INTRODUCTION
Lumbar interbody fusion is a standard surgical treatment for degenerative lumbar spine diseases. Open laminectomy and instrumented fusion with or without an interbody cage have traditionally been the major approaches to treatment of lumbar fusion. Minimally invasive transforaminal lumbar interbody fusion (MIS-TLIF) with microscopic assistance has gained popularity for its advantages over open techniques [1-3]. Spine surgeons have continually sought the least invasive procedures to achieve spinal decompression and fusion at a reduced morbidity risk compared with open techniques, especially in older adults. With the evolution of endoscopic spinal surgery over the past decade, endoscopic lumbar spinal fusion has developed considerably. Endoscopic techniques require a smaller incision than do techniques that require a tubular retractor and result in less blood loss and soft tissue damage. They also involve faster recovery. Endoscopic transforaminal lumbar interbody fusion (endo-TLIF) has gained prominence as a minimally invasive method for reducing postoperative morbidity. The technique is safe and effective and yields favorable outcomes, with postoperative complication and fusion rates similar to those of other techniques [4-6]. Three endoscopic trajectories are commonly employed for interbody fusion: full endoscopic trans Kambin lumbar interbody fusion, posterolateral endo-TLIF or biportal endoscopic lumbar interbody fusion, and endoscopic-assisted oblique lumbar interbody fusion [7]. The biportal endoscopic approach facilitates manipulation of the endoscope and other surgical devices through separation of the working and endoscope channels. A meta-analysis by Luan et al. reported distinct advantages of biportal endoscopic transforaminal lumbar interbody fusion (BE-TLIF) over MIS-TLIF in treating lumbar degenerative diseases, including lower intraoperative blood loss, reduced early postoperative pain, shorter hospital stays, and faster functional recovery [8]. Nevertheless, consensus regarding surgical incision for endo-TLIF approaches is lacking. Percutaneous pedicle screw incision during BE-TLIF reduces invasiveness and shortens the learning curve of the technique. Additionally, this approach is aligned with both patient physiology and pedicle screw design, eliminating the necessity for additional surgical incisions. In this study, we assessed surgical and functional outcomes following BE-TLIF with percutaneous pedicle screw incision through exclusive application of this technique.
MATERIALS AND METHODS
1. Study Design and Patient Population
This retrospective study was conducted at a regional hospital in Taipei, Taiwan and enrolled all patients operated on by one specific surgeon (MHW) from October 2019 to April 2023. The Institutional Review Board (IRB) of Taipei Medical University approved the study (IRB No. N202407044) and waived the requirement for informed consent due to the retrospective design. Patients who experienced numbness or paresthesia below the affected level, gait disturbance, back pain, hyperreflexia, or increased muscle tone upon physical examination with radiological findings were included in the study. Along with participants with poor adherence to a minimum of 1-year postoperative follow-up, patients with high-grade spondylolisthesis (Meyerding grades 3 to 5) were excluded from the study due to the complexity of spinal pathologies.
Participant age, sex, clinical diagnosis, comorbidities, and follow-up times were recorded. Operation-related indices, including operation time and length of hospital stay, were also recorded. Intraoperative blood loss was assessed in accordance with the change in perioperative hemoglobin level due to the infeasibility of blood loss estimation during the endoscopic procedure. Lumbar spine x-ray and magnetic resonance images were obtained preoperatively and at follow-ups for evaluation of interbody fusion and subsidence rates. The extent of new trabecular bone formation between vertebral bodies was assessed following the Brantigan–Steffee classification [9,10]. The total number of reported grade 4 (probably fused) and grade 5 (fused) cases was divided by the total number of participants who underwent BE-TLIF to obtain the fusion rate of vertebral bodies. The degree of vertebral body collapse around the disc space was categorized as grade 0 (0%–24%), grade 1 (25%–49%), grade 2 (50%–74%), or grade 3 (75%–100%) [11]. Subsidence was assessed at follow-ups or during the reoperation period if an additional procedure was required. The total number of cases categorized as grade 2 or 3 were divided by the total number of participants who underwent endo-TLIF to determine the subsidence rate.
2. Outcome Assessments
Outcome data were obtained through a patient-based outcome questionnaire administered during outpatient follow-up visits or through a telephone interview. Back and radicular leg pain were evaluated using a visual analogue scale (VAS). Functional status was assessed using the Oswestry Disability Index (ODI) [12]. Overall quality of life was assessed using the EuroQoL-5 Dimension VAS (EQ5D-VAS) [13] and classified into 4 groups in accordance with modified MacNab criteria (excellent: no pain or functional limitation; good: occasional back or leg pain, mild functional limitation; moderate: improved general function, but patient requires changes in work and daily life activities; poor: no improvement in function or pain) [14]. These assessments were conducted preoperatively and at 1-, 3-, 6-, and 12-month follow-ups.
