I've had chronic sacroiliac pain since the birth of my first child. X-rays didn't show anything but a CT scan shows significant degenerative arthritis in that area. What is it that shows up on a CT scan in this area that you can't see on an X-ray?
It's true that abnormalities of the sacroiliac joint (SIJ) don't show up readily on radiographs. X-rays will show obvious signs of fracture and possibly bone tumors.

But computed tomography (CT) scans are the best way to detect early changes in the SIJ. There are several things the radiologist looks for on a CT scan to indicate SIJ degeneration. These include joint space narrowing, sclerosing (hardening) of the ligaments, and bone spurs or uneven joint surfaces.

Studies of normal adults without symptoms of SIJ problems often have signs of degeneration early in life. It appears that a great deal of force or load is transferred from the spine to the SIJ. The effect this has on people starts in their 20s and progresses through their 50s and 60s.