ILLUSTRATIVE CASE
A 56-year-old female presented with chronic low back pain for 5 years, which had progressively worsened, accompanied by bilateral leg pain and numbness for 2 years. Conservative treatment, including medication and physical therapy, failed to provide relief. On examination, her visual analogue scale (VAS) score was 8 for back pain and 6 for leg pain. Neurogenic claudication occurred after approximately 15 meters of walking. Bilateral calf muscle atrophy was noted, with L5 root weakness graded 4/5. Knee and ankle reflexes were preserved, hip motion was normal, and the Babinski sign was negative.
Plain radiographs demonstrated L4–5 instability on dynamic flexion–extension views as well as disc height loss. Furthermore, magnetic resonance imaging (MRI) revealed severe central canal stenosis at L4–5 with marked lateral recess hypertrophy, resulting in severe bilateral foraminal stenosis (Figure 1).
STEP-BY-STEP DESCRIPTION
In such cases, several surgical options for achieving fusion are available, including conventional open surgery, microscopic tubular-assisted fusion, or biportal endoscopic fusion. The grafting materials typically used are similar, consisting of autologous bone, hydroxyapatite granules, and a transforaminal lumbar interbody (TLIF) cage. However, in this patient, preoperative imaging not only demonstrated severe central canal stenosis but also significant loss of disc height combined with facet hypertrophy, which together led to severe foraminal narrowing affecting both the traversing and exiting nerve roots. Therefore, achieving adequate decompression of the lateral recess and inserting a larger cage would allow indirect foraminal decompression and enhance the decompression effect. For these reasons, we selected the insertion of an OLIF cage via the biportal endoscopic approach, followed by percutaneous pedicle screw fixation (PPSF).
1. Anesthesia, Position, and Instruments
This procedure was performed under general endotracheal anesthesia with the patient placed prone on a Wilson frame. A radiolucent spine table was used, and the height was adjusted to allow optimal anteroposterior and lateral fluoroscopic imaging. Throughout this technique, the surgeon stood on the left side. The surgical instruments included a standard unilateral biportal endoscopic (UBE) set (BONSS, China), a high-speed diamond burr (NSK Primado2, Japan), an ArthroCare bipolar system, a radiofrequency probe for hemostasis, and the OLIF25 instrumentation system (Medtronic, USA). (Figure 2A).
2. Skin Incisions and Creation of Working Space
Under fluoroscopic guidance, 2 portals were established. The working portal was created through a transverse skin incision of approximately 1.5 cm, located about 1 cm lateral to the superolateral border of the caudal pedicle. This position not only provides quick access to the target point but is also sufficiently lateral from the midline to allow an adequate oblique angle for advancing the distal tip of the cage across to the contralateral side. The incision was initially maintained at 1.5 cm to preserve stable irrigation pressure within the surgical chamber, ensuring a clear operative field. Before insertion of the trial and cage, it could be further extended laterally to approximately 2 cm, thereby facilitating smooth passage of instruments and implants. As in standard BE-LIF, the target point for orientation was the spinolamina junction. The scope portal was made as a longitudinal incision along the outer margin of the cranial lamina. Standard triangulation was then established, similar to conventional BE-LIF techniques, to ensure proper orientation before proceeding with decompression (Figure 2B).
3. Laminectomy and Decompression
Following soft tissue clearance with the ArthroCare bipolar, the spinolamina junction was identified as the initial bony landmark. A high-speed 4-mm diamond burr was then used to thin the ipsilateral lamina cranially until the free margin of the ligamentum flavum (LF) was exposed. Drilling was subsequently extended laterally to thin the pars interarticularis. Using an osteotome, the thinned pars and joint capsule were detached, allowing removal of the entire inferior articular process (IAP). Further burring of the superior articular process (SAP) was performed until the superior and lateral borders of the caudal pedicle were clearly visualized, thereby maximizing the working corridor through Kambin’s triangle. Before performing the flavectomy, the facet was partially resected to expose the medial border of the caudal pedicle. A wide ipsilateral flavectomy was then carried out, achieving adequate decompression of the spinal canal and providing direct access to the disc space. At this stage, an over-the-top laminotomy could also be performed, if necessary, to decompress the contralateral lateral recess.
