L5-S1 Anatomic Features Relevant to Minimally Invasive Decompression 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 bony and neural anatomy may restrict transforaminal and extraforaminal approaches, yet few studies have combined cadaveric dissection with radiologic analysis to define relevant morphology in L5-S1 approaches. The purpose of the study is to characterize anatomical and radiological features of the lumbosacral region relevant to MISS planning and execution. Methods: Twelve Thiel-embalmed donor bodies underwent CT imaging (lumbopelvic region) 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) length; maximal and minimal distances between L5 TP and sacral ala; extraforaminal area bounded by L5 TP, L5-S1 facet (zygapophyseal) joint, and sacral ala; iliac crest-based approach angle to the L5-S1 intervertebral disc (IVD); minimal distance between this approach vector and the ventral ramus of the fifth lumbar spinal nerve (VRL5); facet angulation; and iliac crest height. Results: No left-right asymmetry was detected. Except for L5 TP length, all anatomical measurements obtained directly in the donor bodies differed significantly between sexes. A direct IVD access with a uniportal endoscopic working tube was feasible in 25% of cases. On CT analysis, the maximal and minimal distances between the L5 TP and sacral ala were 11.1 (4.0) mm and 5.6 ± 2.9 mm, with a mean extraforaminal area of 202.0 ± 45.9 mm². The mean approach angle was 35.2 ± 5.0°, and an extraforaminal corridor to L5-S1 IVD was feasible in 75% of donated bodies. The median minimal distance between the approach vector and the VRL5 was 5.0 (7.1) mm, with frequent overlap. Conclusions: The results of this study reveal that the L5-S1 segment shows substantial interindividual morphologic variability, compromising the feasibility of transforaminal and extraforaminal MISS approaches, and highlight the need for individualized preoperative planning, neural identification and/or bony resection to create a safe working corridor.