Medial-caudal first access strategy for safe disc-side corridor creation in unilateral biportal endoscopic surgery for lateral lumbar disc herniation

Abstract

Lateral lumbar disc herniation presents a technical challenge because the herniated fragment may elevate the exiting nerve root and dorsal root ganglion within the narrow foraminal-extraforaminal corridor. During unilateral biportal endoscopy (UBE), limited depth perception in the early stages of decompression may increase the risk of premature neural contact and postoperative dysesthesia. We developed a medial-caudal first access strategy supported by O-arm navigation to establish a safe disc-side working corridor before direct neural exposure. Four patients with symptomatic lateral lumbar disc herniation underwent decompression using an anatomy-based strategy. Preoperative computed tomography/magnetic resonance imaging fusion imaging was used to identify a reproducible medial-caudal disc-side corridor according to the anatomical relationship between the disc fragment, pedicle, facet complex, iliac crest, and exiting nerve root. Portal placement was planned to reproduce this corridor, and O-arm navigation was used as an adjunct to facilitate the reproduction of the planned trajectory and portal geometry during surgery. The planned medial-caudal corridor was successfully reproduced in all four patients. Sufficient disc-side working space was established before direct exposure of the exiting nerve root, allowing gradual cranial and lateral decompression under continuous endoscopic observation. Adequate decompression was achieved without neurological complications, postoperative dysesthesia, or conversion to open surgery. Postoperative imaging confirmed sufficient decompression with limited medial bone removal. The proposed strategy may provide a practical and reproducible workflow for UBE treatment of lateral lumbar disc herniation. By prioritizing the creation of a limited disc-side working corridor before neural exposure, this technique may improve early depth control, reduce unnecessary neural manipulation, and preserve the surrounding bony structures. O-arm navigation serves as an adjunct to reproduce the preplanned surgical corridor, rather than as the primary objective of the procedure.

Highlight box.

Surgical highlights

• This article describes a medial-caudal first access strategy for unilateral biportal endoscopic decompression of lateral lumbar disc herniation.

• The key technical concept is to establish a limited medial-caudal disc-side working corridor before direct exposure of the exiting nerve root.

• Stepwise cranial and lateral expansion after disc-side depth acquisition allows controlled exposure and decompression of the exiting nerve root.

What is conventional and what is novel/modified?

• Endoscopic and navigation-assisted techniques have been used for foraminal and extraforaminal lumbar disc herniation to improve anatomical orientation and access to the narrow foraminal-extraforaminal corridor.

• The novel aspect of this technique is not navigation itself, but the sequence of decompression: sufficient disc-side working space is created first, and direct neural exposure is intentionally delayed until a disc-side working corridor is established. Portal placement is planned according to the intended disc-side trajectory rather than fixed skin distances, and O-arm navigation is used as an adjunct to reproduce the preplanned corridor and portal geometry.

What is the implication, and what should change now?

• In lateral lumbar disc herniation, surgeons should avoid searching for or manipulating the exiting nerve root during the early phase of decompression.

• Establishing a medial-caudal disc-side working corridor first may improve early depth control, reduce unnecessary neural manipulation, and allow controlled decompression while limiting bone resection.

• This strategy may provide a reproducible technical workflow for anatomically demanding unilateral biportal endoscopic decompression, although further validation in larger studies is required to confirm its efficacy.

Introduction

The term lateral lumbar disc herniation refers to foraminal, extraforaminal, and combined foraminal-extraforaminal lesions in which the exiting nerve root is the principal neural structure at risk during decompression. These lesions are surgically demanding and account for approximately 7–12% of lumbar disc herniations (1-3). The herniated fragment is often located close to the exiting nerve root and dorsal root ganglion within a narrow foraminal-extraforaminal corridor. In some cases, the disc fragment elevates or displaces the exiting nerve root cranially, increasing the risk of neural irritation during surgical exposure and fragment removal (1-3).

Minimally invasive and endoscopic techniques have been increasingly used to treat lateral lumbar disc herniation (4-8). These approaches can reduce soft-tissue disruption and provide direct access to the foraminal and extraforaminal regions. However, unilateral biportal endoscopy (UBE) is performed through a limited working corridor with a two-dimensional endoscopic view, and spatial orientation and depth perception may be difficult to achieve during the early phase of decompression. In this setting, premature exposure or manipulation of the exiting nerve root before sufficient disc-side working depth is established may increase technical difficulty and the risk of postoperative dysesthesia.

