Stand-alone L5–S1 transdiscal screw fixation and direct foraminal decompression as a minimally invasive fusion method in high grade isthmic spondylolisthesis: technical note and case series

Introduction

Lumbosacral spondylolisthesis can cause significant back and/or lower extremity radicular pain associated with standing and walking. Isthmic spondylolisthesis is the anterior translation of one vertebra relative to the one caudal due to a defect or abnormality in the pars interarticularis (1). Surgical management of high-grade L5–S1 isthmic spondylolisthesis is challenging, and no consensus exists on the most effective approach. A variety of techniques and fixation strategies have been reported, including posterior instrumented fusion with pedicle screw fixation, anterior fusion, circumferential fusion, and transdiscal pedicle screw fixation as the inferior part of the instrumentation construct (2-5). Whichever treatment strategy is employed, the goal is the same: to provide enough stability to achieve fusion despite the high mechanical forces seen at the lumbosacral junction.

The incorporation of transdiscal fixation as part of the instrumentation construct across L5–S1 in the setting of high-grade isthmic spondylolisthesis has shown promising results, both biomechanically and clinically. Transdiscal fixation—which provides three-column stabilization—is nearly twice as stiff as pedicle screw fixation, potentially improving fusion rates (2). In addition, a case-control study comparing the incorporation of transdiscal screw fixation versus posterolateral fusion and pedicle screw fixation among patients with Grade III spondylolisthesis or higher found that patients undergoing transdiscal screw fixation as the inferior part of the construct in conjunction with standard pedicle screws proximally without interbody graft had greater improvements in functional outcomes after surgery (3). Similarly, patients treated with the incorporation of transdiscal fixation without interbody graft had faster improvement of radicular pain and return to daily activities compared to those with posterolateral interbody fusion (PLIF) and pedicle screw fixation (4). Reports of mid-term and long-term outcomes of this method without interbody graft have demonstrated favorable results (6,7).

Multiple variations of this technique exist. The original description by Bohlman was before the advent of pedicle screw instrumentation and involved the transdiscal placement of a fibular strut graft (8). Grob was the first to describe transdiscal fixation using standard 6.5 mm partially threaded cancellous screws without transdiscal graft placement (9). and others have described satisfactory results from this technique (10). There are no reports on the treatment of high-grade L5–S1 isthmic spondylolisthesis with isolated transdiscal fixation using standard 7.3 mm cannulated screws and the technique of resection of the superolateral aspect of the sacrum, which is useful to address symptomatic vertical foraminal stenosis. The purpose of this study is to present the surgical technique of foraminal decompression for vertical foraminal stenosis and stand-alone transdiscal fixation with fully threaded 7.3 mm cannulated screws for grade 2 or 3 L5–S1 spondylolisthesis, our experience with this technique, and the results from five consecutively treated patients. We hypothesize that this technique is a safe and effective minimally invasive method to treat high-grade isthmic spondylolisthesis. We present this article in accordance with the SUPER reporting checklist (available at https://jss.amegroups.com/article/view/10.21037/jss-25-18/rc).

Preoperative preparations and requirements

Study design and participants

All procedures performed in this study were in accordance with the Stanford University Medical Center Institutional Review Board (#7935) and with the Declaration of Helsinki and its subsequent amendments. Publication of this article and accompanying images was waived from patient consent according to the Stanford University Medical Center Institutional Review Board.

Patient demographics, complications, revisions, Visual Analog Scale (VAS) back or leg pain and Oswestry Disability Index (ODI) scores were collected. VAS scores were collected preoperatively, then at 6 weeks, 3 months, 6 months, and 12 months postoperatively. ODI scores were collected preoperatively, then at 3, 6, and 12 months postoperatively. Preoperative and postoperative VAS and ODI scores were compared. Radiographic fusion was assessed at last follow-up as previously described. Fusion was described as radiographs demonstrating Lenke grade A or B for either anterior or posterior fusion grade at L5–S1. All patients had standard upright anteroposterior radiographs as well as flexion/extension lateral radiographs pre-operatively (11). All patients additionally had an MRI of the lumbar spine to assess the degree and location of stenosis. Preoperative anterior translation, Meyerding classification, and difference in flexion/extension anterolisthesis were collected from preoperative radiographs by two spine surgeons (C.R.J., T.F.A.), and any disagreements were resolved via consensus. Due to debilitating and persistent symptoms in the setting of acceptable sagittal balance, each patient was indicated for decompression and in situ instrumented fusion of the L5–S1 segment.

