sacroiliac (SI) joint!

The sacroiliac (SI) joints, the points where the pelvis and sacrum meet, can be the source of great discomfort when out of balance or inflamed. Although pain and stiffness can be caused by a number of reasons, when a student tells you they have diagnosed SI pain, take heed.  Many poses, including backward and forward bends and twists, can make matters worse.

Sacroiliac Joint Anatomy

A joint is where two bones come together. The sacroiliac joint is where the sacrum bone and the ilium bone join one another.

The sacrum is located at the base of your spine. It is composed of five vertebrae that have fused together during development to form a single bone roughly the size of your hand. When you view the sacrum from the front, it looks like a triangle with its point facing down. When you view it from the side, you see that it curves, concave in front, convex behind, and that it tilts, so its top end is well forward of its bottom end. Protruding from the bottom end of the sacrum is the tailbone (coccyx).

Each half of the pelvis is composed of three bones, the ilium, the ischium and the pubic bone, that have fused together during development. The topmost bone (the one that forms the pelvic rim) is the ilium. The sacrum is wedged between the left and right ilium bones. On the upper part of the sacrum, on each side, there is a rough, rather flat surface that abuts a corresponding rough, flat surface on the ilium. These surfaces are called auricular surfaces. The places where the auricular surfaces of the sacrum and ilium come together are the sacroiliac joints.

The sacrum bears the weight of the spine. The SI joints distribute this weight so that half goes to each hip and, from there, to each leg. As gravity wedges the triangular sacrum firmly down between the inclined auricular surfaces of the ilium bones, it tends to force the ilium bones apart, but strong ligaments prevent them from moving. This wedging action and the resistance of the ligaments combine to form a stable joint.

Some of the ligaments that stabilize the SI joints cross directly over the line where the sacrum and ilium meet. Those on the front are called the ventral sacroiliac ligaments, and those on the back are the dorsal sacroiliac ligaments. Other strong ligaments (the interosseous ligaments) fill the space just above the SI joints, holding the ilium bones firmly against the sides of the upper sacrum. The normal, tilted position of the sacrum places its top end forward of the SI joints and its bottom end behind them. This setup means the weight of the spine tends to rotate the sacrum around the axis formed by the SI joints, pushing the top end down and lifting the bottom end up. The sacrotuberous and sacrospinous ligaments are ideally located to oppose this rotation by anchoring the lower end of the sacrum to the lower part of the pelvis (the ischium bones).

The auricular surfaces of the sacrum and ilium are lined by cartilage. The joint space is completely surrounded by connective tissue and is filled with a lubricating fluid called synovial fluid. Like other synovial joints, the SI joints can move; however, their range of motion is very limited. For example, trained chiropractors, physical therapists and other professionals learn to feel the PSIS tilting back slightly relative to the sacrum when a standing person lifts one knee toward the chest as if marching. This rocking action is thought to aid in walking. However, according to one anatomy text,

The sacroiliac synovial joint rather regularly shows pathologic changes in adults, and in many males more than 30 years of age, and in most males after the age of 50, the joint becomes ankylosed (fused, with the disappearance of the joint cavity); this occurs less frequently in females.¹

In other words, with age, the sacrum and the two ilium bones often merge into a single bone. This might explain why some orthopedic surgeons do not believe in SI joint injury. Perhaps they have operated on adults, seen with their own eyes that the sacrum is completely fused to the two ilium bones, and concluded that even the slightest dislocation of this joint is impossible. This may well be true in people whose joints have fused, but that leaves out the rest of us, more women than men, who, through heredity or lifestyle (including yoga), have retained mobility in our SI joints.