Cage design-centric glider approach to full-endoscopic lumbar fusion: optimizing nerve root protection in facet-sparing and facet-resecting techniques
Article information
Abstract
Endoscopic transforaminal lumbar interbody fusion (TLIF) offers substantial advantages in the management of degenerative spinal diseases, including accelerated postoperative recovery. However, its technical complexity and steep learning curve pose risks for nerve root injury. Optimizing nerve root protection in full-endoscopic facet-sparing TLIF (FE fs-TLIF) and full-endoscopic facet-resecting TLIF (FE fr-TLIF) is essential for enhancing surgical safety. This study aimed to improve the nerve root protection in FE fs-TLIF and FE fr-TLIF by optimizing cage glider selection and insertion techniques based on the specific cage shape—banana-shaped or bullet-shaped. The goal was to ensure safe cage positioning and mitigate nerve root injury during discectomy, endplate preparation, and cage insertion. These strategies were validated through cadaveric simulations and clinical implementation. In FE fr-TLIF utilizing bullet-shaped (straight) cages, one-tip and two-tip cage gliders effectively protected the traversing nerve root by facilitating medial cage entry, thereby minimizing irritation of the exiting nerve root. Conversely, in FE fr-TLIF with banana-shaped cages, the lateral tilt of the cage holder during implantation required the use of a two-tip cage glider to protect the traversing and exiting nerve roots, thereby mitigating the potential risk of nerve irritation. In FE fs-TLIF, a one-tip cage glider is preferred for safeguarding the exiting nerve root, while the traversing root is inherently protected by the medial wall of the facet joint. The use of a two-tip cage glider in FE fs-TLIF can cause injury to the nerve root during glider insertion. In addition to the selection of cage gliders, improper cage insertion steps can also contribute to postoperative neurapraxia. The appropriate selection of cage gliders with corresponding insertion techniques is critical for nerve root protection in endoscopic TLIF. Tailoring these choices to the specific approach (FE fs-TLIF or FE fr-TLIF) and cage type (banana or bullet) enhances surgical safety and clinical outcomes.
Introduction
Recent advancements in spinal endoscopic techniques have positioned full-endoscopic lumbar interbody fusion (endo-LIF) as a promising alternative to open and minimally invasive transforaminal lumbar interbody fusion (TLIF), offering comparable long-term outcomes. The endoscopic approach features smaller incisions, minimal muscle disruption, reduced blood loss, and superior short-term outcomes [1,2]. However, the technical complexity and steep learning curve of endo-LIF present a significant risk of nerve root injury, particularly for less experienced surgeons [3,4]. This underscores the importance of employing the correct surgical technique for nerve root protection.
Two commonly employed approaches in endo-LIF are full-endoscopic facet-sparing TLIF (FE fs-TLIF) and full-endoscopic facet-resecting TLIF (FE fr-TLIF) [5–11]. FE fs-TLIF involves a partial facetectomy and discectomy via the Kambin triangle, whereas FE fr-TLIF requires a complete facetectomy and discectomy through an extended Kambin triangle, resembling the approach used in conventional open TLIF [12]. Due to the diverse anatomical considerations of these approaches—including surgical incision, operative route, extent of facet resection, and cage entry point—FE fs-TLIF is associated with a higher incidence of exiting nerve root irritation, while FE fr-TLIF presents an increased risk of traversing root and thecal sac injury [12–14]. Nerve root injury can result in prolonged hospitalization and adversely impact clinical outcomes [9,12,15].
Literature and clinical experience indicate that the highest risk for nerve root injury arises during discectomy, endplate preparation, and cage insertion [9,14]. Therefore, developing effective methods to protect the nerve root during these steps is crucial, although there is a lack of technical guidelines focusing on these skills. Kim et al. [9] introduced a dedicated working glider to prevent nerve root irritation during endo-LIF procedures. A previous cadaveric study revealed that the selection of cage glider designs and surgical steps should be individualized to the specific approach, differentiating between FE fs-TLIF and FE fr-TLIF [16]. These studies suggest that diverse cage designs play a crucial role in nerve root protection during endo-LIF surgery. To ensure tailored nerve root protection, this study discusses the surgical tips and the selection of appropriate cage gliders for these two approaches, focusing on the use of bullet-shaped (straight) or banana-shaped cages during discectomy and cage implantation.
