Biportal endoscopic spine surgery (BESS) has gained significant traction as a minimally invasive alternative for lumbar decompression, combining the benefits of magnified visualization with minimal tissue disruption. The clinical advantages of the procedure, such as reduced postoperative pain, shorter hospital stays, and faster recovery, have been well documented [1,2]. However, the efficacy and safety of BESS are highly dependent on the surgeon’s detailed understanding of lumbar spine anatomy as visualized through endoscope, as well as on meticulous portal planning and precise intraoperative orientation [3].

Compared to conventional open or microscopic techniques, BESS operates in a dynamic and constrained 3-dimensional anatomical corridor. This necessitates real-time adaptation to anatomical variations, such as interlaminar-disc mismatch and the medial curvature of the lamina, often referred to as “banana concept” [4]. Proper portal positioning, informed by these variations, is essential to establish a safe working trajectory, minimize bone resection, and preserve stabilizing structures, including the facet joint and laminar isthmus.

Therefore, this study aims to elucidate key anatomical considerations and technical nuances in BESS, with emphasis on portal trajectory, laminar curvature, and superior articular process (SAP) as a reliable intraoperative landmark. By integrating these principles into surgical planning and execution, BESS can achieve both effective neural decompression and maximal preservation of spinal stability, which are central to successful long-term outcomes.

A retrospective analysis was carried out using intraoperative video recordings of lumbar spine surgeries performed through BESS technique between January 2024 and March 2024. All videos were obtained from a prospectively maintained surgical database. The study included participants with an average age of 45.79±3.01 years, consisting of 65% male and 35% female.

A total of 29 surgical videos were initially reviewed. To ensure consistency and clarity in the data, strict inclusion and exclusion criteria were applied. Videos with high-definition quality, complete procedural coverage, and operations performed specifically for either herniated nucleus pulposus or lumbar spinal stenosis were included, while videos with poor resolution (n=4), involved combined procedures, or presented intraoperative complications that obscured relevant anatomical structures (n=2) were excluded from this study.

After applying these criteria, 23 surgical videos were selected for final analysis. The cases spanned 3 spinal levels, namely L3–4 (n=3), L4–5 (n=17), and L5–S1 (n=3). All patient-identifying information was anonymized before analysis to ensure confidentiality. Ethical approval for this study was obtained from the Ethical and Research Committee of the Medical Faculty at Diponegoro University (approval No. 005/KEPK/FK-UNDIP/I/2025).

All surgeries were performed alternately by 2 experienced neurosurgeons specializing in spine surgery. Patients were positioned in a flat prone position with adequate support using soft cushions to relieve pressure on the abdomen and thorax. Fluoroscopic marking using a C-arm with an orientation of 90° was performed at the cranial and caudal spinolaminar junctions, with a skin incision distance of approximately 2.0 to 2.5 cm (Figure 1). A perpendicular skin incision was made down to laminar surface, followed by cross-incision of the fascia, blunt dissection, and dilation.

Interlaminar space was identified through direct endoscopic visualization and pulsatile palpation. Initial drilling was performed on the cranial lamina to expose the bony surface. This was followed by lateral drilling of the inferior articular facet, guided by the "banana concept" to preserve key anatomical structures. Once the inferior facet was adequately drilled, superficial flavectomy was performed until the deep layer of the ligamentum flavum was identified.

After this procedure, the superior articular facet was exposed, serving as an anatomical landmark to locate the intervertebral disc. Flavectomy of the deep layer was performed as needed to access the disc space while minimizing unnecessary resection. Current indications for removing both layers included cases of ligamentum flavum hyperplasia. However, when this condition was present, preservation or minimal resection of the ligament could be performed. In this study, 2 independent spine surgeons reviewed the videos to identify and score key anatomical landmarks, including the lamina, ligamentum flavum, spinous processes, facet joints, and nerve roots in all 23 a. Visualization quality was rated using a 3-point scale (1=poor, 2=moderate, 3=excellent). Interobserver reliability was assessed using Cohen kappa coefficient (κ).

To support anatomical analysis, mismatch measurements were conducted between the inferior margin of the superior endplate and the midpoint of the inferior edge of the superior lamina. This was also carried out between the superior margin of the inferior endplate and the midpoint of the superior edge of the inferior lamina (Figure 2).

