The Posterior Oblique Lateral (POL) Full-Endoscopic Approach: A Transverse Process Docking Technique to Avoid Early Neural Contact in Challenging Foraminal and Extraforaminal Pathologies - Technical Note and Illustrative Case Series
Article information
Abstract
Endoscopic spine surgery continues to expand in scope. Although the transforaminal route through Kambin triangle remains a key access corridor, its use may be limited by anatomical factors such as disc height loss, spondylolisthesis, nerve root anomalies, or a high iliac crest. This technical note describes a posterior oblique lateral (POL) approach that may facilitate access in anatomically constrained cases. The POL approach begins with docking on the transverse process along a posterior-oblique trajectory. The superior articular process, facet capsule, and pars are then identified stepwise before the working corridor is advanced to Kambin triangle under direct visualization. The technique is designed to create a wider working angle and reduce early contact with neural structures. Its use is demonstrated in 3 patients with foraminal stenosis, spondylolytic spondylolisthesis requiring fusion, and far-out syndrome. The POL approach provided access to the target pathology in all 3 cases, including one case with a markedly narrowed Kambin triangle. Decompression and fusion procedures were completed successfully, including facetectomy, cage placement, and extraforaminal decompression. All patients had symptomatic improvement without perioperative complications. Given the limited sample size, comparative efficacy and safety remain uncertain, but these cases support the technical feasibility of the approach. In anatomically constrained cases, the POL approach may provide a feasible and versatile endoscopic alternative. By establishing a reproducible working corridor, it may complement the standard transforaminal approach.
INTRODUCTION
Over the past 2 decades, endoscopic spine surgery has remarkably evolved, with the interlaminar and transforaminal approaches becoming the 2 primary access routes for lumbar procedures [1]. The transforaminal approach is considered a fundamental technique because it allows access to the intervertebral disc space through Kambin triangle, a natural anatomical safe zone [2].
Interestingly, although the transforaminal approach is widely adopted and highly beneficial, certain limitations may be overlooked. Many studies have defined the anatomical constraints of Kambin triangle, the key corridor for the transforaminal approach [3,4]. Ozer et al. [5] reported that the wide, classical Kambin triangle was present only in 17.6% of patients and 20.8% in a cadaveric study, indicating that most cases have narrow or absent safe space.
Considering these anatomical constraints, alternative access routes have been explored. Echt et al. [6] reported a tubular microdiscectomy technique based on caudal transverse process (TP) docking for far lateral disc herniations to reduce manipulation of the exiting nerve root. Musharbash and Lee [7] further described an intertransverse endoscopic approach using a similar posterolateral trajectory to minimize dorsal root ganglion irritation.
In this context, we describe the posterior oblique lateral (POL) approach, which aligns with these concepts and provides a structured, stepwise method based on sequential anatomical identification. The approach emphasizes docking on the TP as a reliable bony landmark to establish the working corridor, with gradual exposure of neural structures under direct endoscopic visualization.
TECHNICAL NOTE
1. Key Principles
The key concept of this approach is entry through a posterior-oblique trajectory, with initial docking at the lower-level TP to provide a stable and reproducible reference point. From there, the surgeon advances along sequential anatomical landmarks, gradually expanding and securing the working corridor before advancing to the target. Pathology is then addressed from the lateral side, progressing toward the medial structures. The POL approach is illustrated in (Figure 1), in comparison with the transforaminal route.
Comparison of transforaminal (TF, green) and posterior oblique lateral (POL, blue) trajectories.
2. Patient Setup
The patients are positioned prone on a radiolucent table. The operating room set-up is illustrated in (Figure 2A). Endoscope selection is determined by pathology and the working corridor. Two sizes of endoscopic systems are commonly used, as shown in (Figure 2B). The 8-mm system (outer diameter [OD], 8 mm; working channel, 4.1 mm) is suitable for soft disc herniations or limited foraminal decompression, where only a narrow access corridor is required. The 10-mm system (OD, 10.5 mm; working channel, 5.6 mm) is preferred for stenotic pathology or cases requiring extended bony decompression.
Patient setup. (A) Operating room setup for a right-handed surgeon treating left-sided lumbar pathology. (B) Endoscope sizes and working channels.
3. Surgical Technique
1) Target level identification
Confirm the target level with anteroposterior fluoroscopy and tilt the fluoroscope to align with the vertebral end plate of the targeted level. The C-arm is then rotated approximately 15°–25° to obtain an oblique view that shows the junction of the TP and the superior articular process (SAP), as shown in (Figure 3A–C). The skin entry point is marked along a posterior-oblique trajectory directed toward the lower TP of the target level, the initial bony docking site.
