Minimally invasive transforaminal lumbar interbody fusion for scoliosis: surgical technique for focal treatment of the lumbosacral fractional curve

Introduction

Minimally invasive transforaminal lumbar interbody fusion (MIS-TLIF) can be used in the treatment of both adult spinal deformity and lumbar degenerative disease. The umbrella term minimally invasive surgery (MIS) encompasses a broad range of surgical techniques. The MIS approach in general is beneficial for patients with medical comorbidities for which decreased muscular dissection and blood loss may be ideal (1). Multiple studies have shown comparable outcomes for disability, pain, and quality of life between MIS-TLIF and open TLIF (2-4). Recent evidence emphasizes the importance of the lumbosacral fractional curve as a driver of symptomatology in patients with scoliosis (5,6). Often the concavity of a coronal fractional curve creates foraminal stenosis that can lead to pain and radiculopathy. This may lead to intolerable pain that drives patient discomfort, decreased quality of life, and the decision to pursue surgical intervention. Herein, we outline our surgical approach for MIS-TLIF when applied to treating the symptomatic fractional curve. We present this article in accordance with the SUPER reporting checklist (available at https://jss.amegroups.com/article/view/10.21037/jss-24-127/rc).

Preoperative preparations and requirements

The utility of an MIS-TLIF approach targeted at correcting the lumbosacral fractional curve relies on appropriate patient selection. All patients will require a history and physical examination to confirm the specific pain generator. Imaging is required and includes long cassette anteroposterior (AP) and lateral radiographs, lumbar dynamic radiographs, magnetic resonance imaging (MRI), and computed tomography (CT) of the lumbar spine. In post-menopausal women or patients with risk factors, a dual-energy X-ray absorptiometry scan may be obtained to assess bone quality.

Patients who are determined to be symptomatic from the fractional curve (e.g., L4–S1 radiculopathy along the concavity of the fractional curve) and who have a fractional curve Cobb angle >10 degrees may be suitable candidates for an MIS-TLIF approach.

Illustrative case

The procedure performed in this study was in accordance with the ethical standards of the institutional and/or national research committee(s) and with the Helsinki Declaration (as revised in 2013). Written informed consent was obtained from the patient for publication of this article and accompanying images. A copy of the written consent is available for review by the editorial office of this journal.

We describe a case of a 72-year-old with right-sided lumbar radiculopathy and dorsiflexion weakness. His preoperative imaging was notable for lumbosacral transitional anatomy with the left L5 transverse process fused to S1 on the left (Figure 1). His MRI demonstrated clear impingment of the L4 nerve root due to the fractional curve (measuring 10 degrees) with up-down foraminal stenosis at L4–5 and lateral recess stenosis at L3–4. Multiple surgical options were reviewed, and the decision was made to proceed with an MIS L3–5 posterior instrumented fusion, right-sided L3–4 hemilaminotomy with medial facetectomy, and L4–5 right-sided MIS-TLIF. He was discharged on postoperative day 3 without complication, and postoperative full-length plain radiographs demonstrated correction of his fractional curve. He had complete resolution of his radiculopathy and weakness at 1-month follow-up. At 12-month follow-up, he remained symptom-free.

Figure 1 Preoperative full-length AP (A) and lateral (B) radiographs, along with preoperative lumbar AP (C) and lateral (D) radiographs. Postoperative full-length AP (E) and lateral (F) radiographs, along with postoperative lumbar AP (G) and lateral (H) radiographs. Intraoperative fluoroscopic AP (I) and lateral (J) images demonstrate horizontalization of the L4–5 segment, an increase in disc/foraminal height, and an increase in segmental lordosis. AP, anteroposterior.

Step-by-step description

The operative procedure is depicted in Video 1.

Patient positioning and localization

The patient was positioned prone on the Jackson table. The surgical site was then sterilely prepped and draped. A preoperative timeout was carried out with the entire operating room staff to verify the correct patient, surgical site, and procedure.

Placement of posterior superior iliac spine pin, O-arm spin for navigation

A navigation array was placed into the right iliac bone, seated just into the sacrum, via an in-out-in technique. An intraoperative mobile CT was then performed for navigation. First, a navigation wand was used to plan screw trajectories. After the six screw trajectories were planned, each bisecting the skin at 3.5 cm from the midline, an incision was made with a #10 blade. Bovie electrocautery was used to create a separate fascial opening for each respective screw.

Pedicle screw placement across fusion levels

Screws were placed in the following manner. Starting at the most rostral level, furthest from the navigation array, the navigation wand was used to identify a pedicle screw trajectory. Then, a fascial cut using a 10-blade is made in line with this screw trajectory. After this, a navigated drill with a drill guide set to 35 mm is used to make an initial pedicle screw entry point. A K-wire is placed to maintain access to the bony channel, and then an undersized tap is advanced over the K-wire. Following this, the pedicle screws are placed over the K-wire. For most MIS-TLIF cases, a 22 mm tubular retractor can be used. In cases where MIS-TLIF is used to treat the lumbosacral fractional curve, we use a pedicle screw-based retractor system so that its distractive force can increase access to the collapsed endplate along the fractional curve concavity. After screw placement, an O-arm intraoperative CT was obtained to confirm the appropriate location of the six screws.

