Introduction
Meyerding’s classification defined high-grade spondylolisthesis (HGS), i.e., grade 3 to grade 5 spondylolisthesis, as a vertebral body displacement by >50% with respect to an adjacent vertebral body [1]. Approximately 11.3% of patients with spondylolisthesis present with HGS [2,3]. Most patients with HGS present with global spinal malalignment symptoms, such as sagittal imbalance and associated physiologic compensatory mechanisms that cause global backache and impaired health-related quality-of-life measurements [4–7]. Dysplastic lumbosacral (LS) morphology, including pars defects, sacral end-plate anomalies, and transitional vertebrae is frequently related to HGS [8]. The two most vital surgical indications in HGS are pain alleviation and segmental deformity correction [3].
In recent years, the adoption of minimally invasive techniques for transforaminal lumbar interbody fusion has appeared as a promising surgical approach, providing the potential for global alignment restoration with solid fusion while offering reduced perioperative morbidity and faster recovery [4]. The existing literature on the use of minimally invasive surgery–transforaminal lumbar interbody fusion (MIS–TLIF) for treating HGS identifies a rise in interest among spine surgeons but also determines notable areas of knowledge deficiency [9,10]. Most of these studies reveal the overall benefits of MIS–TLIF, such as reduced blood loss, shorter hospital stays, lower infection rates, and improved wound healing compared to traditional approaches [11]. However, these investigations are frequently limited to patients with low-grade spondylolisthesis in the context of spondylolytic spondylolisthesis. Furthermore, we determined that most of the research solely focused on the feasibility and early outcome experience of the authors [12–14]. We revealed that most publications describing minimally invasive surgeries for treating HGS, especially those involving grade 4 or 5 listhesis, used a combined anterior and posterior approach in conducting a comprehensive review [15]. To the best of our knowledge, a detailed report that provides specific technical nuances and subtleties for the optimal use of the single-level posterior-only approach utilizing the MIS–TLIF technique is greatly lacking.
We effectively extended the indications of MIS–TLIF to HGS in the current era of “less is more.” In the present study, we describe a cantilever reduction and fusion technique in HGS with MIS–TLIF.
Materials and Methods
The authors reported the outcomes of consecutively operated patients with HGS (grades 3–5) in whom reduction, posterior instrumentation, and fusion were achieved with the MIS–TLIF technique, performed by a single experienced surgeon, from January 2017 to December 2022. The Institutional Ethical Clearance Committee at Bombay Hospital and Medical Research Centre, Mumbai (IRB approval no., BH-EC-1257) approved this study. We included patients of all age groups having grade 3–5 spondylolisthesis, disturbed global sagittal balance, chronic low backache with or without radicular pain, and >75% collapse of disc height. Informed consent was obtained from all individual participants included in the study, as well as from all patients and their families for publishing their data and photographs.
Patients’ demographics, including age, sex, and gender, were reviewed preoperatively. They were evaluated clinically in terms of low back pain and radicular pain, and both symptoms were scaled by Visual Analog Scale (VAS) score preoperatively as well as postoperatively at each follow-up. Radiographs of the LS spine in a standing position (anteroposterior and lateral views) and whole spine scanogram (anteroposterior and lateral view) were performed to calculate the slip angle (SA) and sacropelvic and spinopelvic parameters preoperatively as well as postoperatively. The LS kyphosis (LSK) or SA and L5 incidence (L5I) were calculated as indices for the LS balance, and the spinopelvic balance was characterized by correlating the sacral slope (SS) and pelvic tilt (PT), where SS>PT indicated a balanced spinopelvic structure [8]. All patients underwent magnetic resonance imaging preoperatively. Intraoperative neuro-monitoring (IONM) was used for all patients with grade 4 and 5 listhesis to continuously monitor bilateral exiting and traversing nerve roots during reduction maneuvers.
Patients were categorized into two groups according to the grade of listhesis for data analysis and preventing confounding of results. Group A consisted of patients with grade III anterolisthesis and group B included patients with grades IV and V anterolisthesis.