Step-by-step description

Cases were performed in the operating room of a tertiary hospital. All patients were positioned prone on a Mizuho OSI ProAxis (Union City, CA, USA) table. Neuromonitoring was not routinely used. A standard posterior approach via a midline incision was used to expose the posterior bony elements of the L5–S1 segment, including the entirety of the bilateral L5 transverse processes and sacral ala. In patients with radiculopathy, foraminal decompression was performed first. The ProAxis table was placed in a slightly flexed position to facilitate decompression. The superior portion of the L5/S1 facet joint was resected down to the superior aspect of the sacrum, which was then resected proximal to the S1 pedicle and posterior to the L5/S1 disc space to a point medial to the medial border of the S1 pedicle. The L5 nerve root was identified and then further decompressed by resecting the proximal part of the pars defect flush to the lower portion of the L5 pedicle and by completing the resection of the posterior aspect of the sacrum and L5/S1 disc-osteophyte complex from underneath the L5 nerve (Figure 1). This was done with the combination of a Midas-Rex (Medtronic, Minneapolis, MN, USA) drill, a spinal osteotome, and a Kerrison rongeur. The L5 nerve root was traced along its route and any remaining dorsal compression was removed with Kerrison rongeurs. At the completion of decompression, a ball tip probe was easily passed from a point medial to the L5 pedicle and then along the L5 nerve root through the foramen. The L5–S1 disc space was identified, partial discectomy was performed, and the space was packed with morselized autograft. The flexion was removed from the ProAxis table for the fusion portion of the procedure. Somewhat more inferior start points than usual for S1 pedicle screws were identified and a pedicle probe was used to cannulate a path across the L5-S1 disc space, angling approximately 5–10° medially into the L5 vertebral body under fluoroscopic guidance, such that it crossed the disc space just anterior to its posterior margin to allow for the most purchase possible into the L5 vertebra. A threaded 2.8 mm guide wire was then placed in each transdiscal path and replaced with a fully threaded stainless steel Synthes (DePuy Synthes, Warsaw, IN) 7.3 mm cannulated screw with a washer. This was again confirmed with biplanar fluoroscopy. The L5 transverse processes and sacral ala were decorticated and a locally obtained bone graft was placed. These steps were performed in a relatively standard fashion as has been previously described, under fluoroscopy guidance (8). Representative preoperative and postoperative radiographs are shown in Figure 2.

Figure 1 Foraminal decompression technique involving resection of posterior sacrum and disc-osteophyte complex. The figure was generated by a medical illustrator commissioned by the authors.

Figure 2 Representative preoperative (A-D) standing lumbar radiographs and postoperative (E,F) radiographs.

Postoperative considerations and tasks

Standard postoperative recovery for lumbar fusion was performed with admission to the ward and monitoring until medically ready for discharge. Bracing was not used in the postoperative period. Standing postoperative anteroposterior and lateral lumbar radiographs were obtained before discharge.

Statistical analysis

Descriptive statistics were analyzed as means and ranges for continuous variables, and frequencies and percentages for categorical variables. All analyses were performed using Microsoft Excel (Redmond, WA, USA).

Tips and pearls

Patient cohort

The mean age of the five patients in this series was 81.6 (range, 69–93) years. There were 2 (40%) female patients and 3 (60%) male patients. All patients had both pre-operative back and lower extremity pain. All patients had pre-operative MRI scans of the lumbar spine and were found to have severe bilateral foraminal stenosis at L5–S1. Three (60%) patients had a previous L5–S1 decompression using the foraminal decompression technique described above without a fusion performed by the same surgeon. Previous decompression was performed 14 months prior to revision decompression with in-situ non-instrumented fusion in one patient and 7 years (84 and 83 months) prior in two other patients. These three patients had the return of both lower extremity pain and back pain after initially doing well after their index decompression, indicating them for revision surgery. The five patients in this series had a mean post-operative follow-up of 18 months with a minimum of 12 (range, 12–24) months (Table 1).

Clinical outcomes

Mean pre-operative ODI was 46 (range, 26–60). Mean postoperative ODI was 26 (range, 4–51). Mean pre-operative VAS was 8 (range, 6–9), which improved to mean 6-week VAS of 3 (range, 0–8), mean 3-month VAS of 1 (range, 0–4), and mean 6-month VAS of 1 (range, 0–4). Mean 12-month VAS was 0.6 (range, 0–3).

Radiographic results

Mean preoperative upright anterior translation was 18 (range, 15.6–20.5) mm. Four patients were qualified as Meyerding grade two spondylolisthesis and one patient was a Meyerding grade three. Mean difference in L5–S1 anterolisthesis between flexion and extension was 4.4 (range, 3.5–5) mm.

Complications and revisions

Mean operative time was 164 (range, 124–233) min. No patients in this series had any reported complications or returned to the operating room. All five patients had a minimum of 6-month post-operative radiographs. No implant complications were noted. All patients achieved solid radiographic fusion at the last follow-up.

Discussion

Transdiscal fixation is a promising technique for treatment of lumbosacral spondylolisthesis and may have favorable outcomes relative to interbody fusion and pedicle screw fixation. Performing the surgery with transdiscal fixation alone without the use of proximal pedicle screws has several potential advantages: involving less operative exposure; less operative time; and no disruption of the proximal facet joint (inherent to some extent in the placement of the proximal pedicle screw), which may be important in minimizing the likelihood of proximal junctional problems after fusion. With one exception, previously described techniques for transdiscal fixation have included pelvic screws and/or cranial pedicle screws (8,12). In this study, we describe our surgical technique for foraminal decompression and standalone L5–S1 transdiscal screw fixation for patients with high-grade isthmic spondylolisthesis using two fully threaded 7.3 mm cancellous screws. We also describe the clinical outcomes of a case series of patients treated with this relatively lower cost technique.

