The specimen shows fine fronds of pigmented tissue, sometimes discrete and sometimes matted
together forming small nodules of brown tissue. These arise from a pale membrane.
This young woman presented at the age of 19 with a six-month history of swelling and
instability in her left knee which had apparently been a sequel to a minor strain. On examination
she had quite considerable effusion and synovial thickening in both knees and subsequently the
lesion seen here. The left knee which clinically was similarly affected, was later explored but this
showed only an effusion and the presence of a large medial synovial plica. This was excised and
both knees are now essentially trouble-free. The histology of the lesion is that of villonodular
synovitis; the pigmentation is due to the deposition of haemosiderin. The aetiology of the condition
is obscure but it is regarded as non-neoplastic.