Tophaceous pseudogout (tumoral calcium pyrophosphate dihydrate crystal deposition disease).

Most cases of calcium deposition seen radiologically in soft tissues are caused by calcium hydroxyapatite and occur either as a complication of trauma with associated necrosis (eg, fat necrosis), generalized connective tissue diseases (eg, scleroderma), metabolic disturbances (eg, hyperparathyroidism, familial hyperphosphatemia), sarcoidosis, myeloma, or metastases. Hydroxyapatite deposits are seen at many soft tissue sites, including joint capsules, ligaments, blood vessels, dermis, etc. On the other hand, deposits of calcium pyrophosphate are seen typically in the meniscus, articular cartilage, ligamentum flavum, and intervertebral disc. They usually are punctate or linear in distribution within the meniscus or parallel to the subchondral bone end plate. We report seven cases of massive focal calcium pyrophosphate dihydrate (CPPD) crystal deposition disease (tophaceous pseudogout) that occurred in atypical locations for CPPD. The ages of the patients ranged from 31 to 86 years (average, 60.7 years). One patient was male and six were female. The temporomandibular joint was involved in three patients and the metatarsophalangeal joint of the great toe was involved in two patients. The hip joint and cervical spine were involved in one patient each. A mass or swelling with or without pain was a common symptom. None of the patients in our series had clinical or radiographic evidence of CPPD crystal deposition disease in any other joints. Roentgenograms showed calcified lesions with a granular or fluffy pattern. Histologically, the lesions showed small or large deposits of intensely basophilic calcified material containing needle shaped and rhomboid crystals with weakly positive birefringence characteristic of CPPD. Foreign body granulomatous reaction to the CPPD deposition was constantly found. Chondroid metaplasia around and in the areas of CPPD deposition was observed commonly. Some of the chondroid areas showed cellular atypia in chondrocytes suggestive of a malignant cartilage tumor. It is important to recognize this rare form of CPPD crystal deposition disease and to identify the CPPD crystals in the calcified deposits, thus avoiding the misdiagnosis of benign or malignant cartilaginous lesions.