Advancements in unilateral biportal endoscopic lumbar interbody fusion: a narrative review

Abstract

Introduction

Lumbar fusion remains a cornerstone for managing degenerative lumbar disorders. Recently, Unilateral Biportal Endoscopic Lumbar Interbody Fusion (ULIF) has emerged as a key minimally invasive technique with promising clinical outcomes. This narrative review analyzes current applications, technical advancements, and the existing evidence base for ULIF. It also highlights the challenges, controversies, and research gaps that may affect its widespread adoption, offering insights to guide future clinical practice.

Methods

A structured literature search of the PubMed, Web of Science, Embase, and Cochrane Library databases was conducted for relevant articles published through August 2025. Search terms included “unilateral biportal endoscopic,” “UBE,” “biportal endoscopic spine surgery,” “lumbar interbody fusion,” “ULIF,” and “BE–TLIF.” This review critically synthesizes findings from clinical studies, technical reports, and systematic reviews focusing on surgical techniques, clinical efficacy, fusion outcomes, and complications associated with ULIF.

Results

ULIF is increasingly utilized for a wide spectrum of lumbar degenerative diseases. Comparative studies indicate that while ULIF yields comparable long-term fusion rates and clinical improvements to minimally invasive transforaminal lumbar interbody fusion (MIS–TLIF), it offers distinct advantages, such as reduced intraoperative blood loss and shorter hospital stays. However, these benefits are often offset by longer operative times, particularly during the initial learning curve. Technical innovations, including novel cage designs and navigation systems, aim to enhance fusion and restore sagittal alignment. Despite its potential, the technique's adoption is challenged by this steep learning curve and a distinct complication profile, including concerns regarding dural tears, epidural hematomas, and nerve root injuries. Questions also remain regarding its cost-effectiveness and its ability to achieve superior radiological outcomes.

Conclusions

ULIF represents a safe and effective minimally invasive alternative for treating various lumbar spine disorders. However, its widespread adoption is constrained by its technical demands, a specific complication profile, and unresolved questions regarding its cost-effectiveness and long-term radiological benefits. Future high-quality research, particularly multicenter Randomized Controlled Trials (RCTs), is necessary to definitively establish its long-term efficacy, standardize surgical techniques, and optimize patient safety.

Introduction

The incidence of lumbar degenerative diseases, such as lumbar disc herniation, spinal stenosis, and spondylolisthesis, is steadily increasing amidst an aging global population, posing a primary challenge in spinal surgery [1]. Lumbar fusion remains a definitive treatment for these conditions when conservative management fails, especially in the presence of spinal instability. Posterior Lumbar Interbody Fusion (PLIF) remains one of the most widely performed traditional open procedures, providing reliable decompression and stabilization [2]. However, this approach is associated with considerable iatrogenic injury to the paraspinal muscles, which may result in complications, such as chronic postoperative back pain and adjacent segment disease [3–5].

To address these limitations, minimally invasive surgery (MIS) techniques emerged. Minimally invasive transforaminal lumbar interbody fusion (MIS–TLIF), performed through a tubular retractor, established itself as a standard of care, mitigating soft tissue injury compared to open approaches [6, 7]. More recently, the Unilateral Biportal Endoscopic (UBE) technique has gained prominence. Consequently, its application in lumbar fusion, known as Unilateral Biportal Endoscopic Lumbar Interbody Fusion (ULIF), is rapidly expanding [8].

The emergence of ULIF marks a significant evolution from 'endoscope-assisted' to 'fully endoscopic' spinal fusion. Unlike previous techniques where endoscopes were used primarily for visualization aids, ULIF enables the entire fusion procedure—including neural decompression, endplate preparation, bone grafting, and interbody cage insertion—to be performed under direct, magnified endoscopic vision [9, 10]. The UBE technique is characterized by its use of two independent portals—one for viewing and another for working. This setup, analogous to arthroscopy, enables triangulation of instruments, affording surgical dexterity and freedom of movement often difficult to achieve with single-portal endoscopy [11–13]. Furthermore, the continuous saline irrigation system maintains a clear surgical field, minimizes thermal injury from drills or radiofrequency devices, and mitigates the risk of infection [14]. This combination permits a complex procedure like spinal fusion to be performed through minimal incisions, minimizing iatrogenic injury while enhancing surgical precision.

