1. Obturator Foramen is a large gap in the hip bone, situated anteroinferior to acetabulum, between the pubis and the ischium.
2. It is large and oval in males, and small and triangular in females.
3. It is closed by the obturator membrane which is attached to its margins, except at the obturator groove where the obturator vessels and nerve pass out of the pelvis.

Obturator Foramen

Ossification

The hip bone ossifies in cartilage from three primary centres and five secondary centres. The primary centres appear, one for the ilium during the second month of intrauterine life; one for the ischium during the fourth month; and one for the pubis during the fifth month. At birth the hip bone is ossified except for three cartilaginous parts. These are:

(i) the iliac crest;

(ii) a Y-shaped cartilage separating the ilium, ischium and pubis;

(iii) a strip along the inferior margin of the bone including the ischial tuberosity.

The ischiopubic rami fuse with each other at 7 to 8 years of age.
The secondary centres appear at puberty, two for the iliac crest, two for the Y-shaped cartilage of the acetabulum and one for the ischial tuberosity.

Ossification in the acetabulum is complete at 16-17 years, and the rest of the bone is ossified by 20-25 years. The anterior superior iliac spine, pubic tubercle and crest and the symphyseal surface may have separate secondary centres of ossification.

Determination of Sex

The following characteristics help in finding out whether a given hip bone belongs to a male or a female.
1. The greater sciatic notch is wider in females (75°) than in males (50°)

2. The acetabulum is large in males, and its diameter is approximately equal to the distance from its anterior margin to the pubic symphysis

3. The chilotic line extends from the iliopubic eminence to the iliac crest. In females, the pelvic part of the chilotic line is longer than the sacral part

4. The curvatures of the iliac crest are more pronounced in males

5. The iliac fossa is deeper in males

6. The pubic crest is shorter in males

7. The lower margin of the ischiopubic rami is more everted in males, for the attachments of the crus of the penis with intumed ischial spine

8. The preauricular sulcus is more marked in females

9. The obturator foramen is large and oval in males, and small and triangular in females

10. The subpubic angle is more in females and is 80°-85°