The sacrum is a large, triangular bone situated at the base of the vertebral column. It is the final segment of the spine, positioned just above the coccyx, or tailbone. This bone is a foundational component of the posterior pelvis, serving as a robust link between the spine and the lower half of the body.
Anatomy and Position in the Pelvis
The sacrum is not a single bone from birth but forms through a developmental process called fusion. It begins as five separate vertebral segments (S1 through S5) that gradually merge into one solid piece. This fusion typically starts in the mid-teens and is usually complete by the early to mid-twenties, creating its characteristic triangular shape.
The bone’s broad upper section connects directly to the fifth lumbar vertebra (L5), forming the lumbosacral joint. Below, the narrow tip articulates with the coccyx. The sacrum is firmly wedged between the two large hip bones, known as the ilia.
This connection point on either side is called the sacroiliac (SI) joint. The sacrum features several pairs of openings, known as the sacral foramina (four anteriorly and four posteriorly). These holes function as exit points for the sacral nerves, which supply the pelvis, buttocks, and legs. The sacrum’s shape and location make it a stable base for the entire upper skeleton.
The Sacrum’s Essential Role in Biomechanics
The primary function of the sacrum is to act as the keystone structure for transferring the body’s weight. It receives the entire axial load from the head, torso, and upper spine, channeling that force downward. This load is then distributed through the two sacroiliac joints to the ilium bones and ultimately into the lower limbs.
The SI joints balance stability with minimal movement, allowing for a small degree of rotation and tilting. The range of motion is quite small, often measured as only about three degrees of flexion and extension, which aids in shock absorption. Strong ligaments and the interlocking shape of the sacrum and ilium stabilize this load transfer, which is crucial for upright posture and walking.
This joint design manages high compression loads and bending moments that occur during daily activities. The stability of the sacrum within the pelvic ring allows it to function as a fixed point, ensuring forces moving up from the legs are efficiently managed. This mechanism prevents excessive shear forces from destabilizing the base of the spine, maintaining structural integrity.
Common Causes of Sacral Discomfort
Pain originating in or around the sacrum often stems from issues with the surrounding sacroiliac joints. Sacroiliac joint dysfunction is a common diagnosis involving either hypermobility (too much movement and instability) or hypomobility (stiffness from too little movement). Both conditions can lead to inflammation, termed sacroiliitis, causing pain in the lower back and buttocks.
The location of the sacrum also makes it a source of nerve-related discomfort. Since the sacral nerves exit through the foramina, any inflammation or misalignment in the area can cause irritation or compression. This can manifest as pain that radiates down the leg, mimicking the symptoms of sciatica. The proximity of the sacrum to other large nerves means that localized issues often produce far-reaching symptoms.
Discomfort can also result from traumatic events or repetitive stress placed on the bone and its surrounding ligaments. Direct trauma, such as a fall onto the buttocks or a car accident, can injure the SI joint or even cause a fracture in the sacrum itself. Repetitive strain from activities like heavy lifting or a significant leg length discrepancy can also lead to chronic irritation and pain over time. Pregnancy is another factor, as hormones relax the pelvic ligaments, increasing joint movement and potential instability.