7 Sacral Spine

10.1055/b-0039-166416

7 Sacral Spine

Antonios Varelas, Fady Y. Hijji, Ankur S. Narain, Philip K. Louie, Daniel D. Bohl, and Kern Singh

7.1 General Information

Begins as five initially unfused vertebrae:
- Fusion begins around age 18 years and is completed by age 30 years.
Concave, inverted triangular shape.
Structural orientation:
- Base: broad, superior end.
- Apex: narrow, inferior end.
Articulations:
- Ilium: with the superolateral articular surface.
- L5 (final lumbar vertebrae): with the superior articular process.
- Coccyx: with the apex of the sacrum.

7.2 Bony Anatomy

(Fig. 7.1)

Dorsal (posterior) surface:
- Contains four pairs of foramina:
  - Allow passage of ventral rami for first four sacral spinal nerves and sacral arteries.
- Median sacral crest:
  - Fusion of first three or four sacral spinous processes.
  - Bony projection at midline of pelvic surface.
- Intermediate sacral crest:
  - Fusion of the sacral articular processes of S2, S3, S4.
- Lateral sacral crest:
  - Fusion of all five sacral vertebral articular processes:
    - Incomplete fusion leads to the formation of the posterior sacral foramina.
Pelvic (anterior) surface:
- Four transverse ridges, marking the traces of the four fused intervertebral disks.
- Sacral promontory:
  - Anterior projection of the pelvic surface.
  - Forms the posterior margin of the pelvic inlet.
  - Less prominent in females than in males:
    - Leads to an oval pelvic inlet in females and a heart-shaped inlet in males.
Ala of sacrum:
- Large superolateral projections of S1 due to fusion of the transverse vertebral processes.
- L5 nerve root runs over the top of the sacral ala.
Sacral canal:
- Inferior extension of the vertebral foramen.
- Contains the filum terminale and cauda equina:
  - Filum terminale is a nonneural, connective tissue filament that provides longitudinal support for the spinal cord:
    - Fibrous extension of the conus medullaris.
    - Filum terminale internum is enclosed within dural sac, while the externum (coccygeal ligament) is extradural and attaches to the first segment of the coccyx.
  - Cauda equina is a collection of spinal nerves and nerve roots of the L1–S5 nerve pairs and coccygeal nerve:
    - Emerges from the conus medullaris.
Sacral hiatus:
- An opening at the inferior border of the sacral canal:
  - Occurs when the lamina of the fifth sacral vertebrae fail to fuse.

7.3 Ligamentous Anatomy

Sacroiliac ligaments:
- Stabilize the sacroiliac joint.
- Comprises three divisions:
  - Anterior (symphyseal ligaments):
    - Weak stabilization of the sacroiliac joint.
    - Resists external rotation.
  - Posterior ( Fig. 7.2 ):
    - Forms the primary bond between sacrum and ilium.
    - Considered by many as the strongest ligaments in the body.
    - Important for pelvic ring stability.
  - Interosseous:
    - Resists abduction of the sacroiliac joint.
Sacrotuberous ligament (pelvic floor):
- Spans the sacrum and tuberosity of the ischium:
  - Helps create a boundary for the greater and lesser sciatic foramina.
- Stabilizes the pelvic girdle.
- Resists shear and flexion.
- Passes posterior to the sacrospinous ligament.
Sacrospinous ligament (pelvic floor):
- Located within the greater sciatic notch:
  - Spans the sacrum and spine of the ischium.
  - Forms the greater and lesser sciatic foramina.
- Resists external rotation of the ilium beyond the sacrum.
Sacrococcygeal ligaments:
- Spans the sacrum and coccyx.
- Closes the sacral hiatus.
- Comprises three divisions:
  - Anterior:
    - A continuation of the anterior longitudinal ligament.
  - Posterior:
    - Consists of two components: deep and superficial:
      - Deep ligament is a continuation of the posterior longitudinal ligament.
      - Superficial ligament completes the inferior portion of the sacral canal.
  - Lateral:
    - Completes the final foramen for the fifth sacral nerve.
Coccygeal ligament:
- Anchors the termination of the dural sac at S2 to the first segment of the coccyx.

**Fig. 7.2 (a,b)** Posterior views of the sacral ligamentous anatomy. (Reproduced with permission from An HS, Singh K, eds. Synopsis of Spine Surgery. 3rd ed. New York, NY: Thieme; 2016.)

7.4 Muscular Anatomy

Anterior sacral muscles ( Table 7.1, Figs. 7.3, 7.4 ):
- Muscular attachments occur along the pelvic (anterior) surface of sacrum:
  - Piriformis.
  - Coccygeus.
  - Iliacus.
Posterior sacral muscles ( Table 7.2, Figs. 7.5, 7.6 ):
- Muscular attachments occur along the dorsal (posterior) surface of sacrum:
  - Multifidus lumborum.
  - Erector spinae.
Lateral sacral muscles ( Table 7.3, Fig. 7.6 ):
- Muscular attachments occur along the lateral edge of sacrum:
  - Gluteus maximus.

**Fig. 7.3** View of the pelvic floor and the muscular attachments of the piriform and coccygeus to the sacrum.

**Fig. 7.4** Anatomical view of the attachment sites for the anterior sacral muscles.

**Fig. 7.5** Posterior view of the muscular attachment of the erector spinae and multifidus lumborum to the sacrum.

**Fig. 7.6** Anatomical view of the attachment sites for the posterior and lateral sacral muscles.

**Table 7.1** Anterior surface
Muscle	Origin	Insertion	Action	Innervation
Piriformis	Sacrum (S2, S3, S4)	Greater trochanter of femur	External rotation, abduction, extension of hip joint	Nerve to the piriformis (L5, S1, S2)
Coccygeus	Ischial spine, sacrospinous ligament	Inferior sacrum, coccyx	Elevation/support of pelvic floor, flexion of coccyx	Anterior primary rami of S4, S5
Iliacus	Ala of sacrum, iliac fossa	Lesser trochanter of femur	Flexion, external rotation of hip joint	Femoral nerve (L2, L3, L4)

**Table 7.2** Posterior surface
Muscle	Origin	Insertion	Action	Innervation
Erector spinae	Sacrum, iliac crest	Varies for each erector spinae muscle	Extension, lateral flexion of vertebral column and neck	Dorsal primary rami of spinal and thoracic nerves
Multifidus lumborum	Sacrum, transverse process of C2–L5	Spinous processes of vertebrae	Extension, ipsilateral lateral flexion, contralateral rotation of spine	Dorsal primary rami of C1–L5

**Table 7.3** Lateral border
Muscle	Origin	Insertion	Action	Innervation
Gluteus maximus	Sacrum, ilium, coccyx	Upper fibers: iliotibial tract Lower fibers: gluteal tuberosity of femur	Extension of hip, external rotation of femur	Inferior gluteal nerve (L5, S1, S2)