3. General Preparation
Before surgery, patients were placed prone on the operating table, with support from a radiolucent spine frame system. A 0° and 30° endoscopy system (Arthrex, USA) was used for general orthopedic joints under general anesthesia for all patients. Normal saline irrigation must remain continuous in the biportal endoscopic spine surgery set, with the endoscope directing irrigation fluid from the viewing portal to the working portal.
4. Surgical Procedure
For single-level surgery, portal sites were designed for pedicle screw entry and BE-TLIF under fluoroscopic guidance. In this procedure, 2 incisions are typically positioned along the medial pedicle line. For a right-handed surgeon operating from the left side, the viewing portal is generally placed at the lateral border of the upper-level pedicle (blue circle) through the incisional wound (red line) with the working portal positioned 2 to 3 cm caudally to allow for screw placement at the lower-level pedicle (blue circle) and cage entry, as illustrated in Figure 1A. On the right side, the viewing and working portals were placed at the lower half of the pedicle (blue circle) to provide favorable accessibility to spinal lesion for delivery of cage through the working portal, as displayed in Figure 1B. Space was created for normal saline to flow through a segment of the proximal lamina and interlaminar space using a muscle detacher. To prevent poor drainage, a small endoscopic retractor can be used to maintain smooth fluid flow, ensuring adequate visibility and minimizing soft tissue swelling. Figure 2 shows the configuration of portals and percutaneous incisions in biportal posterolateral approach employed in this study.
This surgical technique is similar to that used for MIS-TLIF, in which a tubular retractor and microscopy are applied. BE-TLIF involves an endoscopic irrigation system through 2 incisions. In this study, ipsilateral laminectomy was completed using a burr, Kerrison punch, and osteotome. Contralateral sublaminar decompression and then unilateral facetectomy were performed, with osteotomes used to harvest autologous bone. Following the removal of the inferior articular process, the superior articular process was removed using an osteotome and Kerrison punch, with a space retained between the exiting and traversing nerve roots.
After ipsilateral and contralateral decompressions and facetectomies, the ligamentum flavum covering the dura and nerve root was extracted. An incision was made on the disc using a specialized Indian knife designed for endoscopy. Discectomy was performed with a pair of pituitary forceps and a curette. For visualization, the arthroscope was inserted into the disc space. The cartilaginous endplate was thoroughly removed using a curette, exposing the subchondral bone, as displayed in Figure 3. Under fluoroscopy, allogenic bone chips and autologous bone harvested from the lamina and facet were impacted using a specialized cannula. A polyetheretherketone cage (OPAL Spacer System, DePuy Synthes) was then vertically placed under fluoroscopic and endoscopic guidance, as illustrated in Figure 4. A cage-specific instrument was used to transversely position the cage, with a retractor protecting the exiting and traversing nerve roots. For fixation, 2 ipsilateral percutaneous pedicle screws were inserted through the 2 previously used portals. On the contralateral side, 2 percutaneous pedicle screws were inserted through new incisions. Following a similar procedure on the ipsilateral side, the screws were connected through percutaneous rod insertion. The fluoroscopic views after pedicle screw and cage insertion were shown in Figure 5. A hemostatic matrix was applied and a drain catheter was inserted to prevent postoperative epidural hematoma, concluding the operation.
RESULTS
1. Demographic Characteristics
A total of 21 patients who received BE-TLIF were included in the final analysis. Demographic data for these patients are summarized in Table 1. The sample included 7 men and 14 women with a mean age of 69.9 years (range, 41–90 years). The following clinical diagnoses were recorded: degenerative spondylolisthesis (14 patients, 11 with grade 1 scores, and 2 with grade 2 scores), spinal stenosis (3 patients), lumbar facet cyst (1 patient), and reoperation (3 patients, 1 due to adjacent segment instability, and 2 due to recurrent disc herniation). Nineteen patients underwent single-level surgery and 2 underwent double-level surgery.
2. Surgical Outcomes
The mean operation time per level was 281.1±60.8 minutes (range, 180–400 minutes). The mean perioperative hemoglobin level change was -1.02 gm/dL. The mean postoperative hospital stay was 5.0±1.6 days (range, 2–9 days). The interbody fusion and subsidence rates at final follow-up were 87% and 13%, respectively.
