Background Although various risk factors have been reported for adjacent segment degeneration after lumbar fusion, the exact mechanisms and risk factors related to adjacent segment degeneration have not been clear. The present study was conducted to evaluate the risk factors for radiological adjacent segment degeneration in patients surgically treated for single-level L4 spondylolisthesis focusing on a single pathology, a specific fusion level, at a set interval. Methods We assessed preoperative and five-year postoperative radiographs for 72 patients who underwent L4-5 anterior or posterior lumbar interbody fusion for single-level L4 degenerative spondylolisthesis. Adjacent segment degeneration was defined as imaging evidence of one or more of the following conditions at L1-2, L2-3, or L3-4: 1) a loss of more than 20% of the preoperative disc height, 2) anterolisthesis or retrolisthesis greater than 3 mm, 3) or osteophyte formation greater than 3 mm. Results We found adjacent segment degeneration in 21 patients, with 31 discs affected. Multiple logistic regression analysis identified the following significant independent risk factors for adjacent segment degeneration: female gender (odds ratio 10.80; 95% confidence interval 1.20–96.89), posterior lumbar interbody fusion (odds ratio 7.70; 95% confidence interval 1.82–32.66), and pre-existing disc degeneration (odds ratio 12.29; 95% confidence interval 1.69–89.27). Conclusions Female gender, posterior lumbar interbody fusion, and pre-existing disc degeneration were significant independent risk factors for radiologically diagnosed adjacent segment degeneration in patients treated for L4 degenerative spondylolisthesis by interbody lumbar fusion.
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