Compact Navigation-Assisted Transpedicular Approach Using Unilateral Biportal Endoscopy for Caudally Migrated Lumbar Disc Herniation

Abstract

Introduction

Severely migrated lumbar disc herniations are challenging to access using conventional interlaminar or transforaminal approaches, especially in revision cases. A transpedicular route provides direct access but raises concerns regarding accuracy, safety, and radiation exposure when fluoroscopy is required. In this Technical Note, we describe a transpedicular unilateral biportal endoscopic (UBE) approach assisted by a compact navigation system for a caudally migrated disc herniation, highlighting its feasibility and potential advantages.

Technical Note

A 73-year-old man with a history of L1/2 fusion and multilevel laminoplasty presented with severe low back pain, radicular leg pain, and weakness. Magnetic resonance imaging demonstrated a left L2/3 disc herniation that had migrated caudally to the lower border of the L3 pedicle. Based on these findings, a transpedicular UBE approach assisted by a compact navigation system was selected. After intraoperative computed tomography (CT) acquisition and automatic registration, real-time navigation accurately localized the left L3 pedicle and displayed the planned entry point and drilling trajectory relative to the patient's anatomy. Using a navigated drill, a pedicle entry point was created, and the transpedicular channel was prepared. Under UBE visualization, the medial pedicle wall was drilled to expose the L3 nerve root, enabling direct removal of the caudally migrated fragment. The procedure was completed safely with minimal blood loss and required only a single intraoperative CT, with no fluoroscopy. Postoperative CT confirmed an approximately 6-mm transpedicular bone window without pedicle fracture. The patient had immediate symptom relief and remained recurrence-free at 6 months.

Conclusions

This Technical Note demonstrates the feasibility of a transpedicular UBE approach assisted by compact navigation for complete removal of a severely caudally migrated lumbar disc herniation. The technique allowed accurate instrument guidance, effective decompression, and zero radiation exposure to the surgical team. Future studies are warranted to validate its long-term safety and broader applicability.

Introduction

Surgical excision remains a treatment option for lumbar disc herniation¹⁾. Besides open discectomy—often microscope-assisted—percutaneous endoscopic techniques are increasingly popular^2,3). Single-portal procedures performed with a working-channel endoscope, known as full-endoscopic spine surgery (FESS), represent a modern evolution thanks to specialized instruments. Biportal techniques are also gaining traction; unilateral biportal endoscopic (UBE) spine surgery has been widely adopted and further developed in South Korea, offering a wider field of view and improved operability⁴⁾. Although these percutaneous procedures may have advantages over conventional methods due to their minimally invasive nature, they demand advanced skills, are difficult to master, and may involve similar rates of recurrence and complications⁵⁾. At the same time, standard interlaminar or transforaminal trajectories can become challenging when an extruded disc migrates cranially or caudally to lie ventral to the dural sac; in such cases, a transpedicular approach has been described.

Although only a few reports are available, a transpedicular approach can be performed via FESS⁶⁾. Partial resection of the pedicle provides direct access to large, herniated discs; however, identifying the pedicle accurately is difficult, and fluoroscopic confirmation exposes the surgeon to radiation^7-9). Navigation is therefore ideal in these cases. Conventional systems rely on stereoscopic cameras positioned outside the surgical field and require a large, unobstructed workspace, which can interfere with instrument handling and reduce operability.

A compact navigation system has recently been developed that mounts a monocular camera on the instrument to recognize reference points, eliminating the installation space required by conventional systems^10,11). This system appears highly compatible with both FESS and UBE. In this Technical Note, we describe a navigation-assisted transpedicular UBE technique for highly migrated lumbar disc herniations and outline its practical application.

Technical Note

Patient information

A 73-year-old man had a history of L1/2 fusion surgery and laminoplasty at L2/3, L3/4, and L4/5. He presented with low back pain and weakness of the left lower limb, accompanied by radicular pain that began after lifting a heavy object. The Visual Analog Scale (VAS) scores were 8 for low back pain and 7 for leg pain, and the Oswestry Disability Index (ODI) score was 43. Magnetic resonance imaging revealed a caudally migrated disc herniation originating from L2/3, in contact with the left L3 pedicle (Figure 1). The option of microdiscectomy via a UBE transpedicular approach, augmented with navigation, was presented to the patient, and written informed consent was obtained. Preoperative computed tomography (CT) demonstrated relatively large left L3 pedicle dimensions (11 mm cranio-caudally and 13 mm mediolaterally). Bone mineral density was 0.731 g/cm² at the femoral neck (T-score -0.7).

