Hip Bone: Anatomy, Acetabulum, and Clinical Significance

Overview

The hip bone (os coxae, innominate bone) is a large, irregular flat bone formed by the fusion of three primary bones: the ilium (superior), ischium (posteroinferior), and pubis (anteromedial). These three bones meet at the acetabulum — the cup-shaped socket that receives the femoral head to form the hip joint. Fusion is complete by age 25 at the triradiate cartilage.

Components

Ilium

The largest part, forming the superior ala (wing) and body:

• Iliac crest: palpable from anterior superior iliac spine (ASIS) to posterior superior iliac spine (PSIS); useful for bone marrow biopsy and iliac crest graft; the highest point corresponds to L4 vertebral level (Tuffier's line)
• ASIS: attachment of sartorius (most superior) and inguinal ligament; avulsion fractures common in adolescent athletes
• AIIS: attachment of rectus femoris (straight head)
• PSIS: dimples of Venus; marked by skin dimples; sacroiliac joint identification
• Iliac fossa: internal surface; origin of iliacus muscle
• Iliopectineal line (arcuate line): separates greater pelvis from lesser pelvis
• Gluteal surface: posterior external surface; three gluteal lines marking muscle attachment zones

Ischium

Forms the posteroinferior part:

• Ischial spine: attachment of sacrospinous ligament; important obstetric landmark for determining pelvic size; pudendal nerve block site
• Ischial tuberosity: weight-bearing when seated; origin of hamstrings and adductor magnus; ischial bursitis (Weaver's bottom)
• Lesser sciatic notch: between ischial spine and tuberosity; pudendal nerve and vessels pass through here (with sacrospinous ligament below and sacrotuberous ligament behind)

Pubis

Forms the anteromedial part:

• Body: medial flat part forming pubic symphysis
• Superior ramus: projects laterally from body to acetabulum; pectineal line, iliopectineal eminence
• Inferior ramus: joins ischial ramus to form ischiopubic ramus
• Pubic tubercle: attachment of inguinal ligament (medially) and conjoint tendon; hernia examination landmark

Obturator Foramen

• Large oval opening between pubis and ischium
• Largest foramen in the skeleton
• Closed by obturator membrane (except for obturator canal superiorly transmitting obturator nerve and vessels)

Acetabulum

The acetabulum is the lateral cup:

• Formed by ilium (40%), ischium (40%), pubis (20%)
• Lunate surface: horseshoe-shaped articular surface (not complete inferiorly — acetabular notch)
• Acetabular fossa: non-articular central area; contains fat pad and ligament of head of femur (ligamentum teres)
• Acetabular notch: inferior gap converted to foramen by transverse acetabular ligament; transmits vessels to femoral head
• Acetabular labrum: fibrocartilaginous rim that deepens the socket and provides suction-seal stability

Pelvic Architecture

• Greater pelvis (false pelvis): above pelvic brim (arcuate line); supports abdominal viscera
• Lesser pelvis (true pelvis): below pelvic brim; contains bladder, rectum, uterus/prostate
• Pelvic inlet: bounded by promontory of sacrum, arcuate line, pecten pubis, pubic crest; measured in obstetrics
• Pelvic outlet: bounded by coccyx, sacrotuberous ligaments, ischial tuberosities, ischiopubic rami, pubic symphysis

Sex differences: female pelvis is wider, more oval inlet, wider subpubic angle (>90°), shorter pubic symphysis, more circular inlet — adaptations for childbirth.

Clinical Relevance

• Hip fractures: intracapsular (neck of femur — avascular necrosis risk) vs. extracapsular (intertrochanteric, subtrochanteric — better prognosis)
• Acetabular fractures: high-energy trauma; classified by Letournel; require CT; often require ORIF
• Pelvic fractures: Tile classification; associated with massive hemorrhage (venous and arterial from internal iliac branches) and urethral injury
• Avulsion fractures: ASIS (sartorius), AIIS (rectus femoris), ischial tuberosity (hamstrings) in adolescent athletes
• Iliac crest bone graft: standard autograft for spinal fusion and other reconstructions

Study Points

• Name the three bones forming the hip bone and describe their contributions to the acetabulum.
• Identify the key bony landmarks on the ilium and their clinical uses.
• Explain the obstetric importance of the ischial spine and pelvic diameters.
• Describe the structure of the acetabulum and the role of the labrum.
• Distinguish intracapsular from extracapsular hip fractures in terms of vascular anatomy and prognosis.