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.

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.

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.