Technical Notes
Patient informed consent statement
The study was conducted in accordance with the Declaration of Helsinki and approved by the Institutional Review Board of National Cheng Kung University Hospital (A-ER-113-382). Informed consent was obtained from all individual participants included in the study.
Surgical techniques
Surgical indications for uniportal endoscopic lumbar fusion surgery
The indications for FE fs-TLIF and FE fr-TLIF are comparable to those for open TLIF, encompassing conditions such as unstable spondylolisthesis, spinal stenosis with instability, and severe herniated disc [17]. Comprehensive preoperative evaluations employing computed tomography and magnetic resonance imaging were conducted for establishing the diagnosis. All surgical procedures were performed using full-endoscopic techniques with an endoscopic spine system (Vantage Biotech Co., Ltd., Taoyuan, Taiwan).
Surgical procedure
Bullet-shaped cage in FE fs-TLIF and FE fr-TLIF
In FE fs-TLIF, the procedure begins with partial facetectomy of the superior articular process (SAP), followed by discectomy and endplate preparation (Fig. 1). Cadaveric experiments have revealed that, during FE fs-TLIF, the exiting nerve root lies in closer proximity to the surgical field than the traversing nerve root. Improper use of a two-tip cage glider in this context increases the risk of iatrogenic penetrating injury to the nerve root (Fig. 2). The traversing nerve root is generally regarded as protected because of the preserved medial wall of the inferior articular process (IAP) or facet joint. Therefore, the use of a one-tip cage glider is recommended for safeguarding the exiting nerve root. For a bullet-shaped cage, the cage holder is inserted at an angle of approximately 39.2° within the one-tip cage glider [16]. The glider is initially inserted in a caudal direction and then rotated 135° to protect the exiting nerve root effectively (Fig. 3). If lateral recess decompression is necessitated, the medial IAP is resected after cage insertion to ensure nerve safety. Surgical photos and corresponding imaging illustrating this procedure are presented in Fig. 4.
The illustration of surgical technique of full-endoscopic facet-sparing TLIF (FE fs-TLIF) and full-endoscopic facet-resecting TLIF (FE fr-TLIF). (A, B) During FE fs-TLIF, operation is performed within Kambin triangle, formed by exiting root, endplate, and facet joint. During FE fr-TLIF, interbody fusion is performed within extended Kambin triangle after complete facetectomy. (C) In FE fs-TLIF and FE fr-TLIF, the surgical incision is lateral to midline 4–8 centimeters and lateral pedicle line, respectively. (D) The cage entry point of FE fs-TLIF is close to exiting root and FE fr-TLIF is close to traversing root. TLIF, transforaminal lumbar interbody fusion.
The cadaveric photo demonstrates nerve root stabbing injury during two-tip cage glider insertion. (A, B) During full-endoscopic facet-sparing TLIF (FE fs-TLIF), incorrect selection of two-tip cage glider poses risk of exiting root irritation no matter insertion with cranial-caudal or left-right orientation. (C) During full-endoscopic facet-resecting TLIF (FE fr-TLIF), incorrect insertion of cage glider with left-right orientation poses risk of traversing root stabbing. TLIF, transforaminal lumbar interbody fusion.
The photo of one-tip and two-tip cage glider with associated insertion technique. (A) The photograph of one-tip cage glider. (B) The photograph of two-tip cage glider. (C) The surgical steps for one-tip cage glider insertion during full-endoscopic facet-sparing TLIF (FE fs-TLIF). (D) The surgical steps for one-tip cage glider insertion during full-endoscopic facet-resecting TLIF (FE fr-TLIF) with bullet cage. (E) The surgical steps for two-tip cage glider insertion during FE fr-TLIF with bullet or banana cage. TLIF, transforaminal lumbar interbody fusion.
The clinical photograph of full-endoscopic facet-sparing TLIF (FE fs-TLIF) with utilization of one-tip cage glider. (A, B, D, E) The lateral and upper view of one-tip cage glider during FE fs-TLIF before and after 135° rotation. (C, F) The pre-rotation and post-rotation radiographic photo of one-tip cage glider in L5/S1 Kambin’s triangle. TLIF, transforaminal lumbar interbody fusion.
