Geoffrey S. Marecek

Bony Anatomy

The pelvis is composed of two innominate bones and the sacrum. The innominate bone and sacrum are joined at the sacroiliac joint. The joint has complex geometry, and multiple views are required to confirm appropriate reduction and safe implant position.
The sacrum is roughly trapezoidal in the axial plane with the widest point anteriorly and inverted-triangular in the coronal plane with the base at the lumbosacral articulation. In the sagittal plane, the sacrum may be straight or markedly curved but is usually linear through the upper sacral segments.
The sacrum is perforated with tunnels for the sacral nerve roots. The tunnels slope caudally and laterally from the spinal canal to their exit points.

Radiographic Anatomy

In a true AP view, the tip of the coccyx will be aligned with the superior border of the pubic symphysis. This ensures appropriate rotation and pelvic tilt. The pelvic brim gently curves from the symphysis to the sacroiliac joint where it meets the anterior border of the sacrum.
Asymmetry in the AP pelvis will provide information about the displacement of the hemipelvis (Figs. 12-1 and 12-2).

The inlet view provides information and detail about anteroposterior displacement and rotational deformity of the hemipelvis.
A true inlet view is parallel to the anterior cortex of the sacral segment of interest. An estimate of the orientation of the beam can be obtained from the sagittal reformat on the preoperative CT scan.
The indentations formed by the alar slope are easily seen on this view and are the anterior limit of safe screw position in the 1st sacral segment. These indentations may be particularly pronounced with sacral dysmorphism (Figs. 12-3 and 12-4).

The outlet view provides information about craniocaudal displacement of the hemipelvis. Information about flexion-extension can also be gleaned by observing differences in the shapes of the obturator foramina.
A true outlet view provides an en face view of the sacral nerve tunnel exits at the sacral segment desired. Appropriate rotation is achieved when the spinous processes are in line with the pubic symphysis. An estimate of the orientation of the beam can be obtained from the sagittal reformat on the preoperative CT scan.
The lateral borders of the nerve root tunnels are best visualized on the outlet view. A sacral lateral view should be obtained when the drill bit or wire reaches the lateral border of the tunnel (Figs. 12-5 and 12-6).