Target level identification in the posterior oblique lateral approach. (A) Anteroposterior fluoroscopy confirms the level, (B) C-arm rotation approximately 15°–25° to obtain an oblique view. (C) Docking on the transverse process provides a stable bony landmark.
2) Incision and docking
A small skin incision is made laterally over the premarked entry point. A dilator is advanced under fluoroscopic guidance until firm bony contact is established on the TP. During this step, the natural intermuscular plane between the multifidus and longissimus muscles is followed, allowing blunt dissection with minimal muscle trauma. The working cannula is then introduced and secured in a stable dock on the TP. The illustration and endoscopic view are shown in (Figure 4A–C).
Docking in the posterior oblique lateral approach. (A) Schematic illustration showing the working cannula docked on the transverse process (TP) along a posterior-oblique trajectory. (B) Intraoperative fluoroscopy confirming stable docking on the TP. (C) Endoscopic view after docking on the TP. SAP, superior articular process.
At L5–S1 with a high iliac crest, move the entry slightly superior to the iliac crest. Consider docking at the junction between the sacral ala and the base of the SAP, as shown in (Figure 5A–C).
Special docking consideration at L5–S1. (A) Standard docking at L4–5 with the working cannula positioned on the transverse process. (B) At L5–S1, the trajectory is redirected caudally, and docking is performed at the junction between the sacral ala and the base of the superior articular process. (C) Surface marking of the skin entry point relative to the iliac crest.
3) Identification of anatomical landmarks
Under endoscopic visualization, define the superior margin of the lower-level TP and follow medially to the SAP. In this region, identify the medial branch of the dorsal ramus, typically coursing within the mammillary-accessory ligament [8]. While preservation is possible, it is often transected during exposure. Critically, the intermediate and lateral branches of the dorsal ramus, which supply the paraspinal musculature, should be preserved [9]. The inferior articular artery that supplies the facet capsule can be visualized together with the medial branch [10]. This vessel originates from the dorsal branch of the segmental artery. Careful coagulation is recommended when these vessels are encountered to maintain a clear surgical field.
Following hemostasis, the facet capsule and pars interarticularis are identified to form the medial boundary of exposure. Preoperative imaging should be carefully reviewed to assess the relationship between the exiting nerve root and the tip of the SAP, which serves as a key anatomical reference during endoscopic orientation. This allows the surgeon to anticipate the position of the exiting nerve root and maintain safe dissection during the initial exposure. Accurate identification of these structures establishes a reliable anatomical framework, as shown in (Figure 6A–C).
Stepwise identification of anatomical landmarks. (A) Endoscopic view showing sequential identification of the transverse process (TP), superior articular process (SAP), facet capsule, and pars. (B) Fluoroscopic view showing the trajectory toward the transverse process-superior articular process junction. (C) Endoscopic overview showing the transverse process, superior articular process, facet capsule, and surrounding anatomical orientation, with coagulation of the inferior articular artery and medial branch of the dorsal ramus.
4) Working corridor preparation
The working channel is advanced under direct endoscopic visualization towards Kambin triangle. As an initial step, partial release of the intertransverse ligament can be done in some cases. If the triangle remains narrow, a limited resection of the superior margin of the TP and the lateral aspect of the SAP may be performed. These steps widen the corridor before docking at Kambin triangle by tracing the upper edge of the lower pedicle under continuous visualization, as shown in (Figure 7A–C).
Working corridor preparation. (A) Schematic illustration showing final docking at Kambin triangle (*) by tracing the upper edge of the lower pedicle. The dotted line indicates optional partial resection of the superior margin of the lower transverse process and the lateral aspect of the superior articular process in anatomically constrained cases. (B) Intraoperative fluoroscopy showing the endoscope trajectory directed toward Kambin triangle along the pedicle. (C) Endoscopic view showing the pedicle, disc, and endplate margins, confirming safe docking orientation under continuous visualization.
Overresection of the facet joint may cause iatrogenic instability, while aggressive medial drilling poses a risk of pedicle breach.
5) Decompression and removal of pathology
Through the prepared corridor, decompression can be tailored as indicated:
Extraforaminal decompression: Remove extraforaminal disc fragments or osteophytes to decompress the exiting nerve root. Maintain the cannula bevel as a shield and minimize traction.
Foraminotomy: Widen the neural foramen by removing the ventral part of SAP, intervertebral disc or foraminal ligament to free the exiting root.
