The body's shock absorber
The sacroiliac joint is one of the most important joints in the body, yet many people know nothing about it. The sacroiliac joint connects the spine to the pelvis. Without it, you wouldn't be able to stand or walk. As the name suggests, the sacroiliac joint sits between the sacrum and the iliac bones of the lower back, in the area where the spine meets the pelvis. Alterations in function of the sacroiliac joint are a common cause of lower back pain. Sacroiliac dysfunction, or SI dysfunction, is the term used to describe this condition.
Sacrum – Sacral spine anatomy
The sacrum is a large triangular-shaped bone found at the base of the spinal column. It consists of the last four or five vertebrae that by adulthood, fuse together to form a single bone. Located just above the coccyx and wedged between the right and left iliac bones (hip bones), the sacrum forms the back wall of the pelvis. The coccyx, commonly referred to as the tailbone, is the very bottom segment of the vertebral column. The right and left iliac bones are joined together in the front by the symphysis pubis (a small joint that connects the pubic bones).
The sacrum is slightly curved, giving the pelvic cavity more room to house organs and tissue. Interestingly, the sacrum is shorter and wider in females than in males. The term for such differences between males and females of the same species is sexual dimorphism. This is why men tend to have a narrow pelvic cavity, and women, a more hourglass shape for bearing children.
The top of the sacrum is connected to the lumbar vertebrae; the base is joined to the coccyx. Wings on either side of the bone are called the sacral ala. These wings fit—similar to interlocking pieces of a puzzle—between the two halves of the pelvis. It is this junction of the sacral alae and the iliac bones that forms the sacroiliac joint.
Commonly called the SI joint, the sacroiliac joint is really a pair of joints that function in concert with one another. The left and right sacroiliac joints support and transmit the weight of the body to the legs through the pelvis. They also form a strong base for the spine. The SI joint distributes the shock of motion across the pelvis, thereby reducing strain on the spine. It also stabilizes the pelvis, enabling you to maintain an upright position while walking or running.
If you look at your back, you'll notice two small dimples at the base of your spine, just above the buttocks. These dimples correspond with the location of the sacroiliac joint.