Mounting the learning curve in unilateral biportal endoscopic lumbar decompression in an Asian population: experience of a single surgeon’s first 105 consecutive cases with early functional outcomes

Introduction

In recent years, spinal surgery has witnessed a paradigm shift from open towards minimally invasive spine surgery (MISS) (1,2), with the latter now having indications that extend from the common herniated disc to complex intradiscal pain management (3). The benefits of reduced tissue disruption are as follows (4)—small percutaneous incision(s), reduced blood loss and postoperative pain, preservation of segmental stability and mobility, shorter length of stay, and expedited recovery and return to work. This has resulted in the increase in popularity and acceptability of MISS with patients (5), and is particularly relevant in the Asian context, wherein traditional cultural preconceptions are opposed to perceptibly “larger” and more “open” surgeries. Further, MISS permits ambulatory or same-day surgery feasible under local and/or regional anesthesia, which provides a surgical option in increasingly geriatric patient populations (6-8), wherein prolonged open surgery under general anesthesia may pose significant risks of major cardiac adverse events (MACEs). The employment of MISS would hence allow for more patients who have spinal pathologies indicating the need for decompression to be eligible for such procedures. Multiple studies have supported non-inferiority of MISS to standard techniques while achieving the goals of spinal decompression (9-12).

Endoscopic spinal surgery, a pioneering approach, has thus emerged as a promising avenue in MISS. This technique utilizes small incisions, specialized instrumentation, and real-time endoscopic visualization with enhanced clarity, magnification and illumination to precisely access and address spinal pathology. In particular, the unilateral biportal endoscopic (UbE) approach offers distinctive advantages by using two ports for simultaneous visualization and instrumentation, maintaining a unilateral trajectory (13). A recent cohort study by Wu et al. has shown that in the realm of endoscopic surgery, the biportal approach is equivalent to the uniportal approach in addressing lumbar spinal stenosis with UbE having a potentially lowered risk of complications, inadequate decompression, or propensity for conversion to open surgery (14).

We present a comprehensive analysis of a single-center, single-surgeon experience in employing the UbE approach in lumbar decompressive surgery. In presenting the surgical technique, postoperative outcomes, and operative times with surgical experience, we aim to provide a perspective on the efficacy, safety and feasibility of endoscopic lumbar decompressive surgery with respect to the experience of a single surgeon mounting the learning curve of UbE, and to demonstrate a temporal evolution of outcomes through a series of 105 consecutive UbE cases. We present this article in accordance with the STROBE reporting checklist (available at https://jss.amegroups.com/article/view/10.21037/jss-24-153/rc).

Methods

Patient cohort/inclusion and exclusion criteria

A consecutive cohort comprising the first 105 UbE lumbar decompressive surgeries performed at our institution, Sengkang General Hospital, by a single surgeon (Y.H.) from January 2022 to September 2023 was retrospectively analyzed. At enrollment, the senior author had just returned from a fellowship and had no previous experience with endoscopic lumbar decompression. All patients were of Southeast Asian ethnicity and had preoperative lumbar spine magnetic resonance imaging (MRI). These patients were recruited prospectively from outpatient clinics and were elected for surgical management after clinical assessment and diagnostic imaging. Presenting complaints ranged from axial back pain to lower limb radiculopathy, claudication and numbness/weakness. We only included patients with degenerative or prolapsed lumbar disc, or spinal stenosis causing lower limb radicular, claudication and sensorimotor symptoms; the underlying neural compression must be attributable to MRI findings that were clinically and radiologically concordant. We did not enroll or perform surgery for patients with back pain as the sole symptom, or failed back surgery syndrome, or spondylolisthesis with back pain alone. Patients with non-degenerative spinal pathology (such as fracture, neoplasm and infection), or who underwent endoscopic cervical surgery, or did not complete the lumbar surgery, or were lost to follow-up, were excluded from our study.