In a model of high-grade spondylolisthesis, transdiscal fixation without interbody graft had a lower range of motion (ROM) in extension than the Bohlman technique (fibular strut across L5/S1) and modified Bohlman technique (titanium mesh cage across L5/S1), but comparable bending and torsional ROM (13). The incorporation of an inferior transdiscal fixation with L5 pedicle screws without interbody is also 1.6 to 1.8 times stiffer than an L5/S1 traditional pedicle screw construct in all planes, but comparable to anterior lumbar interbody fusion (ALIF) with pedicle screw fixation (2). Additional biomechanical studies of standalone transdiscal fixation have found no difference in ROM or stiffness in any plane between transdiscal screw and pedicle screw constructs alone (14,15). There was also no difference in load to failure (14), but standalone transdiscal fixation had greater ROM and lower stiffness in flexion-extension than transforaminal interbody fusion (TLIF) with pedicle screw fixation (15). These biomechanical studies suggest the feasibility of isolated transdiscal screws as a mode of fixation for spondylolisthesis, which was further supported by our clinical results.

In-situ transdiscal fixation should be reserved for patients with acceptable sagittal balance, as those with spinopelvic imbalance should undergo reduction while increasing segmental lordosis. However, reduction of lumbosacral spondylolisthesis does carry a potential risk of L5 nerve root injury (16). Previous studies incorporating transdiscal fixation have shown favorable results with in situ fusion (4,12). On the other hand, some studies performed partial reduction with transdiscal fixation with good results (3,17). We further limited this technique to patients with high grade isthmic spondylolisthesis in the setting of near-complete collapse of the L5/S1 disc space as these patients are typically inherently more stable than patients with relatively more normal disc height. In our series, in situ fusion was performed in all cases and fusion was achieved in each instance, without the adjunctive use of bracing.

Long-term results of L5/S1 pedicle screw fixation incorporating an inferior transdiscal screw without interbody graft, as far as 5 years postoperatively, have demonstrated large improvements in pain and functional outcomes (7,10). Although this technique was initially described for adult spondylolisthesis (9,18), it has also been shown to be effective for management of adolescent spondylolisthesis and thoracolumbar pathological fracture (19,20). Comparative studies have shown the use of this method without an interbody graft had greater postoperative improvements in functional outcomes relative to pedicle screw fixation (3), and faster improvement of pain and return to daily activities relative to PLIF and pedicle screw fixation (4). Another study demonstrated comparable outcomes between partial reduction with transdiscal fixation and TLIF with posterolateral fusion for lumbosacral spondylolisthesis (21). While transdiscal screw insertion is more technically challenging than traditional pedicle screws, intraoperative navigation and robot-assisted surgery can facilitate this technique, particularly for minimally invasive approaches (22-24). The much more limited soft tissue dissection and smaller incision required for this technique, brought about by obviation of the proximal pedicle screw, less disruption of the anatomy compared to the traditional reduction and posterior pedicle screw approach, qualify the technique described here as a minimally invasive approach for this complicated anatomic pathology.

Limitations of this case series include its small sample size and lack of long-term follow-up. The retrospective nature of data collection limited the reliable collection of functional outcomes data. Furthermore, we did not routinely obtain postoperative computed tomography (CT) scans for these patients, so while radiographic fusion was determined on AP and lateral lumbar radiographs (XR), it was not confirmed on advanced imaging. Nonetheless, this is the first study, to our knowledge, to describe the use of standard stainless steel 7.3 mm cancellous screws for the management of isthmic spondylolisthesis as well as provide a detailed description of the decompression surgical technique. Additionally, we described the outcomes and complications of a case series of patients who underwent this technique. We have found this technique to be useful in elderly patients with high grade isthmic spondylolisthesis, a relatively collapsed disc space, and acceptable sagittal balance. Further studies are needed to determine long-term and functional outcomes of this technique. Additional comparative studies are needed to investigate the differences, if any, between outcomes of standalone transdiscal fixation versus adjunct pedicle screw and/or pelvic screw fixation.

Conclusions

Direct foraminal decompression and standalone transdiscal screw fixation with grafting is a safe and useful minimally invasive method of achieving fusion for patients with high grade isthmic spondylolisthesis, relatively collapsed disc space, and acceptable sagittal balance.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the SUPER reporting checklist. Available at https://jss.amegroups.com/article/view/10.21037/jss-25-18/rc

Peer Review File: Available at https://jss.amegroups.com/article/view/10.21037/jss-25-18/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://jss.amegroups.com/article/view/10.21037/jss-25-18/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All procedures performed in this study were in accordance with the Stanford University Medical Center Institutional Review Board (#7935) and with the Declaration of Helsinki and its subsequent amendments. Publication of this article and accompanying images was waived from patient consent according to the Stanford University Medical Center Institutional Review Board.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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