Introduction
Although conventional open posterior fusion techniques yield satisfactory outcomes in degenerative lumbar diseases, they are often associated with complications such as posterior tension band disruption, significant blood loss, and prolonged hospitalization [1–3]. Minimally invasive (MI) procedures have gained popularity as alternatives designed to reduce these drawbacks. MI lumbar interbody fusion has demonstrated comparable functional outcomes with fewer complications than open techniques, particularly in obese or elderly patients with multiple comorbidities [4–11].
Over the past few decades, several spinal fusion techniques employing different surgical approaches have been developed. Among these, minimally invasive transforaminal lumbar interbody fusion (MIS-TLIF) and oblique lumbar interbody fusion (OLIF) are the most widely studied. MIS-TLIF enables direct decompression of the spinal canal through a posterior approach [12,13]. In contrast, OLIF achieves indirect decompression via ligamentotaxis through vertebral body distraction, using an oblique corridor between the anterior vessels and the psoas muscle [14–16]. Both techniques are also effective in correcting deformity in the sagittal and coronal planes [17]. In single-level disease, comparative studies have shown that OLIF results in lesser blood loss and better radiographic outcomes with similar functional outcomes to MIS-TLIF [18–21].
Two-level spinal fusion is relatively common, with MIS-TLIF and OLIF together accounting for up to 20.37% of all MI spinal surgeries for lumbar degenerative disease [22]. However, technical differences in multilevel procedures may influence outcomes differently than in single-level fusion. Given the limited evidence comparing these techniques in two-level disease, this study aimed to compare the clinical and radiographic outcomes of MIS-TLIF and OLIF. We hypothesized that OLIF would provide superior radiographic correction, while both procedures would yield comparable patient-reported outcome measures.
Materials and Methods
Study design and patient population
This retrospective study analyzed data collected from patients who underwent spinal surgery at a single institution between January 2020 and December 2022. During this period, 448 patients underwent spinal fusion procedures, including 326 MIS-TLIF and 122 OLIF cases. Prior to data collection, ethical approval was obtained from the Institutional Review Board (IRB) of King Chulalongkorn Memorial Hospital, Bangkok, Thailand (IRB no., 1034/64). The requirement for informed consent was waived owing to the retrospective design. The study was conducted in accordance with the Declaration of Helsinki.
Inclusion criteria were patients who underwent two-level MIS-TLIF or OLIF for degenerative lumbar disease. All patients had a minimum follow-up of 12 months. Exclusion criteria included infection, trauma, tumor, prior lumbar surgery, and incomplete follow-up data. Based on data reported by Hung et al. [21], a sample size of 40 patients (20 per group) was estimated to provide 80% statistical power at a significance level (α) of 0.05. To minimize selection bias, propensity score matching was performed at a 1:1 ratio, yielding 20 matched patients per group. Matching variables included age, sex, body mass index, diagnosis, surgical level, and baseline functional scores (Oswestry Disability Index [ODI], Visual Analog Scale [VAS] score for back pain, and VAS score for leg pain). Postoperative outcomes were not included in the matching process. Weighted statistics were derived from preoperative demographic and baseline clinical and radiographic parameters. All procedures were performed by three fellowship-trained spine surgeons from the same academic spine team using standardized operative techniques.
Intervention
The MIS-TLIF procedure involved hemilaminectomy, medial facetectomy, and discectomy on the approach site. An interbody cage (Medtronic, Minneapolis, MN, USA) filled with autogenous local bone graft and allogeneic bone graft was inserted into the prepared disc space under microscopic assistance. Percutaneous posterior instrumentation was subsequently placed under navigation assistance.
The OLIF procedure utilized a retroperitoneal approach through the corridor between the anterior vessels and the left psoas muscle. Discectomy was performed using an orthogonal maneuver, followed by insertion of an interbody cage filled with demineralized bone matrix (Grafton; Medtronic) into the prepared disc space. Percutaneous posterior instrumentation was subsequently placed in the prone position with navigation assistance.
