Posterior Hip Dislocation 

 

·         Hip dislocation accounts for only 5% of all dislocations

o       Posterior hip dislocations are much more common than anterior hip dislocations (90% to about 10%)

·         About 20% of pelvic fractures involve the acetabulum and about 33% of acetabular fractures involve the posterior rim

·         Mechanism in posterior dislocation classically is unrestrained occupant of a motor vehicle accident, especially collisions which are head-on, in which the flexed knee strikes dash with hip flexed and adducted

o       Force is transmitted along femoral shaft to the hip

o       Associated with fractures of the posterior rim of the acetabulum

o       Posterior dislocations can also result from falls from a height

o       Anterior dislocations are more apt to occur if the hip is abducted at the time of injury

·         Imaging findings

o       Conventional radiography

§         In posterior dislocations, the head of the femur is usually situated superior and lateral to its normal position in the acetabulum

·         In anterior dislocations, the head usually rests inferior and medial to its normal acetabular position

§         May be subtle if head lies in AP plane appears as if it still resides in the acetabulum

§         There may be associated fractures of the head of the femur and/or posterior rim of the acetabulum

·         The posterior rim of the acetabulum normally is the more lateral of the two edges (anterior rim and posterior rim) seen on the anteroposterior (AP) view of the pelvis

§         Since the posteriorly dislocated head lies closer to the cassette, the posteriorly dislocated head may appear smaller than the head on the opposite side which lies farther from the cassette and is magnified more

o       Computed tomography (CT)

§         Provides an accurate means of evaluating not only the dislocation but the associated fractures as well

 

Posterior fracture-dislocation, hip. Anteroposterior conventional radiograph of the pelvis (above) shows
that right femoral head (blue arrow) lies more superior than the superior rim of the acetabulum (white arrow). The normal left femoral head appears slightly larger than the posteriorly dislocated right because it is farther from the imaging surface and more magnified. The contrast in the bladder was injected intravenously for a CT scan. The CT scan of the pelvis (below) demonstrates the femoral head (red arrow) well posterior to the acetabulum. There are associated fractures of the posterior rim of the acetabulum (yellow arrow).
For a larger photo of the same image without the arrows, click here
 

·         Treatment

o       Reduction of dislocation

§         Avascular necrosis of the femoral head is more likely to occur if the reduction does not occur before 6 hours