Acetabulum

The acetabulum is composed by the union of the body of the ilium, ischium and pubis to form the socket of the hip joint.

The acetabulum presents a concave, cup-shaped articular surface of the pelvis that receives the head of the femur to form the coxofemoral (hip) joint.

The acetabulum is formed by the fusion of three pelvic bones:

• Ilium (cranial part)

• Ischium (caudoventral part)

• Pubis (medioventral part)

The fusion lines are usually evident during development but become indistinct in adult animals.

Structure

• Acetabular fossa: The central, non-articular depressed region, often filled with adipose tissue and the ligament of the femoral head.

• Lunate surface: The crescent-shaped articular region that surrounds the acetabular fossa. It is smooth and covered with hyaline cartilage for articulation with the femoral head.

• Acetabular notch: A distinct notch on the ventromedial border of the acetabulum, which becomes a foramen (the acetabular foramen) due to the transverse acetabular ligament bridging across it.

• Acetabular rim: The outer edge that is more prominent dorsally and cranially; contributes to joint stability.

Species Variations

Dog and Cat

• The acetabulum is deep and round, allowing extensive flexion and extension.

• The acetabular notch is prominent.

• In cats, the joint is more flexible compared to dogs.

Horse

• The acetabulum is large and deep, reflecting the high mechanical stress from locomotion.

• The lunate surface is more extensive to support the weight of the large femoral head.

• The ligament of the head of the femur and the accessory ligament are present, limiting abduction.

Ruminants (Cattle, Sheep, Goat)

• The acetabulum is deep and adapted to support the heavy body mass.

• The joint allows mainly flexion and extension; rotational movement is restricted.

Pig

• Similar to ruminants, but with a relatively shallower acetabulum.

• Still allows a stable weight-bearing articulation.

Ossification and Development

• The three bones forming the acetabulum each contribute an ossification center that fuses by maturity.

• The triradiate cartilage between the ilium, ischium, and pubis persists until skeletal maturity and plays a role in growth.

Ligaments and Joint Capsule

• Ligament of the femoral head (ligamentum capitis femoris) anchors the femoral head into the acetabulum.

• Transverse acetabular ligament spans the acetabular notch, converting it into a foramen and reinforcing the acetabular rim.

• The joint capsule is relatively tight in most species to prevent dislocation but varies in laxity between carnivores and ungulates.

Clinical Relevance

• Hip dysplasia: Common in dogs, characterized by a shallow acetabulum leading to instability and degenerative joint disease.

• Luxation/dislocation: Occurs due to trauma or ligamentous injury; ventral dislocations are common when the acetabular notch is weak.

• Fractures: Acetabular fractures are serious due to the articulation’s weight-bearing role and complex surgical repair needs.

• Arthroplasty: In small animals, total hip replacement involves the acetabular component, highlighting the importance of acetabular depth and orientation.