Ventral decompression: After partial removal of the ventral aspect of the SAP, the traversing root is identified, allowing medial decompression of the thecal sac and nerve root from a lateral trajectory.
Facetectomy (when indicated): In fusion cases, resection of the SAP above the pedicle facilitates disc space access and allows endplate preparation. Partial resection of the Inferior articular process (IAP) may be considered when combined with decompression, but it is usually preserved to protect the traversing nerve root during cage insertion.
This concept is illustrated in (Figure 8A–C), which demonstrates the lateral working angle and endoscopic visualization of both exiting and traversing nerve roots.
Endoscopic decompression and pathology removal. (A) Illustration of the lateral working angle for foraminal and ventral decompression through the posterior oblique lateral trajectory. (B) Fluoroscopic view showing cannula docking at the lateral facet region. (C) Endoscopic image showing both the exiting and traversing nerve roots and the disc.
6) Closure
After confirmation of hemostasis and placement of a drain, the working cannula and dilators are withdrawn under visualization. The incision is closed in standard fashion.
4. Illustrative Cases
1) Case 1 - right foraminal stenosis at L4–5
A 36-year-old woman presented with persistent right L4 radicular pain for 9 months, described as sharp, activity-related, and exacerbated by prolonged ambulation and positional changes. Conservative treatments, including medication and physiotherapy, had failed to provide relief. The preoperative scores were visual analogue scale (VAS) back pain 6/10 (radicular in nature, not mechanical), VAS leg pain 8/10, and Oswestry Disability Index (ODI) 31. She also underwent a transforaminal epidural steroid injection, which resulted in a temporary improvement of back and leg pain symptoms only one month before symptoms recurred. Neurological examination revealed normal motor power, intact sensation, and symmetric reflexes.
Preoperative magnetic resonance imaging (MRI) and computed tomography demonstrated right-sided L4–5 foraminal stenosis with an endplate osteophyte, as shown in (Figure 9A and B). Due to the focal foraminal compression and failure of conservative treatment, she was scheduled for endoscopic decompression using the POL approach.
Case 1: right foraminal stenosis at L4–5. (A and B) Preoperative magnetic resonance imaging and computed tomography showing right-sided L4–5 foraminal stenosis (arrows). (C) Intraoperative endoscopic view showing removal of an endplate spur compressing the exiting L4 nerve root. (D) Fluoroscopic image showing the working cannula position during pathology removal.
Under general anesthesia, the POL approach was performed by docking on the L5 level TP and stepwise progression to Kambin triangle. Foraminal decompression was achieved by removing the endplate osteophyte, as shown in (Figure 9C and D), without complications.
Postoperatively, the patient was pain-free, with both back and leg VAS reduced to 0 in 1 week. At 3-month follow-up, she developed mild recurrent leg pain during early ambulation (VAS leg pain 3/10), while back pain remained absent (VAS back 0) and ODI was 10. By the subsequent month, her symptoms had gradually resolved again, returning to complete pain relief with VAS 0 for both back and leg pain.
This case highlights that even in relatively simple foraminal stenosis, the POL approach can be effectively applied. It provides decompression results comparable to the transforaminal route, while maintaining direct visualization and controlled anatomical orientation throughout the procedure.
2) Case 2 - spondylolytic spondylolisthesis at L5-S1
A 42-year-old woman presented with mechanical low back pain and neurogenic claudication for 1 year. The walking distance was limited to approximately 500 m, with symptoms aggravated by a change of position. MRI revealed multiple lumbosacral schwannomas together with low-grade spondylolytic spondylolisthesis at L5–S1. She initially underwent laminectomy from L1–4 with tumor removal; pathology confirmed schwannoma.
Although her leg pain improved partially, she continued to suffer from disabling mechanical back pain and claudication attributable to the spondylolytic lesion at L5–S1. The dynamic film showed more severe spondylolisthesis, as shown in (Figure 10A and B). The preoperative scores were VAS back pain 7/10, VAS leg pain 7/10, and ODI 27. She was scheduled for POL-lumbar interbody fusion (POL-LIF) at L5–S1. The POL approach was chosen to enable a controlled facetectomy and to create a safe working corridor while avoiding dissection through the previous surgical site.
Case 2: spondylolytic spondylolisthesis at L5–S1. (A and B) Dynamic flexion-extension radiographs showing a pars defect with instability at L5–S1. (C) Postoperative radiograph obtained 6 months after surgery, confirming satisfactory cage position and fusion. (D) Intraoperative endoscopic view showing that Kambin triangle was extremely narrow, precluding safe direct docking. (E) After partial resection of the transverse process and superior articular process, a wider working corridor was created, allowing safe fusion and decompression.