We enrolled the first 105 consecutive cases that fulfilled the inclusion/exclusion criteria, performed by the senior author, to demonstrate an unbiased perspective of the challenges of transitioning from open or tubular decompression to percutaneous endoscopic approach, based on the anticipated learning curve of 20–60 cases (13) in literature. All 105 consecutive patients subsequently underwent UbE lumbar decompressive surgery and were included in our study.

Anesthesia, surgical positioning and adjuncts

The patient is placed under general anesthesia for greater muscle relaxation and precise positioning, then subsequently prone on a radiolucent spinal surgery table (Pro-Axis, Mizuho OSI, CA, USA). Intraoperative fluoroscopy is used alongside a metal wand for surgical landmarking on the skin. Intraoperative neuromonitoring is employed to readily pick up any nerve irritation.

Landmarking and portal creation

On the preoperative antero-posterior fluoroscopic view, the horizontal meridian is landmarked by the spinolaminar junction, where the base of the spinous process meets the inferior aspect of the lamina, whilst the vertical plane is demarcated just lateral to the midline connecting the spinous processes. The first portal incision is made 1 cm in length at the intersection of the horizontal and vertical lines, whilst the second portal incision of the same length is made approximately 1.5–2 cm distal to the former along the same vertical line. Depending on the laterality of the pathology, the viewing portal is the incision on the surgeon’s left, whilst the instrument portal is on the right with the surgeon standing on the side of the pathology.

Surgical technique

The primary aim of surgery is to decompress the nerve root(s) and/or dural sac at the involved spinal level(s) with degenerative or prolapsed disc, or spinal stenosis. There was no instrumentation, fusion, correction of spondylolisthesis, nor surgery performed for failed back syndrome. The surgical technique employed therefore, is constant for all 105 consecutive cases.

Lumbar decompression first involves dissecting the skin layer down to the bony prominences past all the muscle, fatty and fibrous tissue. Next, the interlaminar space is located and the entry point is confirmed in intraoperative imaging. A mixture of burrs, rongeurs, curettes and chisels is used to perform superior and inferior laminotomy on the side of pathology as well as medial facetectomy and flavectomy to expose and achieve nerve root or dural decompression. Discectomy may subsequently be performed if indicated intraoperatively. Hemostasis is performed with radiofrequency probes and hemostatic matrix (Floseal, Baxter, IL, USA) is injected if required. Adequacy of nerve root and dural sac decompression is checked with ball-tip probes and nerve hooks. After generous irrigation and meticulous hemostasis, the skin is closed in layers and covered with a waterproof dressing. Sutures are removed postoperatively on day 14 after a thorough wound inspection by the surgical team.

Preoperative and postoperative management

Patients underwent similar perioperative regimens as per institutional guidelines detailed below. They were admitted to the day surgery ward on the morning of the operation and were assessed by the physiotherapist for assessment of the 36-item Short Form Survey (SF-36), Visual Analogue Scale (VAS) for pain and Oswestry Disability Index (ODI) scores. A member of the surgical team would review to confirm neurological manifestations, laterality of the symptoms and other relevant perioperative matters. Postoperatively, the patients spent 1 hour in the post anesthesia care unit before being discharged to the short stay ward for an immediate postoperative review by the surgical team. The physiotherapist then sees the patient on the same day postoperatively after clearance from the surgeon to kickstart ambulation and mobilization. Patients are typically discharged on the same day or the next, with postoperative physiotherapist advice and outpatient appointments for up to 3 months, as well as clinic reviews with the surgeon in 2 weeks, 6 weeks, 3 months and 6 months. Patients with intraoperative or postoperative complications were transferred to the inpatient ward for management with a multidisciplinary team.