In FE fr-TLIF, the procedure entails the complete resection of the IAP and SAP using either the outside-in or inside-out approach, followed by discectomy and endplate preparation (Fig. 1) [8,9,18]. Following complete facetectomy, the absence of bony structure shielding the traversing root results in the cage entry point being closer to the traversing nerve root than the exiting nerve root. Consequently, the cage holder is maintained in a relatively medial position at approximately 16.1°, as reported in previous literature. In this context, selecting an appropriate cage glider design becomes essential to prioritize protecting the traversing nerve root over that of exiting nerve root protection [16]. For the bullet-shaped cages, both one-tip and two-tip cage gliders can be employed. The cage glider is initially inserted in a cranial-caudal orientation and subsequently rotated 90° toward the traversing nerve root for providing protection. Direct insertion in the left–right orientation should be avoided as it exacerbates the risk of nerve root stabbing injury, as demonstrated in the cadaveric experiments (Fig. 2). Comprehensive surgical photos and accompanying imaging are presented in Figs. 5 and 6.
The clinical pictures of full-endoscopic facet-resecting TLIF (FE fr-TLIF) with utilization of one-tip cage glider for bullet cage. (A, B, D, E) The lateral and upper view of one-tip cage glider during FE fr-TLIF before and after 90° rotation. (C, F) The pre-rotation and post-rotation radiographic photo of one-tip cage glider in L5/S1 extended Kambin’s triangle. TLIF, transforaminal lumbar interbody fusion.
The clinical photographs of full-endoscopic facet-resecting TLIF (FE fr-TLIF) with utilization of two-tip cage glider for bullet cage or banana cage. (A, B, D, E) The lateral and upper view of two-tip cage glider during FE fr-TLIF before and after 90° rotation. (C, F) The pre-rotation and post-rotation radiographic photo of two-tip cage glider in L5/S1 extended Kambin’s triangle. TLIF, transforaminal lumbar interbody fusion.
Banana-shaped cage in FE fs-TLIF and FE fr-TLIF
In FE fs-TLIF, the initial approach mirrors that used for bullet-shaped cages, beginning with a partial facetectomy of the SAP. However, during the banana-shaped cage implantation, the lateralized trajectory of insertion further elevates the risk of exiting nerve root irritation. A one-tip cage glider remains the preferred choice for protecting the exiting nerve root in this scenario. The glider is inserted in a caudal direction and then rotated 135° to ensure nerve root safety.
In FE fr-TLIF, the insertion of banana-shaped cages differs significantly from that of the bullet-shaped cages because it necessitates a lateralized trajectory. This trajectory heightens the risk of exiting nerve root irritation, thereby making a two-tip cage glider essential for protecting the traversing and exiting nerve roots (Figs. 3, 7). The two-tip cage glider is initially inserted in a cranial–caudal orientation and subsequently rotated 90° clockwise or counterclockwise to a left-right orientation to provide effective protection for both nerve roots. Fig. 6 displays detailed imaging and surgical photographs.
The illustration of bullet-shaped cage and banana-shaped cage insertion steps during full-endoscopic facet-resecting TLIF (FE fr-TLIF). (A) The photo of bullet-shaped cage. (B) The photo of banana-shaped cage. (C) In the comparison, under the protection of traversing root with cage glider, bullet cage would be inserted with axis relatively parallel to central axis into central component of endplate without holder tilting. (D) The initial insertion of banana-shaped cage would be relatively parallel to central axis of vertebrae for correct location. Nevertheless, when knocking the cage into anterior column, the holder must be laterally tilting for cage correct docking. TLIF, transforaminal lumbar interbody fusion.
