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One hundred, twenty-four patients undergoing lumbar or lumbosacral for degenerative conditions were entered into a prospective study. The patients were randomly assigned to one of three treatment groups. Group I underwent posterolateral fusion using autogenous bone graft. Group II had autogenous posterolateral fusions supplemented by a semi-rigid pedicle screw/plate fixation system (Luque II; Danek Medical, Memphis, Tannessee). Group III patients underwant posterolateral autogenous fusion with a rigid pedicle screw/rod fixation system (Texas Scottish Rite Hospital [TSRH]-Danek Medical, Memphis, Tennessee). All the patients were operated on by the same surgeon, Identical bone grafting technique was used in all, and all were treated in an identical fashion postoparatively. Fusion status was determined from the anteroposterior, oblique, and flexion-extension radiographs obtained at 1 year. Clinical results were rated as excellent if the patients were pain-free and had returned to work; good if the patients had mild backache requiring non-narcotic analgesics and had returned to work; fair if continuing back pain prevented a return to work; or poor if the pain was worse than that which the patient experienced preoperatively or the patient required revision surgery.Nine patients who were originally assigned to Group II or Group III were placed in Group I intraoperatively. This change was due to the identification of severe osteopenia and the determination that pedicle screw purchase was poor. One patient was lost to follow-up. Thus, 51 patients were in Group I, 35 in Group II, and 37 in Group III. Follow-up ranged from 9 to 28 months, averaging 16 months. The overall fusion rate for Group I was 65%; for Group II, 77%; and for Group III, 95%. The increased fusion rate in the rigidly instrumented pedicle screw group (Group III) was significant when compared to that of Group I (P = 0.002) or to Group II (P = 0.034). Clinical results in Group I included 71% good or excellent results, with 8% requiring revision surgery. In Group II, 89% had good or excellent results, with two additional surgeries. Group III had 95% good or excellent results, with no additional surgeries. Group III had 95% good or excellent results, with no additional surgeries. No infections or neurologic deficits occurred. No screws broke, although two Luque screws backed out, and one TSRH linkage loosened. Three screws were misplaced laterally to the pedicle (1.1% of screws) without negative sequelae. In all three groups, the fusion rates were lower for smokers than nonsmokers, and lower for revision surgery thanprimary surgery. The lowest fusion rates were seen in revision surgery without instrumentation (48%), degenerative spondylalisthesis without instrumentation (45%). Pseudarthrosis repair was successful in 20% of patients in Group I, and 100% of patients in Groups II and III. Rigid pedicle screw/rod fixation led to a significantly higher percentage of fusions in degenerative lumbar disease than did fusion without instrumentation. The fusion rate was also higher with rigid instrumentation than with semirigid plate/screw flaxation, although clinical results were improved with both fixation systems. I recommend the use of a rigid pedicle screw instrumentation system in patients undergoing fusion for degenerative disc disease or degenerative spondylolisthesis, and in revision surgery.