Among the 23 cases reviewed, a measurable interlaminar-disc mismatch was observed, as shown in Table 1. The mismatch measurements varied by level, showing that at L2–3, the cranial mismatch was the largest (8±1.2 mm), while at L5–S1, the caudal mismatch reached the highest value (-5±1.23 mm). No statistical comparison between levels was conducted. SAP served as a reliable anatomical landmark for identifying the intervertebral disc under anteroposterior (AP) C-arm fluoroscopic guidance (Figure 3A). In a series of 23 cases evaluated with endoscopic visualization, SAP consistently correlated with the disc location, confirming its value as a dependable intraoperative reference point. A limited medial facetectomy involving only SAP (Figure 4), preserving more than 50% of the facet joint, was effective in enlarging interlaminar window.

Interobserver agreement in identifying key anatomical landmarks (lamina, ligamentum flavum, spinous process, facet joint, and nerve root) was assessed, resulting in a Cohen kappa value of κ=0.84. Each case included 5 categorical items rated independently by 2 spine surgeons, and no weighting scheme was applied. This agreement specifically related to the categorical identification of landmarks and did not extend to morphometric distance measurements. Accurate delineation of interlaminar space was crucial for determining the surgical trajectory and intraoperative depth. Under endoscopic view, the lamina could be distinguished from interlaminar zone through pulsatile palpation, direct endoscopic visualization, and its characteristic curved contour (Figure 3B), commonly called the “banana concept.” This anatomical understanding necessitated a cranial-to-caudal approach with a medial-to-caudal trajectory shift (Figure 5), which minimized unnecessary resection in an effort to preserve spinal stability.

The deep layer was resected to the extent necessary to confirm adequate root decompression (Figure 6), while the remaining portion was preserved. Furthermore, the superficial layer, often more hypertrophied, was selectively removed, while the deeper, more elastic layer was preserved whenever feasible to maintain the integrity of the posterior tension band. This selective resection approach was intended to reduce the risk of postoperative instability, fibrosis, and excessive dural exposure [5].

Following flavectomy and minimal facetectomy, adequate space was created to facilitate the identification and evacuation of the intervertebral disc. The anatomical triangle, bounded laterally by the facet joint, medially by the traversing nerve root, and cranially by the endplate, served as an ideal working corridor for discectomy (Figure 6). When performed precisely in this region, discectomy allowed for effective removal of disc material (Figure 7) while preserving as much of the surrounding stabilizing structures as possible (Figure 8).

The effectiveness of BESS relied heavily on comprehensive anatomical knowledge of lumbar spine as visualized endoscopically. Central to this technique was precise portal planning, which enabled safe instrument navigation and targeted decompression [6]. A critical consideration was the anatomical variation between interlaminar window and the intervertebral disc, referred to as interlaminar-disc mismatch, which required intraoperative adaptation and technical refinement [3,6].

As shown in Figure 2, morphometric measurements were used to quantify this mismatch. A total of 2 primary distances were assessed, namely (1) the vertical gap from the inferior border of the superior endplate to the midpoint of the inferior margin of the superior lamina (cranial mismatch), and (2) from the superior border of the inferior endplate to the midpoint of the superior margin of the inferior lamina (caudal mismatch) [7]. These parameters were obtained using intraoperative endoscopic imaging and fluoroscopic alignment. The results supported the importance of morphometric evaluation in preoperative planning and intraoperative orientation, enhancing surgical precision during interlaminar access [8].

Fluoroscopic guidance in the AP view facilitated the identification of the spinolaminar junction, allowing precise alignment of the cranial (viewing) and caudal (working) portals approximately 2.0 to 2.5 cm apart. These portals were strategically placed to straddle interlaminar space at commonly targeted levels, such as L4–5 or L5–S1, ensuring optimal access and visualization.

Intraoperatively, endoscopic view revealed interlaminar space bordered by adjacent laminae and laterally bounded by SAP, as shown in Figure 2B. This interlaminar window provided the essential anatomical corridor for accessing the epidural space and performing neural decompression. Proper portal alignment with the natural anatomy facilitated a linear and controlled surgical path, minimizing soft-tissue disruption and preserving key structures such as the facet joint and laminar isthmus [6]. Establishing this alignment through anatomical planning created a framework for the entire procedure. Any misalignment could lead to awkward instrument angles, excessive drilling, or compromised visualization [9,10].

Accurate spatial orientation and anatomical landmark identification were essential for navigating endoscopic surgical environment [11]. A critical variation encountered was interlaminar-disk mismatch, characterized by a vertical offset between interlaminar space and the underlying disc center. When unrecognized, this could result in incomplete decompression or unnecessary bone resection [12]. Early detection of this mismatch enabled trajectory modification, optimizing surgical outcomes.