Comparative studies suggest that ULIF achieves fusion rates and long-term clinical outcomes comparable to MIS–TLIF, supporting its efficacy [15, 16]. Currently, the primary ULIF approaches include biportal endoscopic transforaminal lumbar interbody fusion (BE–TLIF) and biportal endoscopic extraforaminal lumbar interbody fusion (BE–EFLIF) [17, 18]. This narrative review summarizes the current evidence on ULIF, with an emphasis on its technical developments, clinical applications, and the ongoing challenges and controversies in modern spine surgery.

Literature search strategy

A structured literature search of the PubMed, Web of Science, Embase, and Cochrane Library databases was conducted for relevant English-language studies published up to August 2025. The search strategy utilized a combination of keywords and MeSH terms: “(unilateral biportal endoscopic OR biportal endoscopic spine surgery OR UBE OR BESS) AND (lumbar interbody fusion OR interbody fusion OR ULIF OR BE–TLIF OR TLIF)”.

The initial database search yielded 2432 records. An additional 25 studies were identified through manual screening of reference lists from retrieved systematic reviews and key articles. After the removal of 254 duplicates, a total of 2203 records were screened based on their titles and abstracts. Of these, 1989 records were excluded, because they were clearly irrelevant (e.g., different surgical techniques, basic science/biomechanical studies, and non-English studies). This left 214 potentially eligible studies for full-text review; however, 33 of these articles could not be obtained. The remaining 181 studies were subjected to a full-text review. An additional 108 studies were excluded based on pre-defined criteria: (1) case reports with fewer than 10 patients; (2) conference abstracts or technical notes without clinical data; or (3) lacking relevant clinical or radiological outcomes for this review.

Ultimately, a total of 73 studies met the inclusion criteria and were included in this narrative review. This selection process is summarized in a PRISMA-style flowchart (Fig. 1). This review prioritized clinical studies, comparative trials, technical reports, and systematic reviews to analyze the efficacy, challenges, and outcomes of ULIF for degenerative lumbar diseases.

Fig. 1.

Flowchart of the literature selection process

A PRISMA-style diagram illustrating the study identification, screening, eligibility, and inclusion process for this narrative review.

The development of UBE technology

The origins of spinal endoscopy date to the early 1980s. In 1983, Forst et al. [19] first performed an arthroscopy-assisted lumbar discectomy. In 1996, Kambin et al. [20] applied arthroscopic techniques to lateral recess stenosis, and in the same year, De Antoni et al. [21] first described the posterior translaminar approach using UBE technology.

UBE research then remained relatively scarce amidst advancements in other endoscopic techniques until Soliman et al. [22] refined the technique in 2013. They introduced Irrigation Endoscopic Discectomy (IED), which maintains a clear surgical field in a liquid environment [23]. This innovation sparked renewed interest in UBE. In 2017, Heo et al. [24] were the first to apply the technique to lumbar interbody fusion surgery, reporting favorable clinical results. Its development has since accelerated, expanding the application of UBE globally to encompass various spinal surgeries [25, 26].

The ULIF surgical technique: an overview

The ULIF procedure is typically performed under general or epidural anesthesia with the patient in a prone position. The technique relies on establishing two independent skin incisions (Fig. 2a), which enables triangulation and flexible instrumentation, as illustrated in the axial view (Fig. 3a). The surgical approach is transmuscular, navigating the natural cleavage planes of the paraspinal muscles (e.g., between the multifidus and longissimus muscles) rather than detaching them from the spinous processes [17, 18, 27].

Fig. 2.

