Hip Bone

Introduction

Introduction to the Hip Bone (Innominate Bone)

The hip bone (os coxae) is a large, irregular, flattened bone that forms the bony pelvis together with the sacrum and coccyx. Each hip bone unites anteriorly at the pubic symphysis and articulates posteriorly with the sacrum at the sacroiliac joint. It develops from three primary ossification centres—the ilium, ischium, and pubis—which meet and fuse at the triradiate cartilage of the acetabulum. The hip bone transmits the weight of the trunk from the vertebral column to the lower limbs via the sacroiliac joints, and provides extensive surfaces for muscle attachment. A thorough knowledge of its parts, landmarks, and foramina is fundamental to understanding pelvic architecture, obstetric mechanics, and common fracture patterns.

Study

1. General Features and Orientation

The hip bone has an outer (gluteal) surface and an inner (pelvic) surface. It consists of three fused parts—the ilium above, the ischium posteroinferiorly, and the pubis anteroinferiorly. The acetabulum is the deep cup‑shaped fossa on the lateral surface that articulates with the head of the femur. The obturator foramen, a large oval opening bounded by the ischium and pubis, is closed by the obturator membrane except for the obturator canal. The bone is oriented so that the iliac crest is superior, the pubis is anterior, and the ischium is posterior.

2. The Ilium

The ilium is the broad, fan‑shaped upper part of the hip bone. It presents a body (which contributes to the upper two‑fifths of the acetabulum) and a wing (ala). The superior border is the iliac crest, which runs from the anterior superior iliac spine (ASIS) to the posterior superior iliac spine (PSIS). Below the ASIS is the anterior inferior iliac spine (AIIS); below the PSIS is the posterior inferior iliac spine (PIIS). The crest has an outer lip, intermediate area, and inner lip for attachment of the three layers of abdominal wall muscles. The iliac fossa is the smooth concave inner surface giving origin to the iliacus muscle. The gluteal surface externally bears three curved lines—posterior, anterior, and inferior gluteal lines—which delineate the attachments of the gluteal muscles. The greater sciatic notch is a deep indentation below the PIIS, transmitting the piriformis muscle and other structures to the gluteal region.

3. The Ischium

The ischium forms the posteroinferior part of the hip bone. Its body contributes to the lower two‑fifths of the acetabulum. The ischial spine projects posteromedially from the posterior border, separating the greater sciatic notch above from the lesser sciatic notch below. The lesser sciatic notch lies between the ischial spine and the ischial tuberosity. The ischial tuberosity is the rough, expanded inferior part that bears weight in sitting and gives origin to the hamstring muscles (semimembranosus, semitendinosus, long head of biceps femoris) and adductor magnus. The ramus of the ischium projects anteriorly to join the inferior pubic ramus, forming the ischiopubic ramus.

4. The Pubis

The pubis is the anterior part of the hip bone. It consists of a body, a superior ramus, and an inferior ramus. The body of the pubis articulates with the opposite side at the pubic symphysis, a secondary cartilaginous joint. The superior ramus extends from the body to the acetabulum; its upper border is the pecten pubis (pectineal line), which continues into the arcuate line of the ilium. The iliopubic (iliopectineal) eminence marks the junction of the ilium and pubis. The pubic tubercle is a small projection on the superior aspect of the body, giving attachment to the inguinal ligament. The inferior ramus runs downwards and laterally to join the ischial ramus. The obturator foramen is bounded by the superior and inferior pubic rami and the ischial ramus and body.

5. The Acetabulum

The acetabulum is the deep cup‑shaped socket on the lateral aspect of the hip bone that articulates with the femoral head. It is formed by the fusion of the three component bones: ilium (superior two‑fifths), ischium (posteroinferior two‑fifths), and pubis (anteroinferior one‑fifth). The acetabular fossa is the central non‑articular roughened depression, surrounded by the smooth semilunar articular surface. The acetabular notch is a gap inferiorly in the rim, bridged by the transverse acetabular ligament. The acetabular labrum, a fibrocartilaginous rim, further deepens the socket.

6. The Obturator Foramen and Canal

The obturator foramen is a large oval aperture bounded by the superior and inferior rami of the pubis and the ramus of the ischium. In life it is largely closed by the obturator membrane, except for a small superior opening—the obturator canal—which transmits the obturator nerve and vessels from the pelvis into the medial compartment of the thigh.

7. Ossification and Development

The hip bone ossifies from three primary centres—for the ilium, ischium, and pubis—which appear between the third and fifth months of intrauterine life. At birth, the three parts are separated by the Y‑shaped triradiate cartilage in the acetabulum. Fusion begins around puberty (12–14 years) and is complete by 20–23 years. Numerous secondary ossification centres appear at the iliac crest, ischial tuberosity, anterior inferior iliac spine, pubic symphysis, and other apophyses during adolescence, fusing by 25 years. The triradiate cartilage is an important radiographic landmark; premature fusion can lead to acetabular dysplasia.

8. Sexual Differences and Pelvimetry

The female pelvis, and therefore the female hip bone, exhibits adaptations for childbirth. The female hip bone is lighter, with a wider subpubic angle (80–85° vs 50–60° in males), a broader greater sciatic notch, an everted ischiopubic ramus, and a shallower, smaller acetabulum. The preauricular sulcus (a groove near the sacroiliac joint) is more pronounced in females. These features are used in forensic determination of sex. The inlet, midcavity, and outlet pelvic diameters are critical in obstetrics.

9. Clinical Considerations

Fractures of the hip bone are typically high‑energy injuries and are classified by the Tile or Young‑Burgess systems based on the mechanism and stability. Acetabular fractures are complex and may require surgical fixation to prevent post‑traumatic osteoarthritis. Avulsion fractures of apophyses (ASIS, AIIS, ischial tuberosity) occur in adolescents due to sudden muscle contraction. The hip bone is also a frequent site of primary and metastatic bone tumours, and its marrow is a common site for biopsy.

Summary

Summary of the Hip Bone

The hip bone is a composite bone formed by the ilium, ischium, and pubis, united at the acetabulum. It articulates with the sacrum at the sacroiliac joint and with the opposite hip bone at the pubic symphysis. Key landmarks include the iliac crest, iliac spines, greater sciatic notch, ischial spine and tuberosity, pubic tubercle, pecten pubis, acetabulum, and obturator foramen. The bone ossifies from three primary centres that fuse at the triradiate cartilage. Sexual dimorphism is evident in pelvic dimensions relevant to childbirth. Fractures and avulsions are significant clinical injuries.