Synovial haemangioma of the knee: light and electron microscopic findings.

Venous haemangioma of the synovium of one knee also involved adjacent, skin and muscle of the leg in a 4-yr-old girl. Phleboliths were seen in the knee on X-ray. Histologically, the synovium contained large venous channels and was laden with haemosiderin. On electron microscopy, the synovial cells were morpholgically most like synthetic (B) cells. Erythrocytes were ingested and broken down, stepwise, to siderosomes. The synovial cell cytoplasm also contained electron-dense punctate iron deposits. The synovial response appeared to represent chronic haemarthrosis.