Lumbar degenerative disease (LDD), including lumbar spinal stenosis (LSS), lumbar disc herniation (LDH), and degenerative or isthmic lumbar spondylolisthesis (LS), has been one of the most prevalent and disabling spinal disorders that cause low back and leg pain, disability, and poor quality of life (1, 2). As a result, evolutions in both non-surgical and surgical treatment of LDD continue through the present day.

Among surgical procedures, lumbar interbody fusion is the gold standard for stabilizing spinal instability and decompressing neural elements (3). The most commonly used surgical approach is the posterior approach. Conventional posterior procedures include minimally invasive transforaminal lumbar interbody fusion (MI-TLIF) through microscopic tubular technique and open posterior lumbar interbody fusion (PLIF) (4). However, lumbar interbody fusion has been rated as one of the most painful procedures (5, 6). The main disadvantage of the conventional MI-TLIF or PLIF is the extensive paraspinal iatrogenic damage caused by dissection and retraction, which would induce the risk of chronic pain and delay patients' post-operative recovery and mobilization, placing a substantial economic burden on the public healthcare systems (7, 8). Therefore, there is a significant clinical and economic rationale for improving the management and outcomes of these conditions (9).

The concept of “fast-track” surgery was initiated by Kehlet in the 1990s and further developed as Enhanced Recovery After Surgery (ERAS) (10, 11). ERAS is a multidisciplinary and multimodal perioperative management approach that aims to improve surgical outcomes, reduce complications, and shorten the length of the hospital stay (12, 13). With the increasing application of ERAS protocols in neurosurgery, minimally invasive uniportal endoscopic technique has gained popularity as a key component for the management of lumbar interbody fusion (14–17). However, this technique was restricted by its vision and specific instruments. In recent years, the biportal endoscopic system and unilateral biportal endoscopic lumbar interbody fusion (ULIF) were developed to combine the advantages of conventional and endoscopic surgery (18–20). Through independent viewing and working channels, unrestricted vision, and ample operation space could be obtained while the posterior structure could be preserved. ULIF has been used to treat LDD; however, whether ULIF could enhance recovery compared with conventional procedures remains controversial.

The purpose of this systematic review and meta-analysis was to compare the surgical data, clinical outcomes, laboratory outcomes, radiographic outcomes, and surgical complications between ULIF and conventional MI-TLIF or PLIF for the treatment of LDD.

This study was designed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines and registered with PROSPERO (ID: CRD42021230695) (21, 22).

Enhanced Recovery After Surgery is a multidisciplinary perioperative care pathway designed to achieve early recovery for patients undergoing major surgery. The three phases of ERAS protocol are pre-operative, intraoperative, and post-operative periods, and the key components include optimization of nutrition, emotional support, multimodal opioid-sparing analgesia, antimicrobial prophylaxis, appropriate surgical procedure, and early mobilization (40). Since the first publication of the ERAS consensus statement in 2005, the ERAS Society has now published guidelines in more than 20 surgical specialties, including colorectal surgery (11), pancreatoduodenectomy (41), radical cystectomy (42), gastrectomy (43), bariatric surgery (44), liver surgery (45), lung surgery (46), and cardiac surgery (47). For spine surgery, some cohort studies and a meta-analysis suggested that improved outcomes could be obtained through the implementation of ERAS protocols during the perioperative period (48–51). In 2021, an evidence-based recommendation for lumbar fusion surgery was developed by the ERAS Society (52). Although surgical techniques should be decided on a case-by-case basis, the minimally invasive technique achieved a strong recommendation grade because is paramount for post-operative recovery (52, 53).

Conventional MI-TLIF and open PLIF were effective surgical procedures of lumbar interbody fusion for treating LDD, but the paraspinal muscle damage and blood loss may delay pain relief and functional recovery. With the advancement of optical technologies, water-based endoscopic procedures have gained popularity (54–57). ULIF, combining the endoscope and the minimally invasive spine instruments, has been increasingly used as an alternative to conventional lumbar interbody fusion techniques (18, 58). This systematic review and meta-analysis directly compared the outcomes and complications of ULIF to conventional MI-TLIF or PLIF for LDD. Different from the previous meta-analysis by Lin et al. the current study did not merge the patients who underwent MI-TLIF or PLIF in a single group because these two posterior procedures had very different paraspinal muscle injury levels (59). The results revealed that there was no significant difference in the radiographic outcomes and complications between the ULIF group and MI-TLIF or PLIF group. Nevertheless, enhanced recovery was observed through superior clinical outcomes, surgical data, and laboratory outcomes in patients receiving ULIF.

The LOS was shortened by 1.27 days (P < 0.001) and 4.40 days (P = 0.018) in the ULIF group compared with the MI-TLIF group and the PLIF group, respectively. This effect was associated with enhanced pain relief and function recovery by ULIF. The current study suggests that ULIF has a significantly better short-term improvement in VAS-BP and ODI than both MI-TLIF and PLIF groups. In addition, an enhanced short-term improvement in VAS-LP and long-term improvement in ODI were noted in the ULIF group compared with the PLIF group. These findings may be attributed to the reduced atrophy, denervation, and ischemic paraspinal muscle damage caused by dissection and retraction (60, 61). Furthermore, the endoscope provides a clear and magnified view, allowing more precise manipulation for the decompression of the central canal, lateral recess, and bilateral nerve roots (32). The study by Kim et al. reported that there was no significant difference in the early and final ODI between unilateral biportal endoscopic and open microscopic techniques for lumbar discectomy (61). Therefore, preserving paraspinal muscle and posterior soft tissue may benefit lumbar interbody fusion more than sole discectomy, which requires less muscle retraction. Inevitable systemic inflammatory response due to iatrogenic muscle injury is associated with post-operative pain and disability (34, 62). Thus, effective alleviation or suppression of the inflammatory response is essential for the enhanced recovery of patients. CPK and CRP were presentative biomarkers, which peeked on post-operative 2–3 days, recovering to the normal range weeks after the surgery (39, 63). In the current study, the peek of both CPK and CRP was significantly lower in the ULIF group, indicating that the biportal endoscopic technique produces less systemic inflammatory response than conventional procedures. This advantage may also relate to favorable pain relief, function improvement, and LOS.