Discussion
This study provides a comprehensive review of the surgical techniques utilized in the two approaches to uniportal full endo-LIF. It identifies the most critical timepoints for nerve root irritation during the operation, offering critical guidance for spine surgeons. Furthermore, the study explores a range of surgical techniques for inserting banana-shaped and bullet-shaped cages, expanding upon the existing literature on open TLIF procedures [19]. In FE fs-TLIF, irrespective of whether a banana-shaped or bullet-shaped cage is being inserted, a one-tip cage glider is recommended for protecting the exiting nerve root. This preference is based on the preserved bony structure, minimal traversing root irritation risk, and potential for stabbing injury to the nerve with the use of a two-tip cage glider. During banana-shaped cage implantation in FE fr-TLIF, a two-tip cage glider should be used for protecting both nerve roots. This is necessitated by the lateral tilt of the cage holder during insertion, which heightens the risk of exiting root irritation. In comparison, one-tip and two-tip gliders are both recommended in FE fr-TLIF when using a bullet-shaped cage. The proper insertion of the cage glider tips and the corresponding rotation technique are also elucidated. A summary of these key points is listed in Table 1.
The summary of cage glider uses in FE fs-TLIF and FE fr-TLIF
The choice between FE fs-TLIF and FE fr-TLIF should be guided not by a rigid doctrine but rather by careful consideration of individual patient factors and pathology-specific requirements. In our clinical practice, we consider three primary factors when selecting between FE fs-TLIF and FE fr-TLIF. First, patient comorbidities play a crucial role in the selection process. For older patients or those with high surgical risk, FE fs-TLIF under local anesthesia is often preferred. Second, radiological findings aid in determining the need for direct decompression. FE fr-TLIF is particularly more suitable in patients with facet joint hypertrophy, facet cysts, or ligamentum flavum pathology. Third, sagittal alignment goals are a key consideration influencing cage selection. FE fr-TLIF permits the placement of larger cages owing to the wider surgical corridor, which can enhance spinal alignment [20]. Overall, we advocate for a flexible and tailored approach to each patient’s anatomy and clinical needs to achieve both surgical safety and optimal outcomes.
Numerous strategies for nerve root protection have been developed for both preoperative and intraoperative phases [13]. Preoperatively, well-trained and experienced endoscopic spine surgeons have been demonstrated to achieve better clinical outcomes [21]. In some patients, an adequate Kambin triangle—bounded by the exiting nerve root, facet joint, and lower vertebral endplate—may be absent, especially in the upper lumbar levels. Thus, preoperative comprehensive imaging evaluations with computed tomography or magnetic resonance imaging are crucial for comprehensive surgical planning [15]. Advanced imaging facilitates preoperative planning by helping to assess the extent of facetectomy required, determine the optimal size of the working portal, and individualize the endoscopic approach angle [22]. Optimizing perioperative preparation is also vital for nerve root protection. Precise perioperative local anesthesia and neurophysiological monitoring equipment help mitigate the incidence of nerve root injury [13,23–26]. Surgical skill plays a pivotal role in ensuring nerve protection, with techniques such as the outside-in approach for inferior articular facetectomy, accurate docking points for the guidewire or working cannula, and proper execution of foraminoplasty being strongly recommended [18,27–30]. However, existing literature demonstrates a relative paucity of studies specifically addressing the risk of nerve root injury during the steps of cage insertion despite its clinical significance [9,13]. In our study, we aimed to address this knowledge gap by elucidating the surgical techniques for facilitating appropriate selection and use of cage glider selection.
During TLIF, achieving reliable fusion and restoring lumbar lordosis while minimizing cage subsidence and pseudoarthrosis remains a top priority for surgeons. Hence, numerous studies have examined the influence of diverse artificial disc designs, such as bullet-shaped and banana-shaped cages, on interbody fusion outcomes [31–33]. Although a consensus has not been established, several studies highlight the potential advantages of banana-shaped cages over bullet-shaped ones in restoring disc height, segmental lordosis, and lumbar lordosis. These benefits are theorized to stem from the anterior placement of the banana-shaped cage in the intervertebral space [19]. Furthermore, the anterior placement of banana-shaped cages is believed to lower the risk of subsidence due to the higher mechanical strength of the apophyseal ring [34]. In FE fs-TLIF, there are increased concerns regarding the higher risk of cage subsidence and suboptimal sagittal alignment restoration, which are attributed to the use of smaller cages necessitated by the relatively narrow surgical corridor [12]. The adoption of banana-shaped cages may address these limitations, potentially enhancing radiological and clinical outcomes in FE fs-TLIF and FE fr-TLIF. However, to date, only a limited number of case reports have explored the use of banana-shaped cages in uniportal full-endoscopic fusion surgery [35]. To address this gap, our team developed and introduced a preliminary surgical technique for the application of banana-shaped cages in endoscopic TLIF. This technique is grounded in single-center clinical experience and cadaveric anatomical studies, offering a foundation for future research to evaluate the safety and efficacy of banana-shaped cages in endoscopic fusion procedures.
