The sacroiliac (SI) joints are formed by the connection of the sacrum and the right and left iliac bones. The sacrum is the triangular-shaped bone in the lower portion of the spine, below the lumbar spine. While most of the bones (vertebrae) of the spine are mobile, the sacrum is made up of five vertebrae that are fused together and do not move. The iliac bones are the two large bones that make up the pelvis. As a result, the SI joints connect the spine to the pelvis.
The sacrum and the iliac bones (ileum) are held together by a collection of strong ligaments. There is relatively little motion at the SI joints. There are normally less than 4 degrees of rotation and 2 mm of translation at these joints. Most of the motion in the area of the pelvis occurs either at the hips or the lumbar spine. These joints do need to support the entire weight of the upper body when we are erect, which places a large amount of stress across them. This can lead to wearing of the cartilage of the SI joints and arthritis.
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