The insert and revolve technique: a novel approach for inserting cages during unilateral biportal endoscopic assisted fusion surgery for effective spinal alignment restoration

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Asian Spine J. 2024;18(4):514-521

Publication date (electronic) : 2024 August 21

doi : https://doi.org/10.31616/asj.2024.0066

Ji Soo Ha, Shreenidhi Kulkarni, Do-Hyoung Kim, Chang-Wook Kim, Rajendra Sakhrekar, Hee-Don Han

Department of Neurosurgery, Yonsei Okay Hospital, Uijeongbu, Korea

Corresponding author: Ji Soo Ha, Yonsei Okay Hospital, 107 Gyeongui-ro, Uijeongbu 11696, Korea, Tel: +82-31-848-0885, Fax: +82-31-848-0884, E-mail: Nshjs82@naver.com

Received 2024 February 19; Revised 2024 June 4; Accepted 2024 June 7.

Abstract

Introduction

Lumbar degenerative disease (LDD) is the most prevalent spinal ailment in older adults due to the wear and tear of the spine. The LDD has a spectrum of manifestations ranging from innocent asymptomatic changes to more advanced lumbar canal stenosis, instability, and spondylolisthesis. These developments cascade into the loss of disc height, loss of segmental sagittal alignment, facetal hypertrophy, and foraminal stenosis, ultimately leading to a substantial decline in the quality of life and causing significant patient distress [1–7]. Thus, the restoration of adequate disc height and lumbar lordosis mitigates the progression of adjacent segment disease [8–10]. Notably, lumbar fusion surgeries employing a 360-degree fusion with an interbody cage exhibit superior fusion rates, enhanced alignment restoration, and improved clinical outcomes compared with posterolateral fusion [11].

Minimally invasive (MIS) techniques have reduced the disruption of soft tissues and muscles, resulting in a smaller footprint while achieving comparable results. Endoscopic decompression has distinct advantages over conventional techniques but the same cannot be universally asserted for fusion techniques [12–15]. The challenges in fusion surgeries have been addressed through the use of unilateral biportal endoscopy (UBE), particularly in UBE lumbar interbody fusion (ULIF). A broader range of vision, the utilization of conventionally used instruments for open surgery, and the ability to introduce larger intervertebral cages are features facilitated by UBE [4,16,17].

This study aimed to describe a novel method, called the “insert and revolve technique,” that aids in introducing larger intervertebral cages through ULIF, enabling the correction of spinal alignment. The clinical and radiological impact was assessed at 1-year follow-up.

Materials and Methods

Patients and study design

The study design was retrospective and was conducted in accordance with the Declaration of Helsinki. The place of this study accepted retrospective studies without the approval of the institutional review board. Informed consent was obtained preoperatively from all eligible patients. Preoperative and postoperative radiologic images (X-ray, computed tomography [CT], and magnetic resonance imaging [MRI]) were taken. Patients with symptomatic lumbar spondylolisthesis, spinal stenosis, herniated lumbar intervertebral disc with severe back pain, and adjacent segment disease were operated and included in the study if they met the following criteria: (1) underwent ULIF using the “insert and revolve technique” and (2) completed 12 months of follow-up after surgery.

The demographic and clinical data, including age, gender, diagnosis, neurological deficit, level of surgery, estimated volume of intraoperative blood loss, surgical time, and significant intraoperative events, were recorded. The clinical outcomes were measured using the Visual Analog Scale (VAS) for leg pain and back pain, Oswestry Disability Index (ODI), and modified McNab’s criteria (MMC) postoperatively at regular intervals. CT scans and X-ray examinations were conducted at 6 months and 1 year postoperatively, and segmental lordosis, intervertebral disc height, segmental coronal alignment, cage subsidence, and fusion grade were measured.

The degree of segmental lordosis was calculated by measuring the angle between the superior end plate of the proximal vertebra and the inferior end plate of the lower vertebra (Fig. 1A). The segmental coronal alignment was determined as the angle between the adjacent end plates of the instrumented level (Fig. 1B). The intervertebral disc height was measured as the distance of the anterior border of the instrumented intervertebral disc (Fig. 1C). Subsidence was defined as a ≥2 mm migration of cage into the adjacent vertebra determined by comparing the immediate and 6-month postoperative intervertebral height. Subsidence was classified as grade 0 (0%–24% loss of the postoperative disc height), grade I (25%–49% loss), grade II (50%–74% loss), and grade III (75%–100%). Fusion was graded according to the Bridwell grading system; grades 1 and 2 were considered fused and assessed at 6 months and 1 year if not fused at 6 months.

