CT Findings of Lateral compression type of injury
Type 1 (Figure 1): An impaction fracture of the anterior sacrum on the side of the compression is characteristic of a lateral compression type 1 injury,
with superior and inferior rami fractures most commonly seen in conjunction with a small “buckle” in the sacral ala.
Usually,
lateral compression type 1 injuries are stable and heal without any surgical intervention,
and they are distinct from the high-energy fractures seen in younger patients.
Type 2 (Figure 2): Lateral compression type 2 injuries occur when the force is directed over the anterior portion of the iliac wing.
This force tends to rotate the hemipelvis inward,
with the pivot point being the anterior SI(Sacro-iliac) joint,
and leave a small crescent-shaped segment of posterior ilium firmly attached to the sacrum,
a result of a strong posterior SI ligament that leads to a crescent fracture.
Type 3 (Figure 3): Lateral compression type 3 injuries occur in the setting of high-energy lateral compression,
where the force vector causes internal rotation of the hemipelvis on the side of the impact (as occurs in type 1 and 2 injuries) and a corresponding external rotation of the contralateral hemipelvis.
There is disruption of the sacrospinous,
sacrotuberous and anterior and posterior sacroiliac ligaments.
CT Findings of Antero-posterior compression type of injury
Type 1: Diastasis of the pubic symphysis is less than 2.5 cm and may occur without rupture of the posterior ligaments of the pelvis,
resulting in injuries that are rotationally and vertically stable because of the intact posterior ring.
However,
because pure AP compression type 1 injuries are rare,
patients with any degree of pubic symphysis diastasis must be treated as if a posterior pelvic injury is present until it is proved otherwise.
Type 2: There is posterior instability,
a result of injury to the anterior SI complex and the sacrotuberous and sacrospinous ligaments,
allowing for more than 2.5 cm of symphyseal diastasis and anterior SI widening.
Type 3: there is complete disruption of the posterior SI ligament,
with separation of the iliac wing from the sacrum and both rotational and vertical instability.
The distinction between anterior and posterior SI diastasis is often better appreciated on axial CT images,
with posterior displacement of the iliac bone indicating posterior sacroiliac disruption and associated vertical instability.
CT Findings of Vertical shear injury
Vertical shear injury is caused by cranially directed high-energy forces from violent axial loading of the hemipelvis.
The key imaging finding of vertical shear injury is a cephalad displacement of the iliac crest of the injured hemipelvis relative to the opposite side with associated fractures of the pubis and sacroiliac joints.
Vertical shear injuries are both vertically and rotationally unstable and are often associated with other visceral injuries,
making early detection critical to best direct further imaging.
CT Findings of Combined mechanism injury
This category is for pelvic fractures that demonstrate elements of more than one pattern.
In general,
these injuries result from very high energies,
and patients often present with significant displacement.
The most common type of combined mechanism injury is a lateral compression and vertical shear injury.