Introduction

Degenerative Lumbar Spondylolisthesis (DLS) is a common spinal condition in which the vertebra above slips forward from the vertebra below, resulting in pressure on the thecal sac and spinal nerves in the affected spinal segment. This can result in pain, numbness, weakness in the legs and other symptoms. Amongst the variety of options, Transforaminal Lumbar Interbody Fusion (TLIF) remains the workhorse treatment for patients with DLS with or without stenosis, refractory to nonoperative treatment.
Traditional open techniques for instrumented posterior lumbar fusion procedures, like open transforaminal interbody fusion (O-TLIF), are widely accepted methods for managing DLS. However, the morbidity related to these procedures has become an increasing concern for many surgeons. Excessive intraoperative dissection and retraction of the paraspinal muscles, among other significant iatrogenic morbidities related to the approach, can lead to denervation and atrophy, thus increasing the likelihood of unfavorable outcomes. [1-7]. Minimally invasive transforaminal lumbar interbody fusion (MIS-TLIF) was developed as a potential solution to the problems mentioned above by reducing the amount of iatrogenic soft tissue injury while still accomplishing the same goals as the open TLIF. The incision for the interbody access site is made between 3 and 4 cm lateral to the midline. Incision of the fascia allows blunt dissection between the longissimus and multifidus muscles, like in a standard Wiltse muscle-splitting approach, but with less wound size (Fig. 1A-D) [8]. This minimizes trauma to muscular tissues, minimizes intraoperative bleeding, and creates a plane that allows efficient maintenance of the trajectory of the retractors [9].
The advantages of MIS-TLIF over O-TLIF include less blood loss, lower rates of infection, shorter hospital stays, and faster recovery times. However, there is still debate over whether MIS-TLIF is as effective as O-TLIF in achieving successful fusion and improving patient outcomes [10]. This study aims to evaluate the effectiveness and safety of MIS-TLIF compared to O-TLIF in treating lumbar DS, considering factors such as clinical outcomes, complication rates, and radiological outcomes.
Material and methods

This is a retrospective analysis of prospectively collected data. From January 2018 to June 2022, a consecutive series of 46 patients who underwent one or two levels of instrumented TLIF due to symptomatic Degenerative Lumbar Spondylolisthesis (DLS) were prospectively studied. Patients were divided into two groups according to the access method used: O-TLIF (O group) composed of 24 patients, and MIS-TLIF (MIS group) – 22 patients. The group non-randomized assignment, whether MIS or O, was done by two surgeons, M.R. and R.P., experienced in both techniques and who performed all the operations. It strictly depended on the surgeon’s preference and the discussion of the surgical options with the patient. The mean age of patients at the time of surgery was 63.4 (range 39-79) for the O group and 61.1 years (range 46-78) for the MIS group. The O and MIS groups had a mean BMI of 26.4 and 26.7 kg/m2, respectively, with a minimum one-year follow-up period. On average, the O group had a duration of 33.2 months, while the MIS group had 19.5 months. Most patients in the O and MIS groups had DLS at the L4-L5 level, with 19 patients (79%) and 16 patients (73%), respectively. The Meyerding scale indicated that grade I spondylolisthesis was prevalent in most cases, with 21 (87.5%) in the O group and 20 (90%) in the MIS group. 16 (67%) patients in the O group and 16 (73%) in the MIS group had primary DLS surgery. The remaining patients had revision surgery with previous decompressive procedures for the same reason. 19 (79%) patients in the O group and 18 (82%) patients in the MIS group had one-level surgery, while the rest underwent two-level surgery, mainly due to concomitant spondylarthrosis at the adjacent level. The patient’s preoperative characteristics and clinical data are presented in Table 1. All patients consented to participate in the study. They complained mainly of low back pain and varying degrees of radiating pain and neurologic symptoms and underwent conservative therapy for at least 6 months before surgery. Indication for surgery was considered after the failure of conservative treatment for at least 6 months. Inclusion criteria for study enrollment included the following: one or two segment DLS only, posterior decompression, pedicle screw instrumentation, and transforaminal lumbar interbody fusion (TLIF) performed, minimum follow-up of 1 year with a complete set of radiographs, diagnosis of DLS confirmed with magnetic resonance imaging (MRI) and/or computed tomography scans (CT). Exclusion criteria were as follows: significant concomitant lumbar deformity, history of prior lumbar trauma or infection, posterior instrumentation, and fusion >2 levels.

