The sacrum is a triangular bone at the base of the spine, nestled between the hip bones. It connects the vertebral column to the pelvis, forming a sturdy bridge for stability and lower body movement.
Anatomical Structure
The sacrum forms from the fusion of five individual vertebrae (S1 through S5). It forms a single, triangular bone. Its broadest part, the base, faces upward, articulating with the last lumbar vertebra (L5). The narrow, pointed lower end is called the apex.
The sacrum has two main surfaces: an anterior, concave pelvic surface and a posterior, convex dorsal surface. The pelvic surface includes four pairs of anterior sacral foramina for nerves and blood vessels. The dorsal surface also has four pairs of posterior sacral foramina. The sacral canal runs vertically through the bone, housing the lower portion of the spinal cord’s nerves, the cauda equina.
The sacrum forms two articulations. Superiorly, it connects with the fifth lumbar vertebra (L5) to form the lumbosacral joint, allowing spinal and pelvic flexibility. Laterally, it articulates with the ilium, the pelvic bones, creating the sacroiliac joints. Ligaments reinforce these joints, contributing to the pelvic girdle’s stability.
Key Functions
The sacrum’s primary function is weight bearing, transferring upper body weight to the lower limbs. As the body’s center of gravity shifts, the sacrum distributes these forces. This role helps manage loads from standing, walking, and running, reducing stress on spinal or pelvic structures.
The sacrum provides pelvic stability. Its wedge-like shape and iliac articulations create a stable pelvic base. This foundation helps maintain posture and facilitates lower body movements. The sacrum anchors many muscles and ligaments across the hip and lower back, supporting pelvic structural integrity.
The sacrum protects the nerves of the cauda equina as they exit the spinal canal. These nerves, which control sensation and movement in the lower body, pass through the sacral canal and exit via the sacral foramina. The bone’s solid structure shields these neural pathways from external forces for proper function.
Development and Individual Variation
The sacrum’s formation is a gradual process throughout human development, beginning as five distinct vertebral segments. These separate bones progressively fuse, typically completing ossification by the mid-twenties. This progression allows the sacrum to achieve its mature form.
Natural anatomical variations in the sacrum are common and not a medical issue. For instance, the fifth lumbar vertebra (L5) may partially or completely fuse with the sacrum (sacralization of L5). Conversely, the first sacral vertebra (S1) might not fully fuse, remaining somewhat separate (lumbarization of S1). These differences highlight the variability in human skeletal development.
The degree of fusion between sacral vertebrae can also differ among individuals. These variations in fusion patterns or the number of fused segments are part of normal human anatomy. Such individual differences do not impact the sacrum’s function.