Spinal fusion has become a very popular way to treat spinal instability. Bone graft material and/or rods and screws are used to hold two vertebral bones together. The fusion keeps them from moving and stops the pain and disability.
The problem with this treatment is that the segments above and below the fusion are subjected to increased loads and stresses. Then they start to degenerate and become unstable.
Even if you have the L45 level fused, there is still motion where the last vertebra (L5) meets the sacrum (S1) and at the level above (L34). A study was done at the Oregon Health & Science University in Portland, Oregon to compare an L45 fusion with an L4S1 fusion.
They found overall spinal stiffness increased more for the L4S1 fusion compared to the L45 fusion. It could be that extending the fusion down to S1 is the answer. This was the first study done using cadaver spines. More studies will be needed before fusion extension is done routinely. Still, it's worth asking your doctor this question. Perhaps it will be a good option for you.