Tophaceous pseudogout in the knee joint mimicking a soft-tissue tumour: a case report.

Tophaceous pseudogout in the knee joint is rare. We report an 82-year-old man who presented with a one-year history of pain and swelling of the right knee joint. Treatment with non-steroidal anti-inflammatory drugs and aspiration of the joint effusion had not been effective. The mass continued to enlarge, and the patient had difficulty walking. Radiographs and computed tomography showed meniscal calcification with an abnormal soft-tissue mass surrounded by calcification. After excision, massive calcified deposits were seen both inside and on the surface of the tophaceous pseudogout. The deposits showed birefringence under polarised light, suggestive of calcium pyrophosphate dihydrate crystals. At the 2-year follow-up, the patient could walk independently without knee pain or swelling, although his range of knee motion was slightly limited due to joint contracture that developed before surgery.