The sacrum and the tailbone, or coccyx, are not the same bone, though they are closely related components at the base of the vertebral column. Both bones are located in the pelvic region and are formed by the fusion of smaller vertebrae. The sacrum sits just below the lumbar spine, and the coccyx is attached directly to its lower tip. Understanding the structure and function of each bone reveals their distinct roles in supporting the body and stabilizing the pelvis.
The Sacrum An Overview of Structure and Stability
The sacrum is a large, triangular bone situated at the base of the spine, directly beneath the fifth lumbar vertebra (L5). It is formed by the complete fusion of five separate sacral vertebrae (S1 through S5), a process that typically finishes by the time a person reaches 30 years of age. This fusion creates a strong, single structure with an inverted wedge shape.
Its function is to act as the keystone of the pelvic girdle, connecting the spine to the hip bones (ilium) at the sacroiliac joints. The sacrum carries and distributes the weight of the upper body to the lower limbs, making it a load-bearing foundation. Its curvature helps maintain the natural alignment of the spine and aids in shock absorption during movement.
The Coccyx The True Tailbone and Its Vestigial Nature
The bone commonly referred to as the tailbone is the coccyx, which is the final segment of the vertebral column. It is a small, curved bone composed of three to five tiny, partially fused coccygeal vertebrae. The coccyx is attached to the inferior apex of the sacrum via the sacrococcygeal joint.
The popular name “tailbone” stems from its evolutionary history, as the coccyx is considered a vestigial structure, a remnant of the tail present in the ancestors of humans and other great apes. Despite this, the coccyx retains a modern function by providing anchor points for several muscles and ligaments, including parts of the gluteus maximus and the pelvic floor muscles.
The Functional Difference Between the Sacrum and Coccyx
The difference in function reflects the disparity in size and structural stability between the two bones. The sacrum is large and rigid, bearing the majority of the body’s weight and providing a stable foundation for standing and walking. Its connections form the strong sacroiliac joints, which are generally stable but can be a source of back pain.
In contrast, the coccyx is a smaller, non-weight-bearing anchor point. It contributes to balance and stability only when a person is sitting, distributing weight along with the ischial tuberosities (the sit bones). The joint where the two bones meet, the sacrococcygeal joint, is a slightly movable, fibrocartilaginous joint.
This slight mobility is relevant, particularly in females, where the coccyx can move slightly backward during childbirth to increase the diameter of the pelvic outlet. Pain localized to this area, known as coccydynia, is distinct from pain originating higher up in the sacrum or the sacroiliac joints.