Navigation-assisted endoscopic surgery and preoperative computed tomography (CT)/magnetic resonance imaging (MRI) fusion imaging can improve anatomical orientation and facilitate preoperative trajectory planning (4-6,9-13). However, procedural safety in lateral lumbar disc herniation depends not only on image guidance or localization accuracy but also on the sequence in which a safe working corridor is established relative to the disc fragment, pedicle, facet complex, and displaced exiting nerve root. Although image-guided techniques facilitate anatomical localization, a reproducible stepwise workflow for creating a safe disc-side corridor before direct neural exposure has not been sufficiently described.

Therefore, we developed a medial-caudal first access strategy for unilateral biportal endoscopic decompression of lateral lumbar disc herniation. Rather than initially approaching the exiting nerve root, this strategy establishes a limited medial-caudal disc-side working corridor before direct neural exposure. By first acquiring a stable disc-side working depth and then gradually expanding the corridor cranially and laterally, the technique aims to improve early depth control, reduce premature neural contact, and facilitate controlled decompression of the exiting nerve root.

This surgical technique describes a reproducible stepwise workflow for establishing a medial-caudal disc-side working corridor before direct neural exposure during unilateral biportal endoscopic decompression of lateral lumbar disc herniation.

Specifically, we describe the preoperative planning process, portal design, stepwise decompression sequence, technical pearls, and potential pitfalls of the proposed strategy using O-arm-navigated UBE. The principal innovation lies in the decompression sequence, in which the disc-side working corridor is established before direct neural exposure, rather than in the surgical approach itself or the use of navigation. O-arm navigation is used as an adjunct to reproduce the preplanned corridor and portal geometry. We present this article in accordance with the SUPER reporting checklist (available at https://jss.amegroups.com/article/view/10.21037/jss-2026-0173/rc).

Preoperative preparations and requirements

Patient selection

This technique is intended for patients with symptomatic foraminal, extraforaminal, or combined foraminal-extraforaminal lumbar disc herniation causing radiculopathy corresponding to the affected exiting nerve root after the failure of conservative treatment. The applicability of this technique to lesions extending medially from the foraminal region toward the spinal canal has not been established because these lesions may require a different surgical trajectory and extent of facet resection; such lesions are therefore beyond the intended scope of this technical note. Preoperative CT and MRI should confirm that decompression can be achieved using a unilateral biportal endoscopic approach. Careful patient selection is particularly important in cases of severe bony stenosis, marked instability, or pathology requiring fusion surgery. This study was approved by the Institutional Review Board of Okayama Rosai Hospital (IRB No. 586). This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. Written informed consent was obtained from all patients for surgery and the publication of this article, clinical data, accompanying images, and the video. A copy of the written consent is available for review by the editorial office of this journal.

Preoperative imaging assessment

Preoperative CT/MRI fusion imaging was used to evaluate the three-dimensional relationship among the herniated disc fragment, exiting nerve root, pedicle, facet complex, disc inclination, and iliac crest. In this report, medial refers to the direction toward the facet complex relative to the expected course of the exiting nerve root, whereas caudal refers to the direction toward the caudal pedicle, or the sacral ala at L5/S1, relative to the affected disc space. The disc-side region is defined as the space immediately adjacent to the outer annulus at the affected disc level and beneath the elevated exiting nerve root. The working corridor refers to the endoscopically visualized pathway from the limited medial-caudal bony window to this disc-side region. Particular attention was paid to identifying a reproducible corridor that would allow the instruments to reach the disc-side region before direct neural exposure. This preoperative assessment also determined the planned decompression sequence and portal geometry (Figure 1).

Figure 1.

