L5-S1 Anatomic Features Relevant to Minimally Invasive De-Compression and Fusion: A Cadaveric and Imaging-Based Study

Background/Objectives: The L5–S1 segment presents unique characteristics that make surgical access challenging in minimally invasive spine surgery (MISS) procedures. Variability in the spatial relationships of bony and neural structures may restrict some approaches. However, few studies have so far combined radiologic analysis with ca-daveric dissection to characterize the morphology relevant to transforaminal and extra-foraminal L5-S1 approaches. The purpose of the study is to characterize anatomical and radiological features of the lumbosacral region that influence planning and execution of MISS procedures. Methods: Twelve Thiel-embalmed donor bodies underwent CT im-aging (L4–S2) followed by posterior dissection. Bony landmarks were used to obtain bilateral anatomical measurements. Qualitative anatomical analysis included iliolumbar ligament morphology and extraforaminal access feasibility. CT-based morphometrics included L5 transverse process (TP) dimensions; maximal and minimal distances between L5 TP and sacral ala; extraforaminal area bounded by L5 TP, L5 facet (zygapophyseal) joint, and sacral ala; iliac crest–based approach angle to the L5-S1 disc; minimal distance between this approach vector and the ventral ramus of the L5 spinal nerve; facet an-gulation and iliac crest height. Results: No left–right asymmetry was detected. Except for L5 TP length, all anatomical measurements obtained in the donor bodies differed sig-nificantly between sexes. A direct disc access with a uniportal endoscopic working tube was feasible in 25% of cases. On CT analysis, the maximal and minimal distances between the TP and sacral ala were 11.1 (4.0)mm and 5.6±2.9mm, with a mean extraforaminal area of 202.0±45.9mm². Neither gender nor sacral dysmorphism significantly influenced these imaging parameters. The mean approach angle was 35.2±5.0°, and an extraforaminal corridor to L5–S1 disc was feasible in 75% of donated bodies. The median minimal distance between the approach vector and the ventral ramus of the L5 spinal nerve was 5.0 (7.1)mm, with frequent overlap. Conclusions: The L5–S1 segment shows substantial interindividual morphologic variability, compromising the feasibility of transforaminal and extraforaminal MISS approaches, and highlighting the need for individualized preoperative planning, neural identification and/or bony resection to create a safe working corridor.