INTRODUCTION
Lumbar interbody fusion remains the gold standard for treating patients with unstable degenerative lumbar disease. Minimally invasive surgery (MIS) techniques such as MIS-transforaimnal lumbar interbody fusion have gained popularity due to their advantages of direct neural decompression, simultaneous instrumented fusion, and reduced trauma to surrounding structures. Combining these technologies with endoscopic-assisted direct vertebral endplate preparation and cage insertion minimizes surgical trauma [1-3]. Additionally, inserting 2 cages—a posterior lumbar interbody fusion polyetheretherketone (PEEK) cage and an expandable cage—instead of a single large PEEK cage offers potential benefits. Our institute has adopted unilateral biportal endoscopic transforaminal lumbar interbody fusion (UBE-TLIF) with percutaneous instrumentation and dual hybrid cage insertion. This technical note presents our surgical technique for UBE-TLIF using dual hybrid cages.
MATERIALS AND METHODS
1. Anesthesia and Positioning
The patient undergoes general anesthesia in the prone position, with careful padding over the chest, pelvis, and knees. The C-arm fluoroscopy and an endoscopic monitor are placed on the contralateral side of the surgeon. The incision site depends on the patient’s predominant symptoms.
2. Skin Incision and Triangulation
Two longitudinal skin incisions are marked after surface marking under fluoroscopy guidance, with the inferior part of the cranial lamina in the center, 1.5 to 3 cm apart over the upper and lower pedicles. The upper incision accommodates the scope, while the lower incision serves as the working portal. A Cobb retractor is used to create subperiosteal space for optimal water flow. Triangulation is performed at the spinolaminar junction using a 0° scope. Soft tissue is cleared using a plasma radiofrequency (RF) probe, with hemostasis controlled, and good water outflow is maintained with a half-sleeve in the working portal.
3. Decompression, Facetectomy, and Auto-Bone Graft Harvesting
The facet joint is thoroughly cleared from soft tissue using the RF probe. Unilateral laminotomy and decompression with contralateral decompression are performed to achieve central and contralateral lateral stenosis. Inferior articular process (IAP) facetectomy is done first using an osteotome. The IAP is removed piece-by-piece under scope guidance. After IAP facetectomy, the superior articular process (SAP) is exposed and removed en bloc by osteotome.
4. Discectomy and Endplate Preparation
The ipsilateral transforaminal area is exposed. Hemostasis is achieved by coagulating epidural vessels, and the disc space is identified. A more medial annulotomy is performed with an arrow-shaped blunt blade to facilitate better contralateral endplate preparation, using the same 0° scope for better visualization. Traditionally, surgeons use a 30-degree scope for contralateral endplate preparation; however, a more medial annulotomy allows for the use of the 0° scope to remove contralateral disc material and the cartilaginous endplate, ensuring optimal placement of the first PEEK cage and allowing adequate space for the second cage. For endplate preparation, a serial handheld shaver system is used, followed by the removal of disc material with pituitary forceps and curettes to scrape the cartilaginous endplate. The scope is advanced into the intervertebral space to verify complete endplate preparation.
5. Dual Hybrid Cage Insertion
After thorough endplate preparation, bone grafting and cage insertion are the final crucial steps for interbody fusion. An appropriately sized bullet-shaped PEEK cage is selected based on the lateral view spacer measurement. A putty-like artificial bone graft is applied over the bullet-shaped PEEK cage, with a morselized bone graft packed inside the cage. The first PEEK cage is placed obliquely and pushed to the contralateral side under direct fluoroscopic vision using an impactor. Positioning is confirmed on the anteroposterior fluoroscopic view by visualizing the cage near the contralateral pedicular line. The second device, an expandable cage, is then inserted into the disc space under endoscopic visualization. An expandable cage 2 mm smaller than the PEEK cage is selected, as it can be introduced in a compact form and expanded by 2 mm within the disc space to match the final height of the PEEK cage. This ensures equalized disc height restoration and prevents graft and cage loosening. After expansion to the desired height, a bone grafting tunnel is inserted, and bone chips and demineralized bone matrix are packed inside, then locked with screws. This configuration prevents the PEEK cage from backing out, as it is securely held by the expandable cage. The final cage position is confirmed on C-arm fluoroscopy, ensuring ideal placement at the anterior part of the disc space in the lateral view (Figure 1).
