Unilateral Biportal Endoscopic Transforaminal Lumbar Interbody Fusion (TLIF) Using 3-Dimensional-Printed Titanium Cages Compared With Open TLIF: A Comparison of Clinical Outcomes and Fusion Rates

1. Materials

We retrospectively reviewed patients who underwent TLIF for degenerative lumbar spinal stenosis at our institution between 2021 and 2023. The electronic medical records, operative records, and radiologic images were retrospectively reviewed. In this study, we included patients who underwent single-level TLIF using 3D-printed titanium cages for lumbar degenerative spinal stenosis. We excluded patients who underwent surgery for trauma or infection. Multilevel surgery or revision surgery was also excluded from the study. In general, the surgical method (UBE vs. open TLIF) was determined based on the preferences of the surgeon and the patient. In geriatric patients, we preferred open surgery to minimize operative time and thereby reduce postoperative medical complications associated with prolonged operations.

2. 3D-Printed Titanium Cages

We generally prefer to insert 2 cages whenever possible, because using 2 cages provides a larger footprint, which contributes to greater stability, a higher likelihood of successful fusion, and a reduced risk of subsidence. Cage size and angle were determined based on preoperative radiologic images and intraoperative assessment. Preoperative plain radiographs and computed tomography (CT) images were evaluated to determine the necessity of indirect decompression based on disc height reduction and the presence of foraminal stenosis. The disc height of adjacent segments was also assessed to decide index level cage height. During surgery, the degree of disc space distraction after discectomy was estimated, and the appropriate cage height was determined using trial cages.

We analyzed the morphometric characteristics of 3D-printed titanium cages used in open TLIF and UBE-TLIF. These characteristics, including anteroposterior length, width, height, and angle, were obtained from operative records. These parameters were compared between the open TLIF and UBE-TLIF groups.

3. Clinical Outcomes

Clinical outcomes were assessed using the Oswestry Disability Index (ODI), the visual analogue scale (VAS) for back and leg pain at preoperatively, postoperative day 2, and the 1-year follow-up. Changes in ODI, VAS back and leg pain from preoperative measurements to the 1-year follow-up were calculated to compare the improvement in clinical outcomes between open TLIF and UBE-TLIF groups.

4. Radiographic Evaluation

Radiographic outcomes were evaluated using plain radiographs (preoperatively, on postoperative day 2, and at the 1-year follow-up) and CT (preoperatively and at the 1-year follow-up). At 12 months postoperatively, interbody fusion was assessed using the Bridwell interbody fusion grading system on plain radiographs and the modified Bridwell interbody fusion grading system on CT, as follows: grade I, fused with remodeling and trabeculae present; grade II, graft intact, not fully remodeled and incorporated, but no lucency present; grade III, graft intact, potential lucency present at top and bottom of the graft; grade IV, fusion absent with collapse/resorption of the graft. Grades I and II were considered solid fusion, whereas grades III and IV were considered nonsolid fusion [21].

Cage subsidence was defined as migration of the cage by more than 2 mm into the adjacent vertebral body.

5. Spinopelvic Parameters

Spinopelvic parameters, including pelvic incidence (PI), pelvic tilt (PT), sacral slope (SS), lumbar lordosis (LL), segmental Cobb angle, disc height (anterior, middle, and posterior), and foraminal height, were measured on plain radiographs obtained preoperatively, on postoperative day 2, and at the 1-year follow-up. LL was measured between the upper endplate of L1 and the superior endplate of S1. Segmental Cobb angle was measured between the upper endplate of the superior vertebra and the lower endplate of the inferior vertebra. Foraminal height was measured from the inferior border of the pedicle of the superior vertebra to the superior border of the pedicle of the inferior vertebra on lateral radiographs.

6. Surgical Techniques

1) Unilateral biportal endoscopic transforaminal lumbar interbody fusion

Under general anesthesia, patients were positioned prone on a spinal table. Sterile draping was accomplished using a waterproof surgical drape. Two skin incisions for working and endoscopic portals were made under C-arm fluoroscopy guidance, spaced approximately 2.5 cm apart. Both incisions were located at the lateral margin of the ipsilateral pedicle (Figure 1A). These skin incisions were used as entries for percutaneous pedicle screw insertion after cage insertion. For the left side approach, the cranial portal was used as the endoscopic portal, and the caudal portal as the working portal. After the working and endoscopic portals were established, we created an initial working space on the lamina surface under endoscopic guidance. A saline irrigation system was utilized with natural drainage using gravity.

