Introduction: The decrease in segmental lordosis (SL) at the fused level (ΔSL < 0) following posterior lumbar interbody fusion (PLIF) has been identified as a contributing factor to early adjacent-segment disease (ASD) [1]. However, achieving adequate SL is often challenging, even with measures such as the use of lordotic cages, anterior cage placement and dorsal compression between pedicle screws. We hypothesized that a decrease in SL in the intraoperative prone position may contribute to insufficient postoperative SL. This study aims to investigate the risk factors associated with ΔSL < 0 following PLIF, focusing on SL in the intraoperative prone position.
Methods: We conducted a single-center retrospective cohort study of 97 consecutive patients who underwent primary single-segment PLIF at L4-5 for degenerative lumbar spondylolisthesis from 2017 to 2022. We excluded a total of 8 patients: those with non-lordotic cages and those who experienced vertebral fractures within one week postoperatively, resulting in a final cohort of 89 patients (31 males, 58 females; mean age 71.3 years). Patients underwent surgery in the knee-chest prone position with their hip joints flexed at 60°–70°. Patient demographics and cage specifications (height, length and lordotic angle) were obtained from the medical records. Radiological evaluations included measurements of preoperative, intraoperative (prone position), and 1-week postoperative radiographs, as well as preoperative CT images. The change in SL between pre- and postoperative standing position was defined as ΔSL, categorizing the subjects into the ΔSL ≥ 0 group and the ΔSL < 0 group. The change in SL between preoperative standing and intraoperative prone position was defined as ΔSL-P. We compared patient demographics, cage specifications, preoperative parameters, including SL (standing and lateral flexion), lumbar lordosis (LL), % slip, and intervertebral disc height, as well as intraoperative SL, ΔSL-P, cage positioning, and the ratio of cage height to preoperative disc height (cage/disc ratio) between the two groups.
Results: The ΔSL ≥ 0 group comprised 41 patients, while the ΔSL < 0 group included 48. Univariate analysis demonstrated significant differences in age (68.1 years vs. 72.3 years, p = 0.039), preoperative standing SL (1.7° vs. 6.9°, p < 0.001), preoperative LL (37° vs. 44°, p = 0.018), preoperative % slip (8.7% vs. 6.9%, p = 0.038), preoperative disc height (6.9 mm vs. 8.7 mm, p = 0.002), ΔSL-P (+0.3° vs. -4.1°, p < 0.001), and the cage/disc ratio (1.5 vs. 1.1, p = 0.002) between the two groups. Multivariate analysis identified preoperative standing SL (OR 1.144, p = 0.046) and ΔSL-P (OR 0.684, p < 0.001) as significant risk factors for ΔSL < 0. The cutoff value for ΔSL-P on the ROC curve to identify ΔSL < 0 was 0° (sensitivity 0.98, specificity 0.51).
Discussion: Preoperative standing SL and ΔSL-P are significant risk factors for ΔSL < 0. Patients with large preoperative standing SL and a decrease in SL during the intraoperative prone position warrant cautious management. Optimizing the intraoperative position to increase SL in the prone position may prevent inadequate SL achievement following PLIF and reduce the risk of ASD.