Introduction
Lumbar interbody fusion is a widely employed surgical technique for treating various degenerative lumbar spine conditions [1]. The advent of minimally invasive surgery has sparked interest in reducing surgical morbidity while maintaining or improving clinical outcomes [2]. Biportal endoscopic lumbar interbody fusion (BELIF) represents a recent advancement in minimally invasive spine surgery, offering improved visualization and reduced tissue trauma [3].
Biportal endoscopic techniques have emerged as a promising approach in spine surgery, offering several advantages over traditional open and other minimally invasive methods [4–6]. These techniques involve two small portals, one for the endoscope and the other for instruments, enabling improved maneuverability and visualization [7]. The biportal approach provides a wider field of view compared to uniportal endoscopy and enables the use of conventional instruments, potentially shortening the learning curve for surgeons familiar with microscopic techniques [8]. Furthermore, the continuous irrigation system in biportal endoscopy helps maintain a clear surgical field and reduces bleeding, enhancing visibility and potentially improving surgical outcomes.
The use of large polyetheretherketone (PEEK) cages in lumbar interbody fusion has gained attention due to their potential biomechanical advantages [9]. Larger cages offer an increased surface area for fusion, potentially improving stability and reducing subsidence risk [10]. A cage with a larger axial area can distribute stress more evenly across the internal fixation system and the interface between the cage and endplate, thereby preventing internal fixation system failure, including screws and rods, as well as cage subsidence [11]. Additionally, positioning a large footprint cage across bilateral epiphyseal rings is crucial, as the epiphyseal ring is harder than the center of the endplate [12,13]. The PEEK material, with its elastic modulus similar to that of bone, facilitates even stress distribution across the endplate [9]. However, inserting larger cages through minimally invasive approaches poses technical challenges that require mitigation [10,14].
Despite the potential benefits of BELIF using large PEEK cages, research on its effectiveness and safety is scarce. Current challenges include the steep learning curve associated with endoscopic techniques, concerns about the ability to adequately prepare the disk space and end plates through small portals, and uncertainties regarding long-term outcomes compared to traditional open or other minimally invasive approaches [15].
This study aimed to evaluate the clinical and radiological outcomes of BELIF using large PEEK cages in patients with degenerative lumbar conditions. We hypothesized that this approach would yield satisfactory fusion rates, improved clinical outcomes, and low complication rates compared to traditional techniques.
Materials and Methods
Study design and patient population
This was a retrospective study of 12 consecutive patients (nine men and three women) who underwent BELIF with a large PEEK cage between October 2022 and July 2023. The study was approved by the Institutional Review Board (IRB) of Hallym University Kangnam Sacred Heart Hospital (IRB no., 2024-10-012). Written informed consent was waived owing to the retrospective nature of the study.
Patient selection
The inclusion criteria were as follows: (1) age 50–80 years; (2) single-level lumbar degenerative disease (spinal stenosis or herniated intervertebral disk) requiring fusion; and (3) symptoms refractory to conservative treatment for at least 3 months. The exclusion criteria were as follows: (1) multi-level disease; (2) previous lumbar surgery at the same level; (3) osteoporotic patient with a bone mineral density T-score of −2.5 or less; (4) spinal infection or tumor; and (5) inability to complete follow-up for at least 12 months.
Surgical technique (Supplement 1)
All surgeries were performed by a single surgeon using the BELIF technique. The procedure was conducted under general anesthesia with the patient positioned prone on a radiolucent Jackson spinal table.
Preparation and portal placement
The surgical level was confirmed using C-arm fluoroscopy. Two small incisions (each approximately 1 cm long) were made for the endoscopic and working portals. For interbody fusion, a posterolateral approach was employed, positioning the portals near the pedicle lateral margin. Specifically, the endoscopic portal was placed at the lateral border of the facet joint, while the working portal was positioned just lateral to the pedicle lateral margin of the target level.
Soft tissue dissection
A 30° endoscope (4 mm diameter) was inserted through the endoscopic portal. Soft tissue was dissected using a Cobb’s elevator, radiofrequency (RF) wand (ArthroCare, Austin, TX, USA), and shaver system (CONMED, Largo, FL, USA) under continuous saline irrigation. The facet joint was exposed by careful dissection.
Facetectomy and decompression
The inferior articular process was removed using a small-caliber curved osteotome, with the resulting bone chips preserved for subsequent use as autografts. A laminectomy procedure was then performed using a high-speed cutting burr and Kerrison rongeurs. The ligamentum flavum was carefully removed using a pituitary punch, avoiding a dural tear. Both the ipsilateral and contralateral ligamentum flava were removed. The superior articular process was removed for foraminal decompression using a burr and osteotome.
Discectomy and endplate preparation
Annulotomy was performed using a knife after RF-assisted hemostasis. Discectomy was conducted using serially sized disk reamers, starting with the smallest size. Endplates were meticulously prepared under direct endoscopic visualization as follows: (1) The ipsilateral side was prepared with a 30° scope using straight and ring curettes. (2) The antero-contralateral side was then prepared by maneuvering the scope to the opposite side using angled curettes. (3) A 70° scope was used to remove the posterior-contralateral side and dural ventral side annulus and disk. The cartilaginous endplate was carefully excised, while the bony endplate was preserved. The endoscope was inserted into the disk space to confirm the complete preparation and preservation of the bony endplate.
