It sounds like you have lower back pain from a disorder lower in the spine (L4-S1 area) and a congenital fusion of L1-2. These may or may not be related.
The congenital fusion may have no bearing on the lower back pain. It really depends upon how this fusion aligns. If the relationship between L1 and L2 is “normal” (no significant kyphosis or angulation), then this would not affect the spinal relationships below. If these two vertebrae are significantly angulated or mal-aligned, this malalignment can throw off the relationship of the vertebra above and below and cause premature wear.
To discuss treatment, the disorder causing the pain must be identified so appropriate treatment can be instituted.
Dr. Corenman