Article Degenerative spondylolisthesis Discectomy Endoscopic decompression Endoscopic spine surgery Lumbar spine fusion Oswestry Disability Index Spinal stenosis aged anterior spine fusion clinical article conservative treatment controlled study endoscopic surgery female follow up general condition improvement human lumbar spinal stenosis male posterior lumbar interbody fusion preoperative evaluation range of motion spinal cord atrophy spinal cord decompression spondylolisthesis spondylosis surgical technique treatment failure visual analog scale