Structure
Each hip bone is made of the ilium above, the ischium behind and below, and the pubis in front, which meet at a Y-shaped triradiate cartilage in the acetabulum.
Ilium
The ilium is the broad, fan-shaped upper part. Its upper edge, the iliac crest, runs from the anterior superior iliac spine (ASIS) to the posterior superior iliac spine (PSIS). The tubercle of the iliac crest is the widest point, about 5 cm behind the ASIS. Below each superior spine is an inferior spine: the anterior inferior iliac spine (AIIS) above the acetabulum and the posterior inferior iliac spine at the top of the greater sciatic notch. The inner surface has the smooth iliac fossa in front and, behind, the ear-shaped auricular surface for the sacrum, with the rough iliac tuberosity above it for the strong interosseous sacro-iliac ligament. The arcuate line runs along the inner surface as part of the pelvic brim.
Ischium
The ischium has a body, which forms the back of the acetabulum, and a ramus. The ischial spine projects backwards and separates the greater sciatic notch above from the lesser sciatic notch below. The ischial tuberosity is the large rough mass that bears weight when sitting.
Pubis
The pubis has a body, a superior ramus and an inferior ramus. The pubic crest runs along the upper border of the body and ends laterally at the pubic tubercle. The pectineal line continues from the tubercle along the superior ramus, and the obturator crest runs from the tubercle to the acetabular notch. The inferior pubic ramus joins the ischial ramus to form the ischiopubic ramus.
Acetabulum and obturator foramen
The acetabulum faces laterally, forwards and downwards. Only its horseshoe-shaped lunate surface is articular. The central non-articular acetabular fossa holds fat and the ligament of the head of the femur, and the acetabular notch below is bridged by the transverse acetabular ligament. The obturator foramen, between pubis and ischium, is closed by the obturator membrane except for the obturator canal.
Attachments
Most landmarks of the hip bone are defined by what attaches to them.
| Landmark | Main attachments |
|---|---|
| Iliac crest | Abdominal wall muscles, latissimus dorsi, quadratus lumborum, fascia lata |
| ASIS | Sartorius, inguinal ligament |
| AIIS | Straight head of rectus femoris, iliofemoral ligament |
| Iliac fossa | Iliacus |
| Ischial spine | Sacrospinous ligament, coccygeus, gemellus superior |
| Ischial tuberosity | Hamstrings, hamstring part of adductor magnus, sacrotuberous ligament, quadratus femoris |
| Pubic crest | Rectus abdominis, pyramidalis, conjoint tendon |
| Pubic tubercle | Inguinal ligament |
Sciatic foramina and pelvic openings
The sacrospinous and sacrotuberous ligaments convert the greater and lesser sciatic notches into foramina, the main routes from the pelvis to the gluteal region and perineum.
The greater sciatic foramen is bounded by the greater sciatic notch in front and above, the sacrotuberous ligament behind and the sacrospinous ligament and ischial spine below. Piriformis passes through it. Above piriformis run the superior gluteal nerve and vessels. Below it run the sciatic nerve, inferior gluteal nerve and vessels, posterior cutaneous nerve of the thigh, pudendal nerve, internal pudendal vessels, and the nerves to obturator internus and quadratus femoris.
The lesser sciatic foramen, below the ischial spine, transmits the tendon of obturator internus out of the pelvis, and the pudendal nerve, internal pudendal vessels and nerve to obturator internus back in to the perineum.
The pelvic inlet (brim) is formed by the sacral promontory, the ala of the sacrum, the arcuate line, the pectineal line, the pubic crest and the top of the symphysis. The pelvic outlet is bounded by the coccyx, sacrotuberous ligaments, ischial tuberosities, ischiopubic rami and the symphysis.
Sex differences
The female pelvis is adapted for childbirth. It has a wider, rounder inlet, a wider subpubic angle, a wider greater sciatic notch, everted ischial tuberosities and a more oval obturator foramen. The male pelvis is narrower and heavier, with a heart-shaped inlet and a narrow subpubic angle.
Clinical relevance
The pelvis is a ring, so a significant fracture usually breaks it in two places, and the resulting bleeding can be life-threatening.
- Pelvic ring injuries: an open-book injury disrupts the symphysis and opens the pelvic volume, with bleeding mainly from the pelvic venous plexuses and branches of the internal iliac artery. A pelvic binder centred on the greater trochanters closes the ring.
- Avulsion fractures in adolescents: sartorius pulls off the ASIS, rectus femoris the AIIS, and the hamstrings the ischial tuberosity.
- Bone marrow sampling is taken from the posterior iliac crest near the PSIS.
- Iliac crest bone graft: harvesting near the ASIS can injure the lateral cutaneous nerve of the thigh.
- Surface landmarks: the highest points of the iliac crests lie at about L4, used for lumbar puncture; the dimples over the PSIS lie at about S2.
- Pudendal nerve block: the ischial spine is felt per vaginam to guide the needle as the nerve crosses behind it.
On the specimen
This station is a dry hip bone, so siding it comes first. Hold it with the acetabulum facing laterally, the pubis in front and the iliac crest on top; the greater sciatic notch then faces backwards.
- Greater or lesser sciatic notch: the greater notch is large and lies above the ischial spine; the lesser notch is smaller and smooth, below the spine, where obturator internus turns over it.
- ASIS or AIIS: the ASIS is the prominent front end of the crest; the AIIS is the smaller bump just above the acetabulum.
- PSIS or PIIS: the PSIS is the back end of the crest; the PIIS lies below it at the top of the greater sciatic notch.
- Lunate surface or acetabular fossa: the smooth horseshoe is the lunate surface; the rough central pit is the fossa; the gap in the rim below is the acetabular notch.
- Pubic tubercle or pubic crest: the tubercle is the small lateral knob; the crest is the ridge medial to it.
- Iliac tuberosity: the rough area above and behind the auricular surface on the inner side.