Osseous Structure

The posterior wall of the true pelvis is formed by the ventral sacral surface resulting in intimate contact with the pelvic viscera. Two vertical rows of sacral foramina are normally present for the exiting S1 through S4 anterior motor nerve roots. The anterior foramina are larger than each corresponding posterior foramina, oval in shape, and directed laterally. Each foramen is connected to the central canal by the intervertebral foramina ( Fig. 10-1 ).

Osseous anatomy of the sacrum.

( Frank Netter illustrations from www.netterimages.com . © Elsevier, Inc. All rights reserved.)

The five fused sacral vertebrae are situated in a kyphotic sagittal alignment of approximately 25 degrees with the apex at S3. This configuration has a significant influence on the sagittal alignment of the remainder of the spine. Sacral inclination (slope of the S1 end plate) averages approximately 45 degrees from the horizontal. This is the reference point for standard pelvic inlet and outlet radiographs used in pelvic ring trauma. These orthogonal images are typically obtained with 45-degree caudal and cephalad tilts, respectively, of the x-ray source. However, research has demonstrated that significant variety exists in true sacral inclination and the ideal trajectory for adequate imaging should be based on the sacral inclination noted on sagittal sacral computed tomography (CT) reconstructions. Ricci and colleagues found the average ideal caudal tilt to image the anterior body of S1 (inlet view) should be 21 degrees, and the appropriate cephalad tilt perpendicular to the body of S1 (outlet view) should be 63 degrees. Similarly, Peckmezci found the ideal angle for inlet views to be 25 degrees, and for outlet views to be 63 degrees. Of similar importance, this sacral inclination and resultant pelvic incidence critically influences the shear forces subjected to the lumbosacral junction and has been shown to influence the incidence and severity of spondylolisthesis ( Figs. 10-2 through 10-4 ).

Lateral view of sacrum.

(Used with permission from Marcus Sciadini, MD.)

Inlet view of sacrum.

(Used with permission from Marcus Sciadini, MD.)

Outlet view of sacrum.

(Used with permission from Marcus Sciadini, MD.)

The dorsal cortical surface is vertically marked by three crests. The median sacral crest is formed by the fused, former spinous processes. Typically, the S1 median tubercle is more prominent. The sacral groove lies lateral to the median crest and is formed by the fusion of the rudimentary sacral laminae. The lamiae of S5, and sometimes S4, do not fuse, which gives rise to the sacral hiatus. The former facet joints of the sacral spine are fused to form the intermediate sacral crest, which lies just medial to the posterior neuroforamina. The posterior neuroforamina accommodate the exiting dorsal rami. The intermediate tubercles of S5 form the sacral cornua, which articulate with the coccygeal cornua. The lateral sacral crest is formed by the fused transverse processes, which provide the origin for the posterior sacroiliac ligaments. The lateral sacral crest forms the limit between the body of the sacrum and the lateral sacral mass, also called the pars lateralis. Because the superior parts of these lateral portions resemble wings (Latin: alae ), they are conventionally called sacral alae. The sacral alae form a large anterior, triangular surface.

The ventral surface of the sacrum is concave in the vertical and horizontal planes. The medial portion consists of the five fused sacral segments. The fusion sites are represented as the transverse ridges.

The undulating kidney-shaped articular surface of the sacrum is formed by the lateral portions of S1, S2, and the superior part of S3, and it corresponds with the articular surface of the ilium. The remaining caudal sacral segments remain free. In the ventral midline, the anterior portion of the S1 body forms the sacral promontory, which, in combination with the pelvic brim, separates the cranial, false pelvis from the caudal, true pelvis anterior to the sacrum.

Ligamentous Anatomy

As briefly mentioned, the remarkable mechanical stability of the sacroiliac joint is largely due to its ligamentous attachments. Anteriorly, the relatively weak anterior sacroiliac ligaments connect the sacrum and ilium. The sacroiliac interosseous ligaments stabilize the sacrum posterior to the SI joint. The stronger posterior sacral ligaments originate from the dorsal sacral interforaminal and lateral surfaces and connect the sacrum to the posterior ilium. These strong ligamentous attachments have a significant impact on the location of fractures of the sacrum. Transverse fractures commonly occur at the midportion of S3 at the inferior margin of the sacroiliac attachment. Similarly, vertical fractures tend to occur through the sacral ala, away from the relative strength of the SI complex, and through the relatively weak foraminal portion of the sacrum.

Additionally, the iliolumbar ligaments connect the L5 transverse process and posterior iliac crest. The lumbosacral ligaments connect the L5 transverse process with the sacral ala. As a result, L5 transverse process fractures can indicate pelvic ring instability. Lastly, the smaller sacrospinous and more stout sacrotuberous ligaments act as pelvic stabilizers limiting rotation around the horizontal axis.