Preoperative CT/MRI fusion imaging and anatomical concept of the medial-caudal first access strategy. Preoperative CT/MRI fusion imaging demonstrating the anatomical relationship between the L5 exiting nerve root, pedicle, and lateral disc herniation at the L5/S1 level. The proposed medial-caudal first access strategy establishes a medialized disc-side working corridor beneath the elevated exiting nerve root before direct neural exposure. This corridor is designed to simultaneously avoid premature neural contact and stabilize early endoscopic depth perception and spatial orientation. The star indicates the intended medial-caudal starting point for creation of the disc-side working corridor beneath the elevated L5 exiting nerve root. CT, computed tomography; MRI, magnetic resonance imaging.

Required equipment

The procedure was performed using a standard unilateral biportal endoscopic system with a 30° arthroscope, high-speed drill, Kerrison punch, radiofrequency probe, and pituitary forceps. O-arm-based navigation was used as an adjunct to reproduce the pre-planned access corridor and confirm anatomical orientation during the initial phase of decompression.

Portal and corridor planning

Portal placement was designed according to the planned medial-caudal access corridor rather than fixed anatomical distances. The working portal was aligned with the intended disc-side trajectory, and the viewing portal was positioned to maintain continuous visualization of the decompression corridor. The portal configuration was adjusted according to the affected level, disc inclination, and iliac crest morphology to reproduce the preoperatively planned surgical trajectory.

Step-by-step description

Step 1. Patient positioning and navigation setup

The patient was placed in the prone position under general anesthesia. A percutaneous reference frame was attached to the iliac crest, and intraoperative three-dimensional imaging was performed using the O-arm system. Navigation-guided instruments, including probes and high-speed drills, were registered in a standard manner. Navigation was used as an adjunct to reproduce the preplanned corridor and to confirm the relationship among the disc space, pedicle, facet complex, and exiting nerve root during the initial phase of decompression.

Step 2. Portal placement based on the planned disc-side trajectory

Two small skin incisions were created for the viewing and working portals used for UBE. Portal placement was determined according to a preplanned medial-caudal disc-side trajectory rather than fixed skin distances. The working portal was aligned to allow the instruments to reach the disc-side region beneath the elevated exiting nerve root, whereas the viewing portal was positioned to provide continuous visualization of the medial-caudal corridor. At L5/S1, the portal configuration was adjusted according to the iliac crest height and disc inclination (Figure 2).

Figure 2.

O-arm-based navigation setup and portal planning. Intraoperative setup for O-arm-navigated unilateral biportal endoscopic surgery. Navigation-guided instruments were used to reproduce the preplanned medial-caudal access corridor based on the preoperative anatomical assessment. Multiplanar navigation views were used during portal placement and the initial phase of decompression to maintain the planned disc-side trajectory and spatial orientation.

Step 3. Identification of the medial-caudal starting point

Using a navigation-guided probe, the medial-caudal starting point was identified within the planned decompression field, using the lateral aspect of the facet complex as the medial reference landmark and the cranial surface of the caudal pedicle, or the sacral ala at L5/S1, as the caudal reference landmark. The starting point was located medial and caudal to the expected course of the exiting nerve root and the herniated fragment. The initial target was not the exiting nerve root itself but the disc-side region adjacent to the outer annulus beneath the elevated nerve root. This starting point was selected to avoid premature contact with the exiting nerve root and dorsal root ganglion while allowing early creation of the disc-side working corridor (Figures 3A).

Figure 3.

Stepwise creation of the medial-caudal disc-side working corridor and controlled neural exposure. (A) Initial medial-caudal access toward the disc-side region before direct exposure of the exiting nerve root. (B) Stepwise decompression with gradual expansion of the working corridor under continuous endoscopic visualization. (C) Establishment of sufficient disc-side working depth and space beneath the elevated exiting nerve root, allowing controlled neural mobilization and decompression. The navigation images in the lower panel demonstrate the accurate reproduction of the preplanned medial-caudal access corridor.

Step 4. Limited medial bony undercutting and ligamentous release

From the identified starting point, limited medial bony undercutting and ligamentous release were performed stepwise toward the disc-side region. Bone removal was confined to the minimum window between the facet complex and the cranial surface of the caudal pedicle, or the sacral ala at L5/S1, necessary to obtain instrument mobility and continuous visualization of the corridor. During this phase, the instruments were not directed cranially or laterally toward the expected location of the exiting nerve root, and direct neural exposure or manipulation was intentionally delayed until the disc-side corridor was sufficiently established (Figure 3B).