Clinical data collection

This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. Approval from the ethics committee of Sengkang General Hospital was obtained (CIRB Ref No. 2020/2381). Informed consent for the collection of retrospective data was waived. Comprehensive data, including the patient’s demographics, body habitus, chief complaints, diagnostic imaging results, and patient-reported outcome measures such as SF-36, VAS and ODI scores, were collected. Details pertaining to the operations were also analyzed, including operation side, duration, spinal level(s) and number of level(s) operated on, and intraoperative complications. Postoperative details such as complications, length of stay, improvement in preoperative symptoms, and need for relook surgery were also recorded. Postoperative outpatient functional scores were obtained for a duration of 1–3 months. All data were retrospectively acquired from our institution’s medical and operating theatre records.

Statistical analysis

Demographic, radiologic, and clinical details were analyzed using descriptive statistics. Continuous variables were presented as means with standard deviation (SD) and discontinuous variables were expressed as absolute counts with percentages (%). All data were analyzed on STATA Ver 14. The independent Student’s t-test was used to compare preoperative and postoperative patient-reported outcome measures, with P<0.05 being defined as statistically significant. Pearson correlation analyses were also utilized to assess the independent variables affecting operative time with a similar threshold for significance at P<0.05. There were no missing data for any key demographic, operative or outcome variables, including all patient-reported outcome measures, as all 105 patients completed follow-up and data collection at the specified postoperative time points at 1 and 3 months.

Bias

To mitigate selection bias, all 105 patients who met clearly defined inclusion and exclusion criteria as previously discussed were consecutively recruited into the study over a fixed time period. By maintaining uniform surgical technique by a single surgeon, perioperative regimen and outcome assessment protocols, interoperator variability and measurement bias concerning operative time and outcome is limited. Patient characteristics such as age, body mass index, smoking status, number of levels and laterality of decompression, amongst others—all of which could influence operative time and outcome—were explicitly analysed using regression and correlation analysis to account for possible confounding bias. The surgical times and outcomes of all 105 patients were analysed without attrition bias, as evidenced by the inclusion of patients with operative time that exceeded 3–4 hours, and those that required relook surgery.

Results

Baseline characteristics

The mean age was 57.9 years, with the gender balance comprising 52 males and 53 females. The commonest chief presenting complaint was lower limb pain or radiculopathy (93.3%), followed by lower back pain (75.2%), leg numbness (51.4%) and neurogenic claudication (46.7%). Twenty-seven patients had spondylolisthesis on preoperative imaging (25.7%) and three patients had scoliosis (2.9%). Six patients (5.8%) had previous spinal surgery in the same region, with three being open (2.9%) and three being endoscopic (2.9%). Preoperative patient-reported outcome measures were recorded, with VAS for back pain averaging 5.4±2.7, and VAS for leg pain averaging 6.6±2.4. Mean ODI was 51.7±20.1 preoperatively. The SF-36 score was tabulated, with the physical function scoring 39.2±22.1, physical role scoring 13.2±25.5 and bodily pain scoring 38.0±19.4 amongst others (Table 1).

Operative characteristics

A total of 105 patients had a total of 111 pathological spinal levels operated on, with the majority being single-level surgeries (94.3%) and the remainder being double-level (5.7%). The most frequently operated level was L4/5 (56.2%), followed by L5/S1 (31.4%), L3/4 (13.3%), and L2/3 (4.8%). Nearly half of the cases underwent bilateral decompression via a single unilateral incision (45.7%). The incidence of intraoperative and postoperative complications was low, comprising 3 dural tears (2.86%), 1 wound bleeding (0.95%) and 2 acute urinary retention (1.90%). No cases of infections, foot drop or conversion to open decompression were noted (Table 2). The isolated case of postoperative wound bleeding was arrested with suture reinforcement and dressing. The three cases of dural tears were repaired successfully endoscopically with a dural patch, whilst the two cases of acute urinary retention resolved spontaneously upon removal of catheter following mobilization.