Fig. 1

(A) Segmental lordosis calculated as measuring the angle between superior end plate of proximal vertebra (from point a to b) and inferior end plate of lower vertebra (from point c to d). (B) The segmental coronal alignment calculated as the angle between the adjacent end plates of instrumented level (between e–f and g–h). (C) Intervertebral disc height measured as the distance of anterior border of instrumented intervertebral disc (longest distance between two white lines; i and j).

Analyses were performed using the IBM SPSS software for Windows ver. 25.0 (2007; IBM Corp., Armonk, NY, USA). Data were expressed as the mean±standard deviation, median (25th–75th quartile), or frequency (percentage). The normality of data was assessed using the Shapiro-Wilk test. As the data did not follow a normal distribution, changes in parameters post-operation were assessed using the Friedman test for repeated measures after adjusting for Bonferroni correction or Wilcoxon signed-rank test. A p-value of <0.05 was considered significant.

Surgical procedure

Surgical equipment

Six-degree extreme lateral interbody fusion (XLIF) cages and a specially designed cage glider (Medions Co. Ltd., Seoul, Korea) (Fig. 2A, B), Medynus endoscopic spine system kit (Medynus Inc., Irvine, CA, USA), AnyPlus Pedicle MIS screw system (GS Medical Co. Ltd., Irvine, CA, USA), and ARTeMIS percutaneous MIS screw system (Medyssey Co. Ltd., Jecheon, Korea) were used for ULIF.

Fig. 2

(A) Various sizes of extreme lateral interbody fusion cage (6°/40, 45, 50 mm length; 15, 17 mm width; 10 to 15 mm height). (B) Specially designed cage glider for unilateral biportal endoscopy lumbar interbody fusion. Three-way blades of cage glider provide protection of neural structures from cage. (C) Paprika sign; complete endplate preparation visible under direct endoscopic visualization. (D) Even large cage can be safely introduced into intervertebral space using cage glider without additional retractors.

Procedure

After obtaining informed consent, the patients were placed in a prone position on a radiolucent table with a Wilson frame and administered general or epidural anesthesia. After adequate preparation and draping, the lateral borders of the cranial and caudal pedicles were marked under fluoroscopy, and a 1-cm vertical skin incision was made. Adequate fascial release was ensured to establish a proper channel for the outflow of the continuous saline irrigation system. The spino-laminar junction was identified, and a space was created using a Cobb elevator until the facet joint was reached. A water pressure of 45–50 mm Hg was maintained using an automatic irrigation pump system. The facet joint was exposed through radiofrequency (RF) ablation, and all attachments were cleared. Using an osteotome, the inferior articular process (IAP) was chiseled out from the spino-laminar junction to the lateral border of the isthmus parallel to the end plate line. The excised IAP was recovered for use as a bone graft for the interbody cage. The remaining IAP was drilled to expose the tip of the superior articular process (SAP), and the SAP was osteotomized until its junction along with the pedicle and inferior lamina was reached. The ligamentum flavum was excised until the midline, the disc space was exposed, and bleeding was promptly addressed through RF ablation. Using a scope retractor with the traversing root retracted, annulotomy was performed at a more medial location than conventionally practiced. Serial shavers were inserted under fluoroscopy guidance. Intra-discal preparation was carried out using curettes, freers, and pituitary forceps until complete clearance of the disc was achieved. Petechial hemorrhagic spots (Paprika sign) were examined under endoscopic visualization (Fig. 2C). Clearance of the opposite side disc was facilitated by utilizing an endoscope and implementing a more medial annulotomy.

“Insert and revolve technique”

An appropriately sized trial cage was inserted, and its position was verified under a fluoroscope. Upon confirmation, the cage glider was introduced into the disc space, and its location was confirmed under a fluoroscope. An XLIF cage, matching the trial size, was filled with demineralized bone matrix/autologous bone graft and inserted obliquely into the disc space. Subsequently, the cage was hammered until the lagging end was aligned along the posterior vertebral line in the lateral view and crossed the midline in the anteroposterior view under fluoroscopy monitoring (Fig. 2D). Under fluoroscopic monitoring and endoscopic view, a cage pusher was introduced, the cage was gradually hammered to transition its position from the oblique plane to the transverse plane, and the cage was pushed anteriorly to finally position it in the anterior half of the interbody space (Fig. 3A–D). Once completed, the opposite side was explored if needed, and adequate decompression was performed. The pedicle screws were inserted percutaneously in the same incision on the same side and in similar incisions on the opposite side under fluoroscopic guidance. Connecting rods were inserted and locked in place, and the final position was confirmed (Fig. 3E, F). Closure was performed in layers with a negative drain inserted, followed by the application of a sterile dressing.