Surgical technique

MIS and Open procedures were performed under 3-D cone beam fluoroscopy (the O-arm™ system and Stealth Station™ Navigation, Medtronic).
MIS TLIF procedure: After determining the proper operative level, a spinous process-anchored intraoperative navigation frame is attached. After registration, a bilateral approach is undertaken through a paramedian skin incision 4–5 cm off midline using the Wiltse technique. Using the Navigated Universal Drill Guide, wires are introduced into the proper pedicles. Bi-lateral pedicle screws are placed percutaneously over previously introduced guide wires (Fig. 2 A-F). Then, the Pedicle Based Retractor (Stryker) is attached to the extended screw tulips on the symptomatic side. This system allows for rigid placement and pedicle distraction capabilities. It provides a less invasive access option that helps to directly visualize the surgical site and identify anatomical structures (Fig. 3A). By placing the screw first and docking the retractor second, one can preserve key anatomical landmarks and then use them for orientation. Unilateral decompressive facetectomy with laminotomy is then performed with a high-speed drill to expose the disc, and the ligamentum flavum is removed with Kerrison rongeur under microscopic magnification (Fig. 3B-D). The facetectomy procedure is performed on the side of radicular symptoms. If both legs are symptomatic, the procedure involves approaching from the side with the more severe pathology and decompressing the contralateral lamina and foramina through unilateral exposure. The disc space preparation is performed using standard interbody fusion instruments. The proper placement of annulotomy is marked with UDG navigated device (Fig. 3E). Following the removal of cartilaginous material from the endplates, and after several attempts, an interbody cage filled with autogenous bone graft is then placed anteriorly and contralateral to the annulotomy in the interbody space. Additional autogenous bone graft is packed anteriorly to the final interbody implant (Fig. 3F). After the introduction of both rods, compression is applied on both sides. The accurate position of the hardware is confirmed on AP and lateral fluoroscopy.
Open TLIF procedure
The open TLIF approach is performed as described by Harms et al. [11] using a midline open approach. A midline incision is made, followed by bilateral dissection of the muscles and facet joints. A spinous process anchored intraoperative 3D navigation frame is attached. After registration, the trajectory for the screw is planned, using a sharp awl navigated tool penetrating through the proper pedicle. After bilateral pedicle screw instrumentation, A uni or bilateral decompressive facetectomy and flavectomy is performed, followed by a single intervertebral cage (banana or bullet shape type). Neural structures are released carefully. The accurate position of the hardware is confirmed on AP and lateral fluoroscopy. In addition to interbody grafting, intertransverse fusion is carried out on both sides using the bone obtained from the facetectomies/laminotomies. Suction drains are introduced in all patients and removed after 24 hours.

Clinical evaluation
The analyzed perioperative results were recorded, such as surgical time, blood loss, X-ray dose, length of postoperative stay, size of the postoperative scar, and peri-and postoperative complications. Back and leg pain were evaluated using VAS and ODI questionnaires for PROMs (Patient Reported Outcome Measures). The PROM Minimal Clinically Important Difference (MCID) was defined using thresholds of 14.9 for ODI, 2.9 for VAS for leg pain, and 2.1 for VAS for back pain [12]. All measurements were performed preoperatively, postoperatively during discharge (VAS only), 2-3 months postoperatively, and at the latest follow-up.

Radiologic evaluation
For radiologic assessment, silhouette X-ray images in the AP and lateral projections were assessed before and after the surgery, 2-3 months postoperatively, and after the follow–up period. Evaluation of the degree of DS in the Meyerding scale and its correction in (%), along with correction of segmental and total lumbar lordosis, interbody space angle and height, measurement of sagittal vertical alignment (SVA) and pelvic-spinal relations (PI, PT, SS). (Fig. 4) Two independent observers performed all measurements with Surgimap Nemaris Inc., New York, NY, USA software, and the mean values were adapted for the analysis. All patients underwent CT examination one year after the index surgery to evaluate the degree of fusion. If there was continuity of the trabecular bony bridging at the fixed interbody space, it was termed union. If the trabecular bony bridging was discontinuous, it was termed nonunion. Pseudarthrosis was diagnosed with a combination of the following: a visible halo around the pedicle screws on the CT scan, nonunion, and graft collapse or migration [13].
Statistical analysis was performed with SPSS Version 17.0 (SPSS, Inc., Chicago, Illinois, USA). The Student t-test was used for comparing continuous variables between groups, and the chi-square test was used for comparing binary variables between groups. The Student t-test was used to compare the continuous variables within the group. A P value of less than 0.05 was considered statistically significant.

