The sacrum is a distinctive triangular bone positioned at the base of the human spine. It is a single bone that forms an integral part of the vertebral column. This bone is a component of the posterior aspect of the pelvis, contributing to the skeleton. It connects the spine to the lower body.
Precise Location of the Sacrum
The sacrum is at the bottom of the vertebral column, situated directly above the coccyx. This bone is within the pelvic cavity, positioned between the two large, wing-shaped hip bones, also called the ilia. Its broad, upper portion sits directly beneath the last lumbar vertebra, the fifth lumbar vertebra.
The bone narrows as it descends, creating its triangular or wedge-like appearance. Its front surface, which is smooth and curved inward, faces towards the front of the body. In contrast, the back surface is rougher and curves outward. This unique shape allows the sacrum to fit snugly within the pelvic ring, providing a stable foundation for the spine.
Its Place in the Skeletal System
The sacrum connects with the fifth lumbar vertebra (L5), forming the lumbosacral joint. This connection represents a flexible junction that permits some movement between the lower spine and the pelvis. Below, the sacrum articulates with the coccyx, creating the sacrococcygeal joint. This lower connection is generally more fused, allowing for less movement compared to the lumbosacral joint.
On its sides, the sacrum forms strong connections with the two large hip bones, specifically the ilia. These junctions are called the sacroiliac joints, one on each side. The sacroiliac joints are designed for stability rather than extensive movement, serving to transfer forces between the torso and the lower limbs. Together with the ilia, the sacrum completes the posterior part of the pelvic girdle, providing a stable base for the upper body and anchoring the lower limbs.
Key Roles of the Sacrum
A primary function of the sacrum involves bearing weight, as it is a central point for transferring forces from the upper body. It transmits the weight of the torso, spine, and head downwards, distributing it through the pelvis to the lower limbs. This structural role makes it an important component in maintaining upright posture and facilitating movement.
The sacrum also contributes stability to the entire pelvic region. Its strong articulations with the hip bones help to maintain the integrity of the pelvic ring, which is important for balance and locomotion. Furthermore, the sacrum provides protection for the nerves and blood vessels that pass through its openings. These sacral nerves exit through specific foramina, extending to supply the lower limbs and various pelvic organs.