36 year old male football coach presented with a 6 month history of insidious onset anterior knee pain. The pain was aggravated by squatting. On examination there was full range of passive movement. The patellofemoral joint, medial and lateral joint lines were non-tender. The initial working diagnoses to this stage included patellofemoral pain and patella tendinopathy.
Ultrasound of the right knee showed a non-swollen normal looking patella tendon. There was a large well-defined homogenous swelling 20 x 30mm, deep to the patella tendon that was continuous with the anterior joint. There was significant power doppler activity around the margins of the lesion.
MRI confirmed a normal patella tendon with no evidence of tendinopathy. There was a 38 x 50mm swelling deep to the patella tendon in the fat pad extending into the anterior aspect of the joint centrally. The marked blooming on the T2 images was consistent with a diagnosis of Pigmented Villonodular Synovitis (PVNS). There was no other synovium abnormality noted elsewhere.
PVNS is an uncommon but benign proliferative disease of the synovial membrane, classified as either extra-articular (bursa or tendon sheath involvement) or intra-articular (localised or diffuse synovium involvement). The more common extra-articular variant typically presents as a soft tissue mass, whilst the intra-articular variant most commonly presents with pain. Malignant transformation can occur (but is rare occurring in less that 3% of cases). When this does occur however it can be high grade with a poor outcome. The aetiology is unknown although the condition is linked with repetitive trauma, intra-articular haemorrhage, inflammation and neoplastic origin.
Diagnostic confirmation is important as some lesions can resemble aggressive neoplasms but because of the typical MRI findings, tissue biopsy is rarely required. X-ray may show non-specific joint effusion or bone erosions but where calcification is seen the diagnosis of PVNS should be reconsidered. CT can be diagnostic but may not show the full extent of all soft tissue involvement. Ultrasound does not permit characterisation of the disease and some appearances may overlap with rheumatoid arthritis. Ultrasound is useful alongside MRI in showing any surrounding soft tissue involvement and monitoring response to treatment. MRI is diagnostic in appearance due to the ‘blooming’ effect of hemosiderin deposits. MRI also permits wider identification of the full extent of the disease.
Untreated the condition can give rise to significant joint morbidity. Local steroid injections offer only short-term pain relief whilst the mainstay of treatment is surgical resection with synovectomy. Targeted radiotherapy and radionuclide injections maybe useful adjuncts. Recurrence rate after total synovectomy can be 7-20% but diffuse disease is more likely to recur (50%) when compared with localised disease (5%).
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