A common misconception for many people with low back issues is that their back is not the problem but that the pain comes from their “SI joint”. The SI joint is an acronym used to denote the sacroiliac joint. This joint lies below the low back where the sacrum (your butt bone) and ilium (where your hips are) come together and is shaped like a sideways “L”. There is one on either side of the bottom below the spine and they help to distribute forces form your trunk and your legs.
There is some degree of movement at this joint which causes the sacrum to “nutate” and “counternutate” with up to 8 degrees of rotation and up to 8mm of translation during movements of the legs and spine. When one part of this puzzle gets rotated too much, or does not move when it should, pain can occur.
It would appear that if someone had pain in this region, they would likely have some dysfunction at this joint. For this reason, we see this as a frequent attempted diagnosis. Dysfunction of the SI joint being the causative pain factor is actually a common misconception and has become more of a fad diagnosis than the reality of the situation. Low back pain also presents to this same region and true SI dysfunction is present in a relatively low number of cases in people with symptoms of pain around this region. In addition, most people actually begin to fuse (i.e. NO MOVEMENT) at about age 40, so to have the puzzle-piece mismatch causing pain is unlikely.
How do we know if it is the SI joint? Certain diagnostic tests will help to differentiate the SI joint from the low back. Being able to point to a specific quarter-sized location with tenderness to palpation over the SI ligaments are examples of tests that can lead to the diagnosis. People can also report a deep ache to the area while lying down at night or after or during certain movements. People with too much movement in their joints that have had a trauma and pregnant women (due to the effects of the joint-relaxing hormone "relaxin") are among those most likely to experience SI joint dysfunction. SI joint pain is generally treatable as well as low back pain, and a skilled physical therapist should be able to differentiate and treat your pain.