Results

Preoperative demographic data in both groups did not differ significantly, except for the follow–up period, which was considerably longer for the O group (mean 33.2 months), and 19.5 months for the MIS group (p = 0.003). There was no notable disparity in the duration of the operation between the two techniques. Median blood loss following surgery was significantly higher in the O group, 450 ml vs. 170 mL in the MIS group (p = 0.006). Patients receiving O – TLIF spent significantly more days in the hospital post-surgery than those after MIS –TLIF (3.7 days vs. 2.1 days (p = 0.04). Intraoperative radiation was significantly higher in the MIS group, 2890 cGy/cm2 vs 2120 cGy/cm2 in the O group (p = 0.05). Finally, the length of the postoperative scar was comparable between both groups. Clinical perioperative parameters are shown in Table 2.
There were few complications in each cohort, with only one case of pseudoarthrosis developing in each group, proven radiologically with screw loosening and cage migration. There were two reoperations in each group due to the aforementioned pseudoarthrosis, one due to improper position of the pedicle screw in the MIS group and another due to symptomatic Adjacent Segment Disease (ASD) in the Open group. No statistically significant difference was found in the incidence of reoperations between both groups. Three cases of superficial wound infection occurred in the O group, and none in the MIS group (which was statistically significant p = 0.004). All were cured by intensive dressing and antibiotic treatment. No deep infection occurred in any group. In the O group, there was one case of incidental durotomy, which was repaired during the same surgery. No obvious neurological deficit occurred in any group. Cage mild subsidence was found in two patients from each group, but it was clinically asymptomatic. One patient in the O group and two in the MIS group experienced numbness of the left lower extremity postoperatively. However, they recovered completely after 2 months of conservative treatment. Although the total number of complications was significantly higher in the O group (42%) compared to (27%) in the MIS group (p = 0.04), most of these complications were not clinically significant and resolved without any lasting consequences.

The mean VAS score for back pain was 7 for the O group and 6.8 for the MIS group before surgery. However, it significantly decreased to 4.5 and 3.5 at discharge after surgery (MCID = 2.1). Additionally, at 2-3 months post-surgery, there was a further decrease to 2.9 and 1.8, respectively, and the difference between the two groups was statistically significant (p = 0.04). At the last follow-up evaluation, the VAS score for back pain was 1.8 for the O group and 1.5 for the MIS group, respectively. The mean VAS score for leg pain was 6.8 for O and 7.5 for the MIS group preoperatively, which decreased significantly to 3.2 and 1.7 at the discharge, post-surgery (MCID = 2.9). This difference was statistically significant between both groups, with the better outcome for MIS (p =0.002). At the 2-3 months control, VAS leg pain was 2,1 for O and 1,9 for the MIS group, respectively, with no statistical difference. At the latest follow-up, both groups showed significant improvement compared to preoperative status, with no statistical difference in VAS back and leg pain between the 2 groups (Fig. 5A, B). Analysis of ODI scores demonstrated no statistically significant intergroup differences for preoperative and all postoperative measurements. The overall ODI improved significantly in both groups between baseline and 3 months follow-up. In the O group, it decreased from 48.1 to 26.1 with a difference of 22, which is above MCID (it was defined using thresholds of 14.9 for ODI) p = 0.017. A similar situation was observed for the MIS group where ODI improved significantly from 45.6 to 24.2 (difference of 21.4) after 3 months of surgery (p = 0.009). ODI scores for the last follow-up control were even lower in both groups, reaching 20.6 for the O group and 19.5 for MIS, without significant differences between both groups (Tab. 4).
Radiological sagittal balance parameters are reported in Table 5. Neither group showed a significant change in total lumbar lordosis and for measurements of SVA, PI, PT, and SS at the last follow-up. Segmental lordosis changed significantly as follows: from 8.8° and 9.2° preoperatively to 16.1° and 14.2° at the last follow up in O and MIS groups, respectively. The change between the two groups was not statistically significant. In each group, statistically significant improvement in the Meyerding slip ratio was observed at the time-point immediately after surgery and the last follow-up when compared with preoperative status. The O group showed an improvement in the slip ratio from 18.6% to 4.1%, (p < 0.001), whereas the MIS group showed an improvement in the slip ratio from 14.9% to 5.9% (p = 0.01). The change of the Meyerding slip ratio between the two groups differs significantly (p = 0.04). Both groups showed a significant increase in disc height and intervertebral disc space angle at the last follow-up. Notably, the change in disc space height was greater in the O group than in the MIS but without statistical significance.

