What is the Acetabulum?

The acetabulum is a rounded depression in the hip bone. It forms a socket which contains the ball-shaped head of the thigh bone, or femur, allowing it to rotate quite freely. Together, the acetabulum and the head of the femur form what is known as the hip joint. Acetabular fractures are rare but serious injuries and usually require surgery. Acetabulum is a Latin word meaning a small vinegar cup.

A smooth tissue known as cartilage lines the walls of the acetabulum, apart from the lower margin, where there is a gap in the socket called the acetabular notch. The deepest part of the socket is known as the acetabular fossa. It has no cartilage and does not take part in joint movement. Sometimes, as part of a condition known as developmental dysplasia of the hip, a baby may be born with an abnormally shaped head of the femur or acetabulum, but the outlook is positive if treatment begins early, using methods which may include a special harness, a positioning cast or surgery.

Fractures of the acetabulum generally occur when the ball-shaped head of the femur is driven up through the socket during a fall or as the result of a motor vehicle accident. Symptoms of severe pain and inability to walk typically follow, and X-rays of the pelvis and computed tomography (CT) scans may be used to help diagnose the problem. This type of fracture is difficult to treat because the site is awkward to reach and it is also close to important nerves and blood vessels, the bladder and gut.

Surgery to repair acetabulum fractures first involves piecing together the shattered bits of bone and fixing them in place using screws, pins, plates and wires. Since there is a significant amount of bleeding involved with this kind of injury, surgery is usually carried out after several days have gone by, to allow clotting to take place first. Pain relief is given, and the patient may be put in traction to prevent further damage occurring.

Recovery is gradual after surgery for an acetabulum fracture, with crutches being used for at least the first couple of months. Nerves are likely to have been damaged to some extent, either during the initial injury or later in the surgical operation. Any resulting reduced sensation or decreased movement in a limb can take up to 18 months to get better, and a full recovery is not guaranteed, although most people experience some improvement. Some people will go on to develop arthritis, due to the irreversible and damaging effects of the injury on the cartilage lining the socket.