Pain comes from many sources... in people with (or without) fused SI joints, pain felt in that region could be coming from the SIJs or could be referred from other areas, such as the lumbar spine.
Unfortunately as we see from the responses here, the answer to the question, to quote a wise consultant, is, "it depends."
Jen, 42, happy partner of James and Moma to Evan, 14, & Lucy, 12.5 (Crohn's dx @ age 3; on Remicade since April 2010.) I take piroxicam, Flexeril, & Nucynta ER nightly. 3 anti-TNFs didn't pan out for me.
"Science is the father of knowledge, but opinion breeds ignorance." -- Hippocrates