Figure 1.

Surgical steps of UBE-TLIF at the left L4–5 level. (A) Two skin incisions for the working and endoscopic portals were made under C-arm fluoroscopic guidance and spaced approximately 2.5 cm apart. Both incisions were placed at the lateral margin of the ipsilateral pedicle. (B) After a partial hemilaminectomy, contralateral sublaminar bony decompression was performed using a high-speed burr. (C) The inferior articular process of the upper vertebra and the superior articular process of the lower vertebra were resected using a Kerrison rongeur or osteotome. The ligamentum flavum was then resected to achieve complete epidural decompression. Following exposure of the intervertebral disc, the exiting root, and the traversing root, epidural vessels were coagulated. (D) Annulotomy was subsequently performed using a knife or radiofrequency probe. The intervertebral disc and cartilaginous endplate were removed under endoscopic visualization. (E) After discectomy and endplate preparation, 3-dimensional-printed titanium cages filled with autologous bone graft were inserted. The first cage was advanced to the contralateral side using a cage pusher, followed by the insertion of a second cage. (F) Pedicle screws and rods were then applied percutaneously with C-arm fluoroscopic guidance. UBE-TLIF, unilateral biportal endoscopic transforaminal lumbar interbody fusion. Asterisk (*)=dural sac; double asterisk (**)=intervertebral disc; arrow=exiting root.

After a partial hemilaminectomy, contralateral sublaminar bony decompression was performed using a high-speed burr (Figure 1B). Then, inferior articular process of the upper vertebra and the superior articular process of the lower vertebra were resected with a Kerrison rongeur or osteotome. The ligamentum flavum was then resected to decompress the thecal sac. Following exposure of the intervertebral disc, exiting root, and traversing root, epidural vessels were coagulated (Figure 1C). Then, annulotomy was performed using a knife or radiofrequency probe. The intervertebral disc and cartilaginous endplate were removed under endoscopic visualization (Figure 1D). After discectomy and endplate preparation for cage insertion, a 3D-printed titanium cage filled with demineralized bone matrix (DBM) and autologous bone graft was inserted. The first cage was advanced to the contralateral side using a cage side pusher, followed by the insertion of a second cage (Figure 1E). Pedicle screws and rods were then applied percutaneously under C-arm fluoroscopic guidance (Figure 1F).

7. Statistical Analysis

Statistical analyses were performed using R ver. 4.1.1 (R Foundation for Statistical Computing, Austria). A p-value <0.05 was defined as statistically significant, and all tests were 2-sided. Continuous variables are presented as the mean±standard deviation. Student t-test or Wilcoxon rank-sum test was performed to compare continuous variables between the 2 groups. Normality was assessed, and variables that met the assumption of normal distribution were analyzed using the parametric Student t-test, whereas those that did not were analyzed using the nonparametric Wilcoxon rank-sum test. Categorical variables are presented as a number (%). Chi-square test or Fisher exact test were performed to compare categorical variables between the 2 groups. A paired t-test was used to compare the ODI between preoperative and 1-year follow-up assessments. The VAS back and leg pain were compared between preoperative, postoperative day 2, and 1-year follow-up assessments using 1-way repeated measured analysis of variance, followed by Tukey HSD (honestly significant difference) post hoc multiple comparison tests. Logistic regression analysis was conducted to calculate the odds ratio (OR) and 95% confidence interval (CI) to determine whether the surgical method (open TLIF or UBE-TLIF) significantly influence the interbody fusion. Dependent variable was Bridwell interbody fusion grade (grade I or others [grade II, III, or IV]). Independent variables were demographics and operative characteristics. Although demographics and operative characteristics may not directly influence the fusion rate, they could act as potential confounding variables. Therefore, these factors were incorporated as independent variables in the logistic regression analysis.

1. Demographics

A total of 42 patients underwent TLIF using 3D-printed cages. Among them, 21 patients underwent open TLIF, and 21 patients underwent UBE-TLIF. Table 1 shows the demographics of patients in the open and UBE-TLIF groups. The mean age of the open TLIF group (69.8±8.0) was significantly higher than that of UBE-TLIF group (63.7±8.2) with a p-value of 0.018. The mean operative time was significantly longer in the UBE-TLIF group (198.0±36.7 minutes) than in the open TLIF group (163.6±25.9 minutes) with a p-value of 0.001. No significant differences were observed in sex, operated level, number of cages, follow-up period between the 2 groups. A complication occurred in 1 patient (postoperative epidural hematoma) in the open TLIF group. The hematoma was surgically evacuated, and the symptoms associated with it (severe back and radiating pain) were resolved. No other complications occurred in either the open or UBE-TLIF groups.

