The sacrum is a bone located within the human body, playing a foundational role in the skeletal system. This large, triangular structure forms a central component of the pelvic girdle, connecting the spine to the lower limbs. Its position and characteristics allow it to perform functions related to movement and stability.
Its Place in the Human Skeleton
The sacrum is positioned at the base of the vertebral column, between the lumbar spine above and the coccyx, or tailbone, below. Its uppermost part, the sacral promontory, articulates with the fifth lumbar vertebra (L5). This connection forms the lumbosacral joint.
Laterally, the sacrum connects with the two large hip bones, also called ilia, forming the sacroiliac (SI) joints. These joints create a strong connection between the axial and appendicular skeletons. The sacrum, along with the two hip bones, forms the posterior wall of the pelvis, creating a ring-like structure. This arrangement anchors the lower spine and provides a stable platform for the upper body.
Unique Features and Shape
The sacrum has a triangular or wedge-like shape, broader at its top and tapering towards its bottom. This bone forms from the fusion of five individual vertebrae, labeled S1 through S5. This fusion process begins around adolescence and is completed by early adulthood.
Its curved appearance contributes to the overall shape of the pelvis. The sacrum features several openings called sacral foramina, typically four pairs on both its anterior and posterior surfaces. These foramina serve as passageways for sacral nerves and blood vessels that supply the pelvic region and lower limbs. The uppermost part of the sacrum is called the base, while its lower, narrower end is called the apex.
Its Role in Body Support
The sacrum’s design and central location give it a role in supporting the human body. One of its main functions is transmitting the weight of the upper body to the pelvis and to the lower limbs. This weight distribution helps maintain balance and upright posture during activities like standing, walking, and running.
Functioning as a keystone within the pelvic ring, the sacrum provides stability and strength to the pelvic girdle. The sacroiliac joints, where the sacrum meets the hip bones, are designed for limited movement but are strong, aiding in shock absorption. The sacrum also offers protection for the nerves that pass through its foramina, safeguarding these pathways to the lower body.