Background: Degenerative lumbar spine disease can lead to lumbar spine instability. Lumbar spine instability is defined as an abnormal response to applied loads characterized kinematically by abnormal movement in the motion segment beyond normal constraints. Patients with lumbar spinal stenosis (LSS) typically present with low back pain (LBP), cramping, cauda equine syndrome, and signs of nerve root compression associated by weakness, numbness and tingling in their legs that are worsened with standing and walking. This degenerative condition severely restricts function, walking ability, and quality of life (QOL). Objectives: This study aims to compare clinical and radiological outcomes of posterolateral fusion (PLF) with posterior lumbar interbody fusion (PLIF) with posterior instrumentation in the treatment of LSS and degenerative instability. Study Design: A randomized, prospective, controlled clinical study. Methods: In this prospective study, 88 patients with LSS and degenerative instability were randomly allocated to one of 2 groups: PLF (Group I) or PLIF (Group II). Primary outcomes were the control of LBP and radicular pain, evaluated with visual analog scale (VAS), the improvement of QOL assessed by the Oswestry disability index (ODI) scale, and measurement of fusion rate, Cobb angle, spinal sagittal balance, and modic changes in the 2 groups. Results: At 24 months postoperatively, the mean reduction in VAS scores in Group I was more than in Group II (5.67 vs. 5.48, respectively) and the patients in Group I had more improvement in the ODI score than the patients in Group II (42.75 vs. 40.94, respectively). There was a statistically significant difference between the preoperative and postoperative sagittal balance in the 2 groups. The mean Cobb angle changed significantly in the 2 groups. Limitations: There are few prospective studies of PLIF or PLF in patients with LSS and degenerative lumbar spine instability, and a limited number of studies which exists have examined the safety and outcome of each procedure without comparing it with other fusion techniques. Because most of the studies in the literature have been conducted in the patients with IS, we could not compare and contrast our findings with studies in patients with LSS and degenerative lumbar spine instability. In addition, although in our study the findings at a 24-month follow-up period showed that PLF was better than PLIF in these patients, there were some studies in which the authors reported that PLIF showed better clinical results than PLF at a 48-month followup period. So we suggest that rigorous controlled trials at longer follow-up periods should be undertaken in groups of patients with LSS and degenerative lumbar spine instability who undergo posterior decompression and instrumented fusion to help to determine the ultimate best fusion technique for these patients. Conclusion: PLF with posterior instrumentation provides better clinical outcomes and improvement in the LBP, radicular pain, and functional QOL, more correction of the Cobb angle, more restoration of sagittal alignment, more decrease in Modic type 1, and more increase in Modic type 0, despite the low fusion rate compared to PLIF. Key words: Lumbar spinal stenosis, degenerative instability, posterolateral fusion, posterior lumbar interbody fusion, low back pain, quality of life, cobb angle, fusion rate, mobic changes, sagittal balance

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