Abstract
Background Full-endoscopic decompression for L5 isthmic spondylolisthesis is technically demanding due to the intricate “pars crisscross” anatomy. The inherent visualization limitations of endoscopic surgery—specifically the lack of depth perception and a restricted field of view—can make anatomical orientation difficult, increasing the risk of incomplete decompression or iatrogenic injury.
Objective To describe a navigation-assisted full-endoscopic pars crisscross decompression (FE-PCD) technique designed to overcome these anatomical challenges.
Methods We performed this navigation-assisted FE-PCD technique in 3 consecutive patients with stable L5 isthmic spondylolisthesis. An O-arm-based intraoperative navigation system was utilized to provide real-time 3-dimensional (3D) guidance. A “virtual probing” technique was employed to visualize the drill tip relative to the 3D anatomical reconstruction, facilitating the precise identification of the S1 superior articular process, floating lamina, L4 inferior articular process, and the ragged edge.
Results All procedures were successfully completed without intra- or postoperative complications. The use of navigation effectively compensated for the limited endoscopic view, allowing for the clear differentiation of deep bony structures and precise resection of the impinging ragged edge.
Conclusion Our preliminary experience suggests that navigation-assisted FE-PCD is a potentially feasible and safe technique. It mitigates the risks associated with the complex pars crisscross anatomy by providing a reliable 3D roadmap, thereby preventing disorientation, ensuring adequate decompression, and optimizing the extent of facet resection.
Clinical Relevance Full-endoscopic pars crisscross decompression (FE-PCD) is an effective but technically demanding procedure for stable isthmic spondylolisthesis due to the complex 3D anatomy of the "pars crisscross." This integration enhances surgical safety by preventing intraoperative disorientation, ensuring adequate decompression, and minimizing iatrogenic facet resection, thereby potentially shortening the learning curve for this challenging endoscopic technique.
Level of Evidence 4.
- full-endoscopic spine surgery
- lumbar spondylolisthesis
- full-endoscopic pars crisscross decompression
- navigation system
Introduction
Full-endoscopic pars crisscross decompression (FE-PCD) has been established as an effective minimally invasive treatment for foraminal stenosis associated with stable isthmic spondylolisthesis.1,2 However, despite its clinical benefits, the procedure remains technically demanding due to the unique and complex anatomy involved.
The primary challenge lies in the intricate “pars crisscross” structure. This specific anatomical configuration is formed by the intersection of the true facet joint and the pseudarthrosis, creating a cross-shaped structure composed of 4 distinct elements: the S1 superior articular process (SAP), the floating lamina, the L4 inferior articular process (IAP), and the ragged edge of the pars defect.1 In full-endoscopic spine surgery (FESS), the surgeon is limited to a 2-dimensional view of this fundamentally 3-dimensional (3D) pathology.3–5 This limitation can lead to surgical disorientation, making it difficult to identify these 4 components and maintain spatial awareness. Consequently, there is a risk of incomplete decompression or injury to the exiting nerve root, particularly for surgeons in the learning phase.3,4
To address the risk of intraoperative disorientation, we integrated real-time 3D computed tomography (CT)–guided navigation into the FE-PCD workflow. Navigation allows for “virtual probing” of the anatomy, visualizing the drill tip position relative to the complex bony architecture on a 3D screen.6,7 This technical note describes the details of this navigation-assisted technique and reports on its potential feasibility and safety in our preliminary case series.
Surgical Technique
Patient Positioning
The procedure is performed with the patient prone on a Jackson table. First, using C-arm fluoroscopy, key anatomical landmarks including the L5 and S1 spinous processes, the L5 and S1 pedicles, the L5 transverse process, both iliac crests, and the L5/S disc level are marked on the skin (Figure 1A). Then, the surgical field is prepared by washing with soap, disinfecting with povidone-iodine, and applying povidone-iodine-impregnated drapes.
