of choice, to confirm the cystic nature of the lesion, to find a potential communication between the lesion and the joint and to differentiate the lesion from solid osseous or soft tissue tumours. Ultrasound or CT can be useful to guide interventional treatment.
Although periosteal ganglions are considered benign lesions with good prognosis, treatment includes surgical excision with or without corticosteroids injection . Recurrence after surgical excision can occur, especially when there is a communication between the lesion and the adjacent joint that is not disrupted .
Written informed patient consent for publication has been obtained.
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