Lumbar Adjacent Spinal Stenosis

The goal of this clinical trial is to learn if there are differences in the effectiveness of decompression surgery alone versus decompression combined with extended fusion in patients with a previous lumbar spinal fusion who now present with symptoms of lumbar adjacent segment stenosis (LASS). The main questions it aims to answer are: * Does decompression alone differ from decompression combined with extended fusion with regard to clinical outcomes two years after surgery? * Are there any clinical or radiological characteristics that can predict better or worse outcomes two years after surgery? * Are there any differences between the treatment groups in terms of health economics? Researchers will compare decompression surgery alone versus decompression combined with extended fusion to see if there are any differences in treatment effect. Participants will: * Be randomized to one of the two surgical procedures * Visit the clinic four times postoperatively (at three months, 1, 2 and 5 years) for checkups and tests * Respond to questionnaires before surgery, and three months, 1, 2 and 5 years postoperatively