Sacroiliac Joint Dysfunction or SI joint pain, is discomfort in the joint between the sacrum and the right and left iliac bones. Imagine for a moment that you have a ball in a string. The string is your vertebral column, your spine, and the sacrum is a triangular shaped bone at the very end. While most of the bones on this string are mobile the sacrum is actually made of five different vertebra that are fused together and do not move. The iliac bones are two large bones at the back of the pelvis that complete the circle which makes up the pelvic girdle.
Going back to that string and ball for a minute, the string is attached to the ball where the iliac bones and the sacrum meet. There is very little motion at the joint, but the bones are not fused. Normally there is less than 4° of rotation at the SI joints because most of the motion happens either at the hips or in the lumbar spine.
The job of the Sacroiliac joint is to support the entire weight of the upper body when we are standing or erect. This places a very large amount of stress on that particular joint which can lead to wearing of the cartilage and arthritis. (1)
Most people think of arthritis as a problem that occurs in a highly mobile joint, such as the hip, knee, ankle and hands or wrist. But truth be told arthritis can develop in any joint that has cartilage and any degree of motion or movement. In the past scientists have discovered that the spaces between the bones of the head, which were formally thought to have fused into place, are actually somewhat mobile. However, because they are not true joints and have no cartilage and no significant motion they also do not develop arthritis.
References:
(1) The Journal of Manual & Manipulative Therapy: Evidence-Based Diagnosis and Treatment of the Painful Sacroiliac Joint
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2582420/
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