The fusion of the ilium, ischium and pubis results in the formation of several landmarks. These anatomical features are the acetabulum and obturator foramen.

The acetabulum is a deep cup-shaped cavity on the lateral surface of the hip bone. It articulates with the head of the femur, and both structures form the hip joint. The body of the ilium forms the upper third of the acetabulum, while the body of the ischium comprises the posterior third. And finally, the superior pubic ramus contributes to the formation of the anterior third of the acetabulum. The acetabulum features some anatomical landmarks:

Acetabular fossa - somewhat circular non-articular depression located at the center of the acetabulum; it is occupied by the ligament of the head of the femur; inferior the acetabular fossa continues as the acetabular notch;
Lunate surface - crescent-shaped articular surface surrounding the acetabular fossa; it articulates with the head of the femur;
Acetabular notch - a deep notch at the inferior aspect of the acetabulum; it connects both ends of the lunate surface; the acetabular notch is transformed into a foramen by the transverse acetabular ligament, and nerves and blood vessels reach the hip joint through this foramen;
Acetabular margin (acetabular labrum) - C-shaped thickened cartilaginous margin that goes along the outer border of the lunate surface

Right above the acetabulum, the body of the ilium presents a shallow groove that encircles the upper aspect of the acetabulum. It is known as the supra-acetabular groove.

Anteroinferior to the acetabulum is the obturator foramen - a large opening formed by the fusion of the ischium and pubis. Most of the foramen is closed by a connective tissue membrane known as the obturator membrane. However, a small, narrow space remains open. It is known as the obturator canal. The obturator canal is found at the superior aspect of the foramen, and it transmits the obturator artery, obturator vein, and obturator nerve.

The anterior and posterior margins of the obturator foramen present small bony eminences - anterior obturator tubercle and posterior obturator tubercle.

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Features of hip bone (acetabulum and obturator foramen)