Just thought I'd throw this out there. I didn't search this site, but when I tried to google "fused SI joint" images to compare mine to, I did not have much success.
So here are mine (you'll also note the twin HSR's). The x-ray report called the SI Joints indistinct, consistent with Psoriatic Arthritis, my (incorrect) diagnosis at the time. My rheumatologist simplified it for me by saying, "see those two bright white lines? They should be black. These joints have completely fused."
Anyway that's what I was told, I am not a medical professional. Again, throwing this out there for comments, comparison by others trying to figure out their own x-rays.