Acetabulum

The acetabulum is a deep, cup-shaped cavity located on the lateral aspect of the pelvis, formed at the junction of three pelvic bones: the ilium, ischium, and pubis. It serves as the socket for the head of the femur, forming the hip joint or acetabulofemoral joint. The acetabulum plays a vital role in weight transmission from the axial skeleton to the lower limb and allows for a wide range of motion while maintaining joint stability.

Structure

The acetabulum has a distinctive hemispherical shape with a central non-articular depression and a peripheral articular surface. It opens laterally, slightly inferiorly, and anteriorly. Its architecture ensures a snug fit for the femoral head, allowing for a stable ball-and-socket articulation.

Components

• Acetabular fossa: A central non-articular area with a rough surface, formed mainly by the ischium. It contains a pad of fibroelastic fat and the ligament of the head of femur (ligamentum teres).
• Lunate surface: The articular horseshoe-shaped periphery of the acetabulum covered in hyaline cartilage. It articulates directly with the femoral head.
• Acetabular notch: An inferomedial gap in the rim of the acetabulum that allows passage for neurovascular structures and is bridged by the transverse acetabular ligament.

Ossification Contributions

The acetabulum is formed by the convergence of three bones:

Bone	Contribution
Ilium	Forms the superior portion
Ischium	Forms the posterior and inferior part
Pubis	Forms the anterior part

These bones are separated during development by a Y-shaped triradiate cartilage, which later ossifies by the end of adolescence, typically between ages 16 to 20 years.

Location

The acetabulum is located on the lateral surface of the pelvis, inferior to the iliac wing and superior to the obturator foramen. It faces laterally, slightly anteriorly and inferiorly, providing the socket portion of the hip joint. Its positioning enables efficient load transfer from the vertebral column through the pelvis to the femur during upright posture and locomotion.

Function

• Articulation: Forms the socket of the hip joint, articulating with the spherical head of the femur to enable multidirectional movement.
• Load distribution: Distributes body weight from the axial skeleton to the lower limb efficiently during standing and movement.
• Joint stability: Its concavity, along with the labrum and surrounding ligaments, stabilizes the hip joint while allowing a large range of motion.

Acetabular Labrum

The acetabular labrum is a fibrocartilaginous ring attached to the rim of the acetabulum, deepening the socket and enhancing joint stability. It increases the surface area of contact with the femoral head and helps maintain intra-articular pressure, contributing to joint congruence and load absorption.

• Composition: Fibrocartilage, continuous inferiorly with the transverse acetabular ligament.
• Function: Enhances depth of the socket, reduces joint stress, and acts as a seal for joint fluid retention.

Articulations

The acetabulum articulates with the head of the femur to form the hip joint, a synovial ball-and-socket joint. The articulation allows flexion, extension, abduction, adduction, internal and external rotation, and circumduction of the thigh.

Articulating Surface	Joint Type	Movements Allowed
Head of femur	Synovial (ball-and-socket)	Flexion, extension, abduction, adduction, rotation, circumduction

Relations

• Superior: Iliac wing and gluteus medius muscle
• Inferior: Obturator foramen and associated neurovascular bundle
• Anterior: Pubic body and pectineus muscle
• Posterior: Ischium and piriformis muscle

Blood Supply

The acetabulum is supplied primarily by branches of the obturator artery (especially via the acetabular branch that travels within the ligament of the head of femur), superior gluteal artery, and medial circumflex femoral artery. These vessels ensure adequate perfusion for the acetabular bone and labrum.

Nerve Supply

The innervation of the acetabulum and its labrum comes from the obturator nerve, femoral nerve, and branches from the sacral plexus. These nerves carry both nociceptive (pain) and proprioceptive (position) fibers.

Variations and Development

During development, the acetabulum begins to form in the fetal period with the establishment of the triradiate cartilage. As growth continues, ossification centers in the ilium, ischium, and pubis contribute to forming the acetabular cavity.

• Triradiate cartilage: A Y-shaped growth plate that separates the three bones and contributes to acetabular depth.
• Fusion: Completed by late adolescence, typically around 16–20 years of age.

Clinical Significance

• Acetabular fracture: A serious injury usually resulting from high-energy trauma (e.g., car accidents). May involve posterior or anterior wall, column, or roof. Often requires surgical fixation.
• Developmental dysplasia of the hip (DDH): A congenital condition where the acetabulum is shallow or underdeveloped, leading to improper articulation with the femoral head and risk of dislocation.
• Acetabular labral tear: Common in athletes and individuals with femoroacetabular impingement (FAI). Presents with groin pain and limited hip motion.
• Osteoarthritis of the hip: Degeneration of the articular cartilage of the acetabulum and femoral head, leading to joint stiffness and pain.
• Prosthetic acetabulum: In hip replacement surgery, a prosthetic cup is implanted into the acetabulum to receive the artificial femoral head. The bone must be intact or reconstructed for proper fixation.

Palpation and Surface Landmark

The acetabulum itself is not directly palpable due to its deep location. However, surrounding bony landmarks such as the anterior superior iliac spine (ASIS), pubic tubercle, and ischial tuberosity help estimate its position during physical examination and surgical planning.