Sacro-Iliac (SI) Joint Arthritis
The key bones of the pelvic ring that support the spine are the two wings of the ilium and in the center, the sacrum. (The bones in the front are called the pubic bones.) The weight of the upper body is transmitted through the spine to the sacrum. There are very stong ligaments between the sacrum and ilium, and the joint there moves very little. but sometimes that joint can develop arthritis and be a cause of pain.
The sacrum and ilium are illustrated below, with the SI joints marked in red.
In this view from behind, the strong ligaments that hold the ilium to the sacrum are shown.
An example of an arthritic SI joint is shown below. The study that shows this condition is a CT scan, which takes pictures like slices through the part of which the image is taken, like slicing a salami. In the view of the pelvis below, the yellow line indicates from where in the pelvis the cross sectional view is taken.
The cross sectional view shows bone touching bone on the involved side.