Figure 1.

Sagittal T2-weighted MRI slightly left of the midline and axial image at the level of the L3 pedicle. The L2/3-disc herniation is caudally migrated to the left and is in contact with the left L3 pedicle.

MRI: magnetic resonance imaging

Navigation system setup

We selected a transpedicular UBE approach assisted by a compact navigation system with smart delivery tools (NextAR system, Medacta International, Castel San Pietro, Switzerland) based on intraoperative CT imaging. The patient was placed in the prone position under general anesthesia, and the navigation setup was initiated. First, a target frame was fixed to the L3 spinous process using two percutaneous fixation pins, without the need for any additional skin incisions. A single-use target and a fiducial were then mounted on the frame (Figure 2a), and an intraoperative CT scan was acquired. During imaging, the surgical team left the operating room, ensuring zero radiation exposure. The fiducial contains a metal marker that is captured in the CT dataset and serves as a reference body for registering the physical target frame to the three-dimensional space reconstructed from the CT images on the NextAR control unit. The resulting DICOM dataset was uploaded to the NextAR Spine Software, which automatically recognized the markers. The fiducial was subsequently removed to optimize visualization of the anatomical structures and prevent obstruction of the surgical field. After registration, a monocular camera attached to the surgical instrument recognizes the target frame in the operative field, calculates the positional relationship between the instrument and the target frame, and transmits positional information via Bluetooth and projects the instrument's position onto the CT-based three-dimensional images displayed on the control unit. Real-time tracking of the probe, needle, bone tap, and drill was displayed on the control unit, eliminating the need for fluoroscopy and enabling accurate instrument navigation (Figure 2b and c)¹²⁾. Navigation accuracy was verified prior to use by placing the tip of a camera-equipped instrument on predefined calibration points of the target frame and confirming concordance with the corresponding position displayed in the three-dimensional navigation field (Figure 2d).

Figure 2.

(a) Representative intraoperative photograph showing the single-use target and fiducial markers (green) mounted on the target frame fixed to the spinous process for navigation tracking. (b) While viewing the Control Unit monitor, a navigated awl was used to identify and prepare the entry point of the left L3 pedicle. A monocular camera on the instrument recognizes the target frame (arrows), enabling real-time positional tracking displayed on CT-based three-dimensional images. (c) The surgeon can accurately identify the left L3 pedicle by simultaneously viewing the navigation image on the Control Unit monitor (left) and the endoscopic view (right), enabling accurate and radiation-free percutaneous surgery. (d) Representative intraoperative photograph showing the tip of the camera-equipped instrument placed at the calibration point of the target frame after removal of the fiducial markers for navigation accuracy verification.

CT: computed tomography

Surgical procedure

Discography was performed from the contralateral right side before surgery. A 1:1 mixture of contrast medium and indigo carmine (total volume 2.5 mL) was injected into the target disc. After completion of the navigation setup, the left L3 pedicle was identified. Under navigation guidance, a working portal was created with an approximately 10-mm skin incision over the projected trajectory of the left L3 pedicle. A second 6-mm skin incision was made about 10 mm cranial to the working portal for camera insertion. A navigated drill was used to create an entry hole in the subject vertebra, through which a guidewire was inserted. Along this guidewire, an 8-mm bone tap, selected to create a slightly larger working channel, was advanced along a pedicle-based trajectory to prepare a channel approximately 10 mm deep toward the anterior–caudal aspect of the pedicle, with the depth confirmed in real time using the navigation system (Figure 2b and c).

