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Acetabulum

What Is It, Function, Fractures, and More

Author:Anna Hernández, MD

Editors:Alyssa Haag,Józia McGowan, DO,Kelsey LaFayette, DNP, RN, FNP-C

Illustrator:Jessica Reynolds, MS

Copyeditor:David G. Walker

What is the acetabulum?

The acetabulum is a large socket on the lateral face of the hip bone that articulates with the head of the femur to form the hip joint. 

Anatomically, the acetabulum is formed by the fusion of the three pelvic bones: the ilium, ischium, and pubis. The ilium forms the upper boundary of the acetabulum, the ischium contributes to the posterior wall, and the pubis forms the anterior wall. In addition, the acetabulum is divided into an anterior and posterior column. The anterior column includes the anterior half of the iliac wing and the anterior half of the acetabular surface, whereas the posterior column begins at the greater sciatic notch and includes the posterior half of the acetabulum and ischial tuberosity. This distinction between acetabular walls and columns facilitates the classification of acetabular fractures. 

What is the function of the acetabulum?

The acetabulum is part of the hip joint, which is a large, weight-bearing joint that connects the pelvis to the lower limb. The hip joint is a ball-and-socket joint where the ball-shaped head of the femur sits and rotates within the acetabulum, which is a cup-shaped socket. One of the main functions of the hip joint is to transfer weight from the vertebral column to the acetabulum and lower extremities when standing as well as to allow for the range of motion necessary for walking, running, climbing, or jumping. 

Which bone articulates with the acetabulum?

The acetabulum articulates with the head of the femur, which is covered almost entirely in cartilage to facilitate smooth movement as it slides within the acetabulum. The acetabulum, on the other hand, is surrounded anteroinferiorly by a lower depression known as the acetabular notch. On the outside of the acetabular margin, there is the acetabular labrum, which is a ring of cartilage that surrounds the acetabulum and acetabular notch. The acetabular labrum increases the surface of the acetabulum to allow more than half of the femoral head to fit within the acetabulum, thereby providing more stability to the hip joint.

How do acetabular fractures occur?

Acetabular fractures are pelvic fractures that involve the articular surface of the hip joint. These fractures occur primarily in young individuals who are involved in high-energy trauma (e.g., motor vehicle crashes, falls from height, sporting accidents). Less frequently, they may also occur in elderly individuals with poor bone density after a fall from standing height. Because acetabular fractures are usually caused by high-energy trauma, they are often associated with other life-threatening injuries, including pelvic ring and lower extremity fractures; nervous and vascular injuries; and head, chest, or abdominal trauma.

Diagnosis of acetabular fractures can be made with plain X-ray imaging, although a CT scan is commonly conducted to investigate other potential injuries and for surgical planning. Usually, a series of radiographs is obtained, including an anteroposterior view (AP) of the pelvis and Judet views. The Judet views are obtained to assess the integrity of the acetabular columns, and they consist of an obturator oblique and an iliac oblique view. The obturator oblique view is conducted to assess the anterior column and posterior wall of the acetabulum, whereas the iliac oblique view allows for assessment of the posterior column and anterior wall. 

There are several acetabular fracture classification systems with the most commonly used being the Letournel and Judet classification. This system classifies fractures of the acetabulum into five elementary types and five associated, or combined, types. By far, the most common type of fracture is a posterior wall fracture, which typically occurs when an individual is in a seated position and the knee hits the dashboard during a car crash. This pushes the femur against the posterior wall of the acetabulum, causing an impaction of the femoral head on the posterior acetabular rim. Posterior wall fractures may be accompanied by a posterior hip dislocation and injury to the sciatic nerve, which runs just posterior to the acetabulum.

What is treatment for an acetabular fracture?

Treatment of acetabular fractures depends on the type of fracture and characteristics of the individual. Non-displaced fractures may be treated conservatively with protected weight-bearing (i.e., using walking aids, such as crutches, a cane, or a walker) for six to eight weeks, whereas displaced fractures generally require surgical treatment to prevent instability of the hip joint and development of post-traumatic osteoarthritis. Surgery typically involves open reduction and internal fixation, or ORIF, of the pelvis. ORIF refers to the surgical realignment of a bone and posterior implantation of plates, screws, or pins to hold an unstable fracture. In elderly individuals with severe acetabular fractures or those with pre-existing arthritis, a total hip replacement may be necessary.

What are the most important facts to know about the acetabulum?

The acetabulum is a cup-shaped structure located on the lateral face of the hip bone that articulates with the femoral head, forming the hip joint. Acetabular fractures typically involve the articular surface of the acetabulum, and they occur when the femoral head or neck impacts onto the acetabulum. Diagnosis of acetabular fractures can be made with specific pelvis radiographs (including Judet views) but frequently requires a CT scan of the pelvis. Treatment can be conservative for non-displaced fractures; however, displaced fractures generally require surgical treatment to prevent instability of the hip joint and development of post-traumatic osteoarthritis.

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