By your report, you have an isthmic spondylolisthesis at L5-S1 (“if I’m still in pain after the surgery that he can bring me back in to do a decompression of the pars defect and also add screws and rods for more support”).
I generally am not a fan of the anterior approach for lumbar spine disorders (ALIF). This is due to two problems associated with the anterior approach surgery and a separate problem with your specific disorder.
In any anterior approach to the lumbar spine, muscles in the belly are cut and this can lead to a muscular hernia in the belly wall. In addition, moving the intestines (which has to be done to get to the anterior spine) can occasionally lead to adhesions and sequella.
The second problem is found with males. There is a 4% chance of retrograde ejaculation. This is due to injury to the sympathetic nerves which descend immediately to the side of the lumbar vertebra. Retrograde ejaculation means that semen will eject into the bladder and not out the end of the penis.
The third problem with an ALIF for an isthmic spondylolisthesis is that there already is a defect in the posterior elements. The ALIF does provide some stability with the screw in cage, but without the stability of the posterior elements (due to the pars fractures-see website), this construct is not as stable as with a posterior approach.
If he suspects that there is a chance of continued pain and that he will need to go posteriorly in a second surgery to stabilize the surgery, why not do this entire process posteriorly at one sitting? A TLIF will produce an anterior fusion like your surgeon proposes along with the ability to decompress the nerves in the back of the spine and put in posterior instrumentation which will make the surgery very stable from the beginning.
I don’t want to full dissuade you as there is a reasonable chance that an ALIF will work well but the chances of success are less than with a TLIF.
I assume that if the surgeon noted that he might need to “roll someone over during the same surgery and putting in the back up screws and rods during the same surgery” is that if he performs the anterior approach for an ALIF and finds greater instability than he assumes is present, he will need to perform a poster instrumentation and fusion at the same setting.