Step 5. Disc-side depth acquisition before neural exposure

After creating the limited medial-caudal corridor, the instruments were advanced toward the disc-side region adjacent to the outer annulus beneath the elevated exiting nerve root. In this report, sufficient disc-side working depth was considered to have been established when the affected disc level, outer annulus, and adjacent bony landmarks were directly visualized and the instruments could be manipulated within the corridor under continuous endoscopic observation. Working depth therefore describes a procedural working condition rather than a fixed numerical distance or a predefined anatomically safe zone confirmed before visualization of the exiting nerve root. In cases of marked nerve root displacement or tight compression, preoperative fusion imaging and navigation were used only to estimate the expected location of the nerve root; subsequent cranial and lateral dissection proceeded incrementally under direct endoscopic visualization until the nerve root was identified (Figure 3C).

Step 6. Stepwise cranial and lateral decompression

Once sufficient disc-side working depth and spatial orientation were established, decompression was gradually extended toward the cranial and lateral directions under continuous endoscopic visualization. Herniated disc fragments were removed stepwise, and the exiting nerve root was exposed and mobilized only after an adequate working space was created. Final endoscopic inspection confirmed sufficient decompression and free mobilization of the exiting nerve root. The spatial relationship between the medial-caudal starting point, the intended disc-side corridor, and the exiting nerve root is summarized in Figure 4. The complete surgical workflow is demonstrated in Video S1.

Figure 4.

Spatial concept of the medial-caudal first access strategy. The schematic illustration, three-dimensional fusion image, and corresponding endoscopic views demonstrate the medial-caudal starting point and subsequent corridor creation relative to the facet complex, sacral ala, affected disc, and exiting nerve root at L5/S1. The asterisks indicate the medial-caudal starting point, located medial and caudal to the expected course of the exiting nerve root. The corridor is subsequently developed from this starting point toward the disc-side region adjacent to the outer annulus, which represents the intended endpoint before cranial and lateral decompression and direct neural exposure. The double-headed arrow represents the greatest available separation from the exiting nerve root within the operative corridor, indicating the safest medial-caudal region for initial access.

Video S1.

Download video file ^{(81.3MB, mp4)}

Medial-caudal first access strategy for O-arm-navigated unilateral biportal endoscopic decompression of lateral lumbar disc herniation. This video demonstrates the complete sequential decompression workflow using the proposed medial-caudal first access strategy for lateral lumbar disc herniation, beginning with navigation-guided portal planning and initial docking. Preoperative computed tomography/magnetic resonance imaging fusion imaging and navigation-guided portal planning were used to reproduce a medialized disc-side access corridor beneath the elevated exiting nerve root. Initial decompression was intentionally initiated from the medial and caudal aspects to establish sufficient disc-side working depth before direct neural exposure. Stepwise decompression and limited medial bony undercutting allow gradual expansion of the working corridor toward the cranial and lateral directions while maintaining endoscopic spatial orientation. After sufficient disc-side working space is created, controlled mobilization and decompression of the exiting nerve root are performed under continuous endoscopic visualization. The video demonstrates how delayed neural exposure following stepwise corridor creation may reduce unnecessary early neural manipulation during anatomically demanding endoscopic decompression surgery.

Postoperative considerations and tasks

After decompression, the surgical field should be endoscopically inspected to confirm complete removal of the herniated fragment, adequate decompression of the exiting nerve root, and free mobilization of the nerve root without residual compression. Particular attention should be paid to the cranial and lateral aspects of the decompression field, where residual compression may remain if the disc-side corridor has not been sufficiently expanded.

Postoperative MRI should be used to confirm adequate decompression of the affected exiting nerve root. Postoperative CT is useful for evaluating the extent of bony removal and confirming that decompression has been achieved through a limited medial-caudal corridor with preservation of the surrounding facet structure.

The present surgical technique series comprised the first four consecutive patients treated using the proposed workflow after its clinical introduction, and no eligible patients were excluded. All procedures were performed by a single spine surgeon. The series included three L5/S1 lesions and one L4/5 lesion, with three right-sided and one left-sided lesions. In all patients, the preplanned medial-caudal access corridor was successfully reproduced using O-arm-based navigation. Adequate decompression was achieved without intraoperative neurological complications, dural injury, excessive bleeding, conversion to open surgery, or postoperative dysesthesia. Postoperative MRI confirmed adequate decompression of the exiting nerve root, whereas postoperative CT demonstrated that decompression had been achieved through the intended medial-caudal corridor with localized medial bony undercutting and preservation of the surrounding posterior elements (Figure 5).

