Primary cystic arthrosis of the hip
Subchondral cysts become a challenging diagnostic entity when they present as an epiphyseal lucent lesion in the absence of other readily identifiable degenerative changes, and/or in a younger patient. In such instances, this pathology has been considered by some to represent primary cystic arthrosis of the hip when it occurs about the hip with severe, inexplicable pain and limited range of motion. Further imaging workup may be necessary in these scenarios to exclude other differential possibilities, as was done in this case.
Cystic joint changes are most commonly associated with arthritic processes such as osteoarthritis, rheumatoid arthritis and calcium pyrophosphate dehydrate crystal deposition disease (CPPD), or with osteonecrosis, all of which commonly affect the femoroacetabular joint. Occasionally, large and/or solitary subchondral cysts occurring in these diseases may be confused with a neoplastic process, especially given the right clinical scenario. Periarticular/epiphyseal tumors include chondroblastoma, giant cell tumor, metastasis, histiocytosis X and aneurysmal bone cyst. In the acetabulum specifically, fibrous dysplasia and pigmented villonodular synovitis (PVNS) could also be considered.