Under UBE, the medial aspect of the pedicle was drilled, allowing access to the posterior aspect of the L3 vertebral body through the bone window. A rigid endoscope system (Smith & Nephew) was used, with an outer sheath diameter of 3.8 mm and an endoscope diameter of 2.7 mm. A high-speed drill with a burr diameter of 4 mm was used. Irrigation was performed using gravity-driven saline flow, with the irrigation bag positioned approximately 90 cm above the operative field. At this stage, the bone window was completely covered by the extruded disc fragment, and partial removal of the fragment was first performed to obtain visualization. Subsequently, the remaining disc fragment, which was partially stained blue, and the left L3 nerve root compressed by the disc became visible (Figure 3a). After complete removal of the remaining disc fragment, adequate decompression of the left L3 nerve root was confirmed (Figure 3b), and its mobility was confirmed using a Penfield dissector. Because the extruded disc fragment was immediately encountered after creation of an approximately 6-mm bone window within the pedicle, direct manipulation or retraction of the nerve root was minimal. This technique is therefore considered suitable for highly migrated disc herniations in close contact with the pedicle. After confirming adequate hemostasis, the fascia and skin were closed without drain placement. The total operative time was 70 minutes, with minimal blood loss. Radiation imaging was performed only during navigation setup and preoperative discography. No fluoroscopy was required during the surgical procedure itself. Radiation exposure to the surgical team was limited to these imaging procedures, and patient exposure occurred only during these steps.

Figure 3.

(a) Endoscopic monitor view after creation of a 6-mm bone window and removal of the extruded disc. Slightly bluish-stained residual extruded disc and the left L3 nerve root compressed by the disc are visible. (b) On the endoscopic monitor, the removal of the extruded disc can be visualized, with the left L3 nerve root confirmed to be free of compression.

Follow-up and outcome

A postoperative CT scan confirmed an approximately 8-mm transpedicular channel created to a depth of 10 mm, with a 6-mm bone window at the medial aspect of the left L3 pedicle and preservation of pedicle integrity (Figure 4a and b). The patient's lower limb pain and muscle weakness resolved immediately, and he was discharged on the postoperative day one. At the 6-month follow-up, he remained symptom-free, with VAS scores of 2 for low back pain and 0 for leg pain, and an ODI score of 5. No recurrence or pedicle fracture was observed.

Figure 4.

(a) Postoperative axial CT image at the L3 level, showing an 8-mm channel created to a depth of 10 mm. A 6-mm bone window is visible at the medial aspect of the left L3 pedicle. (b) Three-dimensional CT oblique view from the left posterolateral direction (approximately 30°), showing the entry hole created on the lateral aspect of the left L3 lamina.

CT: computed tomography

Ethical considerations

This study was conducted in accordance with institutional ethical standards. Approval from an institutional review board was obtained, and written informed consent for publication was obtained from the patient. The navigation system is approved in Japan for specific clinical indications, primarily in cases involving spinal instrumentation; therefore, its use in the present case should be interpreted with caution.

Discussion

In this Technical Note, we presented a patient with a severely caudally migrated L2/3 left lumbar disc herniation after prior posterior decompression, successfully treated via a left L3 transpedicular approach using UBE assisted by a compact navigation system. This approach enabled precise instrument guidance, complete removal of the extruded nucleus pulposus, and radiation exposure to the surgical team limited to the imaging procedures, with favorable outcomes maintained at 6 months postoperatively.

The transpedicular approach offers a major advantage for accessing ventral lesions without excessive dural traction and has long been applied in thoracic spine surgery⁶⁾. More recently, it has also been adapted for cases of severely migrated lumbar disc herniation, particularly when the fragment lies in contact with the medial aspect of the pedicle^7,13-15). In such cases, the migrated nucleus pulposus is difficult to visualize through a transforaminal approach because of its deep location within the pedicle. Similarly, an interlaminar approach is often challenging in the upper lumbar spine, where the lamina is narrow, or in postoperative recurrences such as the one described here. Although migrated lumbar disc herniation can often be treated via an interlaminar or transforaminal approach, each technique has inherent limitations, particularly at upper lumbar levels. Extensive laminotomy may increase the risk of pedicle fracture or facet joint violation¹⁶⁾, and transforaminal approaches may endanger the exiting nerve root. When feasible, a transpedicular approach may help address these concerns by allowing direct access to the migrated fragment while preserving posterior bony structures.

