Hip Bone
Os Coxae
location_on Lateral and anterior pelvis, connecting the axial skeleton to the lower limb
The hip bone (os coxae or innominate bone) is a large irregular bone formed by the fusion of three bones: the ilium, ischium, and pubis. These three components meet at the acetabulum, the deep cup-shaped socket that receives the femoral head. The paired hip bones, along with the sacrum and coccyx, form the bony pelvis, which transmits the weight of the upper body to the lower limbs and protects the pelvic organs.
Key Anatomical Features
- Acetabulum is the deep socket formed where all three components meet, receiving the femoral head
- Iliac crest is the palpable superior rim extending from the ASIS to the PSIS
- Obturator foramen is the large opening bounded by the pubis and ischium
- Ischial tuberosity is the weight-bearing point when sitting
- Pubic symphysis is the cartilaginous joint connecting the two pubic bones anteriorly
- Greater and lesser sciatic notches are separated by the ischial spine
Muscle Attachments
| Muscle | Attachment | Action |
|---|---|---|
| Gluteus maximus | Posterior ilium, sacrum, and sacrotuberous ligament | Extends and laterally rotates the hip |
| Gluteus medius | External ilium between anterior and posterior gluteal lines | Abducts the hip and prevents pelvic drop during gait |
| Iliacus | Iliac fossa | Flexes the hip |
| Rectus femoris | Anterior inferior iliac spine (AIIS) | Flexes the hip and extends the knee |
| Hamstrings (origin) | Ischial tuberosity | Extend the hip and flex the knee |
| Adductor muscles | Pubic body, inferior pubic ramus, and ischial ramus | Adduct the hip |
| Obturator internus | Internal surface of obturator membrane | Laterally rotates the hip |
| Tensor fasciae latae | Anterior superior iliac spine (ASIS) | Flexes, abducts, and medially rotates the hip |
Joints and Articulations
| Joint | Type | Connects to |
|---|---|---|
| Hip joint | Synovial ball-and-socket | Femur (femoral head) |
| Sacroiliac joint | Synovial (anterior) and syndesmosis (posterior) | Sacrum |
| Pubic symphysis | Cartilaginous symphysis | Opposite hip bone |
Common Pathologies
Pelvic fracture
High-energy fractures disrupt the pelvic ring and can cause life-threatening hemorrhage from presacral venous plexus and internal iliac vessels. The pelvis can hold several liters of blood.
Hip labral tear
Tear of the fibrocartilaginous labrum lining the acetabulum, commonly from femoroacetabular impingement. Causes groin pain with clicking and catching.
Acetabular fracture
Complex fracture of the acetabulum, classified by Judet and Letournel into 10 types. Often from dashboard injuries where the femur is driven posteriorly.
Avulsion fractures
In adolescents, the unfused apophyses (ASIS, AIIS, ischial tuberosity) can be avulsed by sudden muscular contraction during sports activities.
Clinical Relevance
The pelvic ring functions like a pretzel: a break in one location usually means a break or ligament disruption elsewhere. Open-book pelvic fractures (anteroposterior compression) can cause massive hemorrhage and require emergent pelvic binder application. The ASIS is at the level of S2, and the iliac crest is at the L4 vertebral level, both important surface landmarks. In children and adolescents, the acetabulum is composed of the triradiate cartilage; injury to this growth center can cause progressive acetabular dysplasia.
Development and Ossification
Each hip bone ossifies from eight centers: three primary centers for the ilium, ischium, and pubis (appearing between weeks 8-16 of fetal development), and five secondary centers for the iliac crest, ASIS, ischial tuberosity, pubic symphysis, and acetabulum. The three primary components fuse at the triradiate cartilage in the acetabulum between ages 14 and 16. The iliac crest apophysis fuses by age 25 (Risser staging).
Did You Know?
- The hip bone was formerly called the innominate bone, meaning the bone with no name
- The shape of the pelvic inlet (round in females, heart-shaped in males) is one of the most reliable indicators of biological sex in forensic anthropology
- The acetabulum means vinegar cup in Latin, named for its resemblance to a small Roman vessel used to hold vinegar
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