Another important anatomical concept included “banana-shaped lamina,” a metaphor describing the medial-to-lateral curvature of lumbar lamina, specifically prominent at L4–5 and L5–S1. Unlike a flat medial-lateral plane, the lamina curved medially and inferiorly, forming a concavity. This morphology necessitated a curved surgical trajectory to prevent suboptimal portal placement, excessive drilling, or inadvertent violation of stabilizing structures [13]. As shown in Figure 3B, the cranial lamina inclined medially from its inferolateral margin, guiding the working channel to follow its curvature. Figure 5 showed necessary adjustments to portal positioning, shifting caudally and medially to align with the natural laminar slope, enabling focused decompression while minimizing structural compromise.

In a series of 23 cases, this "banana" configuration was consistently noted, particularly at L4–5 and L5–S1. Interlaminar-disk mismatch was observed in 93% of cases, with a vertical shift ranging from 5.5±1.02 mm to 8.0±1.2 mm. Without recognition, this misalignment led to either insufficient decompression or over-resection. Preoperative imaging, detailed anatomical understanding, and intraoperative endoscopic assessment were crucial to address such variations accurately [14,15].

SAP functioned as a reliable anatomical reference in the procedure. Figure 3C showed consistent SAP visualization even in cases with hypertrophic lamina or overlapping facets. Preoperative fluoroscopic triangulation from SAP allowed accurate disc localization and alignment of the decompression pathway. Figure 6B showed the surgical corridor bounded medially by the traversing nerve root, superiorly by the disc endplate, and laterally by SAP, forming a triangular space facilitating safe and precise discectomy.

These intraoperative cues, specifically SAP, became increasingly significant in anatomically distorted cases. Furthermore, its strong cortical contour provided dependable identification under both endoscopic and fluoroscopic guidance. Portal placement typically began with identification of the inferior articular process and SAP, followed by conservative medial facetectomy (preserving >50% of the facet joint), thereby expanding the operative field, improving visualization, and reducing operative time, particularly in hypertrophic facet joints or narrow interlaminar windows [4,16].

This technique, as displayed in Figure 9, allowed sufficient working space while preserving critical stabilizing structures. Radiographic evidence (Figure 9A and B) confirmed minimal resection of lamina and facet margins, supporting the principle that BESS enabled precise, pathology-targeted decompression in an effort to maintain spinal stability. Such preservation was essential at mobile segments such as L4–5 and L5–S1, aiming to reduce postoperative instability or adjacent segment degeneration [17].

BESS facilitated selective dissection of the ligamentum flavum, allowing for preservation of its deep elastic layer, which contributed to posterior tension band integrity. Under continuous endoscopic guidance, blunt dissectors were used to minimize dural or nerve injury. This layered, anatomically respectful technique was intended to reduce the risks of postoperative fibrosis and instability, supporting smoother recovery and improved outcomes [18].

In summary, the integration of detailed anatomical knowledge with real-time endoscopic navigation was the cornerstone of successful BESS. Concepts such as banana curvature, interlaminar-disc mismatch, and SAP triangulation collectively guided surgical planning, portal placement, and intraoperative execution. These strategies aimed to enhance procedural safety, reduce unnecessary tissue trauma, and preserve critical spinal structures, defining the evolving standard for minimally invasive lumbar decompression [19,20].

In conclusion, BESS shows strong efficacy for lumbar decompression when guided by precise anatomical understanding and thoughtful intraoperative execution. Accurate localization of the intervertebral disc using SAP enables targeted discectomy while minimizing the need for excessive bony resection. In this technical note, no collection of perioperative or clinical outcome data (e.g., complications, visual analogue scale/Oswestry Disability Index, or reoperation rates) is performed. Therefore, any claims of stability or safety must be approached cautiously. The description of “stability preservation” is based solely on the amount of bone and soft-tissue resection observed during surgery, not on long-term patient outcomes. Furthermore, the selective preservation of the deep layer of the ligamentum flavum contributes to stenotic pathology rather than providing the intended mechanical stability. Additional prospective studies with clinical and radiological results are needed to determine whether such preservation offers actual benefits or when a more complete flavectomy is preferable in cases of hypertrophy. The integration of detailed endoscopic anatomy with technical skill plays a significant role in achieving stable and reproducible outcomes in minimally invasive spine surgery [1,8,9].