Surgical technique
Surgery was performed on a radio-lucent surgical table, equipped with two horizontal bolster supports, and under fluoroscopy guidance. The patient was placed in a prone position, with both hips and knees extended to attempt postural correction (Fig. 1A–C). The pedicles of the concerned motion segment were determined and marked using a 22G spinal needle with fluoroscopy. Paramedian skin incisions, approximately 2.5 cm in length, were made on each side to facilitate guidewire, pedicle screw, and tubular retractor insertion. Guidewires were bilaterally docked into the pedicles. Guidewires were passed along the dense superior endplate for the cranial vertebra, and a robust tricortical purchase was aimed at the caudal vertebra. A non-expandable tubular retractor with a 22-mm diameter was docked precisely over the facet-joint complex on the side which was predominantly symptomatic. An operating microscope was utilized for all minimal access surgeries in the present study. The ipsilateral facet joint and the pars and the lamina of the cephalic vertebra were exposed. The inferior facet and lamina were then precisely excised with a combination of high-speed burr and Kerrison’s rongeurs. Pedicle screws were inserted over the contralateral guidewires at the cranial and caudal levels.
A window was established to facilitate discectomy, disc space preparation, and cage insertion through the passage created between the corresponding exiting and traversing nerve roots (Fig. 2A). All harvested bone was utilized as autograft for fusion later in the surgery. Osteotome was hammered into the disc space to confirm the trajectory under fluoroscopy. A dome osteotomy was performed to resect the sacral dome and to shorten the posterior column in patients with grade 4 or 5 spondylolisthesis (Fig. 2B). Similarly, the superior end-plate of the caudal vertebra was osteotomized in patients with grade 3 spondylolisthesis. This helped to access the disc space, following which a discectomy was performed, the end plates were curetted, and disc space was prepared. The cage size was identified using serial sizing trials, and a bullet-shaped cage, packed with the autograft was driven into the disc space (Fig. 2C). This maneuver enabled establishing an effective lordosis of the motion segment locally, and it lifted the cranial vertebra as the cage progressed through the disc space from posterior to anterior, thereby horizontalizing it. The final cage position was confirmed under fluoroscopic guidance. Ipsilateral pedicle screws were now inserted over the guidewires and pre-contoured lordotic rods were fastened to the tulips with inner screw caps under fluoroscopic guidance. The final reduction was achieved by initially tightening the caudal screw cap bilaterally, followed by the cranial screw cap, with the interbody cage acting as a fulcrum. Additionally, cranial pedicle screws not only pulled the respective vertebral body posteriorly but angulated it, thereby manifesting a lordotic effect (Fig. 2D). This maneuver was bilaterally applied simultaneously with an assistant under fluoroscopic monitoring. Final reduction and screw and cage placement were confirmed under fluoroscopy and adequate hemostasis was ensured (Fig. 2E, F). Subfascial drains were kept bilaterally, and closure was performed in layers with subcuticular closure of the skin with 3/0 monocryl. All patients tolerated the procedure well without any neurological complications.
Postoperative protocol
All patients were mobilized with an LS brace on the first postoperative day. Postoperative whole spine radiographs were taken in anteroposterior and lateral views in a standing position, and spinopelvic parameters were compared with the preoperative values. All patients were discharged on postoperative day 4 and were called for follow-up at 6 weeks, 3 months, 6 months,1 year, 2 years, 3 years, and 4 years postoperatively. Radiographs of the LS spine (anteroposterior and lateral views) were taken at each follow-up in a standing position. Physiotherapy was initiated on the evening of the surgery itself in the form of active ankle pumps, static quadriceps strengthening, and isometric hip exercises. Core strengthening and back extension exercises were started at 6 weeks postoperatively after these isometric core exercises. Functional outcome was evaluated based on the VAS score at each follow-up.
Results
A total of 153 patients with spondylolytic spondylolisthesis underwent MIS–TLIF from January 2017 to December 2022. Of these patients, 36 demonstrated HGS and were included in the present study. Group A consisted of 30 patients (83.33%) with grade III spondylolisthesis and group B involved six patients, of whom 2 (5.55%) had grade IV and 4 (11.11%) had grade V spondylolisthesis. The male-to-female ratio in this subset was 1:3 and the mean age of the patients was 42.16±19.7 years. The L4–L5 level was involved in six patients, whereas the L5–S1 was the segment involved in the remaining 30 patients. The complete reduction was achieved in 24 patients, whereas 12 patients achieved an end stage of grade 1 spondylolisthesis with adequate spinopelvic balance. The mean operative time was 225.20±9.12 minutes (range, 210–270 minutes) in group A and 270.25±5.99 minutes (range, 260–300 minutes) in group B. The mean blood loss was 140.55±8.15 mL (range, 80–150 mL) in group A and 190.54±8.14 mL (range, 110–250 mL). No implant-related complications were reported in any of the patients.