Like conventional procedures, ULIF is also frequently accompanied by substantial surgical blood loss, especially when resecting ligamentum flavum and superior articular process, which would postpone the recovery and induce complications (63, 64). In this study, we found that the EBL and post-operative drainage in the ULIF group were significantly reduced than both the MI-TLIF group and the PLIF group. Continuous fluid irrigation played a vital role in controlling epidural and bone surface hemorrhage. However, the pressure of irrigation and constant outflow should be noted to prevent post-operative neck pain and seizures caused by increased intracranial pressure (65). In addition, rather than electrocautery, bipolar radiofrequency ablation could be applied to obtain effective microvascular coagulation around the dural sac (34, 38, 66). Therefore, better bleeding control leads to less post-operative drainage and early mobilization in patients who underwent ULIF.

Evaluating the fusion rate is of paramount importance for patients who underwent lumbar interbody fusion, as failed solid fusion could jeopardize the surgical effect and quality of life (67). This study yielded similar fusion rates between ULIF and conventional procedures (ULIF vs. MI-TLIF, 89.8 vs. 87.7%; ULIF vs. PLIF, 94.3 vs. 90.3%). Some advantages of the biportal endoscopic system might facilitate the fusion rate of ULIF. Meticulous endplate preparation could be performed under the clean and magnified real-time surgical visualization, offering a favorable fusion environment by completely removing the cartilaginous portion (35, 68). Continuous fluid irrigation may disperse the thermal energy, which could induce necrosis of the endplate and further cage subsidence (34, 69). Moreover, unlike the uniportal endoscopic system which only allows small-sized cages to pass the cannula, large-sized or expandable cages could be used in an independent working portal, which obtained a favorable fusion rate as conventional procedures.

Although there are various advantages, longer ORT and a steep learning curve are the potential drawbacks of ULIF. In the current study, the ORT of the ULIF group was 179.63 ± 29.34 min in MI-TLIF studies and 184.43 ± 41.50 min in PLIF studies, which were significantly longer than the 148.01 ± 24.17 min in the MI-TLIF group and 130.87 ± 23.22 min of the PLIF group. The biportal endoscopic technique is just like arthroscopy. During the initial stages of the learning curve, single-handed instrument handling and identification of anatomical landmarks may be factors that increase the ORT for ULIF (38). For less-experienced surgeons, delicate decompression manipulations may become complex and easily induce complications. Although the surgical complication rate was similar between ULIF and conventional procedures (ULIF vs. MI-TLIF, 6.9 vs. 6.7%; ULIF vs. PLIF, 6.8% vs. 5.1%), the slightly higher incidence of dural tear in the ULIF group should be noted (ULIF vs. MI-TLIF, 4.8% vs. 2.1%; ULIF vs. PLIF, 4.7% vs. 2.8%). Therefore, ULIF is recommended for surgeons who have performed at least 54 cases of biportal endoscopic decompression (70). Another complication was the epidural hematoma, most likely due to oozing from the bone trapped under the intact posterior tension band, which usually could be resolved by itself (32). The incidence of this complication was low (ULIF vs. MI-TLIF, 2.9 vs. 2.3%). The incidence of surgical site infection was slightly lower in the ULIF group than in the MI-TLIF group (0.0 vs. 2.6%). This finding may be attributed to the reduced surgical smoke and wound contamination by the bipolar radiofrequency ablation.

This study has several limitations. First, the impact of smoking was not considered due to the missing data, which could overestimate the fusion rate and cage subsidence rate (71). Second, most studies lacked data on comorbidities, which could have influenced some of the outcomes analyzed. Third, the heterogeneity of the included patients should be acknowledged because they had various LDD, including LSS, LDH, and LS. Fourth, the number of research studies focused on the comparison of ULIF and conventional procedures is still limited. Therefore, some pooled outcomes may not be reliable when more results were reported in future studies. Additionally, no randomized controlled study was included at a higher level of methodological quality. Further multicenter randomized controlled trials with longer follow-up periods should be performed to obtain more convincing conclusions.

Compared with conventional MI-TLIF and PLIF, ULIF was associated with reduced EBL, shorter LOS, alleviated inflammatory response, and comparable fusion rate as well as complication management. Compared with MI-TLIF, a better short-term improvement in VAS-BP and ODI was achieved by ULIF; compared with open PLIF, additional enhanced short-term improvement in VAS-LP and long-term improvement in ODI were observed in ULIF. ULIF could enhance the recovery of patients with LDD compared with conventional posterior procedures.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

HY, YL, and AP contributed to the study concept and design, revised, and edited the manuscript. HY, FC, and YL took part in the initial literature search and assessed the eligibilities of feasible studies. HY and FC interpreted the findings and wrote the first draft of the manuscript. HY, FC, and AP prepared the figures and tables. All authors approved the final version of the manuscript, contributed to the article, and approved the submitted version.