The image evaluation was categorized into single-index level and overall lumbar curvature. Measurements were made with the built-in system in the Picture Archiving and Communication System (PACS, INFINITT PACS Ver. 3.011.4 BN2 64Bit). For single-index level assessment, we aimed at disc height, degree of spondylolisthesis, and spinal canal area. The index-level average disc height on the plain lumbar X-ray was measured. On the lumbar anteroposterior view, we measured bilateral disc height and middle disc height. On the lateral view, anterior, middle, and posterior disc heights were measured. A total of six measurements were taken, and the average was defined as the disc height. Plain films of dynamic flexion and extension were also made to detect the degree of spondylolisthesis. The area of the spinal canal was evaluated using lumbar MRI before and after the operation. For the index level, we selected the most stenotic slice from the T2 axial MRI series taken prior to the operation. The change in the area of the spinal canal was then measured based on the relative position of the T2 axial MRI series six and eighteen months after the operation. The lumbar lordotic angle and scoliotic angle were obtained through overall lumbar curvature evaluation. The overall lumbar curvature was defined as the angle between the T12 lower endplate and S1 upper endplate on the lumbar plain radiograph. A line was drawn over the T12 lower endplate and another over the S1 upper endplate and the angle between these two lines was measured. The lumbar lordotic angle was measured on the lateral film for evaluation of kyphosis and the lumbar scoliosis angle was measured on the anteroposterior plain film for evaluation of scoliosis.