4. Endplate Preparation
After adequate decompression was achieved, the annulus fibrosus was incised to create a working window into the disc space. A Kerrison punch was used to enlarge the window medially and laterally under careful protection of the nerve root with a root retractor. Low-energy ArthroCare bipolar was applied to clean the annular surface, thereby improving visualization of the disc space. Disc material was then removed systematically using pituitary forceps, shavers, and curettes. Endplate preparation was performed meticulously, with complete removal of the cartilaginous layer to expose the bony endplate while preserving its cortical integrity. Preparation was extended to both the ipsilateral and contralateral sides to maximize the bone graft contact surface.
5. Bone Grafting and Cage Insertion
Subsequently, a trial cage was introduced obliquely into the disc space to evaluate both the trajectory and the adequacy of the disc window. Under lateral fluoroscopy, the distal tip of the trial was advanced until it reached the anterior one-third of the vertebral body (Figure 3B). The width of the window was then measured relative to the trial cage, and a dimension greater than 2 cm was considered sufficient to allow safe passage of the definitive cage without undue pressure on the neural elements. This step ensured that both the working angle and the available space were adequate before proceeding with final cage insertion (Figure 3A).
Following meticulous endplate preparation, bone grafting was performed through the disc window using a mixture of autologous bone and hydroxyapatite granules. The graft was carefully delivered through the bone tunnel and packed under fluoroscopic guidance to ensure accurate placement and maximal contact with the prepared endplate.
An appropriately sized OLIF cage (10 mm × 45 mm, 6°) was then inserted along the predetermined oblique trajectory. Under simultaneous endoscopic and lateral fluoroscopic guidance, the cage was advanced gradually until the distal tip crossed to the contralateral side. Once in position, the cage was rotated into a horizontal orientation and gently impacted until the distal tip aligned with the border of the vertebral body. Final adjustments secured the cage in the anterior one-third of the disc space, providing robust structural support and indirect foraminal decompression, complementing the direct neural decompression already achieved (Figure 3C).
6. Final Decompression and Fixation
After bone grafting and cage insertion were completed, contralateral decompression was further extended to ensure that both the central canal and contralateral lateral recess were adequately decompressed. The LF was widely removed, and neural elements were checked to confirm complete decompression before closure.
Finally, PPSF was performed under fluoroscopic guidance. Pedicle screws were placed bilaterally, followed by rod insertion and tightening to stabilize the construct.
7. Video Demonstration
The operative procedure is demonstrated in Supplementary Video Clip 1, with a detailed transcription.
8. Postoperative Considerations and Outcomes
The total operative time was 180 minutes, with endplate preparation requiring more time than in conventional BE-LIF because of the need for wider and more meticulous preparation to accommodate the OLIF cage. Intraoperative blood loss was estimated at 150 mL. The surgical drain was removed, and the patient was mobilized at 24 hours postoperatively. She was discharged on postoperative day 5 with significant clinical improvement, reporting a VAS score of 2 for both leg and back pain. Postoperative radiographs and computed tomography (CT) scans confirmed adequate decompression, restoration of disc height, and correct positioning of both the OLIF cage and pedicle screws.
DISCUSSION
BE-LIF has become increasingly acknowledged as a safe and effective minimally invasive approach for the treatment of degenerative lumbar conditions [1-4]. The technique combines the fundamental principles of interbody fusion with the advantages of biportal endoscopy, allowing surgeons to achieve both direct neural decompression and stable fusion. Compared with microscopic tubular-assisted fusion or uniportal endoscopy, BE-LIF provides distinct technical advantages [4-8]. The use of 2 independent portals separates visualization from the working channel. The viewing portal delivers a magnified, well-illuminated surgical field, enabling precise identification of neural and osseous structures, while the working portal offers a wider operative corridor and greater flexibility for handling instruments. Clinically, these technical strengths contribute to meaningful benefits, such as reduced soft tissue injury, less postoperative wound pain, and faster rehabilitation [3,9]. Several clinical series have consistently demonstrated significant pain relief, improved function, and low complication rates following BE-LIF [10-13]. Importantly, these benefits are not limited to the early postoperative period. Radiographic studies with mid-term follow-up have reported encouraging fusion rates and restoration of foraminal height, supporting both durable decompression and long-term stability of the operated segments [10,12].
