SI dislocations are caused by high energy, and are commonly associated with significant intrapelvic bleeding.
There are three types of SI dislocations:
• Pure SI dislocation
• Iliac wing fracture dislocation
• Sacral fracture dislocation
All of these injuries should be addressed together with any other injury to the pelvic ring.
1. Pure SI dislocation
Pure SI dislocation is often caused by AP compression and/or vertical sheer.
In pure SI dislocation, all ligaments are torn and combined with an anterior pelvic injury. The injured hemi pelvis is completely unstable.
2. Iliac wing fracture dislocation (crescent fracture)
In this type of fracture dislocation, the anterior iliosacral ligaments are torn, while the posterior ligaments are intact. The most common fracture is vertical and run posteriorly.
The anterior part of the iliac wing is dislocated from the SI joint.
The posterior iliosacral ligament holds the posterior segment of the iliac wing attached to the sacrum in its anatomic position.
3. Sacral fracture dislocations
In this type of fracture dislocation, the SI joint is dislocated with the avulsed part of the sacrum.