Surgical time

Operative time comprised the duration spanned from skin incision to completion of wound closure. The mean total operative time was 124.3±54.7 minutes, whilst the mean operative time per level was 117.58±48.5 minutes. The shortest case was 45 minutes, while the longest case took 367 minutes. Pearson correlation was utilized in subgroup analysis based on patient characteristics such as age, gender, body mass index (BMI), smoking status, laterality, spinal level and number of levels of decompression, previous lumbar surgery, severity of underlying stenosis and presence of intraoperative complications. The following statistical significance is revealed. Decompression on the left side and on L5/S1 level demonstrated faster operative time (Table 3), but there is no statistically significant difference based on the laterality or spinal level (P>0.05). A significant positive correlation was noted between high BMI and operating time (r=0.209, P=0.03), whilst a negative correlation was noted with increasing number of prior cases performed (r=−0.267, P=0.006) as well as increasing time from the first case (r=−0.254, P=0.009), showing that greater surgical experience resulted in significantly shorter surgical timings (Table 3, Figures 1-3). Using similar correlation analysis, both bilateral and unilateral surgeries performed via the same port demonstrated faster operative duration with increased time from the first case. Likewise, operative duration in each of the three commonest levels (L3/4, L4/5 and L5/S1) demonstrated a significant reduction over time with increased experience. Subset analysis of significant outliers (operative duration exceeding 200 minutes) as noted in Figure 3 revealed associations with large patient body habitus (BMI >30 kg/m²), or previous spinal surgery at the same level resulting in scarring, or presence of severe bilateral stenosis necessitating prolonged decompression. The presence of intraoperative dural tear (3 cases) was successfully managed endoscopically without conversion to open, and did not statistically influence operative time.

Figure 1 Operative time per level (minutes) against time from first case.

Figure 2 Operative time per level (minutes) against number of prior cases (n).

Figure 3 Operative time (minutes) against BMI (kg/m²). BMI, body mass index.

Postoperative characteristics

Patients had an average postoperative length of stay of 1.9±2.6 days, having received early aggressive ambulatory physiotherapy with immediate weightbearing. In the immediate postoperative period, 54.3% of patients experienced complete relief of all symptoms, while the remaining 45.7% had partial relief of preoperative pain levels and/or neurological symptoms. Three patients, however, reported having significant residual symptoms that did not resolve by 3–4 months follow-up and required relook surgery (2.86%). Of these three cases, two were due to recurrent disc herniation, and one was due to incomplete decompression. There were no observed cases of surgical site or spinal infection, hematoma, foot drop, paralysis or sensory loss. None of the patients reported subjective worsening of preoperative symptoms. All 105 patients were successfully followed up in clinic for review and assessment of functional scores at 1- and 3-month mark. Patient-reported outcome measures revealed marked improvement in postoperative symptoms and function, with all components of VAS back pain, VAS leg pain, ODI, SF-36 physical function, SF-36 physical role and SF-36 bodily pain showing statistically significant improvement at the 3-month mark (Table 4). All components except SF-36 physical role also showed statistically significant improvement at the 1-month mark.

Discussion

Lumbar spine decompression is the gold standard surgical treatment for patients with symptomatic lumbar spinal stenosis or prolapsed intervertebral disc (15,16). The outcomes of traditional open decompressive surgery are comparable to minimally invasive surgery. However, given the added benefits of smaller wounds with reduced soft tissue dissection, postoperative pain, blood loss, infection, and adhesions or scarring (17), as well as quicker return to work, surgeons are increasingly employing MISS techniques. Endoscopic decompression, in our experience, when compared to microscopic, confers superior intraoperative technical advantages of a high-definition, magnified view on screen with angled optics allowing for visualization around corners (using 30° and 70° scope), and continuous saline irrigation to clear debris and minimize bleeding in the field of view. With a skin-to-target approach and endoscopic maneuverability, incision size and tissue trauma are further minimized, enhancing postoperative recovery and enabling far lateral (foraminal and extraforaminal) and contralateral access without expanding exposure or disrupting facet joints. In a meta-analysis of 6 randomized controlled trials (646 patients) by Yang et al. (18), endoscopic decompression revealed statistically significant shorter operative time, greater leg pain relief and lower overall complication rates compared to microscopic decompression for lumbar stenosis. Further, Kotheeranurak et al. also demonstrated in their randomized controlled trial (RCT) (60 patients) reduced blood loss, shorter hospital stays, and improved VAS score for back pain, as well as non-inferior ODI at 24 months (19).

