Question: Left-sided hip pain and reduced range of motion. Answer: Intra-articular calcium pyrophosphate deposition disease (CPPD). MRI of the pelvis shows a soft tissue mass at the anterosuperior aspect of the left hip extending to the level of the anterior superior iliac spine, possibly representing an avulsion injury (Fig. 1). Other differential diagnoses for a lowintermediate signal intensity intra-articular mass include t e n o s y n o v i a l g i a n t c e l l t u m o u r a n d s y n o v i a l osteochondromatosis. CT was undertaken for further characterization, this demonstrating a diffusely mineralized intraarticular mass eroding the anterior aspect of the acetabular roof and extending distally to the femoral head-neck junction, with no appreciable bone continuity to either the acetabulum or femoral head (Fig. 2). MRI of the hip showed a heterogeneous, well-defined low signal intensity soft tissue mass appearing to extend from an erosion at the base of the anterior superior iliac spine. The combination of findings was felt to suggest an underlying diagnosis of gout or calcium pyrophosphate dihydrate disease (CPPD), but a mineralizing intra-articular soft tissue tumour such as synovial sarcoma could not be excluded. Therefore, CT-guided core needle biopsy was undertaken. This showed cores of fibrous tissue with diffuse deposits of CPPD surrounded by a granulomatous infiltrate including foreignbody giant cells and macrophages (Fig. 3a). Rhomboidshaped crystals were seen under polarised light (Fig. 3b). CPPD crystals are weakly birefringent compared with

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