It’s not clear yet just what the next level degenerative changes mean when seen on X-rays. Some studies suggest reoperation is needed to fuse and stabilize the next level. Others suggest that increased compressive pressures are defused by changes in lumbar and sacral angulation and pelvic motion.
All these changes appear to protect the lumbar spinal segments from repetitive loading. The fusion does result in a shift in load upward to the segment above the fusion. But whether the changes seen on X-ray are from these increased compressive loads or from natural degenerative changes still remains a mystery.
The decision to have further surgery should be based on several factors. Symptoms and function are the first key factors. These are even more important than what shows up on the X-rays. Your own level of satisfaction must also be considered.
Patients with a fusion at L45 do tend to have more adjacent segmental disease compared to those with an L5S1 fusion. And the degenerative changes do seem to travel upwards (toward the head) to the next level.
The surgeon can also measure the angle of the sacrum in relation to the lumbar vertebra to help predict future problems. A more horizontal orientation of the sacrum seems to result in fewer problems. This information may help in the decision-making process.