Table 1.

Demographics and characteristics of the operations

2. Characteristics of the 3D-Printed Titanium Cages

A total of 77 3D-printed titanium cages were used in the open TLIF or UBE-TLIF. Among them, 39 cages were used in the open TLIF and 38 in the UBE-TLIF. In the open TLIF group, 3 patients received a single cage, while in the UBE-TLIF group, 4 patients received a single cage. Table 2 shows the characteristics of the cages used in the study. The mean length of cages was not significantly different between open TLIF (25.2±1.7 mm) and UBE-TLIF (24.8±2.2 mm) (p=0.252). The mean cage width was not significantly different between open TLIF (11.0±0.0 mm) and UBE-TLIF (11.0±0.2 mm) (p=0.324). The mean angle was also not significantly different between the open TLIF (4.1°±1.5°) and UBE-TLIF (3.8°±0.9°) (p=0.265). In contrast, the mean cage height was significantly higher in the open TLIF (10.3±1.9) than in the UBE-TLIF (9.3±1.5) (p=0.004).

Table 2.

Characteristics of the 3D-printed titanium cages

Regarding the distribution of the cage size, significant differences were observed in both length (p=0.004) and height (p<0.001) between the 2 groups. In the open TLIF, the most commonly used cage lengths were 24 mm (33.3%) and 26 mm, whereas 25 mm was most common in the UBE-TLIF (36.8%). In the open TLIF, the most commonly used height was 10 mm (28.2%), while 9 mm in the UBE-TLIF (34.2%).

1) Clinical outcomes

Figure 2 shows illustrative cases of the UBE-TLIF and open TLIF. Table 3, Supplementary Table 1, and Figure 3 present the clinical outcomes of the open TLIF and UBE-TLIF groups. The ODIs were significantly decreased in both groups at the 1-year follow-up (p<0.001). The preoperative ODIs were significantly higher in the UBE-TLIF (32.0±4.0) than in the open TLIF group (26.6±3.2) (p<0.001). The ODIs at the 1-year follow-up were not significantly different between UBE-TLIF (4.4±3.6) and open TLIF groups (6.3±3.5) (p=0.089). The improvement in ODI was significantly greater in the UBE-TLIF (27.5±6.3) than in the open TLIF group (20.3±4.4) (p=0.020).

Figure 2.

Illustrative cases of open TLIF and UBE-TLIF. The upper row shows pre- and postoperative plain radiographs and CT images of a patient who underwent open TLIF for L4–5 degenerative spondylolisthesis. Sagittal and coronal postoperative CT images demonstrate solid fusion following open TLIF. The lower row shows pre- and postoperative plain radiographs and CT images of a patient who underwent UBE-TLIF for L4–5 degenerative spondylolisthesis. Sagittal and coronal postoperative CT images demonstrate solid fusion following UBE-TLIF. TLIF, transforaminal lumbar interbody fusion; UBE-TLIF, unilateral biportal endoscopic transforaminal lumbar interbody fusion; CT, computed tomography.

Table 3.

Radiologic outcomes for open TLIF and UBE-TLIF

Figure 3.

Clinical outcomes following open TLIF and UBE-TLIF. Clinical outcomes (ODI, VAS Back, VAS Leg) significantly improved from the preoperative assessment to the 1-year follow-up in both groups (p<0.001). In the open TLIF group, postoperative back pain was significantly higher than preoperative back pain (p<0.001). Conversely, in the UBE-TLIF group, postoperative back pain was not significantly higher than preoperative back pain (p=0.339). (A) ODI. (B) VAS back. (C) VAS leg. TLIF, transforaminal lumbar interbody fusion; UBE-TLIF, unilateral biportal endoscopic transforaminal lumbar interbody fusion; ODI, Oswestry Disability Index; VAS, visual analogue scale. ***p<0.001.