(A) Key anatomical markings under C-arm fluoroscopy. We mark the L4, L5, and S1 spinous processes, the L5 and S1 pedicles, the L5 transverse process, both iliac crests, and the L5/S1 disc level. (B) The reference array is fixed to the iliac crest contralateral to the surgical site using two 3.0 mm pins. (C) The endoscopic view after S1 superior articular process (SAP) shaving. The pars crisscross consists of 4 parts: the S1 SAP, the floating lamina, the L4 inferior articular process (IAP), and the ragged edge.
Navigation System Setup
The Medtronic StealthStation S8 system is used for intraoperative navigation. After the patient is positioned, a reference array is fixed to the iliac crest contralateral to the surgical site using two 3.0 mm pins (Figure 1B). An intraoperative scan is then performed with the O-arm Surgical Imaging System. This scan automatically registers the patient’s spinal anatomy in relation to the reference array, allowing for real-time, navigated instrument tracking.
Anesthesia
Anesthesia begins with the administration of 10 mL of 0.5% lidocaine at the planned site for the navigation reference array. Subsequently, the endoscopic portal tract is anesthetized by step-wise injection of 1% lidocaine targeting the skin (3 mL), subcutaneous tissue (5 mL), iliac crest (2 mL), L5/S facet (6 mL), L4 IAP/ragged edge (2 mL), S1 superior endplate (2 mL), and the L5/S annulus (2 mL). Light conscious sedation is achieved with an intravenous injection of 25 mg of hydroxyzine and 30 mg of pentazocine.
Setting of Endoscopes
The surgical technique is based on the FE-PCD method previously reported.3 An 8 mm working cannula is first docked at the base of the SAP of the sacrum (S1). The L5/S disc is then stained with a mixture of indigo carmine and contrast medium to aid endoscopic visualization. Upon insertion of the endoscope, soft tissues are meticulously cleared to identify the “pars crisscross,” which is the key anatomical landmark for this procedure (Figure 1C).
Virtual Probing of the Pars Crisscross
The pars crisscross consists of 4 structures: the S1 SAP, the floating lamina, the L4 IAP, and the pars ragged edge. Correct identification of these structures is critical but challenging due to the limited view. At this initial stage, the tip of the navigated high-speed drill itself can be used as a virtual probe. By placing the drill tip on any bony structure within the endoscopic view, the surgeon can definitively identify each component on the real-time intraoperative 3D scan (Figure 2A and B). This step allows the surgeon to directly visualize the complex 3D anatomy before any bone resection begins, minimizing the risk of disorientation.
(A) The surgical drill is placed on the S1 superior articular process (SAP). (B) On the navigation screen, the tip of the surgical drill is displayed in the same position. (C) The S1 SAP and floating lamina are resected with surgical drill. (D) The display of the surgical drill tip helps to prevent disorientation and makes it possible to determine the amount of resection.
Navigated 4-Step Pars Crisscross Decompression
The decompression follows a 4-step sequence, which is enhanced by the use of a navigated high-speed drill:
S1 SAP resection: Decompression begins with the resection of the S1 SAP. The real-time tracking of the drill tip allows the surgeon to perform this initial bone work with precision, confirming the exact location and depth of resection on the navigation monitor (Figure 2C and D).
Partial resection of the floating lamina: Next, the floating lamina adjacent to the crisscross is partially removed.
Resection of the tip of the L4 IAP: A critical part of the procedure is the resection of the tip of the L4 IAP, which uncovers the entire ragged edge impinging on the nerve root. In conventional techniques, identifying the exact limit of the IAP can be challenging, potentially leading to excessive resection. Navigation provides crucial guidance here, ensuring that only the necessary portion of the IAP tip is removed to expose the inlet of the nerve root and ragged edge. This targeted approach minimizes damage to the facet joint complex, thereby preserving segmental stability better than extensive exploratory resection.
Ragged edge resection: With the target fully exposed, the pars ragged edge is carefully shaved and completely resected using the navigated high-speed drill. The ability to track the drill tip in real-time is paramount for safety, preventing the cannula from slipping and allowing the surgeon to work confidently in close proximity to the L5 nerve root.
