The sacrum is an important bone located in the lower trunk, forming a strong connection between the spine and the pelvis. It provides a foundation for the upper body and contributes to overall stability and movement. Its design allows it to withstand significant forces and facilitate various bodily functions.
Anatomical Position of the Sacrum
The sacrum is situated at the base of the vertebral column, nestled between the two hip bones (ilia) that form the pelvis. It is an inverted triangular bone, wider at the top and tapering towards the bottom. This unique shape results from the fusion of five individual sacral vertebrae, typically designated S1 through S5, a process that generally completes between the ages of 18 and 30.
Superiorly, the sacrum connects with the fifth lumbar vertebra (L5) at the lumbosacral joint, forming an important junction in the spinal column. On its lateral sides, the sacrum articulates with the iliac bones of the pelvis, forming the strong sacroiliac (SI) joints. These joints are important for transferring forces between the spine and the lower limbs. Inferiorly, the narrow end of the sacrum joins with the coccyx, commonly known as the tailbone.
The sacrum presents two main surfaces: an anterior (pelvic) surface and a posterior (dorsal) surface. The anterior surface is smooth and concave, forming the posterior wall of the pelvic cavity. This concavity provides space for pelvic organs. The posterior surface is rough and convex, featuring several bony ridges or crests formed by fused vertebral components. Both surfaces contain pairs of openings called sacral foramina, which serve as passages for nerves and blood vessels.
Key Functions of the Sacrum
The sacrum functions in support, stability, and protection. It plays a main role in weight bearing, transferring the upper body’s weight from the vertebral column to the pelvic girdle and lower limbs. This structure supports significant pressure and motion, allowing for upright posture and bipedal locomotion.
The sacrum provides stability and strength to the entire pelvic girdle. Its articulation with the iliac bones at the sacroiliac joints creates a stable base for standing, walking, and other movements. This stability is important for maintaining proper posture and distributing forces throughout the lower body. The sacrum acts as a central component, linking the axial skeleton to the appendicular skeleton.
Beyond structural support, the sacrum also offers protection for various internal structures. It forms the posterior wall of the pelvic cavity, shielding pelvic organs from external forces. The sacral foramina and the sacral canal, a continuation of the vertebral canal, house and protect the sacral nerves as they exit the spinal cord to innervate the lower body. The sacrum also acts as an attachment point for muscles and ligaments, which contribute to the integrity and mobility of the spine and pelvis.