Researcher(s) performing image assessments will be blinded to the position in which scans were made. Data to be recorded for each image is the Meyerding scale (1-4 scale rating the severity of the spondylolisthesis), amount of slip (mm), anterior and posterior disc space height (mm), foraminal area, anteroposterior (AP) distance at the most narrow part of the foramen, and disc space angulation. Area of the spinal canal and AP distance across the canal will be measured from axial images. The facet fluid sign, associated with spinal instability, will be assessed from axial MRI views using a 3-point scale with "none" indicating no evidence of fluid in the left or right facet joints; "possible" if there is some suggestion of fluid in the joints; or "definite" if there is at least a 2 mm wide layer of hyperintensity within either joint bilaterally at the level evaluated. Additionally, the width of the fluid fill of the facet joint taken perpendicular to the apparent joint line, and the largest value recorded as effusion size.