The sacrum is a singular, complex bone that anchors the spinal column to the pelvis, forming a robust structure at the base of the torso. It handles the immense forces generated by upright posture and movement. Its shape and position allow it to act as a central support system, linking the upper body to the lower limbs.
The Factual Count and Developmental Change
The number of bones in the human sacrum depends entirely on a person’s age. In adults, the sacrum is considered a single, unified bone. This structure is a composite, formed from the fusion of five separate vertebrae known as the sacral vertebrae, labeled S1 through S5.
In infants and young children, these five sacral vertebrae exist as distinct, individual bones separated by cartilage. Fusion of these bones is a gradual process that begins later in life, typically around the onset of puberty in the mid-teens.
Full fusion of all five segments into the single, triangular sacrum bone is usually complete between the ages of 18 and 30. This consistent timeline is sometimes used in forensic science to estimate the age of skeletal remains. While five segments is the standard count, anatomical variations occasionally occur, resulting in a sacrum composed of four or six fused vertebrae.
Location and Unique Anatomy of the Sacrum
The sacrum is a large, triangular bone positioned at the posterior (back) of the pelvic cavity, wedged between the two hip bones. It forms the back wall of the pelvis and connects the upper spine to the lower body. Superiorly, the sacrum articulates with the last lumbar vertebra, L5, at the lumbosacral joint.
Its lateral sides, known as the alae (wings), connect with the ilium portions of the hip bones, creating the paired sacroiliac (SI) joints. Inferiorly, the lowest segment connects with the coccyx (tailbone). The bone exhibits a distinct concave curvature, bowing inward toward the front of the body.
The sacrum reveals a series of four openings on each side called sacral foramina. These passageways allow the sacral nerves and blood vessels to exit the central canal and travel toward the hips, legs, and pelvic organs. The shape and position of the sacrum are essential for forming a strong pelvic ring.
Primary Role in Weight Bearing and Stability
The primary function of the sacrum is to provide a stable foundation for the spine and transfer the weight of the upper body to the lower limbs. It acts as a biomechanical bridge, channeling forces from the head, torso, and arms through the pelvis and down to the legs during movement. This load transfer is important for humans due to our upright posture and bipedal gait.
The fused nature of the five sacral vertebrae allows the bone to perform this high-stress job. This solid structure prevents shearing forces that would occur if the segments remained separate and flexible under the upper body’s weight. Acting like the keystone in an architectural arch, the sacrum distributes weight evenly across the pelvis to maintain balance and posture.
The sacroiliac joints, where the sacrum meets the hip bones, are reinforced by a dense mesh of ligaments, providing stability. These joints allow for only small, limited movements, which helps absorb shock and reduce impact forces during activities like walking and running. The sacrum also serves as an anchoring point for several powerful muscles and ligaments, including the gluteus maximus, necessary for movement and maintaining spinal support.