DadCue, I'm not sure any of us are qualified to interpret MRI results, which is probably why you haven't had much response. I did google "ligamentum flavum hypertrophy" and found the following:
"Ligamentum flavum hypertrophy means that the ligamenta flava (the plural of ligamentum flavum) is inflamed due to stress on the spine. This generally occurs in people over the age of 50 due to the natural degeneration process of the spine."
It's obvious that this rheumy is not the one who originally diagnosed you. How long have you been seeing her? I would ask her (nicely) about the reactive arthritis diagnosis and why she has moved away from that diagnosis.
You might consider getting a second opinion from another rheumy since your current rheumy's diagnosis differs from the original.
On your MRI question, there are several types of MRI's: T1, T2, STIR, with and without contrast. Again, I don't have enough knowledge to talk intelligently about them and when a certain type is appropriate. SAA sponsored a webinar on using MRI to diagnose spondyloarthritis. I need to go back and watch it again.https://www.youtube.com/watch?v=Tucj4XpYR0o