CorpusHuman Anatomy
SkeletalBone

Hip bone

Os coxae

Also known as: os coxae, innominate bone, pelvic bone, coxal bone, acetabulum, obturator foramen

A large, irregular bone formed by fusion of the ilium, ischium and pubis at the acetabulum. The two hip bones form the pelvic girdle, articulating with the sacrum posteriorly, each other anteriorly and the femur laterally.

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Overview

Acetabular contributionsIlium ≈2/5, ischium ≈2/5, pubis ≈1/5
Triradiate cartilage fusion≈14–16 years

The three components meet in the acetabulum, a deep cup on the lateral surface that receives the femoral head; they are separated in childhood by a Y-shaped triradiate cartilage, which fuses at ≈14–16 years. The ilium forms the upper part (≈2/5 of the acetabulum), the ischium the posteroinferior part (≈2/5) and the pubis the anteroinferior part (≈1/5). The acetabulum has a horseshoe-shaped articular lunate surface around a non-articular acetabular fossa and an inferior acetabular notch, bridged by the transverse acetabular ligament.

Below the acetabulum, the ischium and pubis enclose the obturator foramen, the largest foramen in the skeleton. It is almost completely closed by the obturator membrane, leaving the obturator canal superiorly for the obturator nerve and vessels. Posteriorly, the greater and lesser sciatic notches are separated by the ischial spine; the sacrospinous and sacrotuberous ligaments convert them into the greater and lesser sciatic foramina.

Location

Lateral and anterior walls of the pelvis; the hip region.

Parts & subdivisions

  1. Ilium
    Upper, fan-shaped part; iliac crest and ASIS.
  2. Ischium
    Posteroinferior part; ischial tuberosity and spine.
  3. Pubis
    Anteroinferior part; body and superior/inferior rami.
  4. Acetabulum
    Socket for the femoral head; lunate surface, fossa and notch.
  5. Obturator foramen
    Closed by the obturator membrane; obturator canal transmits the obturator nerve and vessels.
  6. Greater sciatic notch
    Becomes the greater sciatic foramen: piriformis, sciatic nerve, superior/inferior gluteal vessels and nerves, pudendal nerve and internal pudendal vessels.
  7. Lesser sciatic notch
    Becomes the lesser sciatic foramen: obturator internus tendon, pudendal nerve and internal pudendal vessels (re-entering).

Anatomy

Articulations

Sacrum (sacroiliac joint), opposite hip bone (pubic symphysis), femur (hip joint).

Ossification

Three primary centres — ilium (≈week 9), ischium (≈4 months), pubis (≈4–5 months fetal). Ischiopubic ramus fuses ≈7–8 years; the triradiate cartilage fuses ≈14–16 years. Secondary centres (iliac crest, ASIS/AIIS, ischial tuberosity, pubic symphysis) appear at puberty and fuse by ≈20–25 years.

Function

  • Forms the pelvic girdle and the socket of the hip joint
  • Transmits weight from the trunk to the femur
  • Protects pelvic organs
  • Attachment for gluteal, abdominal, thigh, back and pelvic floor muscles

Clinical significance

Acetabular fractures

Result from the femoral head being driven into the socket (e.g. dashboard injury with posterior wall fracture); often accompany hip dislocations and threaten the sciatic nerve.

Obturator hernia

Herniation through the obturator canal, typically in thin elderly women; may compress the obturator nerve causing pain down the medial thigh (Howship–Romberg sign).

Compiled from standard anatomy and physiology references — see the Skeletal System sources. For education only; not medical advice.