The Hip Bone (Os Coxae)

The hip bone (os coxae / innominate bone) is a large, irregularly shaped flat bone that forms the lateral wall of the pelvis. Each hip bone is composed of three parts that fuse during development: the ilium, ischium, and pubis. They converge at the acetabulum, the cup-shaped socket of the hip joint.

Lateral View

Anterior View

The Three Parts

1. Ilium

The largest, fan-shaped upper portion. Key landmarks:

Iliac crest — the palpable superior rim, running from the anterior superior iliac spine (ASIS) to the posterior superior iliac spine (PSIS)
Iliac fossa — concave medial surface, origin of iliacus muscle
Gluteal surface (lateral) — divided by the anterior, posterior, and inferior gluteal lines for gluteal muscle attachments
Auricular surface — ear-shaped area on the medial surface that articulates with the sacrum at the sacroiliac joint
Arcuate line — marks the pelvic brim on the medial surface

2. Ischium

The posteroinferior part. Key landmarks:

Body of ischium — fuses with ilium and pubis at the acetabulum
Ischial spine — separates the greater sciatic notch (above) from the lesser sciatic notch (below); attachment site for the sacrospinous ligament
Ischial tuberosity — the bony projection we sit on; attachment for hamstring muscles and the sacrotuberous ligament
Ischial ramus — inferior projection that joins the inferior pubic ramus

3. Pubis

The anteromedial part. Key landmarks:

Superior pubic ramus — bears the pecten pubis (pectineal line), forming part of the pelvic brim
Pubic tubercle — attachment of the inguinal ligament
Symphyseal surface — articulates with the contralateral pubis at the fibrocartilaginous pubic symphysis
Inferior pubic ramus — joins the ischial ramus to form the ischiopubic ramus, enclosing the obturator foramen

The Acetabulum

The three bones meet in the acetabulum, which faces laterally and slightly inferiorly. Their contributions are:

Ilium — 2/5
Ischium — 2/5
Pubis — 1/5

The acetabulum has a smooth lunate surface (articular cartilage) surrounding a central non-articular acetabular fossa, and is bounded by the acetabular rim. The inferior gap is the acetabular notch.

Triradiate Cartilage (Development)

The three bones are separated at birth by a Y-shaped growth plate — the triradiate cartilage. Definitive fusion (synostosis) occurs between 14 and 16 years of life.

The Pelvic Ring

The paired hip bones articulate with each other at the pubic symphysis anteriorly and with the sacrum at the sacroiliac joints posteriorly, forming the bony pelvic ring — a stable structure essential for transferring trunk loads to the lower limbs during gait.

Clinical Relevance

Feature	Clinical Significance
ASIS	Attachment of inguinal ligament and sartorius; surface landmark for measurement
Ischial tuberosity	"Sitting bone"; avulsion fractures in athletes
Greater sciatic notch	Passage of sciatic nerve, superior/inferior gluteal vessels
Ischial spine	Target for pudendal nerve block in obstetrics
Obturator foramen	Obturator nerve and vessels pass through obturator canal
Acetabulum	Site of hip dislocation and acetabular fractures (classified by anterior/posterior column per Letournel)
Pubic symphysis	Disrupted in pelvic fractures and symphysis pubis dysfunction in pregnancy

Source: THIEME Atlas of Anatomy — General Anatomy and Musculoskeletal System, p. 433–434

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