The burning sensation in your lower back is probably from the multilevel degenerative changes your MRI notes. I will assume that you could live with this long standing pain and the new left buttocks and leg pain is the most concerning issue,
The L5-S1 disc is probably responsible for your lower back pain. This disorder is called isolated disc resorption (IDR) and you can read about it under “causes of lower back pain” on the website.
The pars fracture of L4 is most likely new and the reactive bone changes are the results of attempt to repair (which will not occur unfortunately) as well as reaction to the abnormal motion of this level.
A selective nerve root block with a pain diary (see website) will yield the diagnostic information necessary to determine if that is the correct diagnosis.
Donald Corenman, MD, DC is a highly-regarded spine surgeon, considered an expert in the area of neck and back pain. Trained as both a Medical Doctor and Doctor of Chiropractic, Dr. Corenman earned academic appointments as Clinical Assistant Professor and Assistant Professor of Orthopaedic Surgery at the University of Colorado Health Sciences Center, and his research on spine surgery and rehabilitation has resulted in the publication of multiple peer-reviewed articles and two books.