The Lumbar Adjacent Segment Stenosis Trial

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

Inclusion Criteria: Participants must meet all the following criteria to be eligible for the study: 1. Clinical symptoms of spinal stenosis, defined as neurogenic claudication or radiating pain into the lower limbs with duration \>6 months 2. MRI-confirmed adjacent segment stenosis, proximal to a previously fused lumbar segment (fusion of maximum two levels between L1 and S1, any type of previous lumbar fixation procedure). 3. CT-verified solid fusion 4. Minimum time since previous fusion: one year 5. Eligible for both treatment alternatives: decompression with extended fusion and decompression alone. 6. Age between 18 and 80 years. 7. Insufficient improvement after at least 3 months of non-operative treatment. 8. Understand Norwegian language, spoken and written. Exclusion Criteria: Participants will be excluded if any of the following criteria are met: 1. Previous surgery at the actual adjacent stenotic level. 2. More than two fused lumbar levels. 3. Non-union (pseudoarthrosis) at the previously surgically fused level, radiologically verified. 4. Foraminal stenosis grade 3 at the adjacent level. 5. Spinal fracture. 6. Presence of cauda equina syndrome. 7. Complete motor deficit. 8. ASA physical status classification IV or V. 9. Alcohol or drug abuse. 10. Active cancer 11. Contradictions to MRI (e.g., Cardiac pacemaker electrodes, metal implants in the eye or brain, claustrophobia) 12. Disabling chronic neurological disease (e.g., Parkinson's disease, ALS, MS) 13. Ongoing serious psychiatric disease\& 14. Any condition that in the view of the investigator would suggest that the patient is unable to comply with the study protocol and procedures 15. Declining specific treatment arm 16. Are participating in another clinical trial that may interfere with this trial