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Updated: 8/28/2021

SI Dislocation & Crescent Fractures

Review Topic
Images - AP - crescent fx (wheeless)_moved.jpg axial - crescent fx (wheeless)_moved.jpg
  • Summary
    • Sacroiliac (SI) dislocations and crescent fractures include a spectrum of injuries involving the pelvis which can lead to instability and pelvic malalignment. 
    • Diagnosis requires pelvic radiographs with CT scan.
    • Treatment is generally operative with reduction and fixation of both the anterior and posterior pelvis ring as needed.
  • Epidemiology
    • Spectrum of injuries that include
      • incomplete (Sacroiliac) SI dislocation
        • posterior SI ligaments remain intact
        • rotationally unstable
      • complete SI dislocations
        • posterior SI ligaments ruptured
        • vertically and rotationally unstable
      • SI fracture-dislocation (crescent fracture)
        • iliac wing fracture that enters the SI joint
        • injury to posterior ligaments vary
        • combination of vertical iliac fx and SI dislocation
        • posterior ilium remains attached to sacrum by posterior SI ligaments
        • anterior ilium dislocates from sacrum with internal rotation deformity
        • when ilium fragment remains with sacrum it is termed a crescent fracture
  • Pathophysiology
    • Mechanism of injury
      • lateral compression force
      • usually high energy
    • Pathoanatomy
      • degree of injury to posterior structures determines pelvic stability
      • Iliac wing fractures may be associated with open wounds and may involve bowel entrapment
  • Anatomy
    • Ligaments
      • the SI joint is stabilized by the posterior pelvic ligaments
        • sacrospinous
        • sacrotuberous
        • anterior sacroiliac
        • posterior sacroiliac
    • Nerves
      • the L5 nerve root crosses the sacral ala approximately 2 cm medial to SI joint
    • Blood supply
      • the superior gluteal artery runs across SI joint
      • exits pelvis via greater sciatic notch
  • Classification
    • No classification system specifically for SI injury
      • included in Young- Burgess and Tile classification of pelvic fractures
      • crescent fractures described as LC-2 injury according to Young-Burgess
  • Presentation
    • Symptoms
      • pelvic pain
    • Physical Exam
      • assess hemodynamic status
      • perform detailed neurological exam
      • abdominal assessment to look for distention
      • rectal exam
      • examine urethral meatus for blood
  • Imaging
    • Radiographs
      • recommended views
        • AP pelvis
        • inlet and outlet views
    • CT scan
      • evaluation of sacral fractures
      • posterior pelvis better delineated
  • Treatment
    • Operative
      • immediate skeletal traction
        • indications
          • vertical translation of the hemipelvis
      • anterior ring ORIF
        • indications
          • incomplete SI dislocations with pubic symphyseal diastasis
      • anterior and posterior ring ORIF
        • indications
          • complete SI dislocations
            • vertically unstable require anterior and posterior pelvic ring fixation
      • ORIF of ilium
        • indications
          • crescent fracture
            • required to restore posterior SI ligaments and pelvic stability
  • Techniques
    • Closed Reduction and Percutaneous Fixation
      • positioning
        • intraoperative traction may aid in reduction
        • small midline bump under sacrum may assist with SI screw placement
      • imaging
        • inlet view
          • shows anterior-posterior position of SI joint(s) for screw placement
        • outlet view
          • shows cephalad-caudad position of SI joint(s) for screw placement
        • lateral sacral view
          • ensures safe placement of SI or sacral screws relative to the anterior cortex of the sacral ala and the nerve root tunnel
      • complications
        • L5 nerve root at risk with anterior perforation of iliosacral screw as nerve goes inferiorly over sacral ala
    • ORIF
      • approach
        • anterior approach
          • lateral window with elevation iliacus back to SI joint
        • posterior approach
          • for fixation of crescent fragment to intact ilium
      • fixation
        • plates
        • iliosacral lag screws (SI screws)
  • Complications
    • DVT
      • 35%-50%
    • Neurological injury
    • Loss of reduction and failure of fixation
  • Prognosis
    • Primarily based on accurate and stable reduction of SI joint
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Questions (4)

(OBQ10.159) Anterior penetration of an iliosacral screw through the sacral ala would most likely lead to weakness of which of the following movements?

QID: 3247

Hip flexion



Hip adduction



Knee extension



Ankle plantarflexion



Great toe dorsiflexion



L 1 B

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(OBQ07.62) If a percutaneous iliosacral screw is placed too anteriorly, and the screw exits anterior to the sacral ala before re-entering the sacral body, what will be the most likely finding postoperatively?

QID: 723

Lack of ankle dorsiflexion



Lack of ankle plantarflexion



Lack of knee extension



Loss of bowel and/or bladder control



Lack of great toe extension



L 1 C

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(OBQ06.13) A 39-year-old male is thrown from his motorcycle into a fast-food restaurant and sustains a closed pelvic ring injury. During placement of percutaneous iliosacral screws, the outlet radiograph in Figure A is obtained. What purpose does this view serve?

QID: 24

Evaluation of possible injury to L5 nerve root



Evaluation of anterior-posterior position of screw(s)



Best visualization of sagittal curvature of sacral ala



Best visualization of spinal canal



Best visualization of sacral neural foramina



L 2 C

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Evidence (9)
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