7 Sacral Spine • 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. • 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. Fig. 7.1 Bony anatomy of the pelvic, lateral, superior, and dorsal sacral surfaces. (Reproduced with permission from An HS, Singh K, eds. Synopsis of Spine Surgery. 3rd ed. New York, NY: Thieme; 2016.) • 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. ∘ 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. • 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.
7.1 General Information
7.2 Bony Anatomy (Fig. 7.1)
7.3 Ligamentous Anatomy
Musculoskeletal Key
Fastest Musculoskeletal Insight Engine
