I've been told in the past that I was probably experiencing pain at L5/S1, but Dr. Bierstedt said that is highly unlikely since this area is fused naturally. Even when the disc is thin, as it is in my case, since there is no movement then there should be no pain.

Because there's no movement at L5 my vertebrae above have overcompensated over the years, which has caused the disc at L4/L5 to get overworked (and is dried out). This I already knew, but I just assumed that pain was also being generated at L5/S1. A surgeon in Los Angeles said I needed to get fused at L5/S1. Dr. B basically laughed at that since I'm already fused. Even if the disc completely collapsed it wouldn't matter. He and Dr. Clavel both agreed to just leave it alone.

A sacralization basically means that your sacrum really begins at L5 and your lumbar spine begins at L4. Something like 10% of the population have some form of sacralization, but most people never even know it.