When comparing VAS back pain between the 2 groups, the preoperative VAS back pain was not significantly different between the open TLIF group (4.6±2.2) and UBE-TLIF groups (4.1±1.8) (p=0.930). The VAS back pain at postoperative day 2 was significantly higher in the open TLIF (6.5±0.9) than in the UBE-TLIF (4.8±1.3) groups (p<0.001). However, the VAS back pain at the 1-year follow-up was not significantly different between open TLIF (2.3±1.0) and UBE-TLIF (2.3±1.1) groups (p=0.987). When comparing the VAS back pain within groups, the VAS back pain was significantly increased from preoperative (4.6±2.2) to postoperative day 2 (6.5±0.9) in the open TLIF group with a p-value<0.001. In contrast, the VAS back pain was not significantly increased from preoperative (4.1±1.8) to postoperative day 2 (4.8±1.3) in the UBE-TLIF group with a p-value of 0.339. The VAS back pain was significantly decreased from preoperative to the 1-year follow-up in both groups (p<0.001).

When comparing the VAS leg pain between the 2 groups, the preoperative VAS leg pain was not significantly different between the open TLIF group (7.5±0.6) and UBE-TLIF groups (7.4±0.7) (p=0.435). The VAS leg pain at postoperative day 2 was not significantly different between the open TLIF (3.7±0.7) and UBE-TLIF (3.6±0.9) groups (p=0.484). The VAS leg pain at the 1-year follow-up was not significantly different between the open TLIF (2.4±1.1) and UBE-TLIF (2.5±0.9) groups (p=0.731). When comparing the VAS leg pain within groups, the VAS leg pain was significantly decreased from the preoperative to postoperative day 2 in both groups with a p<0.001. The VAS leg pain was significantly decreased from preoperative to the 1-year follow-up in both groups with a p<0.001.

2) Radiologic outcomes

Table 4 shows radiologic outcomes of 2 groups. The Bridwell interbody fusion grade was not significantly different between the 2 groups (p=0.756). The solid fusion was achieved in 21 patients (100%) in the open TLIF group and 20 patients (95.2%) in the UBE-TLIF group, and there was no significant difference between the 2 groups in solid fusion rate (p=1.000). No cage subsidence was observed in the open TLIF group. On the contrary, subsidence was observed in 2 patients (9.5%) of the UBE-TLIF group. However, subsidence did not cause clinical symptoms and did not require surgical management.

Table 4.

Spinopelvic parameters of the patients

4) Logistic regression analysis

Table 5 shows the result of the logistic regression analyses. The univariable analysis showed that body mass index (BMI) was the only significant factor for Bridwell interbody fusion grade I (OR, 0.73; 95% CI, 0.57–0.94; p=0.016). The surgical method (open TLIF or UBE-TLIF) was not significant factors (OR, 0.68; 95% CI, 0.20–2.31; p=0.534) in the univariable analysis. The multivariable analysis also showed that BMI was the only significant factor (OR, 0.70; 95% CI, 0.53–0.92; p=0.011). The surgical method (open TLIF or UBE-TLIF) was not also significant factor (OR, 0.47; 95% CI, 0.10–2.21; p=0.337) in the multivariable analysis.

Table 5.

Logistic regression analysis for Bridwell interbody fusion grade I or others (grade II, III, or IV)

1. 3D-Printed Titanium Cages

Cage material is an important factor to achieve interbody fusion. We used 3D-printed titanium cages for solid interbody fusion. Systematic reviews and meta-analyses have reported comparisons of fusion rates between titanium and PEEK cages, although these studies were conducted in the context of open fusion surgery [22-24]. For lumbar interbody fusion, titanium cages have higher fusion rate than PEEK cages. However, drawback of titanium cage is a higher subsidence rate than PEEK cage (3–4 GPa) due to higher Young modulus of titanium cage (100–110 GPa) than corticocancellous bone of vertebral body (10–40 GPa) [24-27]. A 3D-printed titanium cage (2–2.5 GPa) has been introduced to mitigate subsidence, with a lowered apparent Young modulus due to its porous architecture [28-31]. The porosity of 3D-printed titanium cage also enhances interbody fusion by promoting osteointegration [31-33]. Although some graft loss may occur due to the continuous saline irrigation and porous structure of the 3D-printed titanium cage, the pores are fine, making the likelihood of such loss minimal. In an animal model study, a 3D-printed titanium cage even without graft material showed a higher fusion rate than PEEK cage with autologous iliac bone graft material [31]. Although this animal model study was not conducted using endoscopic surgery, this result indicates that 3D-printed titanium cage may be more appropriate for UBE-TLIF than the PEEK cage, because continuous saline irrigation may wash out graft material. Therefore, we utilized 3D-printed titanium cages to achieve solid interbody fusion for UBE-TLIF, which showed a comparable fusion rate to that of open TLIF in our study.