Figure 5.

Representative postoperative imaging following the medial-caudal first access strategy. Postoperative MRI demonstrates adequate decompression of the exiting nerve root. Postoperative CT confirms localized medial bony undercutting while preserving the surrounding facet complex and posterior elements, consistent with the intended medial-caudal corridor. The arrows indicate the localized medial bony undercutting that formed the intended medial-caudal corridor. CT, computed tomography; MRI, magnetic resonance imaging.

The mean operative time was 116 min (range, 90–145 min), and the mean follow-up period was 9 months. The mean visual analog scale (VAS) score for low back pain improved from 77.5 preoperatively to 36.3 at 3 months postoperatively, and the mean VAS score for leg pain improved from 88.8 to 33.8. No patient developed new postoperative neurological deficits or dysesthesia during follow-up.

Tips and pearls

The key technical pearl of this strategy is to establish a medialized disc-side working corridor before direct exposure of the exiting nerve root. Surgeons should avoid searching for or manipulating the exiting nerve root during the early phase of decompression. Instead, the initial objective should be to create adequate disc-side working space through limited medial-caudal bony undercutting.

Portal placement should be determined according to the planned disc-side trajectory rather than fixed skin distances. At the L5/S1 level, particular attention should be paid to the iliac crest height and disc inclination, as these anatomical factors may restrict the working trajectory. If the initial working angle becomes excessively cranial or lateral, the instruments may approach the exiting nerve root before sufficient disc-side working depth is established, making subsequent decompression more technically demanding.

Bone removal should be limited to the minimum medial bony window necessary to obtain an adequate working depth and instrument mobility. Excessive facet resection should be avoided, particularly in patients with pre-existing degenerative instability. The pedicle and disc space serve as constant anatomical landmarks when orientation becomes difficult and can be used to re-establish the planned medial-caudal corridor before proceeding further.

Navigation should be regarded as an adjunct rather than a substitute for endoscopic anatomical recognition. The navigation-guided trajectory should always be verified using endoscopic anatomical landmarks, particularly during the initial phase of corridor creation. If bleeding, epidural fat, or soft-tissue obstruction compromises visualization, the surgeon should return to the confirmed disc-side and pedicle landmarks before proceeding cranially or laterally.

Direct neural exposure should be intentionally delayed until an adequate disc-side working depth and space are established. Once sufficient disc-side working space is created, the exiting nerve root can be exposed, mobilized, and decompressed in a controlled manner with less need for abrupt neural retraction.

Discussion

Lateral lumbar disc herniation remains surgically demanding because the herniated fragment is located close to the exiting nerve root and the dorsal root ganglion within a narrow foraminal-extraforaminal corridor (1-3). In some cases, the exiting nerve root and dorsal root ganglion may be displaced cranially by the herniated fragment, making premature neural contact or excessive retraction a potential cause of postoperative dysesthesia or neurological irritation.

Various minimally invasive and endoscopic approaches have been developed for the treatment of foraminal and extraforaminal lumbar disc herniation (4-8). In the conventional paraspinal approach, the foraminal or extraforaminal lesion is accessed through a posterolateral muscle-splitting corridor, with decompression centered on the lateral facet region and the affected exiting nerve root (2,7). Transforaminal percutaneous endoscopic discectomy has also been reported to be safe and effective for foraminal and extraforaminal lumbar disc herniations (14). More recently, favorable clinical outcomes of UBE for far lateral lumbar disc herniation have been reported, including comparative studies with other endoscopic techniques (15,16). These previous reports have described the surgical approach, portal placement, or clinical outcomes. The present technique does not introduce a new paraspinal route; rather, it differs in the predefined decompression sequence, in which a medial-caudal disc-side working corridor is established and direct neural exposure is intentionally delayed until sufficient working space has been created.

