An acetabular fracture is a break in the socket portion of the "ball-and-socket" hip joint. Acetabular fractures are not common — they occur much less frequently than fractures of the upper femur or femoral head (the "ball" portion of the joint).
The majority of acetabular fractures are caused by some type of high-energy event, like a car accident. Many times patients will have additional injuries that require immediate treatment.
In a smaller number of cases, a low-energy incident, such as a fall, may cause an acetabular fracture in an older person who has weaker bones.
Treatment for acetabular fractures often involves surgery to restore the normal anatomy of the hip and stabilize the hip joint.
What is an acetabulum?
The hip is one of the body's largest joints. It is a "ball-and-socket" joint. The socket is formed by the acetabulum, which is part of the pelvis. The ball is the femoral head, which is the upper end of the femur (thighbone).
What is the anatomy of the hip?
The bone surfaces of the ball and socket are covered with articular cartilage—a smooth, slippery substance that protects and cushions the bones and enables them to move easily. Bands of tissue called ligaments connect one bone to another. These ligaments help provide both function and stability to the hip joint, allowing it to move without falling out of the socket. Major nerves, blood vessels, and portions of the bowel, bladder, and the reproductive organs all pass within or close to the pelvis. These structures can occasionally be damaged as the result of an injury to the acetabulum.
What is an acetabular fracture?
Acetabular fractures vary. For example, the bone can break straight across the socket or shatter into many pieces. When the acetabulum is fractured, the femoral head may no longer fit firmly into the socket, and the cartilage surface of both bones may be damaged. If the joint remains irregular or unstable, ongoing cartilage damage to the surfaces may lead to arthritis.
How is the severity of an acetabular fracture measured?
The severity of the injury depends on several factors, including:
- The number and size of the fracture fragments.
- The amount each piece is out of place — In some cases, the broken ends of bones line up well; in more severe fractures, there may be a large gap between the broken pieces, or the fragments may overlap each other.
- The injury to the cartilage surfaces of both the acetabulum and the head of the femur.
- The injury to surrounding soft tissues, such as muscle, tendons, nerves, and skin.
If the bone breaks in such a way that bone fragments stick out through the skin or a wound penetrates down to the broken bone, the fracture is called an "open" or compound fracture. This type of fracture is particularly serious because, once the skin is broken, infection in both the wound and the bone can occur. Immediate treatment is required to prevent infection.
Open fractures of the acetabulum are rare because the hip joint is well covered with soft tissues. When they do occur, they are usually the result of very high-energy trauma.
What causes acetabular fractures?
An acetabular fracture results when a force drives the head of the femur against the acetabulum. This force can be transmitted from the knee (such as hitting the knee against the dashboard in a head-on car collision) or from the side (such as falling off a ladder directly onto the hip). Depending upon the direction of the force, the head of the femur is sometimes pushed out of the hip socket, an injury called hip dislocation.
When the fracture is caused by high-energy impact, patients often experience extensive bleeding and have other serious injuries that require urgent attention.
Acetabular fractures are sometimes caused by weak or insufficient bone. This is most common in older patients whose bones have become weakened by osteoporosis. Although these patients do not often have other injuries, they may have complicating medical problems, such as heart disease or diabetes.