History:
A 48 y/o male was injuried by a heavy falling object (a large piece of glass) over the left hip

Questions:

1. What are the findings?
2. What is the differential diagnosis?
3. What is the diagnosis?

Answers:

1. Anteroposterior (AP) radiograph of the pelvis shows superior and lateral dislocation of the left femoral head. Disruption of the line of the posterior acetabular wall is evident. A large displaced bone fragment is superior to the left femoral head. CT of the pelvis (bone window) without contrast after close reduction of the left hip shows accetabular fracture (posterior column) and humeral head fracture. The displaced posterior column of the left acetabulum is located far lateral to the humeral head.
2. Anterior hip dislocation, posterior hip dislocation
3. Posterior hip dislocation with acetabular (posterior wall) and humeral head fracture

Discussion
Hip dislocations result from severe trauma such as motor vehicle accidents. Posterior dislocation with or without acetabular fracture account for 85% to 90% of traumatic hip dislocation. The mechanism of injury is a blow along the axis of the femoral shaft with the hip flexed (as in hitting the dashboard with the knee in a motor vehicle accident). The posterior wall or column of the acetabulum often is fractured, and the femoral shaft or knee also may be injuried. Associated fractures of the femoral head occur occasionally, and intraarticular fragments may be particularly problematic for reduction. The thigh is characteristically adducted after a posterior dislocation. CT can identify intraarticular fragments and confirm relocation of the hip after reduction. The presence of gas bubbles in the hip joint capsule after trauma, in the absence of penetrating injury, is a reliable indicator of recent hip dislocation. Most bubbles are located anterior to the femoral neck, but bubbles also may be found posteriorly. If hip dislocation is unsuspected because of spontaneous relocation or reduction at the scene, the presence of gas should alert the clinician to the possibility of complications. Complications of hip dislocation include avascular necrosis of the femoral head, transient or permanent sciatic nerve palsy, myositis ossificans, and posttraumatic degenerative arthrosis. A posterior hip dislocation strecches and twists the external iliac, common femoral, and circumflex arteries, resulting in changes in extraosseous blood flow. Although collateral circulation from gluteal vessesls may preserve intraosseous blood flow, delayed relocation may produce a progressive and delayed form of arterial damage that leads to osteonecrosis.

Reference
1. K. J. Fairbairn, M. E. Mulligan, M. D. Murphey, C. S. Resnik. Gas bubbles in the hip joint on CT: an indication of recent dislocation. AJR 1995; 164: 931-934