111 patients with isthmic spondylolisthesis were randomized to receive either lumbar fusion with or without pedicle screw instrumentation or exercise therapy. The results from this study indicated that treatment with posterior lumbar fusion (PLF) showed greater disc height reduction, as well as greater percentage of disc degeneration when compared to exercise treatment. In addition, all patients in the PLF groups with sciatica had laminectomy and decompression. Those patients who received a laminectomy showed greater incidence of adjacent segment disc degeneration.