The achievement of successful fusion is a critical determinant of long-term clinical stability across all lumbar interbody fusion techniques, and BE-LIF is no exception [14-17]. A stable fusion ensures lasting neural decompression, maintains sagittal balance, and reduces the risk of recurrent instability. Multiple factors influence the likelihood of fusion success, many of which are closely tied to surgical technique [14,16]. Meticulous endplate preparation is fundamental: the cartilaginous layer must be completely removed while preserving the integrity of the bony endplate, providing a biologically favorable surface for bone graft incorporation without predisposing to subsidence [10,12,17]. The ability to place bone graft material under direct endoscopic visualization and simultaneous fluoroscopic guidance further enhances accuracy, allowing grafts to be delivered precisely into the intended space with maximal endplate coverage [10,12]. This careful placement not only optimizes the surface area of bone-to-graft contact but also reduces the risk of graft displacement in the continuous irrigation environment. Another technical determinant is cage positioning. Placing the cage in the correct position within the disc space facilitates the maintenance of disc height, contributes to sagittal balance, and creates conditions for indirect foraminal decompression [10,12,18]. Beyond technique, the properties of the bone graft materials themselves, including both composition and configuration, also play an essential role [10,14,17]. Autologous bone remains the gold standard for osteogenic potential, while bioactive substitutes such as hydroxyapatite or bone morphogenetic protein contribute osteoconductive support [19]. However, among all variables, cage characteristics particularly footprint size have emerged as one of the most influential factors determining fusion success, maintenance of foraminal height, and the risk of cage subsidence [1,14,20].
Current literature has explored multiple strategies to increase cage footprint in order to enhance fusion and reduce complications [10,12,14,17,20,21]. Some involve high-cost technologies, such as the use of double posterior lumbar interbody fusion (PLIF) cages [10,13], dual-direction expandable titanium cages [22,23], or other expandable cage systems that can be inserted through a narrow corridor but expand within the disc space [24]. Others emphasize cost-saving solutions, including the use of en bloc IAP fragments combined with a TLIF cage to enlarge the contact area [12]. Within this spectrum, the OLIF cage represents another viable option, offering a broader footprint than conventional TLIF cages [25-27]. A larger cage footprint confers several biomechanical and biological advantages. By increasing the surface area of contact with the prepared endplates, the cage provides a more stable foundation for bone healing, thereby promoting higher fusion rates. The broader surface also distributes axial loads more evenly across the endplates, reducing stress concentration and lowering the likelihood of subsidence. Indeed, recent studies [17,20] have demonstrated significantly lower subsidence rates with large cages compared with standard-sized cages. This effect is directly attributable to the wider contact area, which minimizes endplate overload and maintains disc height over time. In addition, large cages support indirect decompression of the neural foramina by restoring disc and foraminal height, an effect particularly beneficial in cases with severe disc collapse and facet hypertrophy [17,18,27].
Although cage material is an important consideration—for example, polyetheretherketone cages provide elastic properties similar to bone and reduce stress shielding, while 3-dimensional-printed titanium cages offer superior osteo-conductivity and promote osseointegration—current evidence indicates that cage footprint has a greater impact on clinical outcomes than material composition [10,14,20]. While material characteristics influence the biological fusion process, it is the size and shape of the cage that directly determine the extent of endplate contact, the degree of disc height restoration, and the risk of subsidence. In this regard, the OLIF cage in BE-LIF, with its large footprint, offers several advantages [11,20,28]. Unlike conventional OLIF performed through an anterior oblique corridor, which primarily achieves indirect decompression of the neural elements, insertion of an OLIF cage under biportal endoscopic guidance enables not only indirect decompression through disc height restoration but also direct posterior decompression under clear visualization [11,28,29]. This hybrid approach combines the precise visualization of endoscopy with the biomechanical benefits of a large cage. Moreover, BE-LIF with OLIF cage insertion requires only minor modification of the working portal incision and optimizes disc space preparation by adequately widening Kambin’s triangle. Importantly, this technique does not require an additional lateral skin incision for cage insertion, as is necessary in the eX-TLIF technique [11,30]. Compared with techniques using 2 smaller PLIF cages to achieve a similar footprint, a single OLIF cage provides equivalent endplate contact while reducing implant-related risks such as migration. Another practical advantage lies in bone graft management: during endoscopic fusion, continuous irrigation poses the risk of graft washout [10,12], and the OLIF cage acts as a physical barrier, securing graft material in the deep zone and thereby improving graft stability.
