Objective: In our study, we aimed to evaluate dynamic interspinous radiology (DIR) as a predictor of early-onset adjacent segment degeneration (ASD) following multilevel lumbar interbody fusion (L2-ilium).

Methods: From 2018 to 2022, 64 patients who underwent L2-ilium surgery for degenerative spinal disease at a single institution were enrolled. The patients were divided into two groups: Group A, consisting of those who developed ASD (n = 32), and Group B, those who remained normal (n = 32). Patients with preoperative high-grade adjacent segment disc degeneration (Pfirrmann grade ≥ III) were excluded. DIR was defined as the difference in the distance between the midpoints of the superior aspects of the spinous processes during flexion and extension. Demographic data and radiological spinopelvic parameters were compared between the two groups. Adjacent segment muscle quality and flexibility were additionally assessed and compared.

Results: The follow-up period was 13.2 ± 6.3 months in Group A and 11.7 ± 6.8 months in Group B (P = 0.522). There were no significant differences in demographic data between the two groups. In both groups, spinopelvic parameters were improved compared to preoperative measurements (P > 0.05), except for cervical lordosis and pelvic incidence. The correction of sagittal vertical alignment (SVA) in Group A was greater than in Group B, but the difference was not statistically significant (P = 0.05). There was no difference in adjacent segment muscle quality between the groups (P > 0.05). Changes in the proximal junctional angle during flexion and extension were also not significantly different between the two groups (P > 0.05). However, Group A exhibited a significantly larger DIR compared to Group B (10.1 ± 4.1 mm vs. 6.0 ± 3.5 mm, P = 0.004). In multivariable models, only DIR L1–2 remained an independent predictor (OR 1.473, 95% CI 1.048–2.070, p=0.026); DIR T12–L1 was not significant (P= 0.153). The optimal cut-off value of preoperative DIR as a risk factor for ASD was >4.4 mm.

Conclusions: Preoperative DIR at the proximal adjacent level was identified as an independent predictor of early-onset adjacent segment degeneration following multilevel lumbar interbody fusion. An optimal preoperative DIR cut-off value of greater than 4.4 mm at L1–2 may serve as a potential risk factor for ASD