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An increasing body of evidence from recently published studies suggests that OLIF therapy brings technical advantages and favorable outcomes for lumbar degenerative diseases, especially in the L2-5 segments2,5,6,7. While recognizing these benefits, efforts have been made to extend the use of OLIF to the lumbosacral junction. However, the technical strategy for performing OLIF at L5-S1 remains subject to controversy within the spine surgery community2,13,14,19,20.
Based on their experience with anterior lumbar interbody fusion, some researchers have suggested that the OLIF L5-S1 procedure should be executed through the central portion of the L5-S1 disc, situated below the iliac vessel bifurcation6,7,8,11,13. Indeed, this technique is viewed as a minimally invasive retroperitoneal ALIF performed in lateral position16. However, this approach necessitates patient repositioning when achieving multi-level fusion involving the L5-S1 segment in a single-stage operation. Furthermore, this technique sacrifices the integrity of the anterior disco ligamentous complex (anterior longitudinal ligament and annulus), potentially compromising the effectiveness of OLIF in achieving indirect neural decompression. The presence of the superior hypogastric plexus directly over the L5-S1 disc space below the iliac vessel bifurcation raises concerns about the risk of damaging this structure and resulting in retrograde ejaculation16. In some cases, with aberrant vascular structures, the low iliocaval junction or the medially located left common iliac vein make this approach challenging and may result in life-threatening vascular injury17,18.
To address these issues, Silvestre et al. proposed accessing the L5-S1 disc through the oblique corridor between the iliac vessel and the Psoas muscle. However, a significant concern regarding potential damage to the iliac vessel and its tributaries, notably the iliolumbar vein, has led some surgeons to abstain from using OLIF at the L5-S1 level2. Zairi et al. recommended ligating the iliolumbar vein as a prerequisite for successful access to the L5-S1 disc, aiming to reduce the risk of injuring these vulnerable structures9. However, this approach entails more extensive dissection and manipulation, potentially resulting in postoperative thigh pain and increased risk of neurovascular complications. Furthermore, the variable vascular anatomy encountered during exposure can complicate the search for the iliolumbar vein.
Based on experience, the distinct nature of the OLIF L5-S1 procedure necessitates surgeons' familiarity with lumbosacral anatomy. The accessible corridor is within the anatomical space between the Psoas muscle and the LCIV. Cadaveric studies by Kai et al. revealed that the average distance from the LCIV to the Psoas muscle was 12.00 mm at the L5-S1 disc space14. Mild Psoas retraction effectively increases the corridor width to 18 mm, sufficient for introducing cages into the disc space. The limited access space to the L5-S1 disc restricts the use of expansion retractors. To counter this, Kirschner wires are preferred in the procedure for maintaining adequate exposure through elastic retraction. The refined technique requires less dissection and retraction to achieve a suitable working zone for OLIF L5-S1, thus offering a simpler method than previously described. We emphasize meticulous dissection and direct visualization for safe exposure. All dissections should be performed under direct visualization of anatomical landmarks. Additionally, the procedure employs a smaller arthroscope to gain a detailed view of the significant anatomy crossing the surgical window, especially the location of the iliolumbar vein. The iliolumbar vein can typically be protected from incidental damage, obviating the need for vascular ligation and the resulting extensive exposure.
Another technical challenge of the OLIF L5-S1 procedure is implant insertion when encountering the iliac crest2,9,11. Iliac crest obstruction at the L5-S1 level can hinder the orthogonal maneuver of the inserted instrument, potentially resulting in postoperative misplacement of interbody fusion cages. In our series, a supplementary insertion approach was adopted to turn the implants orthogonal to the true lateral direction under fluoroscopy guidance, ensuring a satisfactory cage position. This surgical strategy for cage insertion can also be applied to the same OLIF device system for consecutive L2-S1 reconstruction through the same surgical plane, offering a more versatile and widely applicable procedure for clinical practice.
Finally, a thorough preoperative analysis of vascular anatomy is critical when performing OLIF at the L5-S1 level. An anatomical study by Chung et al. using MRI found a strong association between the morphological characteristics of LCIV and the risk of mobilization at the L5-S1 level15. The LCIV position and perivascular adipose tissue should be considered when accessing the L5-S1 disc between the iliac vessels. Although the classification system used in the study by Chung et al. may not fully apply to this approach, we found that the absence of fat around LCIV results in the adventitial layer adhering tightly to surrounding connective tissue, making blunt dissection and retraction more challenging. If the facet line cuts through the LCIV and no fat plane exits under the left CIV, this is a challenging intra-bifurcation access to the L5-S1 disc. Based on our experience, type III LCIV's anatomical features pose a higher risk of venous laceration for surgeons during exposure. Therefore, we emphasize the preoperative evaluation of perivascular adipose tissue on axial MR images. We do not recommend the OLIF technique for a single L5-S1 fusion in cases of type III LCIV. More recently, Berry et al. employed a similar facet line method to assess the relationship between the left LCIV and the L5-S1 disc, further aiding approach selection20.