The sacrum and coccyx are often mentioned together, but they are two separate bones located at the base of the human spine. They form the lowest segment of the vertebral column, nestled within the pelvic girdle. This region connects the spine to the pelvis and the lower limbs. Understanding the distinction between the two is important for identifying sources of pain or instability in the lower back.
How the Sacrum and Coccyx Differ
The primary contrast between the sacrum and coccyx is their size and composition. The sacrum is a large, triangular bone situated beneath the last lumbar vertebra, forming the posterior wall of the pelvis. This bone is made up of five individual vertebrae (S1 through S5) that fuse together over time, typically completing the process by age 30.
The coccyx, commonly known as the tailbone, is a much smaller bone located at the tip of the spinal column, articulating with the bottom of the sacrum. It is composed of three to five small, rudimentary vertebrae that are often partially or fully fused. Unlike the sacrum, the coccyx is considered a vestigial structure, a remnant of a tail found in ancestral species.
Distinct Functional Roles in the Body
The sacrum’s function centers on providing a stable platform for the upper body and transferring weight. Its triangular shape helps distribute the weight of the torso down to the hip bones and into the legs. The sacrum connects to the hip bones (ilia) on either side, forming the sacroiliac (SI) joints. These joints are reinforced by ligaments to provide stability to the lower spine and pelvis.
The coccyx plays a less significant role in weight bearing, but it serves as an attachment point for various muscles, tendons, and ligaments of the pelvic floor. This network of soft tissues supports the organs of the pelvis and assists with functions like defecation. The coccyx also provides minor support when a person is sitting and leaning backward.
Common Conditions Affecting the Lower Spine
Because of their close proximity, both the sacrum and coccyx can be sources of pain. Sacral pain is often associated with the sacroiliac (SI) joints, where the sacrum meets the pelvis. Inflammation or dysfunction in the SI joint, known as sacroiliitis, can cause pain in the lower back, buttocks, and sometimes radiating down the leg. This condition can be exacerbated by trauma, arthritis, or changes during pregnancy.
Pain localized to the tailbone is called coccydynia, which is five times more prevalent in women than in men. The most common cause is acute trauma, such as a direct fall onto the buttocks, which can bruise, fracture, or dislocate the bone. Coccydynia can also result from internal trauma, such as during childbirth, or from repetitive strain caused by prolonged sitting. The pain is typically aggravated by sitting and can impact daily activities.