Lumbar lateral radiograph showing low midline sill. Lumbar lateral radiograph of a 47-year-old female with spondylolytic spondylolisthesis (A). Explanation of the radiograph. The box indicates grade I spondylolytic spondylolisthesis at the L5-S1 level. A sill is shown between the L4-5 interspinous space when drawing a line connecting the spinous processes of the lumbar spine (B).