Horizontally-oriented fractures left superior and inferior pubic rami. Impaction fracture left sacrum. Oblique fracture left posterior ilium extending into the left SI joint.

CT confirms fractures seen on plain radiograph. The right sacrum, right ilium, and right sacroiliac joint are normal. No vertical displacement of the pelvis. Findings indicate a lateral compression 2 injury.

Lateral compression fractures are one of the major types of pelvic injuries in the Young and Burgess classification of pelvic ring fractures. In lateral compression injuries, a force directed from lateral to medial acts on the pelvis. An LC1 injury consists of a crush fracture of the sacrum and ipsilateral horizontal fractures of the superior and inferior pubic rami and is considered stable since the posterior structures responsible for pelvic stability are intact. All LC injuries will have these characteristic sacral and pubic rami fractures.

With more severe trauma and greater forces, the ipsilateral innominate bone is internally rotated, resulting in either a tear of the ipsilateral posterior sacroiliac ligament or, as in our case and in the illustration, fracture of the posterior ilium, often extending into the SI joint. The posterior SI ligament injury and the posterior fracture are considered equivalent injuries: both result in rotational instability of the innominate bone. This is classified as an LC2 injury.

With still more severe trauma and even greater forces, the contralateral innominate bone is rotated externally, tearing the anterior sacroiliac, sacrospinous and sacrotuberous ligaments; the posterior sacroiliac ligament remains intact. Both innominate bones are now rotationally unstable. This is an LC3 injury.

Illustration created by Stefan Tigges