Intraoperative photographs and radiographs of the ULIF procedure. a Clinical photograph showing the two independent portals (viewing and working) for the UBE technique. b Intraoperative C-arm fluoroscopy used for precise localization of the target level. c Postoperative lateral radiograph showing the final placement of the interbody cage and percutaneous pedicle screws

Fig. 3.

Surgical technique of ULIF. a Axial (transverse) view demonstrating the biportal technique. The spine endoscope (viewing portal) and the working instrument (working portal) create a triangulated approach to the lamina and articular processes, passing through the paraspinal muscles without a tubular retractor. b Posterolateral (Kambin's triangle) view showing the key anatomical corridor for cage insertion. The safe working zone (highlighted in blue) is bordered by the exiting nerve root, the dural sac, and the vertebral endplate, allowing for the safe insertion of an interbody cage

After precise C-arm fluoroscopic localization of the surgical level (Fig. 2b), the two portals are established. Typically, the viewing portal is established just lateral to the spinous process, through which a 0° or 30° spine endoscope is inserted. The working portal is created more laterally, providing an angled trajectory for instruments (Fig. 3a). Unlike a tubular retractor system, the working channel's integrity is maintained dynamically by the instruments and a soft-tissue cannula, protecting surrounding tissues during instrument exchange.

Under continuous saline irrigation and direct endoscopic visualization, the surgeon proceeds with decompression. For a BE–TLIF, this involves a unilateral laminectomy and partial facetectomy to expose the neural elements. The anatomical corridor for cage placement is Kambin's triangle (Fig. 3b). This safe working zone is anatomically defined as a right-angled triangle, with the exiting nerve root as the hypotenuse, the dural sac/traversing nerve root as the medial border (height), and the superior endplate of the caudal vertebra as the base. Following adequate decompression, the surgeon performs a discectomy, meticulously prepares the endplates, and packs autologous bone graft into the intervertebral space before carefully inserting the interbody fusion cage (Fig. 3b). Percutaneous pedicle screws, placed under fluoroscopic guidance, provide final stabilization, with the final construct confirmed radiographically (Fig. 2c). A drainage tube is typically placed before wound closure.

Clinical applications and comparative evidence

The indications for ULIF mirror those of other posterior minimally invasive fusion techniques, primarily addressing degenerative lumbar pathologies such as lumbar disc herniation (LDH), spinal stenosis, and low-grade spondylolisthesis (Meyerding Grades I–II) for which conservative management has failed [24, 28–30]. Rather than analyzing each pathology separately, this review synthesizes the comparative evidence against established standards of care, such as MIS–TLIF and open PLIF, as the findings across different indications are largely consistent.

A large body of evidence now exists comparing the perioperative metrics and clinical outcomes of these techniques (Table 1). A consistent finding is that ULIF offers tangible benefits in the early postoperative period [31–34]. Multiple meta-analyses and comparative studies report that while ULIF and MIS–TLIF yield comparable long-term improvements in pain and functional outcomes, ULIF patients tend to experience significantly less immediate postoperative back pain, facilitating earlier ambulation and shorter hospital stays [35–37]. This benefit is attributed to less muscle dissection and retraction. Furthermore, the continuous irrigation in ULIF provides effective hemostasis, resulting in significantly lower intraoperative blood loss compared to both MIS–TLIF and open surgery [3, 12].

Table 1.

Comparative perioperative and clinical outcomes among ULIF, MIS–TLIF, and conventional open PLIF