Intraoperatively, we identified that the triangle was very narrow. Although contralateral pedicle screw fixation was applied intraoperatively to provide distraction and partial reduction, the Kambin triangle remained narrow, necessitating further bony work under endoscopic visualization, as shown in (Figure 10D and E), to safely widen the corridor for cage placement and decompression.
The patient tolerated the procedure well. Postoperatively, her back pain improved, and walking tolerance increased. She was discharged on postoperative day 3 without complications. Follow-up imaging confirmed satisfactory decompression and cage position. Pain gradually improved during the first postoperative week, and at 6 months of follow-up, she was symptom-free with evidence of solid fusion, as shown in (Figure 10C) (VAS 0/10 for both back and leg pain, ODI 15).
This case highlights that Kambin triangle is not universally safe, particularly in patients with anatomical variations or spondylolisthesis where the triangle may be severely narrowed despite intraoperative distraction or reduction. The POL approach extends its utility beyond decompression, serving as a viable fusion route (POL-LIF). As illustrated in (Figure 11), POL-LIF is compared with other endoscopic fusion techniques.
Comparative illustration of Full-endoscopic fusion trajectories. The Posterior Oblique Lateral approach for Lumbar Interbody Fusion (POL-LIF; blue) trajectory approaches the disc space through a controlled oblique corridor, distinct from the more medial transforaminal lumbar interbody fusion (TLIF; pink) trajectory and the steeper oblique trajectory of Kambin triangle lumbar interbody fusion (KLIF; green).
3) Case 3 - far-out syndrome at right L5-S1
A 75-year-old man presented with right L5 radicular pain of 1-year duration, refractory to conservative therapy. The pain was aggravated in all positions and limited ambulation, without associated mechanical back pain. Neurological examination revealed weakness of the right extensor hallucis longus (motor power grade 4) and decreased pinprick sensation along the right L5 dermatome.
The preoperative imaging showed extraforaminal compression consistent with far-out syndrome at the right L5–S1 level, as shown in (Figure 12A–C). The preoperative VAS scores were 6/10 for back pain, 8/10 for leg pain, with an ODI of 25. Prior to surgery, an L5 selective nerve root block provided significant but temporary symptom relief.
Case 3: far-out syndrome at right L5–S1. (A) Preoperative anteroposterior radiograph showing right L5 transverse process-sacral alar pseudoarticulation (arrow). (B and C) Preoperative axial magnetic resonance imaging and computed tomography showing extraforaminal compression of the exiting right L5 nerve root at L5–S1 (arrows). (D) Endoscopic view after docking at the sacral alar-superior articular process (SAP) junction, showing the surrounding anatomical orientation before decompression. (E) Endoscopic view showing the exiting right L5 nerve root after decompression. (F) Postoperative coronal computed tomography scan confirming the decompression corridor (arrow).
The POL approach was selected to achieve a more favorable working angle. Using a posterior-oblique trajectory, the working cannula was docked on the sacral alar and SAP junction and advanced to the extraforaminal L5 nerve root. Extraforaminal decompression was performed, removing hypertrophic osteophytes and compressive elements under direct endoscopic visualization, as shown in (Figure 12D and E).
Postoperatively, the patient experienced an immediate and significant improvement in radicular pain, with VAS leg pain reduced to 2/10 at one week. Motor strength improved to grade 4+, and sensation along the right L5 dermatome recovered. At 3 months of follow-up, he reported sustained pain relief with VAS back pain 0/10, VAS leg pain 1/10 and ODI improved to 17, along with increased walking tolerance and no recurrent symptoms. Radiographic evaluation confirmed adequate decompression, as shown in (Figure 12F). No perioperative complications occurred.
This case illustrates the flexibility of the POL approach, demonstrating that it is not confined to simple foraminal or extraforaminal decompression. Due to the initial step of stable docking at the sacral alar or lower TP, combined with strict orientation to anatomical landmarks, the POL approach can be safely adapted even in complex cases such as far-out syndrome.
All 3 cases demonstrated marked symptom improvement and functional recovery without perioperative complications. A summary of all cases, including patient demographics, pathology, and clinical outcomes, is presented in (Table 1).
