THE IDEAL AMOUNT OF LUMBAR FORAMINAL DISTRACTION FOR PEDICLE SCREW INSTRUMENTATION

Study Design. A cadaveric study was performed to analyze the changes o f foraminal dimensions of the lumbar spine and also to estimate the am ount of foraminal distraction, achieved with pedicle screw instrumenta tion, that will create the greatest improvement in foraminal dimension s. Objectives. To establish the ideal amount of lumbar foraminal distr action using pedicle screw instrumentation and to investigate the chan ges of adjacent segments and lumbar lordosis with distraction maneuver s. Summary of Background Data. In patients undergoing fusions and inst rumentation for foraminal stenosis, distraction of the lumbar spine ma y improve the dimensions of the intervertebral foramen. Results of pre vious studies have shown that anterior interbody distraction of the lu mbar 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 poste rior pedicle screw instrumentation. Methods, Six fresh frozen human ca daveric spines from T5 to the sacrum were used. The ISOLA instrumentat ion system (Acromed Inc., Cleveland, OH) was applied using pedicle scr ews 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 axia l load of 40 pounds was applied to the specimen. Radiographs were used to measure the posterior disc height, foraminal height, foraminal are a, 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. Result s. The posterior disc height, foraminal height, and foraminal area inc reased with distraction. The greatest incremental change in the forami nal area was noted at 6 mm of distraction. The L4-L5 foraminal area in creased 22.6% at 6 mm of L4-L5 distraction, and the L5-S1 foraminal ar ea increased 39.2% at 6 mm of L5-S1 distraction. With distraction abov e 6 mm, the foraminal change plateaued. The greatest decrease in lumba r lordosis was only 4.6 degrees and occurred at 10 mm of distraction. There was no significant effect of distraction on the foramina of adja cent levels. Conclusion. Posterior distraction using pedicle screws in creased foraminal dimensions. The ideal amount of distraction on the r od, or the minimal amount of distraction necessary for improving foram inal dimensions, may be 6 mm using pedicle screw instrumentation.