Vertical instability in spondylolisthesis: a traction radiographic assessment technique and the principle of management.

STUDY
DESIGN: Lateral radiographs of the lumbar spine were taken of 40 patients with lumbar spondylolisthesis. These radiographs were taken in the neutral, flexion, and extension positions for both erect and recumbent postures, and also in the prone and supine positions with traction applied via a traction table.
OBJECTIVES: To define and demonstrate the presence of "vertical instability" in spondylolisthesis, and to determine the most useful radiographic views for clinical purposes and analysis of the surgical principle. SUMMARY OF BACKGROUND DATA: Lateral radiographs of patients in flexion and extension are widely used to obtain quantitative and qualitative data on lumbar spondylolisthesis. Changes in lumbar disc height and segmental translation in a group of patients with spondylolisthesis have been demonstrated with the addition of traction and compression.
METHODS: Lateral and flexion extension radiographs of the lumbosacral spine in 37 patients with spondylolisthesis taken in standing and recumbent positions and under pelvic traction in the prone or supine positions were suitable for analysis. The changes in disc area, intervertebral kyphotic slip angle, and amount of anteroposterior shift (olisthesis) were measured from the radiographs using a computer digitizer. The disc area was normalized against the area of the superior vertebra, and the amount of anteroposterior shift was normalized against the anteroposterior width of the superior vertebra. Inter- and intraobserver error was found to be negligible, and results were analyzed by paired t test.
RESULTS: Maximum slip angle, maximum olisthesis, and minimum normalized disc area were found with the subject under erect flexion. Conversely, prone traction and recumbent extension produced minimum slip angle, whereas the lowest anteroposterior shifts were seen with the subject under prone and supine traction. Prone traction also resulted in a significantly larger normalized disc area than any other posture. The change in kyphotic slip angle between erect flexion and prone traction is correlated with the change in normalized olisthesis and disc area.
CONCLUSIONS: Erect flexion and prone traction radiographs represent the extremes of subluxation and reduction of the olisthesis, respectively, and the change in olisthesis seen between these extremes is correlated with the change in disc area and the intervertebral slip angle. Vertical laxity of the affected functional spinal unit resulting from disc degeneration produces laxity in the ligaments and disc anulus, allowing olisthetic motion. Restoration of disc height in turn restores tension to the soft tissues around the disc and results in a spontaneous reduction of the subluxation. Restoration and maintenance of disc height with a spacer or interbody fusion therefore is recommended as a goal in the treatment of spondylolisthesis. When spondylolytic spondylolisthesis involves a posterior column deficiency, additional reconstruction of this column with posterior instrumentation is recommended. Application of the traction radiographic technique in planning for spondylolisthesis reduction is discussed along with the technique of stabilization.