Sacrum

The sacrum is a triangular bone at the base of the spine, formed from five fused vertebrae. It joins the spine to the pelvis and transfers upper-body weight to the legs.

Sacral vertebrae: The five vertebrae that fuse during development to form the sacrum. Their fusion creates the sacrum’s solid, weight-bearing structure.
Lumbosacral joint: The junction between the fifth lumbar vertebra and the first sacral vertebra. It is the transition where spinal forces enter the sacrum.
Sacral fracture: A break in the sacrum, caused by trauma or weakened bone. Fractures can disrupt weight transfer or injure nerves passing through the sacrum.
Sacral insufficiency fracture: A fracture caused by ordinary stress on weakened sacral bone, often without major trauma. It can produce severe pelvic or low-back pain despite little or no injury.
Sacral promontory: The forward-projecting upper edge of the first sacral vertebra. This landmark marks the sacrum’s upper boundary and helps define the pelvic inlet.
Pelvic ring: The bony ring formed by the sacrum and the right and left hip bones. The sacrum closes the ring posteriorly, helping distribute forces across the pelvis.
Sacral nerve roots: The nerve roots emerging from the lower spinal cord and sacral segments. Their passage near the sacrum explains neurological symptoms after injury or compression.
Chordoma: A rare malignant tumor arising from remnants of the embryonic notochord, often in the sacrum or skull base. The sacrum is the most common site of chordoma.
Sacral foramina: Paired openings on the front and back of the sacrum through which sacral nerves and vessels pass. These openings preserve routes for nerves and vessels through the fused bone.
Axial skeleton: The bones of the head, neck, and trunk, including the vertebral column and rib cage. The sacrum is the vertebral column’s pelvic foundation.

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