Spinopelvic parameters
The mean lumbar lordosis (LL) was 49.2°±4.21° and the mean SA/LSK was 62.4°±5.29° in group A. Additionally, the mean SS and PT were 28.75°±2.58° and 17.43°±4.24°, respectively. Unbalanced spinopelvic parameters characterized by PT>SS were noted in seven patients (23.33%). Postoperatively, the mean LL and SA were 48°±2.23° and 98.12°±1.04°, respectively, both parameters demonstrating significant improvement as compared to preoperative values (p=0.031 and p=0.04). The SS and PT were corrected to a mean value of 36.27°±3.12° and 15.14°±2.89°, respectively, both parameters exhibiting significant correction postoperatively (p=0.027, p=0.035). Spinopelvic balance was restored (SS>PT) in all patients postoperatively. The L5I was constant preoperatively and postoperatively for each patient, with a mean value of 29.21°±1.61°. Table 1 summarizes the sacropelvic and spinopelvic parameters in groups A and B measured preoperatively, postoperatively, and at each follow-up. No significant change in the radiologic parameters was observed at the end of the last follow-up when compared with immediate postoperative values (p=0.71 and p=0.69 in groups A and B, respectively).
Group B demonstrated a mean LL of 55°±9.23° and a mean SA/LSK of 39.83°±8.98°. Additionally, the mean SS and PT were 27.83°±4.89° and 29.66°±6.55°, respectively. Unbalanced spinopelvic parameters characterized by PT>SS were noted in all patients. Postoperatively, the mean LL and SA were 51.66°±7.12° and 96.5°±2.44°, respectively, both parameters exhibiting significant improvement as compared to preoperative values (p=0.042 and p=0.029). The SS and PT were corrected to a mean value of 31.33°±3.01° and 12.42°±3.86°, respectively, both parameters demonstrating significant correction postoperatively (p=0.04 and p=0.048). Spinopelvic balance was restored (SS>PT) in all patients postoperatively. The L5I was constant preoperatively and postoperatively for each patient, with a mean value of 31.08°±1.31°.
Functional outcome in terms of VAS score and ODI score
Excellent functional outcome was achieved with VAS score improvement for back pain as well as leg pain postoperatively in both groups. The VAS score for back pain in group A reduced from a preoperative mean value of 7.14/10 to 3.91/10 on postoperative day 3, 1.72/10 at 6 weeks, 1.29/10 at 6 months, 1.02/10 at 1 year, 0.5/10 at 2 years, 0.21/10 at 3 years, and 0.19/10 at 4 years follow-up postoperatively. VAS score for leg pain reduced from a preoperative mean value of 8.2/10 to 3.78/10 on postoperative day 3, 1.32/10 at 6 weeks, 1/10 at 6 months, 1/10 at 1 year, 0.83/10 at 2 years, 0.43/10 at 3 years, and 0.11/10 at 4 years follow-up postoperatively. A significant reduction in pain was found for both back pain and leg pain at the end of the last follow-up (p=0.048, p=0.04).
Group B demonstrated a reduced VAS score for back pain from a preoperative mean value of 8/10 to 3.6/10 on postoperative day 3, 2.5/10 at 6 weeks, 1.5/10 at 6 months, 1.1/10 at 1 year, 0.66/10 at 2 years, 0.4/10 at 3 years, and 0.29/10 at 4 years follow-up postoperatively. VAS score for leg pain reduced from a preoperative mean value of 7.3/10 to 2/10 on postoperative day 3, 1.1/10 at 6 weeks, 1/10 at 6 months, 1/10 at 1 year, 0.83/10 at 2 years, 0.43/10 at 3 years, and 0.12/10 at 4 years follow-up postoperatively. A significant reduction in pain was observed for both back pain and leg pain at the end of the last follow-up (p=0.03, p=0.04). Table 2 summarizes the clinical outcomes in terms of VAS scores in both groups.
The preoperative median Oswestry Disability Index (ODI) score was 25.50 in group A. A significant reduction in the score was recorded first postoperatively at 6 months (p<0.05). Thereafter, the trend in reduction continued at each follow-up for up to 4 years but the difference at each follow-up did not reach a statistical significance.
The preoperative median ODI score was 28.0 in group B. This reduced at 6 months postoperatively to 13.0 (p<0.05). Further reduction in ODI score was observed at subsequent follow-ups until 4 years but did not demonstrate statistical significance. Table 3 presents the ODI scores at each follow-up for both groups.