Outcome metric	ULIF	MIS–TLIF	Open PLIF	Key findings and notes
Operative Time	Significantly longer	Intermediate	Generally shorter	ULIF is associated with a longer operative time, a finding consistently reported across studies, largely attributed to its steep learning curve [37]
Intraoperative blood loss	Significantly the least	Significantly less than open	Highest	The continuous saline irrigation provides both a clear surgical field and an effective tamponade effect for hemostasis [42]
Length of hospital stay	Significantly the shortest	Significantly shorter than open	Longest	Patients undergoing ULIF typically have the shortest hospital stay, attributed to reduced surgical trauma and less early postoperative pain, allowing for faster recovery [12]
Early postoperative back pain	Significantly the lowest	Lower than open	Highest	Multiple meta-analyses confirm that ULIF provides superior back pain relief in the early postoperative period compared to both MIS–TLIF and open surgery [11, 43, 44]
Surgical trauma	Significantly the lowest	Lower than open	Highest	Postoperative levels of inflammatory markers are significantly lower in the ULIF group than in the Open PLIF group, and generally lower than in the MIS–TLIF group [44, 45]
Fusion rate at final follow-up	Comparable	Comparable	Comparable	At final follow-up (≥ 1 year), all three techniques achieve high and comparable fusion rates with no statistically significant differences reported in most large-scale comparisons [29, 30]
Overall complication rate	Comparable	Comparable	Slightly higher	Meta-analyses show no significant difference in the overall complication rates between ULIF and MIS–TLIF. Specific risks for ULIF require attention, while open surgery may carry a higher risk of infection [35, 46, 47]

Data are synthesized from multiple published studies. Direct comparison of absolute values should be interpreted with caution; emphasis should be placed on the consistent trends reported across the literature

However, these advantages come at a cost. The most frequently cited drawback is a significantly longer operative time, especially during the steep learning curve [15, 38]. Despite the technical challenges, the final fusion rates at 1- or 2-year follow-up are statistically comparable across ULIF, MIS–TLIF, and open PLIF, which validates its efficacy as a reliable fusion procedure [30, 35, 39]. ULIF has also been shown to be particularly useful in challenging scenarios, such as revision surgery for recurrent disc herniation, where the endoscopic magnification aids in navigating scarred tissue planes [40, 41].

Recent technical advancements

Ongoing innovation aims to overcome limitations and enhance fusion outcomes. A key challenge in posterior fusion is the restricted corridor for cage insertion, which can limit cage size and increase the risk of subsidence [49]. ULIF's flexible, triangulated access facilitates the insertion of larger footprint cages that rest on the stronger vertebral apophyseal ring [50–52]. A recent study by You et al. [53] demonstrated that using large-footprint cages in BE–TLIF resulted in a significantly lower rate of subsidence. Expandable cage technology is another key development, allowing for insertion in a compact state to minimize neural injury risk, followed by in-situ expansion to restore disc height and lordosis [54].

The integration of intraoperative navigation systems into the ULIF workflow aims to improve precision and safety [55]. Kang et al. [56] described an O-arm navigation-guided UBE–TLIF technique to precisely insert a large Lateral Lumbar Interbody Fusion (LLIF) cage through a posterior approach. This hybrid technique combines endoscopic visualization with real-time navigation, reducing the reliance on fluoroscopy. Collectively, these innovations in cage design, materials, and surgical guidance continue to refine the ULIF procedure, aiming to maximize fusion rates and long-term stability while preserving the benefits of a minimally invasive approach [57].

Challenges, controversies, and barriers to widespread adoption

Despite its promise, endoscopic spine surgery has struggled to achieve widespread adoption for decades, and ULIF is no exception. Its growth is constrained by significant challenges and controversies surrounding its technical demands, safety profile, radiological outcomes, and cost-effectiveness.

The steep learning curve

ULIF is a technically demanding procedure that requires surgeons to master new skills, including endoscopic triangulation, managing a fluid surgical environment, and performing complex bony work through small portals [58]. Studies quantifying this learning curve suggest proficiency is often achieved after approximately 30–40 cases [38, 59]. A robust multicenter study by Park et al. [60] reported that surgeons required an average of 39 cases to achieve a stable and efficient operative time. During this initial learning phase, operative times are significantly longer, and complication rates, particularly the need for reoperation, are demonstrably higher [61]. This steep learning curve is arguably the single greatest barrier to adoption, requiring a substantial institutional and individual commitment to structured training to ensure patient safety [62, 63].

