Trauma X-ray - Axial skeleton
Sacrum and Pelvis

Key points

  • If there is one pelvic fracture - look for another fracture, or disruption of the pubic symphysis or sacroiliac joints
  • Carefully check the arcuate lines of the sacrum

Anatomy

The bony pelvis comprises the two hemi-pelvis bones which are bound anteriorly at the pubic symphysis and posteriorly at the sacroiliac joints.

As with other anatomical bone rings if a fracture is seen in one place a careful check should be made for a second fracture, or for disruption of the pubic symphysis or sacroiliac joints.

Pelvis anatomy - Normal AP

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Pelvis anatomy - Normal AP

  • The 2 hemi-pelvis bones and the sacrum form a bone ring bound posteriorly by the sacroiliac joints and anteriorly by the pubic symphysis
  • Each obturator foramen is also formed by a ring of bone

Hemi-pelvis anatomy - Normal AP

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Hemi-pelvis anatomy - Normal AP

  • Each hemi-pelvis bone comprises 3 bones - the ilium (white), pubis (orange) and ischium (blue)
  • The 3 bones fuse to form the acetabulum - the pelvic portion of the hip joint
  • ASIS = Anterior Superior Iliac Spine = attachment site for sartorius muscle
  • AIIS = Anterior Inferior Iliac Spine = attachment site for rectus femoris muscle

Pubic ramus fracture

Fractures seen in the superior or inferior pubic ramus are usually accompanied by another fracture of the other ramus on the same side. These are very common fractures in the elderly.

Pubic ramus fracture

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Pubic ramus fracture

  • The obturator ring is incomplete on the right
  • A fracture passes through the superior and inferior pubic rami
  • Compare with the normal left side

Pelvic fractures

Fractures to the pelvis are highly variable in appearance depending on the mechanism and force of injury. Careful examination of the cortical surfaces of the bony rings will reveal most fractures.

The extent of injury is often underestimated on plain radiographs.

Acetabular fracture

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Acetabular fracture

  • A tiny step in the cortical edge of the pelvic ring reveals a fracture which passes into the acetabulum
  • The fracture passes to the obturator ring and then through the inferior pubic ramus

Pelvis diastasis

High force injury to the bony pelvis may result in "diastasis" (separation) at the pubic symphysis or a sacroiliac joint.

Pelvis diastasis

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Pelvis diastasis

  • Both the pubic symphysis and the right sacroiliac joint are widened
  • There is complete separation of the right hemi-pelvis from the axial skeleton

Avulsion injuries

In young athletically active individuals avulsion injuries may occur, most frequently at the ASIS, and occasionally at the AIIS.

ASIS avulsion fracture

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ASIS avulsion fracture

  • A small fragment of bone has detached from the pelvis
  • All or part of the sartorius tendon origin will be attached to the bone fragment
  • Note the normal appearance of the unfused iliac crest apophysis in this male teenager

Sacral injuries

Fractures of the sacrum may be isolated or accompany pelvis fractures. They are easily missed unless a specific check of the arcuate lines is made on every pelvic X-ray.

Pelvis anatomy - Sacrum - Normal AP

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Pelvis anatomy - Sacrum - Normal AP

  • The sacrum and the iliac bones overlap to form the sacroiliac joints
  • The arcuate lines of the sacrum are the roofs of the sacral exit foramina which carry the sacral nerve roots

Sacral fracture- AP pelvis

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Sacral fracture- AP pelvis

  • Loss of the smooth arcuate lines is a sign of sacral fracture

Page author: Salisbury NHS Foundation Trust UK (Read bio)

Last reviewed: February 2016