IONM demonstrated no loss of signals in the respective exiting and traversing nerve root throughout the procedure in any of the patients. None of our patients reported postoperative paraesthetic symptoms related to exiting/traversing nerve root injury as well as events of surgical durotomy intraoperatively. No complications in terms of pseudoarthrosis were encountered. Two patients reported superficial infection of the surgical scar, which was treated with serial dressings and a short oral antibiotic course. The wound went on to heal uneventfully. Fig. 3 presents preoperative and postoperative images at follow-up of a 15-year-old girl with grade V spondylolisthesis at L5/S1, with excellent clinical outcome postoperatively. Fig. 4 illustrates the case of a 36-year-old female patient with grade IV spondylolisthesis at L5/S1, with excellent pain relief postoperatively. Fig. 5 illustrates preoperative and postoperative images of a 40-year-old female patient with grade V spondylolisthesis at L5/S1 successfully treated with MIS–TLIF technique with excellent postoperative pain relief. Fig. 6 demonstrates preoperative and postoperative radiographs of a 51-year-old female patient with grade III spondylolisthesis treated with MIS–TLIF.
Discussion
Surgical treatment has been the mainstay in HGS cases, especially for symptomatic patients with unbalanced spinopelvic parameters. The optimal approach for managing HGS involves addressing the multidirectional deformity at the LS junction while minimizing the potential for neurological complications. Posterior instrumentation, reduction and interbody fusion, posterior instrumentation, and in situ interbody fusion/posterolateral fusion, posterior transdiscal screw arthrodesis with posterolateral fusion are a few techniques that have been frequently described in literature for HGS treatment [16].
However, a debate remains to this day on whether or not to attempt complete reduction or settle for an in situ fixation with a fusion technique. In situ fusion has been a popular choice of treatment over the years due to its relative technical ease. However, literature holds evidence against this approach due to a significant incidence of pseudarthrosis, progressive slip advancement, and everlasting cosmetic deformity [17]. Spine surgeons are now increasingly engaged in achieving total or near-complete reduction of HGS due to the increasing use of pedicle screws, particularly in situations where the pelvis is unbalanced [17,18].
Extensive wide exposure of the affected segment in craniocaudal and mediolateral dimensions is highly crucial to achieving the targeted trajectory of pedicle screws. This becomes a nightmare with conventional open surgery because of the distorted anatomy, especially more so in a patient with obesity. Minimal access surgery provides the surgeon the freedom to percutaneously insert pedicle screws under fluoroscopic guidance, thereby minimizing the need for substantial local exposure and enabling the surgeon to establish an appropriate entry point for adequate medial convergence of the screws. Long convergent pedicle screws with strong purchase in the proximal vertebral body (5.5-mm or 6.5-mm diameter dual threaded screws were employed in the current series) are crucial for confidently performing the reduction maneuver without the fear of screw cut out. One needs to be mindful that a tricortical purchase (penetrating the anterior sclerotic cortical bone) in the S1 vertebra is of utmost importance to manipulate and hold the reduction intraoperatively and prevent loss of reduction postoperatively. The tubular retractor acts as a continuous guide to precisely direct the trajectory of the dome osteotomy and subsequent insertion of instruments, such as chisels, shavers, curettes, and the cage itself, once positioned in the trajectory of the targeted disc space. HGS cases, such as those that were dealt with in the present study, are pathologies with a complex pathology, and getting disoriented and losing the sense of anatomy in due course intraoperatively is not unusual.
A severely displaced L5 vertebra that is reduced and corrected involves lengthening of the LS junction [16]. Performing a dome osteotomy to shorten the LS junction helps prevent traction injury to the exiting nerve root when attempting to reduce the slipping vertebra. It is one of the key steps of the procedure. This osteotomy excises the posterior aspect of the superior end-plate of the S1 vertebra and enables entry into the degenerated and collapsed L5–S1 disc space from the inferior. The advantage is that it shortens the segment and enables maneuverability of instruments required in disc space preparation such as curettes, shavers, discectomy forceps, and so forth, to get a mechanical listhesis reduction. Additionally, shortening of the segment allows relaxation of bilateral exiting roots and the cauda equina. The trajectory of the osteotome is parallel to the inferior end-plate of L5, thereby allowing strategic placement of a tall enough cage in the disc space. The L5 vertebra gets lifted as the cage gets hammered into the well-prepared disc space, thereby aiding additional reduction of listhesis. The sequential steps have to be performed in a highly inclined trajectory due to the L5–S1 segment configuration, and getting disoriented with the anatomy is not unusual. The tubular retractor acts as a continuous guide to navigate the steps in the right trajectory, co-linear with the L5–S1 disc space in such a circumstance. Precise placement of the tubular retractor in mediolateral and craniocaudal planes helps in performing the sacral dome osteotomy right across from one side to the other, and placement of the cage right in the center of the disc space.
