Bone:Pelvis:Sacrum

The sacrum is the terminal portion of the vertebral column.It is formed by the fusion of the five progressively smaller sacral vertebrae which is triangular in outline and curved with the concavity toward the pelvis.

Structure of the sacrum

* The body of S1 is large and wide. it has anterior projection called the sacral promontory.
* Laterally, to the body is the ala of the sacrum on each side consisting of fused costal elements and transverse processes. It articulates with the ilium forming posterior wall
* 3 surfaces (pelvis, dorsal and lateral)
* Sympathetic trunk, lumbosacral trunk and obturator nerve crosses the sacral ala anteriorly (from medial to lateral)

Pelvic surface

This concave surface is smooth and formed by the fused body of the sacral vertebra with four ridges persisting to mark the lines of ossification (intervertebral disc).

* On each side, there is four anterior **sacral foramina**. The bone lateral to the foramina, **the lateral mass**, is formed by fusion of the costal elements. This gives attachment for the piriformis muscle. The **anterior rami of the sacral nerve** exit these foramina and lie under piriformis.

Relationships: peritoneum is draped over the front of the upper two bodies, below lies the retroperitoneal rectum surrounded by mesorectum. Behind the fascia, the median sacral artery and vein lie in the midline with lymph nodes. On each side, the sacral sympathetic trunk lies medial to the sacral foramina.

Dorsal surface

This convex surface is irregular and rough. It is closed in the midline by fusion of adjacent laminae. The gap above the first sacral laminae is closed by the ligamenta flava attach to the laminae of L5 vertebra.

* Failure of fusion of the lamina of the S5 and often S4 vertebra forms the **sacral hiatus.** This is closed by fibrous tissue forming the superficial sacral coccygeal ligament.
* The sacral spinous process is fused to produce the midline ridge, called **median sacral crest**, that project dorsally.
* S1 carries a superior articular process facing backward to articulate with L5 facet. Caudally, series of fused articular processes form the **intermediate sacral crest**.
* Lateral to the intermediate sacral crest is the four **posterior sacral foramina** that transmit the posterior divisions of the sacral nerves.
* Further lateral to this is a prominent bony ridge formed from fusion of transverse process of sacral vertebra called the **lateral sacral crest.** The lower 3rd and 4th sacral crest gives attachment to the sacrotuberous ligament.

Sacral canal

Triangular in cross section and curves with the sacrum. This represents the vertebral foramen of the sacrum. The canal contains meninges which extend down to the S2 vertebra. The posterior root ganglia are contained within the canal and the anterior and posterior sacral nerves emerges separately from the anterior and posterior sacral foramina.

Joints

* Lumbosacral junction
	+ Joins with 5th lumbar vertebra by L5-S1 disc & facet joints
	+ Superior base articulates with L5
	+ Superior articular processes of S1 faces dorsally
* Sacrococcygeal joint
	+ Apex of sacrum & base of coccyx
	+ Contains fibrocartilaginous disc
* Sacroiliac joints
	+ Ventral synovial joint: Between hyaline covered articular surface of sacrum & fibrocartilage covered surface of iliac bone
	+ Dorsal syndesmosis: Interosseous sacroiliac ligament

Soft tissues

* Thecal sac
	+ Thecal sac terminates at S2 level
	+ Extradural component of filum terminale continues from S2 to attach at 1st coccygeal segment
* Nerves
	+ Sacral canal contains sacral & coccygeal nerve roots
	+ Nerves emerge via ventral & dorsal sacral foramina
* Muscles
	+ Piriformis: Arises from ventral sacrum, passes laterally through greater sciatic foramen to insert on greater trochanter; nerves of sacral plexus pass along anterior surface of piriformis muscle
	+ Gluteus maximus, erector spinae & multifidis arise from dorsal sacrum

Ligaments

* Anterior longitudinal ligament passes over sacral promontory
* Posterior longitudinal ligament on dorsal surface of lumbosacral disc forming ventral margin of bony canal
* Sacroiliac joint secured by broad anterior, interosseous & posterior sacroiliac ligaments
* Sacrospinous ligament bridges lateral sacrum to ischial spine
* Sacrotuberous ligament bridges lateral sacrum to ischial tuberosity

Blood supply:

* iliolumbar artery (internal iliac)
* lateral sacral artery (internal iliac)
* median sacral artery (abdominal aorta)

Ossificiation

* Five 1° ossification centers per vertebra
	+ Centrum (1), posterior neural arch (2), costal element remnants (2)
* Four 2° ossification centers
	+ Sacroiliac (SI) joint epiphyseal plates (fuse ≈ 25 years)