This study has several limitations. First, the surgical techniques for nerve root protection during FE fs-TLIF and FE fr-TLIF are primarily derived from cadaveric findings and accumulated clinical experience. While these anatomical studies offer valuable insights into safe cage insertion, their effectiveness in real-world clinical settings remains to be validated. Currently, there is a paucity of clinical outcome data, especially concerning the use of banana-shaped cages under full-endoscopic guidance. To date, only a single well-designed cohort study has demonstrated a reduction in postoperative neurapraxia with bullet-shaped cages in full-endoscopic fusion surgery employing this technique [36]. To overcome this limitation, we conducted prospective clinical studies to assess outcomes such as nerve injury rates and recovery, with a particular emphasis on the use of banana-shaped cages. The results of these studies are expected to be published soon. Second, the precise risk of injury to exiting nerve roots during the insertion of banana-shaped cage is still unknown. Only a few studies have examined the potential risk [14,19]. However, we believe that despite the limited evidence, heightened awareness of this issue may help minimize the occurrence of complications to the greatest extent possible. Third, our study does not cover the full spectrum of currently available cage options. Devices such as OLIF-type cages and expandable cages exhibit distinct biomechanical properties and insertion profiles that may substantially affect the choice of neural protection strategies. To address this limitation, we plan to include these alternative cage designs in future cadaveric and clinical studies on endoscopic lumbar fusion. Lastly, the recommended surgical steps of cage glider insertion were partly based on cadaveric evaluations, which might not fully represent the clinical situation due to differences in soft tissue characteristics. However, given the limited nerve root visualization and the challenges associated with performing repeated trials of cage glider insertion during clinical practice, cadaveric anatomic dissection remains a valuable resource for advancing surgical knowledge.
In conclusion, appropriate cage glider selection and insertion techniques play an important role in safeguarding nerve roots during FE fs-TLIF and FE fr-TLIF. The selection of a one-tip or two-tip cage glider, along with the appropriate rotational technique, should be individualized to diverse approaches and cage types to augment surgical safety and optimize clinical outcomes.
Key Points
Cage glider selection: Employ a one-tip glider for full-endoscopic (FE) facet-sparing transforaminal lumbar interbody fusion (fs-TLIF) to protect the exiting nerve root, and a two-tip glider for FE facet-resecting TLIF (fr-TLIF) with banana cages to safeguard both exiting and traversing roots; bullet cages in FE fr-TLIF may use either glider depending on root proximity.
Insertion technique: Proper orientation and rotational angles (90° or 135°) are essential to minimize nerve irritation and ensure safe cage placement in endoscopic TLIF.
Surgical safety: Optimized glider choice and technique significantly reduce postoperative neurological complications.
Notes
Conflict of Interest
No potential conflict of interest relevant to this article was reported.
Acknowledgments
We thank Skeleton Materials and Bio-compatibility Core Lab, Research Center of Clinical Medicine, National Cheng Kung University Hospital, Tainan, Taiwan, for the assistance of this project. The data corresponding to this study are available upon request from the corresponding author.
Funding
This study was supported by Taiwan National Science and Technology Council (grants: NSTC 112-2314-B-006-075). This study was also supported by National Cheng Kung University Hospital, Tainan, Taiwan (grants: NCKUH-11402019) and Collaboration Project between Industry and Academia at National Cheng Kung University, Tainan, Taiwan (grants: B112-K576).
Author Contributions
Conceptualization: CLL. Methodology: CLL, CJC, YMH, KCL, HSK, CMC, YHH. Formal analysis: YCH, HCC, YFL. Investigation: YCH, CJC, YMH, KCL. Writing–original draft: CLL, YCH, HCC. Writing–review & editing: CLL. Funding acquisition: CLL. Project administration: CLL. Final approval of the manuscript: all authors.