Fig. 3

Schematic illustrations of the “insert and revolve technique” (A–D). (A) The trajectory of cage insertion starts more medially than with traditional transforaminal lumbar interbody fusion. (B) The cage is pushed ventrally and hammered until the proximal markers cross the midline. (C) Once the proximal markers cross the midline, the cage is revolved transversely using a triangular impactor. (D) The cage is pushed further transversely into the anterior half of the intervertebral space. (E, F) The final positions of the cage, pedicle screws, and rods are identified in X-ray images.

Results

One-hundred-and-twelve patients underwent ULIF from July 2019 to September 2022. Eight patients with a short follow-up period and who were lost to follow-up were excluded from the study, resulting in 104 patients completing the study. Patient characteristics and surgical levels are presented in Table 1. Postoperative pain at the surgical site was managed by intravenous analgesics, which was eventually switched to oral analgesics on postoperative day 2 or 3. All patients were mobilized with safety support on day 1 following surgery. Drains, inserted during surgery to reduce the risk of postoperative epidural hematoma, were removed on postoperative day 1 or 2. The patients were deemed fit for discharge usually by day 3. However, since inpatient therapies were often perceived by patients as opportunities for unwinding, recuperating, and recovery, the average postoperative hospital stay was not assessed in this study. The average duration of return to work was 23±6.7 days.

Table 1

Patient characteristics

Clinical parameters

The VAS back pain score improved from 7.1±0.4 preoperatively to 2.9±0.8 at 3 months, 1.1±0.4 at 6 months, and 0.9±0.4 at 1 year (p>0.05). Similarly, the VAS leg pain score improved from 6.7±0.5 preoperatively to 2.4±0.6 at 3 months, 1±0.4 at 6 months, and 0.8±0.4 at 1 year (p>0.05). The preoperative mean ODI score decreased from 39.8±2.1 to 15.2±1.6 at 12 months following surgery (p<0.05) (Table 2). According to the MMC, 97 patients had an excellent outcome, while seven had a good outcome at the end of 12 months.

Table 2

Comparison of changes in VAS and ODI score

Radiological parameters

Segmental lordosis increased from a mean of 9.3°±11.5° preoperatively to 18.4°±10.4° at the immediate postoperative period and 17.9°±7.8° and 17.78°±8.1° at 3 months and 1 year, respectively, showing a significant improvement after cage insertion. The mean segmental coronal alignment showed a significant improvement from 7.7°±2.1° preoperatively to 3.4°±1.2° postoperatively (Table 3). The intervertebral disc height increased from a mean of 6.3±2 to 10±2.1 mm at the immediate postoperative period and to 10±1.1 mm at 6 months, thus showing a significant improvement (Table 4). Five patients experienced cage subsidence, but none had clinical significance. At 6 months, 65 patients showed grade 1 fusion, 37 had grade 2 fusion, and two had grade 3 fusion based on the Bridwell grading. At 1 year, 92 patients, 11 patients, and one patient showed grade 1, 2, and 3 fusions according to the Bridwell grading, respectively (Table 5). During surgery, three patients experienced dural tears, all of whom underwent revision surgeries, were managed with TachoSil and vascular clips, and required active bed rest for 24–48 hours. These patients recovered with no complications. One patient developed foot drop (Medical Research Council grade III) post-surgery, was closely followed up, and gradually recovered to grade IV at 3 months and grade IV+ at 6 months. Grade 0 cage subsidence occurred in five patients and in four patients on postoperative day 1 and at 6 months. Four patients demonstrated poor bone quality on a dual X-ray absorptiometry scan. However, these observations were not considered clinically significant. They exhibited successful fusion at a 1-year follow-up. Two cages exhibited rotational changes on CT images at 6 months, without resulting in any clinical signs. One cage demonstrated fusion, while the other cage showed grade 3 Bridwell fusion at a 1-year follow-up.

Table 3

Comparison of changes in SL and SCA

Table 4

Comparison of changes in IVDH

Table 5

Perioperative long-term radiological outcomes and symptomatic satisfaction

Discussion

The emergence of MIS fusion techniques signifies a paradigm shift, minimizing injuries to the musculo-ligamentous structures, shortening recovery times, and facilitating an early return to activities of daily living with minimal to no muscle atrophy and associated postoperative back pain [18–22].

