Massive solitary tophus containing calcium pyrophosphate dihydrate crystals at the acromioclavicular joint.

A massive solitary tophus containing calcium pyrophosphate dihydrate (CPPD) crystals was resected from the distal clavicle and proximal acromion of a 62-year-old man who presented with a painful shoulder mass. The diagnosis was confirmed histologically and by X-ray diffraction. CPPD crystal deposition was found within bone, however, the articular cartilage was not involved. Other unusual features of the case include the size of the lesion, the monoarticular presentation and a peculiar discoloration of the overlying skin. The clinical presentation and radiographic features were initially consistent with a malignant process; thus the case demonstrates that CPPD arthropathy should be included in the differential diagnosis of periarticular tumors.