Case of the Month March 2011

27 year old female with an atraumatic mobile lump lateral aspect right patella tendon on extension. ? Ganglion ? Other.

Diagnosis

Nodular type of pigmented villonodular synovitis (PVNS).

Investigation

Initial plain xrays are essentially normal, with only very subtle blurring of the posterior aspect of Hoffa' fat pad (images 1 and 2).

MRI demonstrates a well circumscribed heterogeneous ovoid mass posterior to the lateral aspect of the infrapatellar (Hoffa's) fat pad (images 2 and 3).

Gradient echo imaging reveals low signal areas of haemosiderin blooming within the mass. The MRI appearance is typical for nodular type Pigmented villonodular synovitis (PVNS). Post contrast T1 imaging (not shown) confirms enhancement of the mass.

Discussion

Pigmented villonodular synovitis (PVNS) is a monoarticular synovial proliferative disorder. It predominantly affects adults in their second to fourth decades. It occurs in two forms, presenting as either a localised or nodular type of mass posterior to Hoffa's fat pad, or a diffuse type with distribution through the joint recesses.

The nodular form typically presents as a non-painful soft tissue mass.

PVNS can affect any joint, bursa, or tendon sheath; however, the knee is the most commonly involved (followed by hip, elbow, and ankle)

Imaging features

• On radiographs, PVNS almost never calcifies but can erode into bone, creating large erosions/cystic cavities. Joint space narrowing occurs only late in its course.
• MRI demonstrates low signal masses (representing synovial hypertrophy) on T1 and T2 weighted imaging, haemosiderin deposition, and joint effusion. The nodular type of PVNS is typically well circumscribed, lobular, and usually less than 4cm in diameter. It lacks the diffuse frond-like synovial projections and abundance of haemosiderin found in diffuse PVNS.