The authors have achieved adequate loosening of the motion segment with a unilateral approach to the disc space in all the cases so far. A bilateral TLIF method is not required unless the desired results cannot be achieved with a unilateral osteotomy. The authors believe that the entire mobility of a listhetic segment occurs at the defective pars interarticularis level. Hence, the facet joints of a listhetic segment in the case of lytic spondylolisthesis are non-functional and virgin. These joints do not develop arthritis and consequential stiffness to prevent listhesis reduction or trap exiting nerve roots.
Our results are congruent with those of Labelle et al. [18], and the success of the surgery is not only identified by the maximal reduction of the slipped vertebra but also by restoring spinopelvic and sagittal alignment and ensuring a solid fusion mass. The sagittal balance in the present series was restored in all patients with the C7 plumb line that passes within 2 cm anterior to the posterosuperior corner of the S1 endplate and SS being higher than PT. The LL was restored within the normal range (mean: 48°±2.23° and 51.66°±7.12° in groups A and B, respectively) and the SA was successfully corrected (mean: 96.5°±2.44° and 98.12°±1.04° in groups A and B, respectively). The vertical sacrum was corrected, and pelvic retroversion was reduced (indicated by the increase in SS and decrease in PT). Fusion was documented in all patients by the end of 1 year, and none of our cases reported the loss of reduction or implant-related complications.
The literature demonstrates evidence of numerous studies inclusive of grade 2 spondylolytic spondylolisthesis successfully treated with minimal access technique [9–14]. Min et al. [16] described posterior instrumentation with reduction and interbody fusion along with sacral dome resection. They conducted a retrospective review of 15 patients with HGS. These patients underwent posterior reduction and interbody fusion along with dome resection. However, theirs was a conventional open surgical approach and they advocated posterior instrumentation from L4 to S1. Our study remains unique in performing the entire procedure as a single staged surgery with mono-segmental fusion utilizing the MIS–TLIF technique, considering the sagittal profile and spinopelvic and sacropelvic balance, with no neurological injury incidence reported postoperatively. To the best of our knowledge, this is the first study to describe the unilateral dome osteotomy technique through the tubular retractor to help in loosening the disc space and aid in the seamless listhetic segment reduction.
Most spine surgeons predominantly embraced “Less is More” in their present-day practice, especially more so with the advent of technological advances such as navigation guidance systems, O-arm, and so forth. Ongoing research is crucial to continuously refine MIS–TLIF procedures as surgical tools and techniques evolve. Over the years, we have successfully endeavored to extend minimally invasive surgery to achieve posterior instrumentation, reduction, and interbody fusion as a single staged procedure in HGS cases [19].
One of our study limitations is the single-center, single-surgeon study design. The reproducibility of the results with this minimally invasive corridor may vary based on the operating surgeon’s experience, as we know that MIS–TLIF, especially in such challenging cases, involves a learning curve. We hope to conduct a multi-centric multi-surgeon study in the future with a larger sample size, considering the present study as the foundational research. This would enable us to evaluate the variability in outcomes with surgeons of differing experiences and provide a strong framework for clinical decision-making and guideline development.
Conclusions
Minimal soft tissue trauma, robust screw purchases in the proximal and distal vertebrae, easy maneuverability to achieve the desired spinal alignment, and ample bone graft placement with cage insertion are all factors that contribute to MIS–TLIF superiority in patients with HGS. It helps the surgeon in achieving an excellent clinical outcome while restoring spinopelvic balance, with mono-segmental fusion.
Key Points
Every step of the surgery is of utmost importance for correction, including the strategic patient positioning by keeping both hips and knees extended.
A long convergent trajectory of the pedicle screws in the caudal vertebra along with a robust tricortical purchase helps in the stability of the construct with easier maneuverability for a successful listhesis reduction.
Dome osteotomy at S1 facilitates the shaver and curette insertion and cage positioning with lesser chances of neurological injury.
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