![]() ![]() Fig 1.9-3 Lumbosacral plexus from anterior. The median sacral artery and sympathetic component of the autonomic nervous system are closely related to the anterior sacral surface in the area of the promontory. The major pelvic vessels (internal iliac artery and vein) follow the course of the lumbosacral trunk. b In contrast, the anterior sacroiliac ligaments are much weaker. a Posteriorly, the strong sacroiliac interosseous ligaments stabilize the sacrum within the pelvic girdle. Fig 1.9-2a–b Ligaments around the sacrum. Therefore, fractures in this area or cranial displacement of the hemipelvis can be associated with a stretch injury of the L5 nerve roots. The lumbosacral trunk (L4–5 nerve root) is closely related to the lateral upper sacral alar surface. Therefore, the S1 or S2 nerve roots are more endangered for injury by fractures involving these neuroforamina than at lower levels. The cross-section of the nerve roots S2–5 amounts to 80%, 60%, 20%, and 15% of the cross-section of the sacroiliac foramen, respectively. The diameter of the S1 and S2 nerve roots are about one-third to one-fourth of the diameter of the surrounding foramina, decreasing to one-sixth at the level of S3 and S4. In addition to the nerve roots forming the sciatic plexus, the pelvic floor also contains pelvic splanchnic nerves, which are mixed parasympathetic nerves that control involuntary sphincter muscle action of the rectum and bladder, and the nervi erigentes, which supply the penis and clitoris, and are important for sexual function. The S2–4 roots exit and join the sciatic nerve in front of the piriformis muscle. The S1 root exits to join the L4 and L5 roots in front of the sacroiliac joint. The sacral canal holds the spinal nerves of the sacral and coccygeal plexus, which leave the sacrum through the anterior sacral foramina ( Fig 1.9-3). The dural sac ends at the level of S2 in 84% of patients. Laterally, the sacral ala slopes from posterosuperior to anteroinferior in direct association with the common iliac vessels and lumbosacral trunk. In the midline the S1 vertebral body forms the sacral promontory. The upper sacrum is formed by the superior surface of the S1 body. The lateral surface of the upper three sacral vertebrae forms the kidney-shaped articular surface of the sacroiliac joint, which corresponds to the articular surface of the iliac wing. The sacral hiatus takes up the dural sac at the S1 vertebra. The dorsal rami of the nerve roots exit through the posterior sacral neuroforamina. The lateral sacral crest is formed by the fused transverse processes. The smaller intermediate sacral crest is visible medial to the neuroforamina, representing the former facet joints of the sacral spines. The rudimentarily visible former spinal processes are fused within the median sacral crest. The thin dorsal cortical surface is rough and marked by three major vertically oriented crests. On the lateral side the articular surface of the S1 joint is visible. The sacrum is roughly and irregularly formed and marked with three major vertically orientated crests. Fig 1.9-1a–c Osseous anatomy of the sacrum. The anterior foramina are larger in diameter than the corresponding posterior foramina. The foramina are connected with the central canal via the intervertebral foramina. Two rows of sacral foramina are present, normally four on each side for the exiting S1–4 anterior nerve roots. The ventral pelvic sacral surface forms the posterior wall of the true pelvis with a close relationship to the rectum. Spinal orientation changes from a more horizontal to a vertical shape at the third sacral vertebra. The sagittal contour has a slight kyphosis with an average anterior inclination of 47°. Additionally, it acts as an important stabilizing part of the pelvis by its ligamentous attachments. ![]() It consists of five fused sacral vertebrae and the rudimentary coccygeal bones. The sacrum ( Fig 1.9-1) is a median triangular bone, connecting the two hemipelves to the spinal column. ![]()
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