Despite these advantages, OLIF cage insertion through BE-LIF remains technically demanding and should be undertaken with caution [11,24,27,28]. Several technical considerations are worth highlighting. Patient selection plays a crucial role. We currently apply this technique only at the L3–4 and L4–5 levels, and not at L5–S1, where the high iliac crest often restricts the working corridor, particularly when the surgeon stands on the left side of the patient, thereby limiting the oblique trajectory required for safe cage insertion. The indication should be limited to lumbar stenosis with instability or low-grade spondylolisthesis (Meyerding grade I) [11]. Higher-grade spondylolisthesis is typically associated with severe disc height loss and posterior facet overhang, which narrow Kambin triangle and make it difficult to prepare a sufficiently wide and safe disc space window for cage insertion.
Proper portal planning is another key factor for safe and efficient cage insertion. Two-portal placement must be individualized according to patient anatomy. The working portal should be aligned parallel to the endplates on lateral fluoroscopy to ensure that the cage can be advanced along the correct trajectory [11,28]. In addition, the working portal should be optimized to allow both cage insertion and subsequent percutaneous pedicle screw placement without the need for an additional lateral skin incision, as in the modified technique described by Heo et al. [11] or in the UBE-extreme transforaminal lumbar interbody fusion approach using a large cage as reported by Tian et al. [30]. The incision design must also maintain adequate outflow and preserve irrigation pressure within the surgical field. Specifically, during the initial steps of laminectomy, decompression, and endplate preparation, the working portal incision should be kept minimal, approximately 1.0–1.5 cm to preserve irrigation pressure, facilitate venous hemostasis, and ensure a clear endoscopic view. Skin extension should be performed only at the stage of trial and cage insertion. Making the working portal incision too wide at the beginning can lead to loss of irrigation pressure, increased venous bleeding, and impaired visualization.
Finally, several intraoperative safety considerations should be emphasized to minimize the risk of neural injury during cage insertion [11,28]. First, attention should be paid to the size of the disc window and its correlation with the dimensions of the trial and definitive cage. Ensuring a window width of more than 20 mm is critical for safely accommodating the oblique trajectory of the OLIF cage. Second, decompression of the spinal canal should be completed, or at least a full unilateral flavectomy performed, before root retraction and cage insertion to provide adequate space and reduce neural structures tension. Lastly, the cage insertion process should be carefully monitored simultaneously under both fluoroscopic and endoscopic visualization to ensure precise control of trajectory and depth, while protecting neural elements throughout the procedure.
Summary, these considerations emphasize both the potential and the challenges of OLIF cage insertion via BE-LIF. The approach combines the minimally invasive benefits of biportal endoscopy with the biomechanical advantages of a large-footprint cage, potentially enhancing fusion rates, reducing subsidence, and providing durable clinical outcomes. At the same time, its technical demands highlight the need for careful patient selection, individualized portals planning, and meticulous surgical execution. Our case demonstrates feasibility and provides a step-by-step illustration of the technique. However, further clinical studies with larger cohorts and longer follow-up are essential to validate its safety, efficacy, and reproducibility.
CONCLUSION
This report describes the technical feasibility of inserting an OLIF cage via BE-LIF, combining the advantages of a large cage footprint with the minimally invasive benefits of the biportal endoscopic approach. While the technique is technically demanding, it appears feasible and may enhance fusion potential and long-term outcomes. Further clinical studies with larger patient cohorts and longer follow-up are required to validate its safety, efficacy, and reproducibility.
