• Masashi Tsujino, Akira Matsumura, Shoichiro Ohyama, Minori Kato, Takashi Namikawa, Yusuke Hori, Masaki Kawamura, and Hiroaki Nakamura.
• Scoliosis Center, Osaka City General Hospital, Osaka, Japan.
• Eur Spine J. 2025 Jan 24.

PurposeThis study aimed to compare the incidence of radiological adjacent segment disease (R-ASD) at L3/4 between patients with L4/5 degenerative spondylolisthesis (DS) who underwent L4/5 posterior lumbar interbody fusion (PLIF) and those who underwent microscopic bilateral decompression via a unilateral approach (MBDU) at L4/5. Our ultimate goal was to distinguish the course of natural lumbar degeneration from fusion-related degeneration while eliminating L4/5 decompression as a confounder.MethodsNinety patients with L4/5 DS who underwent L4/5 PLIF (n = 53) or MBDU (n = 37) and were followed for at least 5 years were retrospectively analyzed. Various radiographic parameters at L3/4 and L4/5 were measured before surgery and at last follow-up. Progression of facet degeneration was measured on computed tomography (Japanese Orthopaedic Association [JOA] classification); disc degeneration and spinal stenosis were measured on magnetic resonance imaging (Pfirrmann and Imagama classifications, respectively). R-ASD on plain radiography (X-ASD) was defined as reported by Okuda et al. [1]. R-ASD on CT or MRI (C/M-ASD) was defined as at least a one-grade progression in the relevant classification. JOA score for low back pain and incidence of reoperation were also evaluated.ResultsThe mean parameters at L3/4 in the PLIF group were as follows (before surgery/at last follow-up): (1) % slip: 0.8%/1.9%, (2) change in slip: 0.7/0.4 mm, (3) segmental lordosis: 11.9°/12.1°, (4) disc arc: 7.7°/7.5°, and (5) disc height: 8.6/7.7 mm. Corresponding data in the MBDU group was: (1) % slip: 1.8%/2.4%, (2) change in slip: 0.6/0.5 mm, (3) segmental lordosis: 9.6°/10.8°, (4) disc arc: 7.7°/8.7°, and (5) disc height: 7.8/6.5 mm. Disc height at last follow-up significantly differed between the groups (p = 0.002). Progression of facet degeneration was detected in 55.1% of PLIF patients and 77.8% of MBDU patients. Progression of disc degeneration and spinal stenosis was observed in 45.2% and 36.8% of PLIF patients, respectively, and 58.9% and 36.0% of MBDU patients, respectively. Overall, the incidence of X-ASD was 17.0% in the PLIF group and 16.2% in the MBDU group. Among the patients who underwent plain radiography plus CT or MRI, the total incidence of R-ASD was 70.6% in the PLIF group and 60.0% in the MBDU group. The above rates did not significantly differ between the groups. The mean improvement rate in the JOA score for low back pain was 52.8% in the PLIF group and 52.1% in the MBDU group (p = 0.867). The incidence of revision surgery at L3/4 was 1.9% in the PLIF group and 5.4% in the MBDU group (p = 0.62).ConclusionThe 5-year incidence of R-ASD at L3/4 after PLIF and MBDU in patients undergoing surgery for L4/5 DS is similar, indicating that naturally occurring lumbar degeneration is probably responsible, not fusion.© 2025. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.

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