The sacrum is a large, triangular bone situated at the base of the spine, positioned like a shield at the back of the pelvis. This wedge-shaped structure is composed of five individual vertebrae that fuse together over time. The sacrum is a foundational component of the pelvic girdle, serving as a robust link between the spine and the lower limbs. Its form and location are fundamental to human posture, stability, and the transmission of forces throughout the body.
Placement in the Skeletal System
The sacrum is located immediately beneath the final lumbar vertebra (L5), forming the lumbosacral joint at the bottom of the movable spine. This position situates the sacrum deep within the pelvic cavity, where it acts as the posterior wall of the pelvis. It connects the central axial skeleton to the lower appendicular skeleton.
Laterally, the sacrum articulates with the two large hip bones, the ilia, at the L-shaped sacroiliac (SI) joints. These joints are heavily reinforced by the body’s strongest ligaments, creating a stable, minimally mobile ring structure. This connection allows the entire weight of the upper body to be transferred down to the legs.
At the inferior tip of the sacrum, the coccyx, or tailbone, attaches. The sacrum and coccyx together complete the terminal end of the vertebral column. The complex is tucked between the wings of the pelvis, providing structural support and protection to the pelvic organs.
Detailed Structure and Formation
The sacrum is not a single bone at birth, but develops through the fusion of five separate sacral vertebrae, designated S1 through S5. Fusion typically begins during adolescence and is complete by the third decade of life, usually between the ages of 18 and 30. This merging results in the characteristic single, inverted triangular bone.
Several distinct anatomical features mark the fused sacrum. The dorsal surface features the median sacral crest, a central ridge formed by the fusion of the spinous processes of the upper sacral vertebrae. On both the anterior and posterior surfaces, there are four pairs of openings called sacral foramina. These openings allow the sacral nerves and associated blood vessels to exit the central bony structure.
Running vertically through the center of the sacrum is the sacral canal, a continuation of the vertebral canal. This canal houses the lower portion of the spinal nerves. The fusion of the sacral vertebrae provides a strong, fixed structure that contrasts with the flexible nature of the rest of the spinal column.
The Sacrum’s Role in Stability and Movement
The primary function of the sacrum is the transmission of weight. It serves as the bridge that transfers the entire load of the upper body—including the head, torso, and arms—from the lumbar spine down to the pelvis and lower limbs. This function is accomplished through its firm articulation with the iliac bones at the sacroiliac joints.
The sacrum’s wedge shape and its connections to the pelvis provide stability, particularly during standing and walking. This structure helps maintain proper posture by anchoring the spine to the pelvic girdle. The stable positioning of the sacrum minimizes movement at the SI joints, which is necessary for efficient force transfer.
The sacrum also provides protection for the nervous tissue passing through its canal. The sacral canal shields the nerve bundle known as the cauda equina, which controls sensation and movement in the lower body, including the legs, bowels, and bladder. The sacrum is central to both musculoskeletal mechanics and neurological integrity.
Common Issues Involving the Sacrum
The sacrum is frequently implicated in lower back and hip discomfort, with Sacroiliac Joint (SIJ) dysfunction being a common issue. SIJ dysfunction results from either excessive movement (hypermobility) or restricted movement (hypomobility) at the joint connecting the sacrum and the ilium. This alteration in motion can cause pain localized to the lower back and buttock, often radiating down the thigh.
Sacral fractures are serious injuries that typically result from high-energy trauma, such as car accidents or falls, especially in younger individuals. Low-energy insufficiency fractures can occur in older adults with osteoporosis. The risk depends on their location, particularly whether they involve the sacral foramina or the central sacral canal.
Damage or disease near the sacrum can lead to nerve compression, or radiculopathy, due to its association with the lower spinal nerves. Cauda Equina Syndrome, which involves compression of the nerve roots in the sacral canal, is a medical emergency. This condition can lead to permanent loss of bowel, bladder, and lower limb function. Pain that mimics sciatica, caused by irritation of the sacral nerve roots, is also a frequent clinical complaint.