Perioperative complications were reported in 3 patients. In 1 patient, incidental durotomy was noted intraoperatively. After immediate application of intraoperative hemostatic agents, the patient recovered, with optimal clinical outcomes and no further complications. Cage migration was noted in 2 patients, one of whom underwent reoperation. That patient reported severe back and bilateral leg pain 1 week after surgery. Following additional symptoms and signs of cauda equina syndrome and radiological studies revealing pseudocyst formation, the patient underwent revision discectomy for interlaminar decompression.
3. Functional Outcomes
Functional outcomes are summarized in Table 2 and Figure 6. All patients completed more than 6 months of follow-up. The mean VAS scores for back and leg pain and ODI were significantly lower after surgery and remained consistent throughout the 12-month follow-up period (back VAS, 3.3–0.9, p<0.001; leg VAS, 3.4–1.8, p<0.001; ODI, 18–2, p=0.049). EQ5D-VAS scores significantly improved over the 12-month follow-up period (79.8–98.2, p=0.009). Mean MacNab scores were 3.62, 3.77, 4, and 4.18 at 1, 3, 6, and 12 months after surgery, respectively. Table 3 presents clinical outcomes assessed in accordance with modified MacNab criteria. A successful outcome (rated as good or excellent) was achieved by 90% of study participants. All patients demonstrated symptomatic improvement (ranging from fair to excellent).
DISCUSSION
Traditional surgical approaches involving open posterior fusion effectively treat lumbar degenerative disease but may cause postoperative back pain and muscle atrophy due to the disruption of muscle and ligamentous structures, lengthening recovery periods [15]. MIS-TLIF yields comparable clinical outcomes to traditional open transforaminal lumbar interbody fusion in addition to distinct benefits, including reduced blood loss and shorter postoperative recovery. However, discectomy in MIS-TLIF requires an open incision involving partial separation of paravertebral musculature, partial laminotomy, and facetectomy [16,17]. In endo-TLIF, adequate surgical space and high-quality visualization provided by the endoscopic system allow for optimal decompression [18]. Endo-TLIF preserves the posterior spinal structure and causes less distraction to the cauda equina and bilateral nerve roots without stripping paravertebral muscles. This approach minimizes the load on the internal fixation system and iatrogenic instability [19]. Although endo-TLIF involves muscle stripping to the extent comparable to MIS-TLIF, the employment of wide-angle endoscope optimizes the surgical field with minimum tissue disruption and compromise of bony structures. BE-TLIF combines the advantages of MIS-TLIF with endoscopy. BE-TLIF provides high-quality surgical view, enabling greater surgical precision and direct visualization of the endplates without damaging the subchondral bone [20]. Compared with bipolar cauterization, use of a radiofrequency thermo-controlled ablator causes less thermal damage to paravertebral muscles and neural elements, minimizing systemic inflammation and allowing for efficient vascular cauterization to prevent bleeding around the microvasculature near the dural sac with low nerve injury risk [21]. Fan et al. [22] reported significantly shorter operative times, longer hospital stays, and a higher area of bone grafting in patients who underwent BE-TLIF than in those who underwent endo-TLIF. Intraoperative bleeding did not differ significantly between both groups. Both intraoperative bleeding and operative time may be affected by the specific surgeon’s approach, habits, and proficiency; operating conditions; and the extent of muscle stripping and bone removal. Despite greater muscle stripping and bone removal in the BE-TLIF group, several patients in the endo-TLIF group experienced greater intraoperative bleeding. VAS scores and ODI indices between both groups differed significantly at 1 week, 3 months, and 12 months after surgery compared with preoperative baseline values. These findings suggest that both BE-TLIF and endo-TLIF effectively alleviate pain and promote functional recovery, demonstrating similar treatment effects, thereby indicating equal safety and efficacy between both procedures. In this study, we exclusively performed BE-TLIF through incisions for pedicle screw insertion without additional incisions. The insertion of pedicle screws through percutaneous incisions is aligned with both patient physiology and pedicle screw design.
Multiple meta-analyses have revealed that fusion rates do not significantly differ between endo-TLIF and MIS-TLIF performed to treat lumbar degenerative diseases [23-25]. Another study reported no significant differences in fusion rate between endo-TLIF, MIS-TLIF, and open transforaminal lumbar interbody fusion [23]. Further long-term studies are required to validate the fusion rate and overall clinical efficacy of endo-TLIF. In 2 meta-analyses, operating time was shorter for endo-TLIF than for MIS-TLIF [26,27]. Nevertheless, in a third meta-analysis, the opposite was the case [28]. This discrepancy is potentially attributable to the use of local infiltration anesthesia versus epidural anesthesia. Undeniably, endo-TLIF is difficult to learn, comes with high instrumentation costs, and requires the surgeon to have extensive endoscopic experience and a profound understanding of foraminal anatomy [29]. A surgeon with limited experience or proficiency in performing endo-TLIF poses a risk factor for perioperative complications and the need for revision surgery [29,30].