Complications and safety controversies

While generally safe, ULIF is associated with a distinct complication profile (Table 2). A major point of controversy surrounds the rate of nerve root injury and dural tears [64]. Pooled data suggest a dural tear incidence between 1.9% and 5.2%, a rate often reported as comparable to MIS–TLIF [65, 66]. However, critics argue that the risk may be higher in less experienced hands, as instruments operate in close proximity to unprotected neural elements within the spinal canal. The risk is elevated in revision surgeries or severe stenosis with dural adhesions [3, 67].

Table 2.

Summary of common complications associated with ULIF

Complication	Reported incidence	Key risk factors and clinical notes
Dural tear	Pooled rate: 3.3% [65]Cohort rates: 1.6% [38]	Risk factors include severe stenosis, dural adhesions, and the early learning curve. Most cases are managed conservatively
Symptomatic epidural hematoma	Reoperation rate: 1.9% [68]Pooled rate: 1.2% [65]	Risk factors: female sex, advanced age, anticoagulant use, high irrigation pressure. Asymptomatic hematoma rate on MRI can be as high as ~ 23% [68]
Neurological complications (palsy, dysesthesia)	Transient palsy/deficit: 1.8% [59]Permanent deficit: 0.6% [71]Dysesthesia/radicular pain: 4.1–4.7% [71]	Risks: excessive nerve root retraction during cage insertion, thermal injury. The overall incidence of transient nerve injury decreases significantly after the learning curve [60]
Surgical site infection	0.08% [63]	Incidence is exceptionally low, widely attributed to continuous saline irrigation which reduces bacterial load
Cage-related complications (subsidence, migration)	Subsidence: 1.1% [38]Migration: 1.6% [60]Non-union: 1.9% [71]	Risk factors: osteoporosis, aggressive endplate preparation. Rates for cage subsidence (6.0%) and migration (2.6%) are higher during the early learning curve [60]
Reoperation (all-cause, 90 days)	Overall rate: 3.7%[60]Learning curve effect: 5.1% (early phase) vs. 1.4% (Late phase) [60]	Common causes: symptomatic hematoma, implant failure, inadequate decompression. The reoperation rate is highly correlated with surgeon experience
Incomplete decompression	0.24% [65]	Incomplete decompression is a risk during the learning curve. However, Park et al. [60] showed similar rates in the early (4.3%) and late (4.2%) phases, suggesting it is a persistent technical challenge

Incidence rates are compiled from systematic reviews and cohort studies and may vary based on surgeon experience

Postoperative spinal epidural hematoma (POSEH) is another significant concern, with a reported symptomatic incidence ranging from 1.1% to 8.4% [68, 69]. This risk is attributed to inadequate hemostasis on cancellous bone and the loss of the tamponade effect from fluid irrigation after surgery [70]. The 90-day reoperation rate for complications such as symptomatic hematoma or incomplete decompression reportedly decreased dramatically from 5.1% during the learning phase to 1.4% after proficiency was achieved, which highlights the critical impact of experience on patient safety [60]. While the rate of surgical site infection is exceptionally low (< 0.6%) due to continuous irrigation [15], the unique risks associated with the technique remain a point of debate and a barrier for many surgeons.

Debates on radiological outcomes

While clinical outcomes are comparable, the radiographic advantages of ULIF remain controversial [72]. A key goal of fusion is restoring segmental lordosis, but some studies suggest ULIF is less effective at this and at restoring disc height compared to other techniques such as oblique lumbar interbody fusion (OLIF) or even MIS–TLIF with hyperlordotic cages, primarily due to the posterior-only approach and limitations on cage placement angles [4, 46, 73].

Furthermore, the risk of cage subsidence is a persistent concern. Although meticulous endplate preparation under magnification is an advantage, the technical challenge of inserting a large cage through a small portal may necessitate the use of smaller cages, increasing contact stress and subsidence risk; innovations like large-footprint cages aim to mitigate this specific issue [52, 74]. Finally, while reported fusion rates are high, they derive largely from short- to mid-term follow-up, and long-term data are needed to confirm the durability of ULIF constructs [48].