The preservation of coronal and sagittal alignment, coupled with foraminal decompression, is a pivotal consideration in modern spine surgeries, with various techniques revolving around this fundamental principle [8,9,23]. Achieving adequate decompression, disc height restoration, and maintenance of alignment are crucial aspects, and interbody cages play a significant role by providing distraction and maintaining the tension until union occurs. Anterior lumbar interbody fusion (ALIF) and lateral lumbar interbody fusion (LLIF) eliminated the need for posterior element manipulation and completely preserved the integrity while enabling alignment correction, disc height restoration, foramen widening, and indirect decompression, albeit with limitations in direct posterior decompression. Transforaminal lumbar interbody fusion (TLIF) allows the introduction of large cages with minimal dural sac handling, direct decompression, and relatively easier learning curves [16,18]. ULIF, with its two ports, allows flexibility, provides convenience, and enables the introduction of larger cages with minimal cord handling [16]. The utilization of conventional open instruments, a working portal for introducing instruments and intervertebral cages, adequate end plate preparation under full vision, continuous saline irrigation for enhanced hemostasis, and the prevention of infection risk are among the advantages of ULIF [16].

TLIF effectively improves spinal alignment after surgery, as reported in multiple studies. In a study by Jagannathan et al. [24] in 2009 involving 87 patients with 107 levels who underwent TLIF, the immediate restoration of sagittal imbalance was observed, with most improvement at the surgical level. Lumbar lordosis significantly improved in multilevel surgeries, which correlated with the ability to restore a sagittal imbalance of <10 cm. The study reported a 91% complete correction of all spondylolisthesis cases [24]. In 2022, Leveque et al. [25] conducted a study involving 474 patients from 18 centers, comparing the changes in spinopelvic parameters after 1–2 level TLIF, posterior lumbar interbody fusion (PLIF), ALIF, and LLIF (632 levels). Anteriorly placed grafts (ALIF and LLIF) tended to maintain alignment, while posteriorly placed grafts (TLIF and PLIF) worsened lordosis. This observation was consistent with the report of a 2018 study conducted by Ahlquist et al. [26]. In 2015, Wang et al. [27] reported the use of a bullet cage in MIS-TLIF by rotating it transversely. The technique proved to be effective with excellent outcomes and was deemed safe and efficient, even with small cages. A study by Park et al. [17] in 2019 compared the clinical and radiological outcomes of ULIF with those of PLIF. The ULIF group showed earlier improvement in VAS scores for leg and back pain, with no significant differences in complication rates, fusion rates, and ODI scores at 1 year. They concluded that ULIF is as effective as conventional PLIF in improving clinical outcomes and achieving fusion while being less invasive [17].

Taking into consideration the above parameters, the “insert and revolve technique” was devised to facilitate the placement of large XLIF cages through a smaller posterior window and effectively position them transversely in the anterior intervertebral disc space. The study found improved lordosis and coronal alignment with larger XLIF cages placed transversely in the anterior intervertebral body space, allowing for greater contact between the cage and endplates. These cages have a horizontal saw-blade pattern at the superior and inferior contact surfaces and provide a railing effect for revolving along the axis, minimizing the risk of end plate disruption. The marginal loss of alignment and the absence of clinically significant subsidence probably were attributed to the larger surface area of contact, limiting the amount of subsidence. Two patients demonstrated cage rotation with no clinical signs. The cages did not displace into the canal as the amount of rotation required for displacement is unlikely to occur spontaneously, owing to the size and insertion technique. Polyetheretherketone cages are preferred for patients with severe osteoporosis, as titanium cages may cause fractures and increase the risk of subsidence and loosening. Perioperative complications, including three dural tears and five epidural hematomas, were promptly addressed and had no impact on the long-term clinical outcomes. None of the tears necessitated abandoning the technique and converting to open surgery for repair. In the early days of UBE fusion, three dural tears were found near the root during cage insertion without a glider due to the cage size. The use of a cage glider for cage insertion proved beneficial in protecting the structures, eliminating tears during subsequent cage insertions.

The current technique is limited by the ability to introduce large cages in patients with high-grade spondylolisthesis due to the risk of root injury. Additionally, the revolving technique poses challenges in cases involving the L5–S1 with a steep sacral slope due to the angle. In such instances, oblique placement of cages is the more feasible and safer alternative to mitigate the risk of endplate damage. Another limitation is the inability to use cages with various lateral side angles, which could enhance the sagittal angle after revolving.

Conclusions

The “insert and revolve technique” is a safe, feasible, and effective approach for introducing a large interbody cage through a smaller window. It has excellent sagittal, coronal correction, good union, and low subsidence rates as well as excellent clinical outcomes. Further comprehensive studies on a larger sample are warranted to validate the ease of adoption and overall outcomes of this technique.

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