We will first start with the preexisting disorders in your lumbar spine prior to your accident. Most likely, you had degenerative changes of your lumbar spine that were asymptomatic prior to your accident. You might not have had any symptoms after your motor vehicle accident if not for the presence of these degenerative changes.
Now to your current symptoms. You see to have two degenerative spondylolistheses at L4-5 and L5-S1 with foraminal stenosis at each level. In my opinion, this would call for a TLIF at both levels (a two level PLIF would suffice).
Anterior fusions (ALIFs) might work for one level but not two. In addition, ALIF surgeries have more potential complications, especially in males. Finally, the decompression of the nerve roots is indirect. This means that you open up the disc space and spread it which should give the nerve root more room. You cannot visualize this opening so it is called indirect.
TLIF or PLIF fusions do not disturb any important back muscles (the erector spinae muscles). These surgeries do move or remove the small segment to segment muscles (multifidi and rotators) but these muscles will be useless in the face of a fusion anyway. Why some individuals advertise preservation of these muscles when they will have no use or function after surgery is beyond me.
There is a chance that fusion can increase the stress of the level above but doing nothing can also cause increased stress. There is probably little motion of these two degenerative levels now which currently increases the stress at L3-4. Fusing these levels might increase the stress a little more but not significantly in my opinion compared to right now.
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.