The sacral area refers to the region encompassing the sacrum, a large, triangular bone situated at the base of the vertebral column. This area is important because it serves as a junction connecting the spine to the lower limbs. The sacrum supports the upper body’s weight and provides passage for nerve pathways.
Precise Anatomical Location
The sacral area is a wedge-shaped region positioned directly beneath the last of the five lumbar vertebrae (L5). This bone forms the connection point between the mobile spine and the stable pelvis, sitting just above the coccyx, or tailbone. Externally, this area corresponds to the lower, central back of the hips, located just above the intergluteal cleft.
The sacrum is situated between the two large hip bones, the ilia, completing the bony ring of the pelvic girdle. It acts as the posterior wall of this pelvic structure, providing stability to the torso. This placement means the bone is deeply embedded, forming a base for the attachment of muscles and ligaments.
The superior portion of the sacrum, called the base, articulates with the L5 vertebra, while the inferior tip, or apex, connects to the coccyx. This triangular positioning allows the sacrum to distribute forces traveling down the spine efficiently toward the legs. The landmark where L5 meets the sacrum (S1) is the sacral promontory, which juts slightly forward into the pelvic cavity.
The Sacrum Bone Structure
The sacrum is formed by the fusion of five sacral vertebrae, labeled S1 through S5. This fusion begins in adolescence and is usually complete by early adulthood, resulting in a single, strong bone. This merged nature provides the strength necessary for its role as a weight-bearing foundation.
The bone exhibits a kyphotic curve, meaning it is convex at the back and concave at the front, curving slightly inward. This shape contributes to the natural S-curve of the spinal column, aiding in shock absorption and maintaining posture. The surface is marked by four pairs of small openings called sacral foramina, remnants of where the individual vertebrae once separated.
These foramina, both anterior and posterior, serve as passageways for the sacral nerves and blood vessels to exit the central sacral canal. Along the posterior midline, the fused spinous processes form a bony ridge known as the median sacral crest. This crest provides an attachment point for ligaments and tendons that stabilize the lower spine.
Primary Functions and Nerve Connections
The sacrum’s primary function is to serve as a load-transfer mechanism, transmitting the weight of the upper body from the spine to the pelvis and lower limbs. This transfer occurs through the sacroiliac (SI) joints, the L-shaped articulations where the sacrum connects laterally to the ilium. The SI joints are stable, allowing minimal movement, which maintains pelvic rigidity during walking and standing.
The sacral area is the source of the sacral plexus, a network of nerves formed by the anterior rami of the sacral spinal nerves (S1-S4) and contributions from the lumbar nerves (L4 and L5). This plexus lies on the anterior surface of the sacrum, with nerve roots exiting through the sacral foramina. The nerves control motor function and sensation across the lower body, including the posterior thigh, foot, and lower leg.
A branch of the sacral plexus is the sciatic nerve, the largest nerve in the human body, which innervates much of the lower limb. The sacral nerves also regulate involuntary functions, providing motor control to the pelvic floor muscles, bladder, bowel, and sexual organs.