Discussion

MIS-TLIF and open-TLIF procedures have been demonstrated to have an equivocal medium and long-term improvement in pain and functional disability in treating lumbar problems. The theoretical advantages of MIS-TLIF include decreased tissue damage, fewer intraoperative and postoperative complications, and decreased duration of hospitalisation[14]. Besides good theoretical support, there is still a lack of high-quality evidence to determine whether open or MIS TLIF has better clinical efficacy in treating symptomatic low-grade DLS.This study aimed to evaluate and compare the clinical and radiological outcomes and safety between those two methods of the treatment of low-grade DLS. We studied 46 surgically treated patients divided into two groups according to the access method used: O group – 24 patients, MIS group – 22 patients. Clinical and radiological outcomes were evaluated before and after the surgery and after the follow–up period in both groups. Generally, we found both methods highly effective in the treatment of DLS. The results of this study suggest that MIS-TLIF and O-TLIF have comparable clinical and radiological outcomes and complication rates, with a few exceptions where the first method had some advantages. In our study, MIS-TLIF was associated with a lower rate of blood loss and shorter hospital stays with fewer complications, which could lead to faster recovery times. In a study by Park and Ha [15] and several other papers [16,17], comparisons of cohorts of patients who underwent MIS TLIF with those who underwent open TLIF showed a significant reduction in operative blood loss and length of hospital stay, although the minimally invasive technique required more operative time than the open technique. The results of our study regarding the above aspects were consistent with their findings, except for the operative time, which was comparable between groups in our study. Regarding the comparison of complication rates between MIS-TLIF and O-TLIF, our findings revealed that while the total number of complications was significantly higher in the O group (42%) compared to the MIS group (27%), most of these complications were not clinically significant and resolved without any long-term consequences. Also, we found no statistically significant difference in the incidence of reoperations between both groups. In a systematic review by Hu et al., they found the total incidence of complications 11.87 % for MIS and 14.35 % for open, respectively. The proportion of patients with wound infection in their study was 12% in the MIS group and 25% in the Open group, which is consistent with our results. This may demonstrate that the reduced soft tissue injuries in the MIS-TLIF positively affect the wound healing process [18]. Radiation exposure was a potential health hazard to the operating surgeon during MIS-TLIF, which exposes the surgeon to significantly more radiation than open lumbar surgery [19]. In our study, we used 3-D navigation and intraoperative cone beam fluoroscopy for all surgeries, which significantly reduced the operating surgeon’s exposure. However, the patient still received a high dose of radiation. It was significantly higher in the MIS group. In relation to PROM, our study found that patients in both groups achieved MCID at hospital discharge for VAS leg pain and VAS back pain, which continued until the last follow-up visit. The disparity in leg pain between the two groups was deemed statistically significant, showing improved outcomes exclusively for MIS patients upon discharge and for back pain at 2-3 months of follow-up. At the final follow-up, all the PROMs were comparable. Most published studies compare VAS and ODI between MIS and open procedures for functionality and disability. These authors presented evidence indicating that MIS patients exhibited lower postoperative VAS and ODI scores compared to those who underwent open TLIF in the short- and medium-term, albeit without statistical significance [20-22]. Our study is largely consistent with these findings. In our study, ODI scores demonstrated no statistically significant intergroup differences for preoperative and all postoperative measurements. The overall ODI significantly improved in both groups from baseline to 3-month follow-up, with a non-significantly lower score in the MIS group at the last follow-up. Several studies reported that MIS-TLIF was superior to O-TLIF in VAS and ODI scores at long–term follow-up (more than 1 year of follow-up) [23,24]. We do not support these findings as our VAS and ODI outcomes were comparable between both groups after more than one year of follow-up.Regarding sagittal parameters, Carlson et al. [25] found that average total lumbar lordosis and segmental lordosis improved postoperatively. Modi and coauthors, in their series, also demonstrated that lumbar lordosis and segmental lordosis at the index level improved postoperatively without significant difference between open TLIF and MIS-TLIF [26]. They also proved that most lordosis was corrected at the index level with the TLIF procedure. Our findings agree with these results, indicating similar radiological outcomes in both groups. The two groups differed significantly only in their Meyerding slip ratio.In the O group, the slip ratio improved from 18.6% to 4.1%, while in the MIS group, it improved from 14.9% to 5.9%.Both groups underwent significant changes. The Meyerding slip ratio between the two groups differed significantly, with the O-group showing better slip reduction. However, this fact did not translate into better clinical outcomes for the O-group. At the final follow-up, both groups showed a significant increase in disc height and intervertebral disc space angle.
Finally, it is essential to note the limitations of the study. Firstly, the study was a retrospective analysis of prospectively collected data, which may have introduced potential methodological defects and risks of bias. Secondly, there was no true randomization as the group assignment (whether MIS or O) strictly depended on the surgeon’s individual preference and discussion of surgical options with the patient. Thirdly, the follow-up period was significantly longer in the O-TLIF group due to our center’s recent shift towards MIS techniques. In combination with the small sample size in each group, these factors may have influenced the results. Therefore, these limitations should be considered when interpreting the outcomes of the current study.

Conclusion

In conclusion, both MIS-TLIF and O-TLIF are equally effective, leading to significant clinical and radiological improvement in patients with DLS. In certain aspects, MIS has some advantages over the O-TLIF technique: reduced blood loss, shorter hospital stays, and fewer complications. However, evidence for superiority in achieving successful fusion and better outcomes is limited. The ability to reduce the slip was better in open TLIF but did not reflect better clinical outcomes in the O-group. Clinicians should consider patient factors and surgeon expertise when choosing between MIS-TLIF and O-TLIF for the treatment of degenerative spondylolisthesis. This study is to be continued with a larger patient population and longer follow-up, which would make it more acceptable.

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