These cannot be determined by plain films, as the x-ray passes through the soft tissue. A study by Giles et al., stated that sixteen of the thirty patients (53%) had retrolisthesis of L5 on S1 ranging from 2–9 mm; these patients had either intervertebral disc bulging or protrusion on CT examination ranging from 3–7 mm into the spinal canal. Fourteen patients (47%) without retrolisthesis (control group) did not show any retrolisthesis and the CT did not show any bulge/protrusion. On categorizing x-ray and CT pathology as being present or not, the well positioned i.e. true lateral plain x-ray film revealed a sensitivity and specificity of 100% ([95% Confidence Interval. = [89%–100%]) for bulge/protrusion in this preliminary study.” (7)Spinal cord compressions are also possible with patients experiencing pain, rigidity and neurologic signs that may follow some distance along nerves to cause symptoms at some distance from the location of the retrolisthesis. Diagnosis
Complete Retrolisthesis - The body of one vertebra is posterior to both the vertebral body of the segment of the spine above as well as below.Stairstepped Retrolisthesis - The body of one vertebra is posterior to the body of the spinal segment above, but is anterior to the one below.Partial Retrolisthesis - The body of one vertebra is posterior to the body of the spinal segment either above or below. (3)

Grading
Since the vertebral body in a retrolisthesis moves in a posterior direction, the grading used for spondylolistheses is of little use.
It is however useful to divide the anterior to posterior dimension of the intervertebral foramina (IVF) (4) into four equal units. A posterior displacement of up to ¼ of the IVF is graded as Grade 1, ¼ to ½ as Grade 2, ½ to ¾ as Grade 3, ¾ to total occlusion of the IVF as Grade 4. Alternatively, a measurement of the amount of displacement can also made by measuring the bone displacement in millimetres.Retrolistheses can be caused by injury and the resulting instability of the connecting soft tissues especially ligaments, discs, muscles, tendons and fascia. They may also involve muscles through spasm as a result of nerve malfunction due to pressure caused by the posterior displacement of the vertebra encroaching on the contents of the IVF. The IVFs contents include spinal (sensory and motor) nerves, arteries, veins and lymphatic vessels which cater to the nutritional and waste removal needs of the spinal cord.Degenerative spinal changes are often seen at the levels where a retrolisthesis is found. These changes are more pronounced as time progresses after injury, and are evidenced by end plate osteophytosis, disc damage, disc narrowing, desiccation and disc bulging. “A retrolisthesis hyperloads at least one disc and puts shearing forces on the anterior longitudinal ligament, the annular rings, nucleus pulposus, cartilage end plates and capsular ligaments. The bulging, twisting and straining tissues attached to the endplates pull, push and stretch it. It is worsened with time, becoming irreversible.”  This is the etiology of degenerative joint disease.