A concern with the transpedicular approach is the potential compromise of pedicle axial strength. Meisterhans et al¹⁷⁾ reported in a cadaveric biomechanical study that the narrow pedicles at L1 and L2 may carry a higher risk of fracture. However, in a report of 35 cases by Jiang et al¹⁵⁾, in which caudally migrated herniations were removed via transpedicular FESS, no pedicle fractures occurred. Likewise, Krzok et al¹³⁾ reported preserved pedicle integrity in 21 cases involving cranially or caudally migrated herniations treated with transpedicular FESS and up to 8 mm of bone removal. Choi et al¹⁴⁾ also demonstrated bone remodeling and pedicle reconstruction at 1 year in a series of five patients. In our case, the relatively large L3 pedicle dimensions (11 mm cranio-caudally and 13 mm mediolaterally) may have contributed to the absence of fracture or lumbar instability, despite reaming with an 8-mm–diameter bone tap and creation of a 6-mm bone window, during the six-month follow-up. Careful preoperative radiological assessment of pedicle dimensions and conservative bone removal are advisable until further clinical experience is accumulated.

Although the transpedicular FESS approach is minimally invasive through a single portal, it poses challenges in achieving hemostasis, particularly when relying solely on radiofrequency coagulation⁷⁾. In contrast, UBE allows the use of adjunctive hemostatic agents such as bone wax, commonly used in open surgery, enabling more effective bleeding control. It also accommodates a broader range of instruments not feasible through a single-portal FESS. We believe these advantages outweigh the drawbacks of creating two small portals instead of one.

Finally, while anatomically rational, percutaneous approaches such as FESS or UBE require precise orientation. Most previous reports have relied on X-ray imaging to confirm pedicle position^7,13-15), but repeated imaging is time-consuming and increases radiation exposure for the surgical team⁷⁾. Navigation-assisted surgery overcomes these limitations; however, conventional systems often constrain surgeon posture and reduce operability. This is because conventional navigation systems rely on a stereo camera positioned outside the surgical field to detect both the target and the surgical instruments, with positional information calculated and projected onto the control unit, thereby requiring a wide, unobstructed line of sight between the camera, target, and instruments. In contrast, the compact navigation system employed in this case uses a different tracking principle, determining positional information using only a monocular camera attached to the surgical instrument in conjunction with the target, without concern for camera placement outside the surgical field (Figure 2b and c)^10,11). We therefore consider this system highly compatible with percutaneous surgery. However, the relative accuracy of this system compared with conventional navigation systems cannot be determined based on the present study. According to publicly available regulatory documentation, the system was cleared by the U.S. Food and Drug Administration based on substantial equivalence to an existing navigation system¹⁸⁾. Nevertheless, direct quantitative comparison of accuracy was beyond the scope of this report.

In conclusion, we presented a case of complete removal of a severely caudally migrated lumbar disc herniation in a patient with a history of prior surgery, successfully treated via a transpedicular approach using UBE combined with a compact navigation system. Although the findings from a single case cannot establish the overall utility of this approach, larger series are warranted to evaluate the risk of pedicle fracture and to further validate its safety and effectiveness. Similarly, the potential advantages of integrating UBE with compact navigation require confirmation in broader clinical studies. Nevertheless, this report highlights how advanced assistive technologies may enhance the precision and feasibility of minimally invasive spinal surgery.

Author Contributions: Designed the study, T.U. and Y.H.; performed the surgical procedures, T.U., T.K., N.K., T.H., and D.K.; drafted the manuscript, T.U. and Y.H.; critically revised the manuscript, T.K., N.K., T.H., K.T., S.K., and D.K.; supervised the study, T.U.

Conflicts of Interest: Payment/services: No financial support was received from any organization for the submitted work. Financial relationships: Takeshi Umebayashi: Personal fees from Medacta International. Serves as Director of the Tokyo Spine Clinic, a reference center for the M.O.R.E. Education Spine Program, which receives an annual contractual fee for hosting observers. Has received lecture and speaker fees on multiple occasions. Daichi Kawamura: Personal fees from Medacta International for assisting in a cadaver training session (April 2024) and delivering a lecture at the M.O.R.E. Japanese Symposium (October 2024). Shinji Kumamoto: Personal fees from Medacta International. A family-owned company managed by a relative has received lecture and speaker fees on multiple occasions. Yasukazu Hijikata: Personal fees from Medacta International as an invited speaker at a symposium in June 2023. The lecture was unrelated to this research; honorarium and travel/accommodation expenses were provided. Other relationships: None declared.

Ethical Approval: This study was approved by the Institutional Review Board of the Institute for Health Outcomes & Process Evaluation Research (Approval code: 202401).

Informed Consent: Written informed consent for publication was obtained from the patient.