6. Percutaneous Pedicle Screw Insertion Under C-Arm Fluoroscopy
Percutaneous pedicle screw insertion is performed bilaterally, and bilateral precontoured rods are applied. Screw positioning and rod application are secured and confirmed under fluoroscopy. After instrumentation, a 1/8-inch Hemovac drain is placed in the disc space under endoscopic guidance, followed by layered skin closure.
RESULTS
1. Case 1
A 64-year-old female with chronic lower back pain and bilateral lower leg radiculopathy presented with a 15-minute claudication time. She had a history of previous surgery elsewhere. Radiographic imaging revealed L2–3 and L3–4 spondylolisthesis with disc space narrowing and prior instrumentation at L4–S1 with PEEK cages at both levels. Magnetic resonance imaging (MRI) confirmed adjacent segment disc degeneration at L2–3, L3–4, bilateral lateral recess and foraminal stenosis, and spinal canal compromise. The patient underwent unilateral biportal endoscopy (UBE) lumbar fusion extension surgery using our dual hybrid cage insertion technique. Endoscopic-assisted laminectomy and decompression were performed, followed by dual hybrid cage placement at L2–3 and L3–4. Previous screws were removed endoscopically and replaced with percutaneous screws. New pre-contoured rods were applied bilaterally, extending fusion from L2 to S1 (Figure 2).
2. Case 2
An 83-year-old male presented with severe back pain and bilateral claudication. He walked with a forward stooped posture. X-rays showed degenerative scoliosis with 12.16° Cobb angle. MRI revealed ruptured disc fragment with degeneration, both facetal disruption and severe central canal stenosis at L3–4. She underwent endoscopic UBE-TLIF with dual hybrid cage insertion (Figure 3).
3. Case 3
A 75-year-old female presented with back pain and radicular pain from the buttock to the calf region for 1 year. She had a history of previous spine surgery at L4–5. X-rays indicated adjacent segment degeneration with collapsed disc space at L3–4 and collapsed foramina on extension imaging. MRI showed foraminal stenosis. She underwent fusion extension surgery at L3–4. Portals were made, and triangulation was performed, followed by laminectomy, flavectomy, and osteotomy of the SAP and IAP. The transforaminal area was exposed, hemostasis was achieved, and the traversing nerve root was identified. A more medial annulotomy was performed, disc space preparation was done, and contralateral disc preparation was performed. The cage slider was inserted, followed by the insertion of the appropriate-sized PEEK and expandable cages. Percutaneous cement-augmented screws were inserted due to osteoporosis (Figure 4).
All included patients were informed that data related to their cases would be submitted for publication, and written consent was obtained. All surgeries and postoperative courses were uneventful, with patients discharged between postoperative days 7 and 10 (Table 1). Follow-up MRI scans demonstrated well-decompressed dura, while postoperative radiographs revealed improved scoliosis correction, optimal sagittal alignment, and restored lumbar lordosis.
DISCUSSION
Although the use of 2 PEEK cages or one large cage, such as the transforaminal lumbar interbody fusion or oblique lumbar interbody fusion cage, is considered the gold standard for posterior approach fusion surgeries [2-6], we believe that UBE-TLIF using the dual hybrid cage insertion technique offers several advantages. The expandable cage allows surgeons to pass the implant through a small working portal, minimizing surgical trauma [6]. Controlled disc space expansion aids in decompressing neural structures. Additionally, expandable cages can accommodate more bone graft material, enhancing the potential for successful fusion without graft loosening. The increased surface area within the cage promotes bony ingrowth, fusion, and the development of a robust structural bridge between adjacent vertebral bodies [7]. Moreover, placement of 2 cages is less burdensome, providing nearly the same contact surface as compared to large single PEEK cage as shown in previous studies [8]. Authors have shown insertion of extreme lateral interbody cage through one medial annulotomy with good lordosis restoration through their novel “insert and revolve technique” [9]. We can add more bone chips in between the 2 cages to enhance the fusion bed.