Bone graft and cage insertion
The saline inside the disk space was removed before bone grafting. A mixture of local autograft bone, allograft bone, and demineralized bone matrix was prepared. The bone graft was impacted into the anterior disk space using a specially designed funnel-shaped device (Fig. 1). A large PEEK cage (Lospa Is DLIF cage system; Corentec Co. Ltd., Seoul, Korea; dimensions: 40 mm length, 18 mm width, 6°, and an appropriate height selected intraoperatively) was filled with the bone graft mixture. The annulus was resected to the maximum extent feasible to accommodate the large cage. The exiting and traversing nerve roots were protected using a root retractor inserted through the working portal. The cage was inserted through the working portal under endoscopic visualization and fluoroscopic guidance. The cage position was monitored to prevent compression of the nerve roots. The cage was positioned parallel to the coronal plane using a cage impactor.
Percutaneous pedicle screw fixation
Percutaneous pedicle screws (Lospa Is MIS spinal system, Corentec Co. Ltd.) were placed bilaterally under fluoroscopic guidance. The existing endoscopic and working portals were utilized for ipsilateral screw insertion, while two additional incisions were made on the contralateral side. During rod insertion, a maximal lordotic curve was achieved using a compressor after appropriate rod bending.
Closure and postoperative care
Hemostasis was achieved under endoscopic visualization. A surgical drain was inserted through the working portal. The incisions were closed in layers using absorbable sutures. Patients were mobilized on the first postoperative day under physiotherapist supervision. The drain was removed on the second postoperative day.
Key technical points
The posterolateral approach offers advantages for both ipsilateral and contralateral decompression. Meticulous endplate preparation under direct endoscopic visualization is crucial to prevent subsidence and promote fusion. The use of different angled endoscopes (30° and 70°) enables comprehensive disk removal and endplate preparation. The large PEEK cage provides increased stability and potentially improved fusion rates. However, cage insertion demands caution to avoid neural injury, which can be mitigated by using the quarterback portal for nerve root protection.
Clinical and radiographic evaluation
Demographic, perioperative outcome (operation time, surgical drainage, and length of hospital stay), and clinical outcome data were collected. Clinical outcomes were evaluated using the Visual Analog Scale (VAS) for back (VAS-LBP) and leg (VAS-LE) pain, the Oswestry Disability Index (ODI), and the European Quality of Life-5 Dimensions (EQ-5D) value. Assessments were conducted preoperatively and at 3, 6, and 12 months postoperatively. Complications were recorded and classified as intraoperative (e.g., incidental durotomy) or postoperative (e.g., surgical site infection and hematoma).
Fusion status was evaluated using computed tomography (CT) scans at 12 months postoperatively, based on the Bridwell criteria. Successful fusion was defined as Grades I and II, while Grades III and IV were considered non-union. Two observers independently evaluated the CT images, and any discrepancies were resolved by consensus. Cage subsidence was defined as >1 mm loss of disk height compared to the immediate postoperative images.
Statistical analysis
All variables were summarized using descriptive statistics. Continuous variables were expressed as mean±standard deviation, while categorical variables were expressed as frequency (percentage). Repeated measure analysis of variance was conducted to evaluate changes in clinical outcomes over time. This method allowed for examining correlations between repeated measurements among the same participants. All statistical analyses were performed using Stata/MP ver. 17.1 (Stata Corp., College Station, TX, USA). Two-tailed p-values <0.05 were considered indicative of statistical significance for all tests.
Results
Demographic and perioperative data
The mean age of patients in this series was 69.1±7.2 years. The mean body mass index was 23.6±2.7 kg/m2. The majority of operations were performed at the L4–5 level (10 cases, 83%), while the remaining two cases (17%) were at the L5–S1 level. Spinal stenosis was the primary diagnosis in 11 patients (92%) and one patient (8%) had a herniated intervertebral disk. The mean operation time was 149.7±37.4 minutes. The average volume of surgical drainage was 201.4±59.7 mL. The mean length of hospital stay was 5.2±4.2 days.
Clinical and radiographic outcomes
The study revealed significant improvements in all measured clinical outcomes over the 12-month follow-up period. Statistically significant improvements were observed in VAS-LBP, VAS-LE, ODI, and EQ-5D scores when comparing preoperative and 12-month postoperative values (p<0.05). Fusion was achieved in 10 patients (83.3%) at 12-month follow-up, as evaluated through CT using the Bridwell criteria. Cage subsidence >1 mm was observed in one patient (8.3%).
Intraoperative complications included one case each of incidental durotomy (8.3%) and wrong-site surgery (8.3%). Postoperative complications included three cases (25%) of asymptomatic hematoma, all of which resolved without intervention. Additionally, there was one case of wound dehiscence (8.3%), but no instances of surgical site infection were reported.