Confirmation of Decompression and Technical Pearls
Finally, the complete decompression of the exiting L5 nerve root is confirmed along its entire path from the inlet to the outlet of the foramen through the direct endoscopic view (Figure 3A and B). Note that the surgical sequence may be adjusted based on the individual patient’s anatomy, as the presentation of the pars crisscross can vary. Furthermore, the ragged edge must be pursued thoroughly to its lateral extent, as it can also cause compression at the outlet of the foramen. The significant advantage of real-time navigation lies precisely in its ability to manage these anatomical complexities and ensure a complete decompression.
(A) Decompression of the L5 nerve root is confirmed from the inlet to the outlet. (B) The probe was used to confirm that the L5 nerve root is decompressed along its course.
Case Presentation
Initial Experience
We performed this navigation-assisted FE-PCD technique in a consecutive series of 3 patients (all men; mean age, 65.3 years) with symptomatic stable L5 isthmic spondylolisthesis at the L5/S1 level. Stable spondylolisthesis was defined as slippage without dynamic instability on preoperative radiographs (Figure 4A and B), characterized by <3 mm translation and <10° angulation.1,2 Based on the clinical strategy that distinguishes between pseudarthrosis-related radiculopathy and discogenic instability, we targeted adult patients (age >40) with radicular symptoms and excluded those with predominant instability-related low back pain (VAS ≥5).8 This ensures that the primary pathology is static physical compression from the ragged edge rather than segmental motion.
(A) Flexion sagittal view of plain radiograph of the lumbar spine. (B) Extension sagittal view of plain radiograph of the lumbar spine. (C) Plain T2-weighted image magnetic resonance imaging of the lumbar spine. Parasagittal view of the right side. (D) Plain T2-weighted image magnetic resonance imaging of the lumbar spine. Axial view of L5/S level. L5/S foraminal stenosis and epidural lipomatosis at L5/S level could be confirmed.
The procedure was successfully completed in all cases. The mean total operative time was 121.7 ± 5.25 minutes. The mean estimated blood loss was 5.3 mL. Crucially, no intraoperative or postoperative complications occurred, such as nerve root injury, dural tear, or wrong-level surgery. All patients achieved immediate relief of leg pain postoperatively.
Representative Case
A 75-year-old man presented with a chief complaint of right leg pain that had persisted for approximately 3 years. He had been previously diagnosed with epidural lipomatosis at another institution and was told that surgery was not an option. Seeking a minimally invasive solution, he was referred to our department. Physical examination revealed neuropathic pain in the right L5 nerve root distribution, with a positive straight leg raise test at 50° on the right side. Plain radiographs and magnetic resonance imaging confirmed a diagnosis of stable L5 isthmic spondylolisthesis with right-sided foraminal stenosis at the L5/S1 level (Figure 4).
The patient underwent the navigated FE-PCD under local anesthesia and light sedation. The total operative time was 117 minutes, with an estimated blood loss of 5 mL. The use of real-time navigation facilitated the identification of the complex pars crisscross anatomy and allowed for safe and precise resection of the impinging bony structures.
Postoperatively, the patient’s right leg pain improved immediately, and he was able to ambulate immediately after the surgery. Postoperative CT scans confirmed adequate bony decompression of the L5 nerve root (Figure 5).
(A) Reconstruction of a 3-dimensional preoperative computed tomography image of the lumbar spine. (B) Postoperative image. Sufficient bony decompression could be confirmed.
Discussion
This technical note describes our initial experience with navigation-assisted FE-PCD for stable L5 isthmic spondylolisthesis. Our preliminary results suggest that the integration of real-time 3D navigation appears to be a feasible and safe approach. Despite the inherent complexity of the pars crisscross anatomy, which typically poses risks for disorientation and wrong-level surgery, the procedure was successfully performed under local anesthesia in all 3 cases without any such complications.
