Can an mri of the sacrum show a non union after a 4 year injury?

CT or X-ray is bette: If there's a documented injury 4 yrs prior, then very often you will show non-union later on. The spine bones don't heal well unless they are immediately put close together (ie., had to have done 4 yrs ago) in order to get "union". While the MRI can detect it, it's not the best test. An X-ray or CT scan of just the sacrum would suffice. In any case, what to do depends on symptoms you have now.TTYD

Yes but expensive.-: -I have found CT scans give much better bone detail, but we usually start with plain imaging & proceed from there