Regarding the length and height of cages, larger cages were inserted in open TLIF than in the UBE-TLIF. This is attributed to the limited and narrow endoscopic view of the UBE-TLIF, which may increase the risk of injury to the exiting, traversing nerve roots, or thecal sac during large cage insertion. The size of cages has significant influence on interbody fusion [34-36]. Using a longer cage allows it to span the ring apophysis of endplate, reducing the risk of subsidence. Additionally, using a cage with higher height allows it to restore disc height and segmental lordosis. Therefore, the open TLIF would have advantages in terms of cage size, longer length, and higher height than the UBE-TLIF. Nevertheless, the spinopelvic parameters measured preoperatively, on postoperative day 2, and at the 1-year follow-up showed no significant differences between UBE-TLIF and open TLIF. Additionally, LL, segmental Cobb angle, disc height (anterior, middle, and posterior), and foraminal height increased significantly from the preoperative period to the 1-year follow-up in both groups. Kang et al. [18] also reported findings consistent with ours, showing significant increases in LL, segmental Cobb angle, and disc height in both UBE-TLIF and open TLIF, but no significant differences between the 2 groups.

2. Clinical Outcomes

Previous articles consistently reported that postoperative VAS back pain was significantly lower in the UBE-TLIF than in the open TLIF groups [10,14-18]. These results are in line with our results. These consistent results would be due to advantages of the minimal invasive characteristics of the UBE-TLIF as endoscopic spine surgery. With minimal muscle injury, UBE-TLIF can preserve multifidus muscles, which would be injured in the open TLIF due to detached from origin (mammillary body) and insertion (spinous process) point of the multifidus muscle. Preservation of multifidus muscles is paramount in the minimally invasive spine surgery due to function of multifidus muscle as a stabilizer [37,38]. In the cadaveric study, multifidus muscle has a high cross-sectional area and a low fiber length-to-muscle length ratio. This finding indicates that the multifidus muscle is a unique stabilizer of the spine [38]. Moreover, minimal muscle dissection with less injury leads to lower postoperative back pain in the UBE-TLIF than in the open TLIF. Previous literature comparing UBE-TLIF and open TLIF consistently reported significantly lower postoperative back pain in UBE-TLIF than in the open TLIF [15,16,39,40]. Lower postoperative back pain facilitates enhanced recovery after surgery (ERAS) [41]. After spine surgery, prolonged bed rest may cause medical complications such as pneumonia, pulmonary embolism, or deep vein thrombosis. Lower postoperative back pain owing to UBE-TLIF, and application of the ERAS would facilitate early ambulation and prevent medical complications [15,16]. Nevertheless, one drawback of UBE-TLIF is that it requires a longer operative time than open TLIF. In our study, the operative time for UBE-TLIF was significantly longer than that for open TLIF. Similarly, previous studies comparing UBE-TLIF with open fusion surgery have consistently reported longer operative times for UBE-TLIF [14-16,18,39,40]. As the operative time increases, the duration of anesthesia is also likely to be prolonged. In elderly patients, the risk of pulmonary and cardiovascular complications tends to be higher [42-44]. Therefore, we often prefer open TLIF in older patients to minimize operative time. This tendency may explain why the mean age of patients in the open TLIF group was significantly higher than that in the UBE-TLIF group in our study.

Regarding ODI, the ODI at the 1-year follow-up did not show a statistically significant difference between the open TLIF and UBE-TLIF groups. However, the degree of ODI improvement from preoperative to 1-year follow-up assessment was significantly greater in the UBE-TLIF group. This appears to be attributable to the difference in preoperative ODI, which was significantly higher in the UBE-TLIF group. The improvement in ODI was significantly greater in the UBE-TLIF group, suggesting that UBE-TLIF achieved a greater improvement in functional outcomes.

Although postoperative day 2 VAS back pain and the degree of ODI improvement were better in the UBE-TLIF group, the 1-year follow-up clinical outcomes, including VAS back and leg pain, were comparable between the 2 groups. Nevertheless, UBE-TLIF would still offer several advantages over open TLIF. Continuous saline irrigation provides a clear operative field and may help reduce the risk of infection. Additionally, the highly magnified endoscopic view allows for meticulous inspection of the surgical field. Furthermore, in patients with a high BMI, open surgery requires a larger incision, which can lead to greater muscle injury, increased bleeding, and a higher likelihood of infection. In contrast, UBE-TLIF would minimize these risks even in obese patients.

