The sacrum is a large, triangular bone located at the base of the spine, centrally within the pelvis. It connects the upper body’s axial skeleton to the lower body’s appendicular skeleton. The sacrum acts as a strong, stable anchor for the entire torso, playing a fundamental role in balance, posture, and the distribution of body weight.
Anatomy and Position of the Sacrum
The sacrum is a singular bone formed through the fusion of five separate vertebrae (S1 through S5). This fusion creates a solid, unmoving unit, unlike the mobile vertebrae above it. The bone is triangular, with a broad base at the top and a narrow apex pointing downward.
It is situated directly below the last lumbar vertebra (L5), forming the lumbosacral junction. The bottom connects to the coccyx, or tailbone. Laterally, the sacrum is wedged between the two large hip bones (ilia) to form the back wall of the pelvic girdle.
The lateral surfaces, known as the auricular surfaces, articulate with the corresponding iliac bones. The superior surface, or base, features a prominent forward-jutting edge called the sacral promontory. The sacrum has a gentle curve, which contributes to the natural curvature of the spine.
Functions of the Sacrum
The primary function of the sacrum is to act as a central anchor for weight transfer. It receives the weight of the upper body from the lumbar spine and distributes that force across the pelvis and into the lower limbs. The sacrum’s wedged shape helps lock the pelvis together, providing stability when standing or moving.
The sacrum provides a stable attachment point for powerful muscles and ligaments necessary for maintaining posture and facilitating movement. Muscles, including the gluteus maximus and piriformis, attach directly to the sacrum, influencing hip rotation and balance. By linking the spine and the hips, the sacrum ensures a smooth transmission of forces during activities.
The bone also serves a protective role for the nerves that exit the spinal cord. The sacral canal runs through the center of the sacrum, shielding the lower spinal nerves. These nerves exit the bone through four pairs of openings called the sacral foramina. They control sensation and movement in the hips, legs, and feet, and innervate organs like the bladder and bowel.
The Sacroiliac Joints
The connections between the sacrum and the two iliac bones are called the sacroiliac (SI) joints. These are strong, weight-bearing articulations stabilized by a dense network of powerful ligaments. The SI joints are not designed for wide-ranging mobility but rather for subtle, limited movement.
The primary role of the SI joints is to absorb shock and reduce stress on the upper spine. They allow only a small amount of gliding and rotational movement, which is necessary for gait mechanics and accommodating forces during walking or running. The joint surfaces develop ridges and depressions that allow the two bones to interlock for stability.
Dysfunction or inflammation in these joints, known as sacroiliitis, is a frequent source of discomfort. When the supporting ligaments become irritated or strained, it often results in pain that radiates across the lower back, buttock, and sometimes down the leg.