["Mouse" in Baker's cyst].

Calcified bodies may arise in a joint due to trauma, arthropathy resulting in joint destruction or synovial osteochondromatosis. These calcified loose bodies may pass into a popliteal cyst though posterior joint-bursal communications, or can arise in a popliteal cyst de novo on account of chondrometaplasia. A case history illustrates the differential diagnostic considerations. X-ray combined with ultrasound investigation will exclude soft tissue tumors and vascular lesions.