Discussion
This study investigated the effectiveness and safety of BELIF using large PEEK cages. The results showed significant improvements in clinical outcomes, including VAS scores for back and leg pain, ODI scores, and EQ-5D values, with the most substantial improvements occurring within the first 3 months after surgery. The fusion rate of 83.3% at 12 months after surgery is comparable to rates reported for traditional transforaminal lumbar interbody fusion (TLIF) techniques in the literature [4].
BELIF offers several advantages over traditional open TLIF and other minimally invasive techniques. The biportal approach provides enhanced visualization of the surgical field [4], facilitating more precise decompression and implant placement. This is achieved through the ability to maneuver the endoscope independently of surgical instruments. BELIF also minimizes tissue damage compared to open procedures [3], by utilizing two small portals instead of a large incision, resulting in less muscle retraction and dissection. This may lead to less postoperative pain and faster recovery. This is reflected in our results, which showed significant pain reduction and functional improvement as early as 3 months after surgery, consistent with previous reports [4,10]. Furthermore, BELIF enables direct central and foraminal decompression of neural elements, unlike some other minimally invasive techniques, such as minimally invasive TLIF with tubular retractors [4]. The ability to directly visualize and decompress the neural structures may have contributed to the significant improvements in leg pain and function observed in our study.
Despite its advantages, BELIF has some limitations. The technique has a steep learning curve, requiring surgeons to develop proficiency in endoscopic visualization and instrument manipulation through separate portals [8,16,17]. Initially, this leads to longer operation times, as reflected in our mean operation time of 149.7 minutes. However, as surgeons gain experience, operation times are expected to decrease [17]. Technical challenges include instrument manipulation through separate portals, potential endoscope fogging, or obstruction of the surgical field by bleeding. These challenges can be mitigated with experience and the use of continuous irrigation systems. Furthermore, BELIF may not be suitable for treating highly degenerative or deformed spines, where more extensive exposure might be necessary. Therefore, patient selection is crucial for achieving optimal outcomes with this technique [18].
The large PEEK cages used in our study offer several potential benefits. The increased surface area of the cage provides a larger footprint for fusion, potentially contributing to improved fusion rates [19]. The fusion rate of 83.3% at 12 months is encouraging, although longer follow-up is necessary to confirm the durability of these results. Large PEEK cages also improve load distribution across the endplates, potentially reducing the risk of subsidence [19]. This is supported by a study comparing subsidence rates according to cage surface area in stand-alone lateral lumbar interbody fusion, which found that wider cages resulted in lower subsidence rates and better restoration of segmental lordosis [20]. The low rate of cage subsidence (8.3%) in our study supports this hypothesis, although a direct comparison with studies using smaller cages is required to confirm this advantage. Furthermore, the ability to insert a larger cage through a minimally invasive approach facilitates the restoration of disk height and indirect decompression of neural elements. This may contribute to the significant improvements in the back and leg pain observed in our study. The biomechanical properties of PEEK, which closely mimic those of bone, may provide additional advantages regarding stress distribution and long-term stability [9]. A biomechanical study using finite element analysis found that the use of larger cages in lumbar interbody fusion reduced endplate and internal fixation system stress without increasing the risk of adjacent segment disease. This was because it does not increase the range of motion and intervertebral disk pressure of adjacent segments [11,21]. However, further biomechanical studies are required to fully elucidate these benefits in the context of BELIF.
In biportal endoscopic spine surgery, the surgeon chooses from various angled scopes depending on the surgical situation. Beginners often start with the 0° scope, which provides an endoscopic image closest to the actual view, minimizing distortion. Accordingly, after forming a triangulation between the endoscope and surgical instrument, surgery can be performed in almost the same environment as conventional microscopic surgery. Conversely, the 30° and 70° scopes used in this study offer the advantage of illuminating every corner during endplate preparation [22]. It is important to select an appropriate type of scope considering the surgeon’s experience and the location and severity of the lesion.
Some limitations of this study should be acknowledged. The sample size was relatively small, which limits the generalizability of our results and the statistical power of our analyses. The follow-up period of 12 months, while sufficient to demonstrate short-term efficacy, is inadequate to assess long-term outcomes and fusion stability. The occurrence of wrong-level surgery, while not specific to BELIF, highlights the importance of meticulous preoperative planning and intraoperative confirmation of the correct level in minimally invasive procedures where anatomical landmarks may be less visible [23]. Future studies should address these limitations by including larger patient cohorts and longer follow-up periods. The authors plan to collect data and report mid-term outcomes at 3 or 5 years to evaluate fusion stability and adjacent segment degeneration. Direct comparison with traditional or minimally invasive TLIF techniques and other minimally invasive approaches through prospective studies would provide valuable insights into the relative effectiveness and safety of BELIF with large PEEK cages.
Conclusions
This study demonstrates the potential of BLIF with large PEEK cages as a promising technique for lumbar interbody fusion. By combining the benefits of minimally invasive surgery with the ability to perform direct decompression and utilize larger interbody implants, BLIF offers a valuable treatment option. Although technical challenges exist, the potential benefits in terms of clinical outcomes and fusion rates justify further investigation of this technique.
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