Pelvis Anatomy Notes

Pelvis Formation

  • Formed by: Sacrum, coccyx, and 2 innominate bones.

Innominate Bone

  • Formed by the fusion of three bones: Ilium, Pubis, and Ischium.
  • These bones fuse to form the acetabulum (articulates with the femoral head to form the hip joint).

Ilium

  • Largest and most superior portion of the innominate bone.
  • Consists of the body and large wing-like ala.
  • Superior ridge (iliac crest) is at the level of L4.

Pubis

  • Most anterior bone of the pelvis.
  • Forms the infero-anterior portion of the acetabulum.
  • Consists of a body and superior and inferior pubic rami.
  • The 2 bodies articulate in the midline at the pubic symphysis.

Ischium

  • Inferior portion of the innominate bone.
  • Body forms the infero-posterior portion of the acetabulum.
  • Extends antero-inferiorly to the ischial tuberosity, then turns antero-medially to join the inferior pubic ramus.

Sacrum

  • Triangular bone formed by the fusion of five vertebral segments.
  • The 1st sacral segment has a prominence on the anterior surface (sacral promontory).
  • Transverse processes fuse to form lateral mass (ala).
  • Ala articulates with the ilium to form the sacroiliac joint (synovial plane).
  • The final sacral segment articulates with the coccyx.

Pelvic Inlet

  • Bounded by: Sacral promontory and sacral ala, superior border of pubic symphysis, and the margin of the inner border of the iliac bones (ilio-pectineal line).

Pelvic Outlet

  • Bounded by: Tip of coccyx, inferior border of symphysis pubis, and ischial spines laterally.

Articulations

  • Sacro-Iliac Joints (SIJ): Synovial joint between the auricular surface of the sacrum and the iliac bone.
  • Symphysis Pubis: A cartilaginous joint between the bodies of the two pubic bones.
  • Hip joints: Synovial joint between the acetabulum of the innominate bone and the femoral head.

Muscles

  • 3 functional groups: Extra-pelvic, Pelvic wall, and Pelvic diaphragm.

Extra Pelvic Muscles

  • Rectus Abdominis: Extends vertically from the pubic symphysis to the xiphoid process & costal cartilages of ribs 5,6,7.
  • Psoas: Attaches to the lateral borders of the bodies of lumbar vertebrae, joins with the iliacus muscles and attaches to the lesser trochanters as the iliopsoas muscles
  • External and internal oblique: Extend from the anterior border of the iliac crests to the inferior borders of ribs 10-12.
  • Gluteus group

Pelvic Wall Muscles

  • Piriformis: Extends from ilium & sacrum, passes through the greater sciatic notch and attaches to the greater trochanter.
  • Obturator internus & externus: Attach to the greater trochanter and obturator foramen; the obturator internus also attaches to the pubic bone.
  • Iliacus: Attaches to the iliac crest and sacrum and attaches to the lesser trochanter after merging with the psoas tendon.

Major Arteries

  • Abdominal aorta bifurcates at L4 into left and right common iliac arteries.
  • Common iliac arteries bifurcate into internal and external iliac arteries at the level of the superior SIJ.
  • External iliac artery becomes the femoral artery after extending along the pelvic brim, courses deep to the inguinal ligament.
  • Internal iliac artery extends posteromedially and has branches that supply the pelvis, muscles, and pelvic viscera.

Pelvic Veins

  • Similar pattern to arteries.
  • Internal iliac veins and tributaries drain pelvic organs and lie medial to the internal iliac arteries.
  • External iliac veins drain legs and lie medial to the external iliac arteries.
  • Common iliac veins drain into the inferior vena cava (IVC) which forms at the level of L5.

Bony Pelvis Differences

  • MALE: Thicker, heavier bone; acute sub-pubic arch; longer, narrower sacrum; narrow, heart-shaped pelvic brim.
  • FEMALE: Lighter, thinner bone; rounded, broader sub-pubic arch; shorter, wider sacrum; wide, oval-shaped pelvic brim.

Normal Apophyses

  • Iliac crest (abdominal muscles)
  • ASIS (sartorius)
  • AIIS (rectus femoris)
  • Lesser trochanter (iliopsoas)
  • Ischial tuberosity (hamstrings)

Conditions

  • Sacroiliitis: Begins with widening SIJ, erosions, sclerosis, and can end with total ankylosis of the SIJ.
  • Diastasis of the symphysis pubis: Normal width should not exceed 10mm; Causes include trauma, post partum, septic arthritis.
  • Open-book pelvic fracture: Diastasis of symphysis pubis and/or fracture of the pubic rami; posterior pelvic disruption of the sacro-iliac joint/s.
  • Paget’s disease: Localized bone disorder that disturbs growth of new bone, resulting in thickened, irregular, soft, and weak bone. Can cause fractures with minimal trauma.