The sacrum:

•  In both male and female is triangular in shape and is typically formed from five fused sacral vertebrae.

•  Has a superior surface (or base) that articulates with the fifth lumbar vertebrae at the lumbosacral angle, and an inferior apex that articulates with the coccyx.

•  Has dorsal, pelvic (ventral), and lateral surfaces.

•  Is curved, so that in the erect individual the upper part of the pelvic surface is almost horizontal, and the lower part almost vertical.

•  Has sacral alae (lateral masses) that are enlarged lateral parts of the sacral vertebrae. They consist of costal elements fused with the body, pedicles, and transverse processes.

•  The apex of the sacrum is formed by the inferior surface of the fifth sacral vertebra, and has an oval facet for articulation with the coccyx via the sacrococcygeal intervertebral disc.

Pelvic surfaces 

•  The pelvic surface is concave transversely and vertically. The smooth pelvic surface of the lateral mass gives attachment to the piriformis muscle.

•  The upper part of the pelvic surface is in contact with the peritoneum, the lower part with the rectum.

•  The dorsal surface is convex and irregular with four pairs of dorsal sacral foramina. It gives attachment to parts of the erector spinae and gluteus maximus muscles, and the thoracolumbar fascia.

•  The triangular lateral surface of the sacrum is broad superiorly and divided into a smooth, upper anterior auricular surface for articulation with the ilium, and a rough, lower posterior area with three deep impressions.

•  Above and behind the L-shaped auricular surface is a roughened area for attachment of the strong interosseous sacro-iliac ligament.

Sacral canal 

•  The triangular sacral canal is formed by the sacral vertebral foramina.

•  The fused dorsal aspects of the sacral vertebral bodies form the sacral anterior wall.

•  The lateral wall is formed by the pedicles and intervertebral foramina.

•  The fused laminae, spines, and ossified ligamenta flava form the dorsal wall.

•  Between the sacral cornua, the dorsal wall is deficient inferiorly; this lower opening is called the ' sacral hiatus ' and is closed by fibrous tissue.

•  The sacral canal contains the lower dural sac (to the level of S2) with the cauda equina, filum terminale, cerebrospinal fluid (CSF), the internal vertebral venous plexus, spinal nerves, and fat.

•  There are paired rows of sacral foramina, which emerge on the dorsal and pelvic surfaces.

•  The pelvic (ventral) and dorsal sacral foramina communicate with the corresponding intervertebral foramina at each level, and convey ventral and dorsal rami of the sacral nerves, respectively.

Variations 

•  Transitional vertebrae occur at the regions of the spine where the morphologic characteristics of the vertebrae markedly change from one area to the next.

•  Incorporation of the last lumbar vertebra into the sacrum (sacralization) reduces the number of lumbar vertebrae to four, and separation of the first sacral vertebra from the sacrum (lumbarization), typically increases the number of lumbar vertebrae to six.

•  The transitional segment may become partially or completely fused to the adjacent segment.

Ossification 

•  Ossification of the sacral vertebrae is similar to that of a typical vertebra, ossifying from three primary centers which appear in the body and in the two halves of the vertebral arch in-utero, between the tenth and twentieth weeks.

•  Centers for the costal elements of the upper four segments appear between the sixth and eighth prenatal months.

•  Secondary centers for the upper and lower surfaces of the bodies, spinous processes, and transverse tubercles and the costal elements appear during puberty.


Pathology


Clinical landmark 

•  The sacral hiatus arises due to the failure of posterior fusion of the S5 vertebra. It is a palpable landmark.

•  The hiatus is covered by the sacrococcygeal membrane which is penetrated with a needle to reach the extradural space when performing caudal anesthesia.

Shoulder dystocia 

•  During normal labor, the fetus progresses downwards through the pelvis rotating as it descends (internal rotation).

•  The fetal head is delivered in the anteroposterior plane with the neck under the pubic arch allowing delivery of the face across the perineum by extension of the fetal neck.

•  The shoulders also rotate in a similar fashion.

•  Shoulder dystocia is said to occur when there is delay in delivering the shoulders as this rotation fails to occur.

•  In the majority of cases, the shoulders are lying in the anteroposterior position with the anterior shoulder overriding the pubic symphysis.

•  Various maneuvers forming an 'Obstetric Drill' for shoulder dystocia have been described including McRobert's maneuver (flexion of the mothers legs at all joints followed by firm abduction at the hip joints), supra pubic pressure, extension of the episiotomy, and Wood's maneuver (rotating the shoulders off the anterior posterior position).

•  Profound shoulder dystocia can lead to trauma of the mother and baby and is associated with significant physical and emotional morbidity.

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