Summary of patients treated with the posterior oblique lateral approach
5. Ethics Statement
This study was approved by the institutional review board (IRB) of Siriraj Hospital, Mahidol University (COA No. 747/2025), and was conducted in accordance with the principles of the Declaration of Helsinki. Informed consent was waived by the IRB due to the retrospective nature of the study and the absence of identifiable patient information.
DISCUSSION
The management of foraminal and extraforaminal lumbar pathologies has long been a surgical challenge. The conventional open posterior approach in these regions often requires wider exposure and usually leads to instrumented fusion. The Wiltse paraspinal approach was originally developed to minimize midline dissection by exploiting the natural muscle-splitting plane [11]. The POL approach was inspired by this same philosophy. This technique adopts a posterior-oblique entry that preserves the muscle plane while docking at the TP to prevent early contact with neural structures. Similar bony docking strategies have been described to minimize neural irritation in prior studies. Tubular microdiscectomy techniques demonstrated the feasibility of caudal TP docking [6]. In a similar context, the intertransverse endoscopic approach reported by Musharbash and Lee [7] showed a low incidence of postoperative dysesthesia (9.1%) compared to classic transforaminal approach literature, suggesting that a posterolateral trajectory with bony docking at lateral aspect of the facet joint may reduce dorsal root ganglion irritation.
Beyond the challenges of direct access to Kambin triangle and the risk of nerve root irritation, anatomical variations further complicate the transforaminal approach, as the “classical” triangle is frequently narrowed or absent [5]. Disc space narrowing, often caused by degenerative disc disease, reduces the dimensions of Kambin triangle. Spondylolisthesis can further distort the normal anatomical relationships. Notably, Choi et al. [12] highlighted that the risk of neural complications increased by 23% for every 1-mm decrease in the distance of the safe zone, and recommended considering alternative approaches when this space is critically narrow. In addition, multiple studies reported the prevalence in 1.9%–4% of imaging studies and as high as 8.5%–30% in cadaveric series [13-15]. Variations in nerve root anatomy may occupy or traverse the safe zone [16]. Additionally, at the L5–S1 level, the presence of a high-riding iliac crest can obstruct the lateral trajectory and restrict instrument passage [4,17].
Importantly, this region contains the medial branch of the dorsal ramus, the target for radiofrequency ablation in facetogenic back pain patients [18]. This branch is frequently transected during exposure. Therefore, awareness of its course is critical not only for safe dissection but also for the integration of therapeutic strategies. Indeed, the same corridor has been used in endoscopic medial branch rhizotomy [19]. Thus, docking on the TP is not just a technical step, but also an anatomical safeguard and a flexible working space that can be adapted to different situations.
POL appears to be most useful for (1) extraforaminal and foraminal lesions with a narrowed or effaced Kambin triangle, (2) L5–S1 when a high iliac crest restricts lateral trajectory, and (3) cases requiring facetectomy to prepare a fusion corridor. As demonstrated in the case illustrations, the POL approach can address various anatomically constrained pathologies. Beyond these indications, routine use for foraminal and extraforaminal pathology may theoretically provide a greater margin of safety. The benefits of wider access must be weighed against potential risks related to anatomy and corridor preparation [20]. The risk of retroperitoneal collection may arise if the working corridor extends too far laterally or breaches the intertransverse membrane [21]. This technique prioritizes anatomy orientation and avoids neural injury over rapid entry.
This study presents a descriptive technical experience illustrated by a limited number of cases, without a comparator cohort and with short follow-up. As such, selection and operator biases are inherent, and long-term durability, including recurrence and reoperation rates, cannot be determined. Future studies may include prospective, multicenter registries and matched comparisons with the standard transforaminal approach to better define indications, complication profiles, and learning-curve effects.
CONCLUSION
This technical note presents the posterior oblique lateral (POL) approach as an alternative corridor for endoscopic spine surgery. By docking initially on the transverse process and advancing medially along sequential landmarks (TP → SAP → Facet capsule → Pars), the surgeon can create a controlled working space before entering Kambin triangle under direct visualization (TP-SAP junction → pedicle → disc). This technique focuses on anatomical orientation and workspace preparation, aiming to minimize early neural contact.
Notes
Conflicts of interest
P Sriphirom, a member of the Editorial Board of Journal of Minimally Invasive Spine Surgery & Technique, is the author of this article. However, he played no role whatsoever in the editorial evaluation of this article or the decision to publish it. The other authors have nothing to disclose.
Funding/Support
This study received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.
Acknowledgments
The authors would like to thank the Division of Neurosurgery, Department of Surgery, Siriraj Hospital, Mahidol University, for their support and contribution to this work.