Unilateral Biportal Endoscopic Trans-Kambin Lumbar Interbody Fusion at the L5/S1 Level: A Consecutive Case Series With 1-Year Clinical and Radiological Outcomes

Background Trans-Kambin lumbar interbody fusion at L5/S1 is technically demanding because of reduced disc height, a narrow safety triangle, anatomical variability of the exiting nerve root (ENR), foraminal pathology, and interference from the iliac crest. This study evaluated the feasibility and 1-year clinical and radiological outcomes of unilateral biportal endoscopic (UBE) trans-Kambin lumbar interbody fusion at L5/S1 in a consecutive case series. Methods This retrospective consecutive case series included 25 patients who underwent single-level UBE trans-Kambin lumbar interbody fusion at L5/S1 between January 2023 and December 2024. Clinical outcomes were assessed using the Oswestry Disability Index and numerical rating scale scores for low back pain and leg pain. Radiological outcomes included segmental lordotic angle, slip distance, and preliminary computed tomography (CT)–based fusion assessment at 12 months. The operative strategy emphasized continuous endoscopic visualization of the ENR during corridor creation, foraminoplasty, disc space preparation, and cage insertion. Results Diagnoses included isthmic spondylolisthesis in 11 patients and foraminal stenosis in 9 patients. The mean preoperative disc height at the safety triangle was 2.6 ± 1.3 mm. At 12 months, segmental lordotic angle increased from 12.4° ± 6.6° to 14.6° ± 5.8° (P = 0.0046), and slip distance decreased from 5.5 ± 2.8 mm to 3.1 ± 2.9 mm (P = 0.001). Oswestry Disability Index improved from 32.5 ± 11.2 to 15.7 ± 12.8 (P < 0.0001). Numerical rating scale scores improved from 5.8 ± 1.6 to 2.8 ± 2.3 for low back pain (P < 0.0001) and from 3.8 ± 2.7 to 2.0 ± 2.2 for leg pain (P = 0.0014). The preliminary CT-based fusion rate at 12 months was 88.0%. No ENR injury, infection, or reoperation occurred. Conclusions A structured, level-specific strategy for UBE trans-Kambin lumbar interbody fusion was feasible at L5/S1 and was associated with significant clinical improvement, modest radiological correction, and an 88.0% preliminary CT-based fusion rate at 12 months. Further studies with larger cohorts and longer follow-up are needed to validate these findings. Clinical Relevance This study provides a level-specific framework for performing UBE trans-Kambin lumbar interbody fusion at L5/S1, a level with substantial anatomical and technical constraints. Level of Evidence 4.