The sacrum is a triangular bone formed by the fusion of five sacral vertebrae (S1–S5), connecting the lumbar spine (via L5-S1 articulation) to the pelvis (via sacroiliac joints) and coccyx. It serves as a keystone for pelvic stability, transmitting axial loads from the spine to the lower limbs. Key functions include protecting sacral nerve roots, anchoring spinal meninges, and providing attachment points for pelvic ligaments and muscles.
Parts and Structures
- Sacral promontory: anterior superior margin of S1, forming the posterior boundary of the pelvic inlet. Critical in obstetrics for measuring pelvic dimensions.
- Sacral canal: Continuation of the vertebral canal, containing the cauda equina, filum terminale, and meninges. Terminates at the sacral hiatus (S4–S5 level), which is covered by the sacrococcygeal ligament.
- Sacral foramina: Anterior (4 pairs): Transmit S1–S4 ventral rami and lateral sacral arteries.
- Posterior (4 pairs): Transmit S1–S4 dorsal rami.
- Ala of sacrum: Lateral “wings” formed by fused transverse processes of S1. Articulates with ilium via sacroiliac joints.
- Crests: Median sacral crest: Midline ridge from fused spinous processes.
- Intermediate sacral crest: Lateral ridges from fused articular processes.
- Lateral sacral crest: Remnants of transverse processes.
- Sacral cornua: Bony projections at S5, flanking the sacral hiatus. Palpable landmarks for caudal epidural access.
- Auricular surface: Lateral articular surface for sacroiliac joints.
- Sacral plexus:Formed by ventral rami of S1–S4 and lumbosacral trunk (L4–L5). Gives rise to sciatic, pudendal, and gluteal nerves.
High-yield concepts
Sacral hiatus: Landmark for caudal epidural anesthesia (needle insertion at S4–S5). The dural sac typically ends at S2, reducing risk of dural puncture.
Spinopelvic Alignment: Pelvic Incidence (PI): Anatomical constant (PI = Sacral Slope + Pelvic Tilt). Critical for sagittal balance
Abnormal PI correlates with spinal pathologies (e.g., spondylolisthesis, degenerative disc disease).
Vascular supply:
Lateral Sacral Arteries (from internal iliac) and median sacral artery (aortic bifurcation).
