Osteology Pelvic ring formed from 2 innominate bones articulate posteriorly with the sacrum and anteriorly through pubis symphysis Each innominate bone is composed of three fused bones: ilium, ischium, and pubis ilium - prominences ASIS AIIS PSIS iliopectineal eminence - region union between ilium and pubis sciatic notch ischium pubis Ligaments & Stability Stability no inherent stability of articulations stability comes from ligament complexes Ligament complexes posterior complex sacroiliac ligaments posterior stronger than anterior iliolumbar ligaments transverse process of L4/L5 to posterior ilium anterior complex pubis syphysis fibrocartilaginous disc between innominate bones pelvic floor complex sacrospinous ligament transversely oriented resists external rotation sacrotuberous ligaments longitudinally oriented resists vertical translation Blood Supply Abdominal aorta bifurcates at L4 into common iliac system external iliac artery courses along pelvic brim branches into common femoral artery (distal to inguinal ligament) internal iliac artery divides distal and posterior near the SI joint into posterior division leads to superior guteal artery and other branches anterior division leads to obturator artery Corona mortis connects iliac and obturator systems Posterior venous plexus injury in pelvic fractures can account for majority of blood loss Neurologic Lumbosacral plexus L1-S4 nerve roots Lateral femoral cutanous nerve L2-L3 nerve roots deep to inguinal ligament near ASIS Obturator nerve Femoral nerve Sciatic nerve Imaging AP pelvis standard radiograph for all trauma patients Inlet view beam perpendicular to the S1 end plate (caudal tilt) Outlet view cranial tilt demonstrates cranial-caudal displacemnt of the pelvic ring and sacral morphology CT provides excellent detail of bony anatomy and can confirm pelvic ring / acetabular fractures that are not always visible on plain radigraphs.