Cost-effectiveness and clinically meaningful benefit

A crucial, often overlooked, barrier is cost-effectiveness. The initial capital investment for endoscopic towers, cameras, and specialized instruments is substantial. This investment, combined with longer operative times, may elevate the overall procedural cost above that of MIS–TLIF, potentially offsetting savings from shorter hospital stays [74, 75]. Furthermore, a critical question remains: do the observed benefits of ULIF—primarily reduced early postoperative pain and blood loss—constitute a clinically meaningful benefit sufficient to justify the steep learning curve, unique complication risks, and high costs? For many surgeons proficient in MIS–TLIF, the incremental advantages of ULIF may not be compelling enough to warrant a transition [15].

Limitations of current evidence and future directions

The current body of evidence on ULIF is constrained by its reliance on retrospective, single-center series, which are susceptible to selection bias. Consequently, a clear need exists for well-designed, multicenter randomized controlled trials (RCTs) to definitively compare the long-term efficacy and safety of ULIF against MIS–TLIF. Furthermore, most studies are limited to 1–2-year follow-up; data on long-term outcomes (> 5 years), such as rates of adjacent segment disease and the durability of fusion, are currently lacking [76].

Future research should prioritize the controversies highlighted in this review. Rigorous cost-effectiveness analyses are needed to clarify the economic impact of ULIF. Further investigation should also target granular radiological outcomes, particularly the ability to restore and maintain sagittal alignment compared to other MIS techniques. Finally, research into optimizing structured training programs is essential to mitigate the learning curve and ensure consistent, high-quality patient care.

Conclusion

ULIF has established itself as a technically feasible and effective minimally invasive technique. Current evidence indicates it can achieve clinical and radiological outcomes comparable to MIS–TLIF, offering advantages in reduced surgical trauma and accelerated early recovery. However, these benefits must be weighed against significant challenges, including a steep learning curve, a specific complication profile, and unresolved controversies regarding its radiological superiority and cost-effectiveness. For surgeons and institutions, adopting this technique requires a profound commitment to training and patient selection. High-quality RCTs with long-term follow-up are imperative to definitively solidify ULIF's role in the armamentarium of modern spine surgery and to guide its continued refinement.

Abbreviations

BE–EFLIF

Biportal endoscopic extraforaminal lumbar interbody fusion

BE–TLIF

Biportal endoscopic transforaminal lumbar interbody fusion

BESS

Biportal endoscopic spine surgery

IED

Irrigation endoscopic discectomy

LDH

Lumbar disc herniation

LLIF

Lateral lumbar interbody fusion

MIS

Minimally invasive surgery

MIS–TLIF

Minimally invasive transforaminal lumbar interbody fusion

OLIF

Oblique lumbar interbody fusion

PLIF

Posterior lumbar interbody fusion

POSEH

Postoperative spinal epidural hematoma

RCTs

Randomized controlled trials

TLIF

Transforaminal lumbar interbody fusion

UBE

Unilateral biportal endoscopic

UBE–TLIF

Unilateral biportal endoscopic transforaminal lumbar interbody fusion

ULIF

Unilateral biportal endoscopic lumbar interbody fusion

Author contributions

LPH and LL conceived the study, conducted the literature search, interpretation, and manuscript writing; CSC assisted in drafting the manuscript; BJC helped verify the data; CJJ served as the corresponding author, reviewing and editing the methodology and manuscript. All authors contributed to the article and approved the submitted version.

Funding

This study was supported by the Natural Science Foundation of Shanxi Province, China (202403021221251).

Data availability

The data analyzed in this review are derived from publicly available studies, which can be accessed through the databases searched in our methodology, including PubMed, Web of Science, Embase, and the Cochrane Library.

Declarations

Ethics approval and consent to participate

This review uses data from published studies; therefore, ethical approval and individual consent to participate were not required. All included studies were conducted in accordance with ethical standards.

Consent for publication

Consent for publication was obtained from all subjects.

Competing interests

The authors declare no competing interests.