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Os Coxae (Hip Bone) Overview

The os coxae (hip bone) is a large, irregular bone that forms the pelvic girdle. It is composed of three fused bones:

  1. Ilium – the superior, fan-shaped part

  2. Ischium – the posterior, lower part

  3. Pubis – the anterior, lower part

These three bones fuse at the acetabulum, the cup-like socket for the femur.

Key Landmarks of the Os Coxae

1. Ilium (Superior Portion)

  • Iliac Crest – the upper ridge (important for muscle attachment).

  • Anterior Superior Iliac Spine (ASIS) – a bony projection at the front (attachment for inguinal ligament & sartorius muscle).

  • Anterior Inferior Iliac Spine (AIIS) – below the ASIS (attachment for rectus femoris).

  • Posterior Superior Iliac Spine (PSIS) – posterior projection (used for surface anatomy landmarks).

  • Posterior Inferior Iliac Spine (PIIS) – below the PSIS, near the greater sciatic notch.

  • Greater Sciatic Notch – passageway for the sciatic nerve.

2. Ischium (Posterior & Inferior Portion)

  • Ischial Spine – a small pointed projection (separates the greater & lesser sciatic notches).

  • Lesser Sciatic Notch – passageway for nerves & tendons.

  • Ischial Tuberosity – a roughened area where the hamstring muscles attach (also the "sit bones" you rest on).

  • Ramus of Ischium – extends toward the pubis and contributes to the obturator foramen.

3. Pubis (Anterior & Inferior Portion)

  • Pubic Body – the main anterior portion, where the left and right os coxae meet at the pubic symphysis (cartilaginous joint).

  • Superior Pubic Ramus – extends from the pubic body toward the acetabulum.

  • Inferior Pubic Ramus – connects to the ischium (forming part of the obturator foramen).

  • Pubic Tubercle – a small bump where the inguinal ligament attaches.

4. Acetabulum (Hip Socket)

  • Formed by all three bones (ilium, ischium, pubis).

  • Deep, cup-like depression that articulates with the femoral head.

  • Acetabular Notch – an incomplete inferior section of the rim.

  • Lunate Surface – the smooth, articular part where the femur fits.

5. Obturator Foramen

  • Large hole formed by the pubis and ischium.

  • Covered by the obturator membrane, with a small opening for the obturator nerve and vessels.