The sacrum is a large, triangular bone situated at the base of the spine, serving as a connection point between the vertebral column and the pelvis. It provides a stable foundation for the entire upper body and is fundamental to the mechanics of the lower back and hips. Although it appears as a single bone in adults, the sacrum is complex, transitioning from multiple independent bones in early life to a single, unified mass. This article clarifies the exact number of vertebrae that form the sacrum and explores the process of their fusion.
The Five Fused Vertebrae
The sacrum is composed of five individual bones, designated S1 through S5, which are known as the sacral vertebrae. This numerical answer can be confusing because in an adult, these five originally separate bones are permanently fused together to form a single structure. The process of this fusion, known as synostosis, is a gradual developmental change that begins early in life.
Fusion typically begins in adolescence, generally starting around the mid-teens. The process is usually complete sometime between the ages of 18 and 30, though the fusion of the first and second segments (S1 and S2) can sometimes continue into the mid-thirties. While the sacrum is classified as a single bone in adult anatomy, the remnants of the fusion lines are still visible, marking where the five individual vertebral segments once met.
Location and Structure of the Sacrum
The sacrum is situated directly below the last lumbar vertebra (L5) and above the coccyx, or tailbone. Its distinctive inverted triangular or wedge shape allows it to fit perfectly between the two large hip bones, the ilia, forming the back wall of the pelvis. This connection with the iliac bones creates the sacroiliac (SI) joints on either side of the sacrum.
The surface of the sacrum contains four pairs of small openings called sacral foramina, which are present on both the anterior and posterior sides. These foramina allow for the passage of the sacral nerves and blood vessels, which descend from the spinal cord to supply the lower limbs and pelvic organs. Along the posterior midline of the bone is the median sacral crest, a ridge formed by the fused spinous processes of the original five vertebrae.
Role in Spinal Stability and Movement
The primary function of the sacrum is to act as a weight-bearing foundation for the spine. It receives the entire weight of the upper body from the fifth lumbar vertebra and efficiently transfers that load downward to the pelvis and the lower limbs. This weight transfer is crucial for maintaining an upright posture and enabling bipedal movement.
The sacroiliac joints, formed by the sacrum’s articulation with the hip bones, provide significant stability to the pelvis. While they are not highly mobile, the subtle movements allowed at the SI joints are necessary for shock absorption during activities like walking and running. The sacrum also serves as the attachment point for numerous muscles and ligaments, including the gluteus maximus, which are integral to movement and overall spinal support.