The Hip Bone (Os Coxae)

The hip bone is a large, irregularly shaped bone that forms the lateral wall of the pelvis. It is composed of three bones that fuse during development: the ilium, ischium, and pubis. These three bones meet at the acetabulum - the cup-shaped socket that articulates with the femoral head to form the hip joint.

The Right Hip Bone - Lateral and Medial Views

Right hip bone - lateral and medial views with full labeling

Right hip bone: lateral view (top left), medial view (top right), and column principle (bottom) — THIEME Atlas of Anatomy

The Three Component Bones

1. Ilium

The largest of the three parts, forming the upper, fan-shaped portion of the hip bone. Key features:

Iliac crest - the curved upper border, palpable along its entire length; has a prominent tubercle of the iliac crest anterolaterally
Anterior superior iliac spine (ASIS) - attachment for the inguinal ligament and sartorius
Anterior inferior iliac spine (AIIS) - attachment for the rectus femoris (straight head)
Posterior superior iliac spine (PSIS) and posterior inferior iliac spine (PIIS)
Iliac fossa - the smooth concave medial surface where iliacus muscle attaches
Iliac tuberosity - posterior roughened area for interosseous sacroiliac ligaments
Auricular surface - ear-shaped articular surface for the sacroiliac joint
Gluteal surface (lateral) - divided by three gluteal lines (anterior, posterior, inferior) where the three gluteal muscles attach
Greater sciatic notch - below the PIIS, converted to the greater sciatic foramen by the sacrospinous ligament

2. Ischium

The posteroinferior part, forming the "sit bone." Key features:

Body of ischium - contributes to the posterior wall of the acetabulum
Ischial spine - projects medially between greater and lesser sciatic notches; attachment for the sacrospinous ligament
Lesser sciatic notch - below the ischial spine, converted to the lesser sciatic foramen by ligaments
Ischial tuberosity - the large rough prominence you sit on; attachments for hamstring muscles and sacrotuberous ligament
Ramus of ischium - joins with the inferior pubic ramus to form the ischiopubic ramus

3. Pubis

The anteromedial part. Key features:

Body of pubis - bears the symphyseal surface, which articulates with the opposite pubis at the pubic symphysis
Superior pubic ramus - projects laterally from the body; has the pectineal line (pecten pubis) and the pubic tubercle
Inferior pubic ramus - projects inferolaterally to join the ischial ramus

The Hip Bone - Anterior View

Right hip bone, anterior view — THIEME Atlas of Anatomy

The Acetabulum

The acetabulum is the deep hemispherical socket on the lateral surface of the hip bone. It is formed by contributions from all three component bones:

Ilium - 2/5 (superiorly)
Ischium - 2/5 (posteroinferiorly)
Pubis - 1/5 (anteriorly)

AP radiograph of right hip showing acetabular contributions - ilium (red), ischium (yellow), pubis (blue)

AP radiograph of the right hip: ilium (red), ischium (yellow), pubis (blue) — Imaging Anatomy, Vol. 3

Parts of the acetabulum:

Acetabular rim - the bony margin
Lunate surface - the C-shaped articular cartilage-bearing surface (covers ~2/3 of a sphere)
Acetabular fossa - the central non-articular depression
Acetabular notch - the inferior gap in the rim, bridged by the transverse acetabular ligament

Triradiate Cartilage

During childhood, the three bones are separated at the acetabulum by a Y-shaped growth plate called the triradiate cartilage. This fuses definitively between ages 14-16 years.

Triradiate cartilage (child): ilium, ischium, and pubis junction at the acetabulum — THIEME Atlas of Anatomy

Obturator Foramen

The large oval opening bounded by the pubis and ischium. It is the largest foramen in the skeleton but is almost entirely closed by the obturator membrane, leaving only the obturator canal superiorly for passage of the obturator nerve and vessels.

The Pelvic Girdle and Pelvic Ring

The two hip bones together form the pelvic girdle, joined:

Anteriorly - at the pubic symphysis (cartilaginous joint)
Posteriorly - to the sacrum at the sacroiliac joints

This creates the bony pelvic ring, which is highly stable and allows minimal motion.

The pelvic girdle and pelvic ring - anterior view

Pelvic ring: hip bones (gray), sacrum (tan), sacroiliac joints, pubic symphysis, and arcuate line — THIEME Atlas of Anatomy

Column Principle (Letournel Classification)

For the purpose of fracture classification, the hip bone is divided into:

Anterior column - runs from the iliac crest through the ASIS and superior pubic ramus to the pubic symphysis
Posterior column - runs from the greater sciatic notch through the acetabulum to the ischial tuberosity

Major trauma transmits force through the femoral neck to the acetabulum and then into these columns. The pattern of fracture depends on femoral neck position at impact.

Key Landmarks Summary

Structure	Part	Clinical Relevance
ASIS	Ilium	Inguinal ligament attachment; surface landmark
Iliac crest	Ilium	Bone marrow biopsy site
PSIS	Ilium	Dimple of Venus; sacroiliac joint reference
Ischial tuberosity	Ischium	Hamstring origin; ischial bursitis
Ischial spine	Ischium	Pudendal nerve block landmark; obstetric diameter
Pubic tubercle	Pubis	Inguinal canal landmark; hernia surgery
Acetabulum	All three	Hip joint socket; fracture classification
Obturator foramen	Pubis + ischium	Obturator hernia site

Sources: General Anatomy and Musculoskeletal System, THIEME Atlas of Anatomy; Imaging Anatomy Text and Atlas, Vol. 3 - Bones, Joints, Vessels and Nerves

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