Acetabulum

The acetabulum is formed by the union of the ilium, ischium, and pubis to form the hip joint cavity. Its articular surface is concave and cup-shaped and receives the head of the femur to form the hip joint.

The acetabulum is formed by the fusion of three pelvic bones: the ilium, ischium, and pubis. The lines of fusion are generally visible 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:

• Dogs and cats: The acetabulum is deep and round, allowing for significant flexion and extension. The acetabular notch is prominent. In cats, the joint is more flexible than in dogs.

• Horse: The acetabulum is wide and deep, reflecting the high mechanical stresses associated with locomotion. The lunate surface is larger in order to support the weight of the large femoral head. The femoral head ligament and accessory ligament are present, limiting abduction.

• Ruminants (cattle, sheep, goats): The acetabulum is deep and adapted to support the large body mass. The joint mainly allows flexion and extension; rotational movement is limited.

• Pigs: Similar to ruminants, but with a relatively shallower acetabulum. Nevertheless, it allows for a stable joint that supports weight.

Ossification and Development:

The three bones forming the acetabulum each contribute to an ossification center that fuses at maturity. The triradiate cartilage between the ilium, ischium, and pubis persists until skeletal maturity and plays a role in growth.

Ligaments and Joint Capsule:

The ligament of the femoral head (ligamentum capitis femoris) anchors the femoral head in the acetabulum. The transverse acetabular ligament spans the acetabular notch, transforming it into a foramen and reinforcing the acetabular rim. The joint capsule is relatively tight in most species to prevent dislocation, but its laxity varies 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.