Outcome assessment and evaluation
Outcomes were categorized into three domains: perioperative, clinical, and radiographic. Perioperative outcomes included intraoperative blood loss, operative time, and length of hospital stay. Functional outcomes were assessed using the ODI and the VAS scores for back and leg pain. These assessments were conducted by a clinical research coordinator independent of the surgical team, at baseline (preoperatively) and at 1, 3, 6, and 12 months postoperatively. Radiographic outcomes were evaluated at both operative levels on pre- and postoperative plain radiographs. On lateral radiographs, anterior disc height (ADH) and posterior disc height (PDH) were defined as the anterior and posterior distances between adjacent vertebral endplates, respectively, while foraminal height (FH) was measured as the vertical distance between the inferior border of the upper pedicle and the superior border of the lower pedicle. Segmental lordotic angle (SLA) was defined as the angle between the superior endplate of the upper instrumented vertebra and the inferior endplate of the lower instrumented vertebra, whereas lumbar lordotic angle (LLA) was measured between the superior endplates of L1 and S1. Additionally, magnetic resonance imaging (MRI, 1.5-Tesla) was performed preoperatively and at 3 months postoperatively. MRI parameters included foraminal area (FA), measured on sagittal slices at the mid-pedicle level on both ipsilateral and contralateral sides relative to cage insertion at each operated level; cross-sectional area (CSA), defined as the thecal sac area at the facet joint level on axial T2-weighted images; and spinal canal diameter (SCD), defined as the distance between the posterior surface of the annulus and the anterior surface of the ligamentum flavum at the midline. All measurement criteria followed previously published methods [16,20,23]. Intraoperative and postoperative complications were also recorded from medical records.
Statistical analysis
Statistical analyses were performed using IBM SPSS ver. 22.0 (IBM Corp., Armonk, NY, USA). Categorical variables were expressed as frequency (percentage) and continuous variables as mean±standard deviation. Demographic categorical variables were analyzed using Fisher’s exact test. Perioperative outcomes (intraoperative blood loss, operative time, and length of hospital stay) were analyzed using analysis of covariance. Changes in functional (ODI, VAS back, and VAS leg) and radiographic outcomes between preoperative and postoperative assessments were analyzed using generalized estimating equations. A p-value <0.05 was considered indicative of statistical significance. Data analyses were performed by an independent biostatistician.
Results
A total of 40 patients were included in the analysis, comprising 20 who underwent OLIF and 20 who underwent MIS-TLIF. The mean age was 67.75±9.95 years in the OLIF group and 68.75±7.28 years in the MIS-TLIF group. Baseline demographic characteristics, preoperative clinical variables, and radiographic parameters were comparable between the two groups (Table 1).
Perioperative outcomes are summarized in Table 2. There were no significant between-group differences with respect to intraoperative blood loss (115.3±17.5 mL for OLIF vs. 113.5±15.3 mL for MIS-TLIF), operative time (165.4±33.4 minutes vs. 146.9±32.9 minutes), and length of hospital stay (3.6±0.6 days vs. 3.7±0.5 days).
Functional outcomes, detailed in Table 3, showed significant improvement from baseline in both groups across all postoperative timepoints. At 1-month postoperatively, the MIS-TLIF group exhibited a greater reduction in VAS score for leg pain (mean change: −6.0 vs. −3.8; p=0.003). However, no significant differences were observed between groups for ODI, VAS back pain, or VAS leg pain at subsequent follow-ups. No intraoperative or postoperative complications were recorded in either group during the 12-month follow-up period.
Radiographic outcomes are presented in Table 4. Both groups demonstrated significant postoperative improvement in all measured parameters, including ADH, PDH, FH, FA, SLA, LLA, CSA, and SCD. However, the OLIF group showed significantly greater improvements in several parameters: ADH restoration (mean change: 4.52 mm vs. 1.19 mm), PDH (4.66 mm vs. 1.20 mm), FH (4.29 mm vs. 1.68 mm), FA (121.20 mm2 vs. 35.62 mm2), segmental lordosis (8.55° vs. 4.65°), and lumbar lordosis (9.9° vs. 5.4°) (p<0.001 for all). No significant between-group differences were found in CSA or SCD improvement.
