Huge tophaceous pseudogout associated with tenosynovial chondromatosis arising from flexor digitorum tendon sheaths of the foot: a case report.

Synovial chondromatosis (SC) is a benign proliferative process of synovial tissue creating multiple cartilaginous nodules in joints. It most commonly occurs in the large joints of the knee, hip, and shoulder, uncommonly in the small joints of the hand and foot, and only rarely in the tenosynovial membrane of tendon sheath, termed tenosynovial chondromatosis (TC). Unlike SC, TC predisposes to the foot or hand. The rarity and unfamiliarity of imagers with TC, as well as the variability of its histologic features often lead to an erroneous diagnosis of extraskeletal chondroma or even chondrosarcoma as in the present case. Calcium pyrophosphate dehydrate (CPPD) crystals are usually deposited in the articular cartilage or periarticular structures such as synovium and capsule, and rarely in other soft tissue structures including bursa, tendon, subcutaneous tissue, and dura mater. CPPD crystals may also be deposited in extraskeletal chondroma and SC. We present an exceptionally rare case of huge tophaceous pseudogout associated with TC that is considered to arise from the flexor digitorum longus tendon sheaths of the foot, initially mistaken for a chondrosarcoma.