In the realm of endoscopic surgery, since both uniportal and biportal approaches have demonstrated favourable surgical outcomes (20,21), the choice between the two is individualized based on surgeon preference and patient factors. The biportal approach offers the surgeon the convenience of utilizing two ports for enhanced visualization and maneuverability within the spinal canal, granting improved accessibility (22). Moreover, irrigation fluid can flow in and out from separate ports, thereby reducing overall pressures within the spinal canal intraoperatively, and hence the risk of compressive neural injury (23).

As with most new techniques, a steep learning curve exists in learning the UbE approach. The surgery is technically challenging and is reported to take at least 15–47 cases of close mentoring to be suited as “free to fly” (24,25). The surgeon in our series was fellowship-trained under prominent senior surgeons at a separate institution over the course of a year. Upon return, the first 10 UbE cases averaged 137.5 minutes per level decompressed. In just a year, this timing was shaved down to 108.9 minutes with a span of 90-odd cases in between. Specifically, the authors observe improvement in surgical time over an initial learning curve of 60 cases in our series (Figure 4), which thereafter tended to plateau. Park et al. likewise reported that UbE technique demands a substantial learning period in an endoscope-naive surgeon and in contrast to our study, utilized an advanced statistical method known as cumulative summation (CUSUM) test to demonstrate surgical competency at the 58^th case (13). This result is similar to ours, although the sample size is smaller (first 60 cases enrolled into the study) and a predefined threshold of 75 minutes was chosen based on the mean operative time of a senior surgeon, along with a priori of failure rate set by internal expert discussion. Thus, an arbitrary benchmark for success without external validation assumes universal achievability, introduces bias and is applied as a binary outcome onto a continuous variable on CUSUM plot. We attribute the improvement of operative time to increased familiarity with the endoscope setup, identification of spinal anatomy and improvement in manipulative skills. The same surgeon has also anecdotally reported average timings of approximately 45 minutes to 1 hour for single-level UbE now, a year after the latest case in this cohort. The potential for endoscopic surgeons to operate as efficiently as traditional open or microscopic surgery, whilst still maintaining patient safety and providing excellent outcomes, is a key driver for its success. The authors hence believe that the initial training phase should be intensive to allow rapid mounting of the learning curve.

Figure 4 Operative time (minutes) for single-level surgery against consecutive cases: plot in orange (actual values) and in green (moving average with interval of 20 cases).

With regards to surgical outcomes, the rates of complications observed in our cohort, such as infection, dural tears, bleeding or neural injury, are comparable and arguably lower than global averages for non-endoscopic decompressive surgery (13). Dural tears that occurred intraoperatively were successfully repaired without the need for conversion to open. Clinically, most patients experienced partial to complete relief of symptoms in the immediate postoperative period. The marked improvement in validated patient-reported outcome measures such as SF-36, VAS and ODI (Table 4) across the first 3 months demonstrates sustained functional results over time and can be used as a clinical surrogate to routine postoperative imaging. Length of stay averaged only a day or two, compared to the longer durations in open cases in literature (26), which may bid to reduce costs for the patient and institution.

