Acetabular pneumatocyst containing air-fluid level

The presence of intraosseous gas most commonly occurs in osteomyelitis, vac uum phenomenon, and postsurgery or posttraumatic states. Several cases of s ubchondral gas-filled lesions, called pneumatocysts, have also been describ ed in the sacroiliac joint and clavicle, none of them with intralesional ai r-fluid level. These pneumatocysts are innocuous lesions of uncertain origi n. We describe one case of acetabular pneumatocyst containing air-fluid lev el in a 62-year-old man with long-standing ankylosing spondylitis involving hip joint. To our knowledge, this is the first reported case of a pneumato cyst in an acetabular location containing air-fluid level.