Discussion
This study demonstrated that OLIF and MIS-TLIF significantly improved functional and radiographic outcomes in patients with two-level degenerative lumbar disease. Perioperative parameters, including blood loss, operative duration, and hospital stay, were comparable between the two groups. This finding differs from results in single-level procedures, where OLIF has been associated with lower blood loss. Zhu et al. [19] reported that OLIF resulted in significantly less blood loss than MIS-TLIF (123.1±39.8 mL vs. 232.0±83.2 mL, respectively) for single-level disease, while operative time and length of hospital stay were similar between techniques. Likewise, Yingsakmongkol et al. [20] and Hung et al. [21] observed reduced intraoperative blood loss with OLIF (90.48±19.74 mL) compared with MIS-TLIF (167.32±35.93 mL). In contrast, Passias et al. [24] noted higher average blood loss (286±320 mL) in multilevel MIS procedures. In the present study, blood loss was comparable between groups for two-level fusion. The higher blood loss reported in multilevel OLIF procedures may be attributed to increased soft-tissue dissection and retraction, as described by Zhu et al. [25]. In MIS-TLIF, hemilaminectomy and medial facetectomy performed under magnification can damage posterior bony structures and paraspinal muscles, potentially leading to increased bleeding. However, the extent of blood loss and operative time in these procedures can vary depending on the surgeon’s experience [22].
Previous studies have reported that direct and indirect decompression techniques produce comparable functional recovery. In OLIF, indirect decompression is achieved by restoring FH and disc space and by reducing the thickness of the ligamentum flavum, thereby alleviating nerve root compression [14,15]. In contrast, direct decompression via the posterior approach, allowing for direct visualization of neural structures, is a key advantage of MIS-TLIF [12]. Zhu et al. [19] found that OLIF and MIS-TLIF yielded significant improvements in VAS scores for back and leg pain, while Hung et al. [21] reported similar reductions in leg pain with the two techniques. Clinical outcomes may, however, vary with preoperative symptom severity and the degree of neural compromise.
In the present study, both approaches produced significant improvement in functional scores, with no long-term difference between groups. The transiently superior leg pain relief observed in the MIS-TLIF group at 1 month likely reflects the immediate benefit of direct decompression, whereas the effect of indirect decompression may manifest more gradually. By later follow-ups, outcomes converged, consistent with findings from previous studies.
Radiographically, OLIF demonstrated greater improvement in disc and foraminal dimensions as well as sagittal alignment (SLA and LLA), likely due to the ability to insert larger interbody cages and achieve greater disc height restoration [19,21,26]. No significant difference was found in SCD or CSA, reflecting the distinct decompression mechanisms of each technique: MIS-TLIF achieves direct canal expansion by removing posterior elements, while OLIF relies on indirect decompression through ligamentotaxis and disc height restoration. These results align with the randomized controlled trial by Isaacs et al. [27].
Some limitations of this study should be acknowledged. First, its retrospective design and the small sample size may have introduced selection bias and limited statistical power to detect subtle differences. Second, the follow-up period of 12 months may not capture long-term differences in fusion rates, adjacent segment disease, or late complications. Future multicenter studies with larger cohorts and longer follow-up durations are warranted to confirm these findings and further delineate the relative advantages of each approach in multilevel degenerative disease.
Conclusions
For two-level lumbar degenerative disease, OLIF and MIS-TLIF resulted in significant improvements in patient-reported and radiographic outcomes. Although OLIF did not demonstrate a perioperative advantage in terms of blood loss, it provided superior radiographic correction, whereas MIS-TLIF offered faster short-term relief of leg pain. These findings highlight the complementary strengths of the two approaches. Larger, multicenter prospective studies with longer follow-up are required to validate these results and to guide surgical decision-making.
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