In this prospective study, 40 patients in total (13 men and 27 women; mean age, 62.14±6.17 years) have been enrolled between the time period of January 2020 and December 2021 in Daejeon Woori Hospital. Unilateral biportal endoscopic lumbar interbody fusion with long PEEK cage was done for all patients. The patient information collected was demographics, diagnosis, preoperative visual analogue scale (VAS) and Oswestry Disability Index (ODI) score (Table 1). A thorough documentation of the patient’s clinical history, examination findings, preoperative investigations including imaging studies, operative details, follow-up time, if any postoperative difficulties and functional scores were done. Patients who qualified the inclusion and exclusion criteria set were selected. The patients with nonresolving low back pain with radiculopathy even after giving a minimum of 4 weeks of conservative trial, combination of medical history and magnetic resonance imaging (MRI) rooting for a diagnosis of lumbar degenerative disease especially lumbar spinal canal stenosis (central and lateral recess stenosis) and spondylolisthesis (lower than grade II), involvement of single-segment/level pathology and chronic cases of lumbar degenerative disease not improved even after nonsurgical treatments that failed or were more than 6 months were included while patients with metastatic disease, acute extruded disc herniation, symptoms or signs not correlating to the imaging studies, patients with coagulation abnormalities or those who had previously undergone instrumentation surgery for the lumbar level and those who were not willing to undergo surgery or could not to complete follow-up criteria were excluded.

This study was approved by the Institutional Review Board (IRB) of the Daejeon Woori Hospital.

The PEEK cage is a high molecular weight thermoplastic material which has got a modulus of elasticity closer to that of bone thereby resulting in less stress shielding, better transfer of load, decreased chance of subsidence and more importantly has got a higher rate of fusion [18]. Radiologically on computed tomography (CT) scan, the fusion status can be better assessed with a PEEK cage as it is radiolucent. Other added advantage is that it mostly avoids any chance of infection as PEEK is an inert material that resists cell adhesion [19,20]. These cages have metallic markers for identification purpose. The cages used for this study was slightly smaller than the conventional oblique lumbar interbody fusion cages, larger than the posterior lumbar interbody fusion (PLIF) cages and TLIF cages [21,22]. The cage dimensions used in this series were of length (40 mm, 45 mm, or 50 mm), height (9 mm, 11 mm, or 13 mm), width (17 mm), and 6° lordotic angle (Table 2. Figure 1), when compared to the conventionally used PLIF cage (25 mm/12 mm/9 mm/0°) and TLIF cage (10 mm/12 mm/30 mm/0°) sizes [22].

Patients were given general or epidural anaesthesia and taken in prone position. More often the right-handed surgeon prefers to approach from left side as it makes it relatively easy to get the surgical instruments from the scrub nurse. A right sided approach is however opted in some cases like-in L5–S1 level or those with a high lordotic angle or even when their demands a direct neural decompression of right foraminal stenosis is warranted. Two transverse skin incisions which are approximately 3 cm apart are made, with inferior part of the cranial lamina at midline being the center of upper pedicle and lower pedicle. The cranial incision is for the endoscopic portal while the caudal one is for the working portal. In obese patients, the skin incisions may need to be further lateral. Serial tube dilators are inserted initially so as to make an easy pathway for endoscopic sheath insertion subsequently. These make triangulation at spinolaminar junction. Soft tissue is cleared off by using radiofrequency (RF) probe. Unilateral laminotomy with bilateral decompression is done to achieve central canal decompression. Ipsilateral complete facetectomy done by removing both the inferior and superior articular processes using multiple osteotomies to save the autograft material. The disc space of ipsilateral side is exposed (Figure 2A), epidural vessels coagulation is done precisely. RF probe or Indian knife may be used for performing annulotomy (Figure 2B). After performing annulotomy, pituitary forceps are inserted to remove the disc material (Figure 2C). A meticulous disc space, endplate preparation is done under vision using a Kerrison punch, angled endplate removers, curettes (Figure 2D), and pituitary forceps so as to achieve a good fusion bed. Atmost care is taken for removing most part of cartilaginous endplate without any bony endplate injury so as to prevent any subsidence of cage into the vertebral body. A 30° scope is used for contralateral side endplate preparation, adequate disc material and cartilaginous endplate removal so that the long PEEK cage sits comfortably. Multiple bleeding spots from the bone marks the end of endplate preparation (Figure 2E). Through the working port, trial size of cage is inserted until a proper size is achieved. Using a bone graft funnel and under fluoroscopy guidance, bone grafting is done by compacting it into anterior portion of disc space. Under direct visualization, the long PEEK cage filled with mixture of autograft and allograft is inserted (Figure 2G) after adequate protection of thecal sac and nerve root by a specific cage guider (Figure 2F). By using a cage impactor, the inserted cage is then placed across the prepared disc space (Figure 2H), and confirmed under the fluoroscopy. Percutaneous pedicle screw insertion is done under fluoroscopy guidance after taking the required skin incisions which marks the end of surgery. A 100-mL surgical drain is inserted via working portal skin incision to prevent any sort of complication arising due to postoperative hematoma formation.

On day one after surgery, patient is mobilized with physical activity and surgical drain is removed on day 2 postoperatively. Postoperative standing radiographs are taken to see the cage and screw placements. MRI scan done after drain removal shows neural decompression in detail.

Clinical assessment of all 40 patients included in the study were done on an outpatient basis for back and leg pain VAS scores and ODI scores at post operative 6 months and 12 months. A minimum of 12-month follow-up period after surgery wherein the preoperative VAS scores for overall back and leg pain, and ODI score at the final follow-up showed significant change.

The radiological assessment for this study were done using radiographs (x-rays) and CT scan. For determining the disc height, segmental lordosis and lumbar lordosis, x-rays taken at 1 week postoperatively and at 1-year follow-up were included. Segmental height was calculated from 1-week postoperative radiograph was compared to the final follow-up radiograph to assess for any cage subsidence. A difference of >2 mm if present between the two was to be considered positive for subsidence [23]. Fusion outcome was assessed by using CT scan and the scan parameters were similar for all the patients. This was preferred as CT scans have better reliability in assessing bony fusions [24,25]. At a follow-up period of minimum of 12 months, the bony fusion was radiologically assessed using Bridewell fusion grading system. The rate of fusion calculated was sum total of grades 1 and 2.

Criteria for fusion status assessment was discussed by investigators and also by an independent musculoskeletal radiologist. The assessment of fusion was initially done independently by each investigating individual and the radiologist. Those cases which were doubtful were reviewed conjointly to reach a consensus. Grading score on CT scan were done according to the guidelines mentioned in Table 3.

For categorical variables, Wilcoxon sign-rank test was used to find the mean and standard deviation. A p-value of <0.05 was considered statistically significant. The statistical analysis was performed using the R ver. 3.1.2 (R Foundation for Statistical Computing, Vienna, Austria).