The ALIF generally depends upon the intact side and back walls of the annulus as well as the posterior elements for stability until the fusion “sets up”. With an isthmic spondylolisthesis, the posterior elements are “disconnected” so the stability of the fusion is not as strong as with an ALIF with intact posterior elements. This is why a posterior construct is stronger as pedicle screws take the place of the disconnected posterior elements.
If you have no leg pain, the need for decompression is less imperative. Nonetheless, I have seen patients with an ALIF and no previous leg pain develop leg pain after an ALIF surgery. You do have to remember that the patients who seek me out with a failure of a previous surgery seek me out due to that failure. This means that the patients with success obviously do not come into my office.