Anatomy Of The Sacroiliac (SI) Joints

In order to understand what’s causing your SI joint pain, it may help to become familiar with the parts of your anatomy that are contributing to the pain. Your SI joints connect the triangular bone at the base of your spine, or your sacrum, to the iliac bones of your pelvis. They absorb shock and provide stability for your body. Normally, your SI joints have very little movement. While limited to only a couple of millimeters, this movement may include rotation, sliding and stretching, or gapping of the joint. This movement allows the pelvis to twist during motions such as walking or climbing, and protects the spine from impact during movement such as jumping.

As with most other joints in the body, the SI joints have a cartilage layer covering the bone, some synovial fluid to lubricate the joint surface, and a capsule surrounding the joint to hold the fluid in place and increase joint stability. This combination is called a true synovial joint—just like a knee or hip—that allows for smooth movement and acts as a shock absorber between the bones. The SI joints have many nerve endings, which can lead to significant pain if degeneration occurs or inflammation affects their ability to move properly.

Common Causes of SI Joint Pain

Your SI joints absorb shock and provide stability for your body. As discussed above, they have very little movement and are surrounded by strong muscles and ligaments that assist in stabilizing the joints. But, like other joints in your body, they can become damaged by inflammation, wear and tear or a single incident. This can change their normal movement and create chronic and often debilitating low back pain.

SacroiliitisSacroiliitis is the term used to describe inflammation of the SI joint.

Osteoarthritis When the cartilage layer between the bones of the SI joint is damaged or worn away, the bones begin to rub on each other, and degenerative arthritis [osteoarthritis] occurs.

InjuryA fall, car accident or simple misstep can damage your SI joints. Awkward lifting or twisting, especially if these actions are repetitive, can also cause harm.

Altered Walking PatternAny condition that alters the body’s normal walking pattern (leg length discrepancy, scoliosis) can put increased stress on the SI joints. Pain while walking is a frequent sign that a patient is experiencing an altered walking pattern and may be at risk of developing SI joint dysfunction.

InfectionAlthough infection of the SI joint is uncommon, it is serious when present and can cause significant SI joint pain.

Prior Lumbar FusionFusion of the lumbar spine alters its natural movement and ability to absorb shock. Sometimes this “transfers” forces to the SI joints, resulting in excessive stress or degeneration.

PregnancyThe ligaments that stabilize the SI joints stretch to allow for delivery. If they remain loose post-pregnancy, it may cause painful movement of the SI joints.

While SI joint pain has several potential causes, getting to the bottom of the pain is important. If you suspect your pain is being caused by a problem with your SI joint, timely evaluation and treatment by a specialist can help.

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Common causes of SI joint pain