3. Interbody Fusion

Achievement of solid interbody fusion is imperative to avoid pseudoarthrosis and subsequent revision surgery. Djurasovic et al. [45] reported that solid interbody fusion has correlates with patient-reported health-related quality of life in patients undergoing instrumented posterolateral lumbar fusion. Therefore, the achievement of solid interbody fusion with a sufficient decompression has to be the ultimate goal of the TLIF for spinal stenosis. The UBE-TLIF has advantage regarding endplate preparation for interbody cage insertion. With a clear operative view in the disc space by endoscopy, surgeon can accurately denude cartilaginous endplate for interbody fusion and preserve osseous endplate to prevent cage subsidence. Park et al. [14] reported that fusion rates were not significantly different between UBE-TLIF and open PLIF. Heo and Park [15] reported that fusion rate was 78.3% (18 of 23) in the UBE-TLIF and 73.9% (34 of 46) in the open TLIF with no significant difference (p>0.05). Kim et al. [10] reported that fusion rate was 93.7% (30 of 32) in the UBE-TLIF and 92.7% (51 of 55) in the open TLIF with no significant difference (p=0.43). Kang et al. [18] reported that fusion rate was 87.7% (57 of 65) in the UBE-TLIF and 88.4% (38 of 43) in the minimally invasive surgery (MIS)-TLIF with no significant difference (p=0.473).

Although comparative studies between UBE-TLIF and open TLIF (or MIS-TLIF) showed no significant differences in fusion rates, there are still concerns for fusion rate of the UBE-TLIF. Because there are also disadvantages of UBE-TLIF that may influence solid interbody fusion. First, cage insertion would be challenging because trajectory angle of cage insertion may be limited due to fixed working portal, although skin has movability to change trajectory angle. If cage insertion angle is not parallel with disc angle, cage subsidence into vertebral body may occur. Furthermore, disc space distraction is not feasible due to intact contralateral facet joint and posterior ligamentous complex (supraspinous and interspinous ligament). Contralateral facetectomy could be performed to release both facet joint for disc distraction with shaver. Another disadvantage of UBE-TLIF is continuous saline irrigation. Although continuous saline irrigation provides clean operative field, continuous saline irrigation may washout local cytokine and growth factors, which are released from bone bleeding. These factors may promote interbody fusion [11,12].

Regarding Bridwell interbody fusion grade, Park et al. [14] reported that proportion of Bridwell grade I was significantly higher in the open PLIF (92.1%) than in the UBE-TLIF (74.1%) with a p-value of 0.013. Kim et al. [10] reported that Bridwell grade I, II, III, IV consisted of 19, 11, 2, and 0 in the UBE-TLIF and 39, 12, 4, 0 in the open TLIF, respectively. The proportion of Bridwell grade I was higher in the open TLIF (70.9%, 39 of 55) than in the UBE-TLIF (59.4%, 19 of 32).

In our study, solid interbody fusion was achieved in 95.2% (20 of 21) of UBE-TLIF group with no significant difference compared to 100% (21 of 21) of open TLIF group (p=1.000). The proportion of the Bridwell interbody fusion grade was not significantly different in the UBE-TLIF and open TLIF (p=0.756). The multivariable logistic regression analysis showed that the surgical method (open TLIF or UBE-TLIF) was not a significant factor (OR, 0.47; 95% CI, 0.10–2.21; p=0.337) for Bridwell interbody fusion grade I.

Notably, in the multivariable logistic regression analysis, BMI was the only significant factor for Bridwell interbody fusion grade I (OR, 0.70; 95% CI, 0.53–0.92; p=0.011). Previous articles also reported that greater BMI was a risk factors for pseudoarthrosis following interbody fusion [46,47]. Obesity may cause excessive biomechanical loading of index operation level, and subsequent cage subsidence. Consequently, it may lead to nonsolid fusion and pseudoarthrosis.

4. Limitations

This study had some limitations. First, its retrospective nature and limited sample size have limitations to compare the open TLIF and UBE-TLIF. Because this study was a retrospective analysis with strict inclusion criteria for single-level degenerative cases, the number of eligible patients was limited. In addition, selection bias could be a potential limitation, as the surgical decision for UBE or open TLIF was based on the patient’s preference or the surgeon’s discretion. A future prospective study with a larger sample size is warranted. Second, we mentioned that open TLIF would have advantages regarding larger cage insertion than the UBE-TLIF. However, this comparison has limitation due to the lack of consideration for the length and height of preoperative intervertebral disc space at the index segment.