Getting to Know Your SIJ

From the Healing Bridge Physical Therapy

Summer 2006 Newsletter

By Katie McLaughlin, MSPT


“But Doc, if it isn’t my disc, then what is causing this pain in my back?” You’ve had an x-ray, and the MRI showed nothing. Is the pain just all in your head? NO! There are many other structures that could be causing symptoms. A common culprit is in the pelvis, or Sacro-iliac Joint, otherwise known as “SIJ.”

The central bone in your pelvis, the SACRUM, is actually a continuation of the spine. It doesn’t look like the other vertebrae because it has fused together to create a single triangular shaped bone. The tip points down and continues as your tailbone. The sides are beveled and connected through ligaments to the part of your pelvis called the ILIUM, which is often the site of your problems or symptoms. Thus you have your SACRUM and your ILIUM as part of your pelvis, creating the Sacro-Iliac Joint: SIJ.

In most joints, you want lots of smooth movement but the SIJ doesn’t work like that. Instead, you want very little movement; there is little natural joint lubricant, and the SACRUM and ILIUM do not have slick surfaces. Instead, there is roughened bone on bone to create a high amount of friction and help increase stability. The architecture is really amazing given that all the weight bearing forces from your legs are transferred up to your trunk through these joints. All of the weight of your spine and trunk transfers down into the SACRUM which wedges between the two ilia snuggly creating the two SIJs.

So what’s the problem if this is such a stable joint? The problem is, if it moves a little bit incorrectly, it can cause a LOT of pain. That’s because the joint is covered by broad, flat ligaments, and anytime you stretch a ligament, you get a sprain (which can range from very mild to severe). The area is also home to the biggest nerves in the body, the sciatic nerves, so the area is rich in pain receptors. If the SIJ moves a little too far and sprains, then you get inflammation which can press on the nerves or chemically irritate them. Sciatica is a huge topic unto itself and will be discussed in a future issue.

The ilia themselves can move in several complex ways, when this movement gets out of balance, it will often result in a sprain. The SACRUM also has several axes of motion, making the combination of these two bones and, all of their significant muscular attachments, susceptible to movement imbalance and a likely source for pain.

For healthy, proper motion, there should be only a small amount of movement at the SIJ. For example, when you are walking and one of your feet goes behind you, there is forward rotation of the ILIUM. You can feel this if you put your hands on your hips with your fingers around the front and your thumbs in back. Let your thumbs slide down until you feel a bump at the back part of your pelvis. Walk around and feel how your bones and joints move and work together.

People with SIJ pain wonder “did the tight muscle pull the bones out of position? Or, did the bone being out of position create the muscle spasm?” This can usually be determined with appropriate assessment . So, if your doctor has diagnosed you with “back pain” of unknown origin, don’t despair! It may be the SIJ which can be mobilized back into the proper position after a skilled diagnosis.

Our PT can help identify which muscles are too tight, too loose or weak, and how your postural habits may be contributing to the problem. With knowledge there is hope. With education and changes in movement patterns, there is healing