The ideal amount of lumbar foraminal distraction for pedicle screw instrumentation.

STUDY DESIGN: A cadaveric study was performed to analyze the changes of foraminal dimensions of the lumbar spine and also to estimate the amount of foraminal distraction, achieved with pedicle screw instrumentation, that will create the greatest improvement in foraminal dimensions.

OBJECTIVES: To establish the ideal amount of lumbar foraminal distraction using pedicle screw instrumentation and to investigate the changes of adjacent segments and lumbar lordosis with distraction maneuvers.

SUMMARY OF BACKGROUND DATA: In patients undergoing fusions and instrumentation for foraminal stenosis, distraction of the lumbar spine may improve the dimensions of the intervertebral foramen. Results of previous studies have shown that anterior interbody distraction of the lumbar spine significantly increased the space in the lumbar foramen. No previous study has assessed the ideal amount of distraction, changes in the adjacent segments, or the effect on lumbar lordosis using posterior pedicle screw instrumentation.

METHODS: Six fresh frozen human cadaveric spines from 15 to the sacrum were used. The ISOLA instrumentation system (Acromed Inc., Cleveland, OH) was applied using pedicle screws at L4, L5, and S1, L4-L5 and L5-S1 rods were contoured to the same degree of lumbar lordosis as in the specimen. The spine was potted in the upright position with the sacrum secured in the mold, and an axial load of 40 pounds was applied to the specimen. Radiographs were used to measure the posterior disc height, foraminal height, foraminal area, and lumbar lordosis. Two mm, 4 mm, 6 mm, 8 mm, and 10 mm of single level distraction were added to L4-L5 and L5-S1. The dimensions of the specimen were digitized and calculated after each distraction.

RESULTS: The posterior disc height, foraminal height and foraminal area increased with distraction. The greatest incremental change in the foraminal area was noted at 6 mm of distraction. The L4-L5 foraminal area increased 22.6% at 6 mm of L4-L5 distraction, and the L5-S1 foraminal area increased 39.2% at 6 mm of L5-S1 distraction. With distraction above 6 mm, the foraminal change plateaued. The greatest decrease in lumbar lordosis was only 4.6 degrees and occurred at 10 mm of distraction. There was no significant effect of distraction on the foramina of adjacent levels.

CONCLUSION: Posterior distraction using pedicle screws increased foraminal dimensions. The ideal amount of distraction on the rod, or the minimal amount of distraction necessary for improving foraminal dimensions, may be 6 mm using pedicle screw instrumentation.