CASE DESCRIPTION
0:16 Case Description
A 56-year-old female with 5 years of worsening low back pain and 2 years of bilateral leg pain and numbness, unresponsive to conservative treatment. Examination showed VAS 8 for back pain, 6 for leg pain, neurogenic claudication at fifteen meters, bilateral calf atrophy, and L5 root weakness grade 4 out of 5. Reflexes were normal and Babinski sign was negative.
SURGICAL TECHNIQUE
Anesthesia & Position
1:05 We performed a biportal endoscopic lumbar interbody fusion using an OLIF cage for this patient. The procedure began under general endotracheal anesthesia, with the patient placed prone on a Wilson frame. A radiolucent spine table was used, with height adjusted for both anteroposterior and lateral fluoroscopy. The surgeon stood on the left side. Standard UBE instruments were employed, including the BONSS set, a high-speed diamond burr, the ArthroCare bipolar system, and an RF probe for hemostasis.
Skin Incisions & Creating Working Space
1:42 Under C-arm guidance, 2 skin incisions were marked. The working portal was made as a transverse incision of about 1.5 centimeters, starting one centimeter lateral to the superolateral border of the caudal pedicle, allowing direct access to the facet joint and convenient cage insertion.
2:09 The target point was the spinolamina junction.
2:17 Sequential dilators were then introduced to establish the working portal.
2:28 The scope portal was created as a longitudinal incision along the outer margin of the cranial lamina, about 3 centimeters apart from the working portal.
3:00 Standard triangulation was then established as in conventional BE-LIF and confirmed under lateral fluoroscopy. Finally, the gravity irrigation system was connected, and both inflow and outflow were checked before inserting the scope and instruments.
Laminectomy
3:10 Next, after introducing the scope, the first view is the surface of the cranial lamina, where we identify the spinolaminar junction, target point. We used a 4-mm burr to open the lamina. The bone was drilled upward until the edge of the LF was seen, then widened laterally to thin the pars.
3:42 The LF was preserved during this step.
3:56 Next, we used an osteotome to separate the thinned pars and cut the joint capsule, allowing removal of the entire IAP.
4:35 We then used the burr to enlarge the bony window and maximize access through Kambin triangle. Drilling continued along the pars until the lower border of the cranial pedicle was exposed, and the apex of the SAP was drilled until the full superior and lateral borders of the caudal pedicle were clearly seen.
5:15 Before removing the LF in Kambin triangle, we resected the facet to expose the medial border of the pedicle.
Endplate Preparation
5:32 We widely removed the LF on the ipsilateral side to expose the spinal canal and reach the disc space. Then, the annulus was incised to create a window for entry into the disc space.
5:52 A 2-mm Kerrison punch was then used to enlarge the annular window medially and laterally, while gently retracting the nerve root for protection.
6:11 A low-energy ArthroCare bipolar probe was used to clean the annulus surface, providing a clearer view of the disc space.
6:28 Through this window, shavers and curettes were used to remove the disc material and carefully prepare the endplates.
This step was performed meticulously, removing as much disc material as possible. The cartilaginous endplate was carefully removed to expose the bony endplate without damaging it. Preparation was extended widely to both the ipsilateral and contralateral sides. Before inserting the trial cage, we measured the width of the disc space window. A width greater than 20 mm was considered safe for OLIF cage insertion.
Inserting Trial OLIF Cage
7:20 The trial cage was inserted obliquely at 30° and inserted under endoscopic and lateral C-arm view. Once its tip passed the anterior one-third of the vertebral body, the angle was increased until the tip aligned with the anterior border. The nerve root and canal were protected throughout with a root retractor.
Inserting OLIF cage
7:40 Then, bone grafting with a mixture of autologous bone and hydroxyapatite was placed through the bone tunnel. Next, the appropriately sized OLIF cage was inserted. As with the trial, it was advanced obliquely, and once the distal tip reached sufficient depth, the angle was gradually opened toward the contralateral lateral border.
8:15 The proximal end was then advanced to bring the cage into a horizontal position. The entire process was carefully observed under both endoscopic and C-arm guidance.
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