Chapter 3

### PELVIS

The pelvis is formed by the ischium, the pubic bones, and ilium, which through the sacroiliac joints (SI joints) connect to the sacrum. This forms a ring structure. The pubic bones are joined anteriorly by the pubic symphysis and form the anterior ring. The posterior ring is formed by the sacrum, the SI joints, and iliac bones. To disrupt this ring usually requires significant force, which can occur in motor vehicle accidents or similar high-energy trauma. A ring structure usually breaks in more than one place, so it is important to carefully examine the entire ring for a second injury once a fracture is encountered. The second injury does not need to be a fracture; it can be disruption of the SI joints or pubic symphysis (Figure 3-1).

###### Figure 3-1.

Open book injury. There is marked widening of the right SI joint (curved large white arrow) and disruption of the pubic symphysis (curved small black arrow). The right hemipelvis is inferiorly displaced. The left SI joint is disrupted (curved small white arrow). There is a comminuted fracture of the right iliac wing (gray arrow), and there are fractures of both pubic rami (white arrows), as the obturator ring in itself comprises a ring structure.

For diagnosing acetabular fractures (Figure 3-2), it is important to differentiate between the acetabular wall, column, or a combination of wall and column fractures. In short, the anterior column extends from the iliac crest to the symphysis pubis and includes the anterior wall. The posterior column extends from the superior gluteal notch to the ischial tuberosity and includes the posterior wall. The acetabular roof is the superior weight-bearing portion of the acetabulum and contributes to the anterior and posterior column.13 For adequate radiographic assessment, bilaterally angled oblique views, so-called “Judet views,” are obtained in addition to the standard AP radiograph because the anterior and posterior columns are better visualized on the Judet views. All acetabular fractures will be evaluated with CT, including cross-sectional reformats, as it provides detailed information about the number, size, and displacement of fracture fragments. Acetabular fractures result from high-energy trauma, such as motor vehicle collisions or a fall from height. The most complex of the acetabular fractures is when both anterior and posterior acetabular columns are involved. Both columns are separated from each other and from the axial skeleton, resulting in a “floating” acetabulum. The spur sign4 seen on an obturator oblique view is diagnostic of both columns being fractured. It is caused by a triangular bone fragment with an inferiorly directed apex.

###### Figure 3-2.

Acetabular fracture. The fracture extends through the posterior wall and the junction of the anterior wall with the superior pubic ramus (white arrows). There is a fracture through the inferior pubic ramus (curved arrow). Therefore, the fracture involves both columns. ...

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