Despite the encouraging results in our series, a surgeon embarking on UbE approach must take caution. It is preferable that the surgeon has acquired adequate experience and familiarity of spinal anatomy from open and tubular decompression approaches, as well as the management of intraoperative complications such as bleeding and dural tears, before transitioning to UbE. Although uncommon, operative times may exceed 3–4 hours in challenging cases and one must be prepared for the possibility of conversion to open. In particular, we experienced an isolated case that took 367 minutes (longest duration encountered) to complete the surgery, due to an underlying chronic, thickened, and calcified ligamentum flavum. Careful patient selection, assessment of preoperative imaging, and surgical planning are hence advocated to avoid prolonged anesthesia time and overrun of the operative list. We recommend starting with single-level UbE at L4/5 or L5/S1 level from a left-sided approach (for a right-handed surgeon) in patients with small body habitus and unilateral pathology (such as prolapsed intervertebral disc in a younger patient), in the absence of previous lumbar surgery at that level. Based on our experience, these factors are important considerations as they are associated with greater technical ease and hence reduced operative times. In amateur hands, operating endoscopically on severely stenotic spinal canals, calcified ligamentum flavum, or significant scarring may lead to excessively protracted surgical time and complications. Supervision by an experienced endoscopic surgeon may further facilitate the training phase during the initial learning curve. In addition, regular postoperative follow-up is essential to look out for rare cases of persistent symptoms due to inadequate decompression or recurrent disc herniations, in which case the surgeon must be ready to repeat imaging and perform relook surgery if indicated.

The limitations of our study include its retrospective nature, relatively short follow-up period, small cohort size, and potential bias of the data given that it was based on a single surgeon. To demonstrate the experience of mounting the learning curve of a new surgical approach, a single-surgeon series reduces data heterogeneity. However, given that the learning curve of any surgery is naturally dependent on individual as well as external factors, these results may not be generalizable across different surgeons with different learning habits, background or innate surgical proficiencies, scrub team and availability of mentorship. Future studies may be undertaken to encompass not only one but a number of surgeons, of varying seniority and background knee and/or shoulder endoscopic experience, assisted by the same scrub team and under the supervision of a constant mentor or senior endoscopic surgeon, in order to enhance the generalizability of results. Moreover, the success of surgical competency is not simply based on operative time alone but also more importantly outcomes. Our present study did not assess for a trend in improvement of outcomes or reduction in complications over time. Further, we did not compare data directly against open decompression or tubular microdiscectomy, which may illustrate the superiority of one approach over the other. The lack of routine postoperative imaging also precludes our ability to radiologically determine if the spinal surgery was successful in decompression, especially in patients who were still symptomatic. The increased operative costs of endoscopic surgery due to the use of specialized instruments were also not accounted for but this is beyond the scope of this article. Future studies to investigate this important topic of cost-effectiveness by weighing increased operative costs against reduced length of stay and improved quality-adjusted life years (QALYs) may be conducted. Hence, in summary, future prospective comparative studies should be designed to include a multi-surgeon cohort with larger patient samples and longer follow-up periods across different surgical approaches (open versus microscopic versus endoscopic), taking into account multiple surgeons of varying proficiencies and trending not just operative times with surgical experience but also outcomes and cost-effectiveness. Such studies would practically be more difficult to conduct but if executed well, the results would help consolidate and shape the role and future of endoscopic surgery.

Conclusions

The UbE approach to lumbar decompressive surgery is one that boasts encouraging subjective and objective outcomes. In our Asian population comprising 105 patients, most patients benefited from a significant improvement of their pre-operative symptoms, with more than 50% of patients describing a complete resolution of all symptoms. Patient-reported outcome measures also showed statistically significant improvement across the board till 3 months of follow-up. Surgical time per level decompressed also saw marked improvement over time, whilst maintaining a low rate of complications. Despite a steep initial learning curve, surgeons can expect to see a marked improvement in efficacy and surgical time whilst maintaining or even improving results. In experienced hands, patients can be discharged within one to two days with minimal symptoms and continue to observe symptomatic improvement in the short term.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://jss.amegroups.com/article/view/10.21037/jss-24-153/rc

Data Sharing Statement: Available at https://jss.amegroups.com/article/view/10.21037/jss-24-153/dss

Peer Review File: Available at https://jss.amegroups.com/article/view/10.21037/jss-24-153/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://jss.amegroups.com/article/view/10.21037/jss-24-153/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by the ethics committee of Sengkang General Hospital (CIRB Ref No. 2020/2381) and individual consent for this retrospective analysis was waived.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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