Pedicle screw insertion through percutaneous incisional wounds has demonstrated effectiveness in endo-TLIF, but this technique differs from the approaches followed in other studies. Traditional endo-TLIF often involves single or dual portal systems positioned either medially or laterally to the midline, with additional lateral portals occasionally used for cage insertion and better visualization [31]. For instance, in one study, the view of the disc space and surrounding anatomy was improved through an accessory medial caudal portal, which allowed for clearer visualization of the disc and the use of retractors to maintain adequate exposure, potentially reducing the risk of iatrogenic durotomy [32]. Portal placement—whether medial or lateral—plays a critical role in procedural outcomes. Medial portals are often advantageous for accessing the central disc space, and lateral portals facilitate proper cage insertion and verification of disc position [32]. In this study, we did not use these additional portals, potentially limiting visualization and manipulation of the disc space, thus affecting the subsidence rate and other outcomes. Other studies on endo-TLIF have highlighted the benefits of incorporating medial and lateral portals for better procedural control, reducing complications and improving overall outcomes by providing surgeons with better access and visibility. By contrast, our approach involves the use of a biportal system comprising percutaneous pedicle screw incisions. This approach is less invasive but may limit the ability to place additional portals for greater visibility or retraction, possibly contributing to the observed 13% subsidence rate through compromised visualization and cage placement. Portal position relative to the pedicle line also affects surgical outcomes. Portals positioned laterally, away from the pedicle line, often allow for better cage placement and disc space visualization, which may reduce complications and improve fusion rates. The lack of additional lateral portals in our approach could have resulted in suboptimal interbody cage placement, potentially contributing to the fusion and subsidence rates observed. This finding warrants further comparison and study.
Technological advancements have addressed the inefficiencies associated with cage insertion. These advancements include the development of a surgical tubular retractor for cage insertion [33], specialized instruments for endplate preparation [34], expandable cages [35], and robotic-assisted pedicle screw placement [36]. Emerging technologies have been designed to facilitate the safety and precision of endoscopic interbody procedures [37], such as robotic guidance systems that assist with both percutaneous pedicle screw placement and trajectory localization for endoscopic discectomy and percutaneous interbody delivery [36]. However, the U.S. Food and Drug Administration has currently only approved robotic assistance for determination of the ideal trajectory for pedicle screw placement. Akbary and Kim [38] reported transforaminal endoscopic lumbar discectomy assisted by robotic navigation, which is expected to streamline three-dimensional navigation for O-arm and electromagnetic imaging. Although the application of mixed reality technologies for image guidance serves as a valuable tool in transforaminal lumbar interbody fusion, conventional fluoroscopy remains necessary for assistance [39]. Magnetic resonance imaging and ultrasound fusion may be feasible for puncture guidance with needle tail intelligent positioning in percutaneous transforaminal endoscopic lumbar discectomy. Additional clinical studies with larger sample sizes are necessary for more comprehensive evaluation of the advantages of this method [40].
This study has several limitations. First, the study included a limited sample size with a short follow-up period and no control group for comparison of the treatment outcomes. Second, all surgical procedures were performed by a single spine surgeon proficient in minimally invasive and endoscopic spine surgeries in a regional hospital setting. Treatment outcomes may vary by surgeon dexterity. Third, intraoperative blood loss was assessed on the basis of perioperative hemoglobin level change, which does not directly reflect actual blood loss. Last, interobserver differences may arise during radiologic interpretation, potentially introducing bias in the assessment of bone fusion and cage subsidence.
CONCLUSION
Compared with traditional techniques, endoscopic lumbar interbody fusion through percutaneous incisions for pedicle screw insertion has demonstrably reduced incisional wounds and minimized perioperative complications, speeding up recovery after surgery. This technique will likely become more prevalent in future practice as surgeons increasingly adopt minimally invasive approaches. Although the biportal percutaneous approach with only pedicle screw incisions has demonstrated favorable outcomes in terms of functional improvement and complications, the absence of additional portals may limit certain aspects of the procedure. Future studies should explore the integration of medial and lateral portals into this technique to assess their effect on visualization, cage placement, and overall surgical outcomes. Such modifications could increase the effectiveness of endo-TLIF, further optimizing surgical results and patient recovery.
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