Exploring the impact of mild-to-moderate foraminal stenosis at L5−S1 on clinical outcomes following L4−5 posterior lumbar interbody fusion

•Patients with L5-S1 FS showed poorer clinical outcomes after L4-5 PLIF.•These patients had higher back pain and ODI scores at final follow-up.•Mild-to-moderate FS at L5-S1 worsens sagittal balance.•L5-S1 FS group had more severe central and foraminal stenosis. Patients scheduled for L4−5 PLIF often have FS at L5-S1. However, data on the clinical and radiographic outcomes of cases with mild-to-moderate L5−S1 FS are lacking, which may affect clinical outcomes or require additional surgery after L4−5 fusion. To evaluate the clinical and radiographic outcomes of L4−5 PLIF in patients with and without mild-to-moderate L5−S1 FS, with a primary focus on the association between L5−S1 FS and postoperative clinical outcomes including back pain, leg pain, and scores on the oswestry disability index (ODI) and EuroQol 5-dimension (EQ-5D). Retrospective comparative study. A retrospective review of patients who underwent L4−5 PLIF from 2014 to 2018. The patients were divided according to the presence of mild-to-moderate FS at L5−S1. Clinical assessment included the pain visual analog scale (VAS), ODI, and EQ-5D score. Radiographic assessments included spinopelvic parameters and grades for central and foraminal stenosis. Clinical outcomes were assessed using validated outcome measures at preoperative, 6-month, 12-month, and 36-month follow-up visits. Radiographic evaluations were performed using preoperative and postoperative radiographs. Foraminal stenosis was assessed qualitatively using MRI with a grading system from none to severe and quantitatively by measuring changes in the foraminal area on CT. Among 186 patients, 55 were categorized as the FS group and 131 as the non-FS group. The FS group was older (p=0.039) and had more severe central stenosis at L5−S1 (p=0.007) as well as more severe FS at both L4−5 and L5−S1 (both p