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.
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.