The Challenge: Anatomical Complexity and Visualization Limitations of FESS
The anatomy of the L5/S1 foramen in isthmic spondylolisthesis presents unique surgical challenges.1,2 The “pars crisscross”—formed by the intersection of the facet joint and the pseudarthrosis—creates a complex 3D stenosis. In full-endoscopic surgery, these anatomical difficulties are compounded by inherent visualization limitations.3,4 The procedure is limited by the lack of stereoscopic depth perception inherent to monocular endoscopic images.3 Additionally, the restricted field of view within the endoscopic cannula prevents simultaneous visualization of the entire crisscross structure.1 Although Sairyo et al established a systematic approach to identify the 4 components of the crisscross, anatomical variability requires the surgeon to constantly maintain a precise mental 3D map to ensure complete decompression.1,2 This is particularly challenging when pursuing the ragged edge to its lateral extent, where the loss of spatial orientation can lead to incomplete decompression or inadvertent injury to adjacent structures.
The Solution: Navigation-Guided “Virtual Probing”
Our experience suggests that navigation directly addresses these challenges by providing a “virtual probe” function. By visualizing the drill tip relative to the 3D reconstruction on the navigation monitor, the surgeon can definitively identify the boundaries of the S1 SAP, floating lamina, and L4 IAP before any resection begins. This real-time feedback compensates for the limited endoscopic field, preventing disorientation during the initial approach.6,7 Furthermore, navigation enables pinpoint resection of the ragged edge, thereby avoiding excessive removal of the facet joint. Although postoperative instability is generally not a primary concern in FE-PCD, the biomechanical rationale for decompression-alone must be considered in the context of the specific pathology. In adult isthmic spondylolisthesis, symptoms typically arise from 2 distinct pathological entities: radiculopathy caused by nerve root compression at the pseudarthrosis site, and discogenic pain associated with segmental instability.8 Surgical strategies should be tailored to these entities; while spinal fusion is indicated for instability, decompression alone is sufficient for resolving radicular symptoms in stable cases.8
Traditional nonfusion decompression, such as Gill’s procedure, often leads to postoperative instability because it involves the removal of the loose lamina and surrounding ligaments, including the ligamentum flavum and interspinous/supraspinous ligaments.1,2,8 Biomechanical analysis using the finite element method has demonstrated that such extensive resection leads to an approximately 2-fold increase in intradiscal pressure and annulus stress during flexion, which may facilitate progressive slippage.9 In contrast, our FE-PCD technique is designed to be “biomechanically minimally invasive.” It leaves the paraspinal muscles and the posterior ligamentous complex intact, thereby maintaining segmental mechanics. Finite element method analysis confirms that endoscopic decompression does not significantly alter spinal stability compared with the preoperative state, resolving radicular pain without the risk of progressive slippage.9 By integrating real-time navigation, we achieve pinpoint resection of the “ragged edge”—the primary source of compression—while further minimizing iatrogenic impact on the facet complex. This targeted approach may help ensure that the procedure remains within the scope of decompression-alone for stable spondylolisthesis.
Limitations
This study has several limitations inherent to a preliminary technical note. First, the sample size is small (n = 3), and the design is retrospective. Second, while navigation facilitates anatomical identification, its benefits must be weighed against the increased radiation exposure from the intraoperative CT scan and the extended setup time, although these factors may be justified by the enhanced safety in anatomically complex cases. Further prospective studies with larger cohorts are warranted to validate the clinical efficacy and long-term outcomes of this technique compared with conventional methods.
Conclusion
Our preliminary experience suggests that navigation-assisted FE-PCD is a safe and feasible technique for the treatment of stable L5 isthmic spondylolisthesis. The integration of real-time 3D navigation effectively overcomes the visualization limitations inherent to FESS, providing critical guidance through the complex “pars crisscross” anatomy. By functioning as a “virtual probe,” this technology enables the precise identification and resection of the ragged edge while minimizing the risk of disorientation and excessive facet removal. Although further validation is required, this technique offers a promising potential solution for the safe execution of one of the most anatomically challenging endoscopic procedures.
Footnotes
Funding The authors received no financial support for the research, authorship, and/or publication of this article.
Declaration of Conflicting Interests The authors report no conflicts of interest in this work.
Institutional Review Board Approval This study was approved by the Institutional Review Board of Tokushima University Hospital (approval no. 3642).
- This manuscript is generously published free of charge by ISASS, the International Society for the Advancement of Spine Surgery. Copyright © 2026 ISASS. To see more or order reprints or permissions, see http://ijssurgery.com.
References
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