Although these techniques reduce surgical invasiveness and improve access to anatomically constrained regions, spatial orientation and depth perception may remain difficult during the early phase of decompression, particularly when working close to neural structures. Navigation-assisted endoscopic surgery and CT/MRI fusion imaging can improve anatomical orientation and surgical planning (4-6,9-13). However, procedural safety depends not only on image guidance or localization accuracy, but also on the sequence by which the initial working corridor is created relative to the displaced exiting nerve root, dorsal root ganglion, and disc fragment.

The main technical concept of the present strategy is the controlled timing of neural exposure. In the proposed workflow, decompression is initiated through a medial and caudal disc-side corridor rather than directly approaching the exiting nerve root from the lateral aspect. This sequence allows sufficient disc-side working space to be established before direct neural exposure. After this working space is created, the corridor can be gradually expanded cranially and laterally, allowing the exiting nerve root to be exposed and mobilized in a more controlled manner.

A key feature of this strategy is the limited medial bony undercutting to reach the disc-side region beneath the elevated exiting nerve root. This does not represent wide facetectomy but rather the creation of the minimum medialized bony window required for safe corridor formation and instrument mobility. Because excessive facet resection may increase segmental mobility and mechanical stress (17), bone removal in this strategy is intentionally limited to the medial-caudal corridor necessary for neural avoidance and early depth stabilization. Conversely, limited partial facetectomy reportedly achieves foraminal decompression without clinically significant postoperative instability (18).

In the present four-case series, the preplanned medial-caudal access corridor was reproduced using O-arm-based navigation, and adequate decompression was achieved without neurological complications, dural injury, conversion, or postoperative dysesthesia. Postoperative imaging confirmed adequate decompression with limited bone removal. These findings support the feasibility of this stepwise decompression workflow, although the small number of cases precludes definitive conclusions regarding its superiority over other approaches.

O-arm navigation should be interpreted as an adjunct rather than the principal innovation of this procedure. Its role is to facilitate reproduction of the preplanned anatomical corridor and portal geometry, particularly during the initial phase of decompression when endoscopic depth perception may be limited. However, O-arm-assisted surgery has practical limitations, including limited equipment availability, additional radiation exposure, and increased operative cost. Because the essential concept of the present technique is the anatomy-based decompression sequence rather than navigation itself, the proposed workflow may also be reproducible using conventional fluoroscopic guidance by surgeons familiar with the relevant anatomy and surgical steps. Navigation may nevertheless facilitate anatomical orientation and reproduction of the planned corridor, particularly during the early learning phase.

This study had several limitations. First, although this series comprised the first four consecutive patients treated after the clinical introduction of the technique and no eligible patients were excluded, the small sample size and single-surgeon setting limit the generalizability of the findings, and potential selection bias cannot be excluded. Further validation involving larger cohorts, multiple surgeons, and multiple institutions is required to confirm the reproducibility and broader applicability of the proposed workflow. Second, clinical outcome assessment was limited primarily to VAS scores and neurological complications. Postoperative functional outcomes, including Oswestry Disability Index (ODI) and Macnab classification, as well as reoperation and return-to-work outcomes, were not systematically evaluated. Moreover, the mean follow-up period of 9 months was insufficient to adequately evaluate longer-term outcomes, including recurrence and postoperative segmental instability. Studies with longer follow-up are required to evaluate the durability and long-term clinical outcomes of the proposed technique.

Third, this surgical technique study did not include a quantitative comparison with other surgical approaches or non-navigation-assisted procedures. Finally, this strategy may not be universally applicable across all anatomical variations, and its applicability to lesions extending medially from the foraminal region toward the spinal canal has not been established.

Conclusions

The medial-caudal first access strategy may provide a practical and reproducible framework for unilateral biportal endoscopic decompression of lateral lumbar disc herniation. By establishing a limited medial-caudal disc-side working corridor before direct neural exposure, this approach may reduce premature neural contact, stabilize early endoscopic depth perception, and allow controlled cranial and lateral decompression while limiting bone resection.

Supplementary

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Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. This study was approved by the Institutional Review Board of Okayama Rosai Hospital (IRB No. 586). This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. Written informed consent was obtained from all patients for surgery and the publication of this article, clinical data, accompanying images, and the video. A copy of the written consent is available for review by the editorial office of this journal.