UBE-TLIF minimizes disruption of paraspinal muscles and ligamentous structures, preserving their function and aiding faster recovery. Additionally, smaller incisions and reduced muscle trauma decrease postoperative pain and shorten hospital stays [10]. Continuous endoscopic magnification allows for precise identification and preservation of neural elements, especially during decompression and cage insertion, in contrast to traditional minimally invasive techniques.
The biportal approach accommodates a wide variety of surgical instruments, including those designed for traditional open or minimally invasive techniques, providing flexibility, control, and reduced surgery costs.
The rationale behind the dual hybrid cage insertion technique is to provide better structural support, a wider fusion surface area, and controlled, efficient deformity correction. A more medial annulotomy allows the surgeon to use a single scope for thorough endplate preparation under clear visualization. This enables surgeons to assess the completeness of fibrocartilage decortication, aiding in optimal cage placement. Adequate endplate preparation is a crucial factor in achieving successful interbody fusion.
The combination of a PEEK cage and an expandable cage optimally distributes load and enhances segmental stability. The radiolucent PEEK cage facilitates fusion monitoring and provides immediate structural support, while the expandable cage allows for customizable height restoration and precise coronal and sagittal alignment. This dual-cage strategy maximizes the fusion surface area while maintaining a minimally invasive profile. Furthermore, the expandable cage enables intraoperative adjustments in lordosis and height, making it particularly beneficial for achieving sagittal balance [11], especially in cases of degenerative lumbar disease with segmental kyphosis or loss of disc height [12].
Additionally, while a titanium TLIF cage may be a viable alternative to PEEK, the use of 2 titanium cages (a titanium TLIF cage combined with an expandable titanium cage) may impose an excessive mechanical burden on elderly patients with relatively weak bone quality. Given the potential risks of increased subsidence and inadequate bone integration, the use of a titanium TLIF cage is recommended only for carefully selected patients with normal bone quality, as verified by preoperative bone mineral density measurement.
UBE-assisted dual hybrid cage insertion requires advanced surgical skills and familiarity with both endoscopic techniques and cage insertion systems. In contrast to Zhu et al. [13], where the authors used a third portal to insert the second cage, we successfully inserted both cages through a single working portal [13-15]. However, inserting an expandable cage from one side, advancing it contralaterally, and expanding it within the disc space is technically unfeasible. Excessive manipulation during expansion to achieve contralateral placement poses a significant risk of dural injury and potential neurological complications.
Furthermore, creating an additional contralateral portal to facilitate cage placement would not only prolong operative time but also increase the likelihood of disrupting normal anatomical structures, which should be preserved whenever possible in minimally invasive procedures. Given these constraints, a hybrid technique—in which a PEEK cage is first inserted through a single approach and advanced deeply to the contralateral side, followed by the insertion of an expandable cage through the same approach—offers superior biomechanical stability and safety compared to the bilateral placement of expandable cages in a minimally invasive setting.
Achieving precise cage placement under endoscopic visualization demands meticulous planning and execution. While this technique may prolong the procedure, particularly for surgeons still learning UBE techniques, efficient workflow, and proper instrumentation are critical for minimizing operative time. Furthermore, proper retraction and careful handling of instruments are essential to avoid neural injury during cage insertion.
However, long-term studies are needed to validate these findings and assess the durability of the unique hybrid construct. Additionally, advancements in endoscopic tools, real-time imaging, and implant design will likely further refine this technique.
Future research should focus on comparative studies between single and dual-cage systems in UBE spinal fusion to determine the cost-effectiveness, complications, and overall benefits of this novel technique. Integration of robotic assistance and navigation technologies may also enhance precision and reduce the learning curve for surgeons adopting this technique [14]. Developing standardized protocols and specialized training programs for young surgeons adopting this technique could help overcome technical challenges.
CONCLUSION
Unilateral biportal endoscopic spinal surgery has evolved from a decompression-only technique to a fusion technique incorporating advanced interbody cage technology. In our limited series, UBE-assisted dual hybrid cages was associated with favorable pain relief, functional improvement, and satisfactory fusion status, suggesting potential advantages over single-cage systems; however, further studies with larger cohorts are needed to validate these findings. This innovative technique offers a minimally invasive solution for complex lumbar degenerative diseases, promising robust biomechanical and clinical outcomes.
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