Facetectomy

The pedicle screw-based retractor system is only gently distracted to help minimize the risk of screw cut-out and damage to the pedicle (Figure 2A). Of note, further safe distraction can be performed after the discectomy is initiated and while a trial or shaver is used to simultaneously distract the intervertebral disc space itself. To facilitate performance of the TLIF, a right-sided laminotomy and facetectomy are necessary. First, an inferior facetectomy is performed. With use of the microscope, the right L4–5 lamina-facet junction is exposed. An L-cut is made using a high-speed drill: a horizontal cut just below the L4 pedicle followed by a vertical cut in the midline. Following this, a straight osteotome is used to disconnect the inferior facet. This bone graft is saved for autograft. After this, a superior facetectomy is made. This is done first by making a horizontal cut just rostral to the L5 pedicle, followed by use of an osteotome to disconnect the superior facet. The superior facet is also saved for autograft. Next, a Kerrison rongeur is used to remove the ligamentum flavum. The thecal sac, traversing, and exiting nerve roots are identified and determined to be fully decompressed. A Woodson helps check to see if the L5 pedicle is appropriately skeletonized—this ensures there is enough room for placement of the cage. Sometimes, it may be required to intentionally drill into the L5 pedicle to increase the surgical corridor for the TLIF.

Figure 2 Equipment and operative considerations for TLIF. (A) The pedicle screw-based retractor system. (B) The green region illustrates the wide lateral to medial working angle used with the up-angled curette to perform the contralateral discectomy. The red region highlights the dorsal-contralateral “TLIF blind spot” that requires extra attention during discectomy. The arrows indicate the deep to superficial motion of the curette to ensure the ALL is not violated. (C) Instrument used to compress rods during TLIF. TLIF, transforaminal lumbar interbody fusion; ALL, anterior longitudinal ligament.

Transforaminal lumbar interbody cage placement

After the disc space is exposed, a 15-blade is utilized to make a large box annulotomy. Initial shavers are used and only rotated gently to scrape the endplate. Aggressive, repeated rotation is avoided to mitigate the risk of endplate fracture. Instead, custom straight and up-angled curettes with serrated edges are used to remove the cartilaginous endplate. The up-angled curette is passed in vertical rows, systematically from deep to superficial, such as when mowing a lawn. This permits efficient and thorough discectomy. Starting at the depth of the disc space permits the blunt side of the curette to identify the anterior longitudinal ligament (ALL) on each pass so that (I) the ALL is not violated and (II) disc material is not inadvertently left next to the ALL. Navigation may also assist with confirming the location of the ALL and ensuring an adequate discectomy.

A wide lateral to medial angle is used with the up-angled curette to perform the contralateral discectomy. The dorsal-contralateral aspect of the intervertebral disc is often the area where disc material is insufficiently removed (“the TLIF blind spot”). Thus, great care is taken in this quadrant of the disc space.

The loosened disc and cartilaginous endplates are removed with a straight and up-angled pituitary rongeur. Again, great attention should be paid to the dorsal-contralateral aspect of the disc space with the up-angled rongeur (Figure 2B). After sufficient discectomy, a straight trial can be passed into the disc space to distract the disc space. This new position can be held open by slightly expanding the pedicle-based retractor. The screws are protected from plowing out of the pedicles by load sharing with the trial. This is a critical step in fractional curve TLIF for scoliosis given that the asymmetric collapsed portion of the disc space will need to be sufficiently opened to accommodate the ultimate cage and avoid (I) an asymmetrically contralaterally placed cage (which may insufficiently horizontalize the fractional curve); (II) a narrow TLIF cage that has higher risk of graft subsidence and pseudarthrosis; and (III) a short TLIF cage that does not result in enough foraminal height restoration and lordotic correction.

A trial is then passed to ensure that the collapsed, non-expanded banana-style TLIF cage will be able to enter the disc space. A wide banana-style, expandable TLIF cage is preferred by the surgeon (A.K.C.) because of several advantages. Firstly, the wide graft size increases the endplate contact for arthrodesis. Secondly, the banana configuration places the cage along the strong ring apophysis of the vertebral endplate and helps to prevent subsidence. Thirdly, anterior placement of the cage, with the entirety of the cage in front of the segmental axis of flexion/extension, ensures that the TLIF induces lordosis. This is to be contrasted by straight “bullet” type TLIF cages which are often unable to be placed as anterior, limiting the anterior fulcrum to maximize lordosis. Furthermore, a majority of the cage can be placed on the ring apophysis—avoiding the weaker central vertebral body endplate. Lastly, an expandable cage is preferred, as this maximizes cage compression onto the endplate—important for fusion given Wolf’s law—and avoids graft migration. Since open TLIF-style rod compression is not possible, the expandable cage ensures optimal cage-endplate loading. Additionally, increases in anterior disc height will result in an increase in segmental lordosis. Additionally, increases in anterior disc height—ventral to the segmental axis of rotation—will result in an increase in segmental lordosis.

Midline graft placement is crucial to maximize segmental horizontalization and correct the fractional curve. Asymmetric placement may result in unintended coronal tilt of the segment. The cage can bias toward the convexity of the fractional curve, so care must be taken to adjust for this bias throughout the discectomy, trialing, and cage placement phases. The cage often follows where the trialing occurred; hence, imprecise trialing can lead to suboptimal cage placement. In other cases, asymmetric placement or use of an asymmetric graft may be preferred if the vertebral body itself is asymmetric in the coronal plane or if global coronal alignment requires asymmetric correction of the fractional curve. The Qiu or Obeid classification schemes may be used to guide the surgeon in this regard (7,8).

Once the distraction is released, and with the aid of gentle manual pressure on the back of the lumbar spine, lordosis can be further induced before rodding. This is aided by the lower placement of the hip pads on the open bottom Jackson table. In appropriate cases, a contralateral facetectomy may be performed to further induce lordosis. Manufacturers also have designed instruments to help to compress rods during MIS-TLIF (Figure 2C).

Facet decortication and packing

A second MIS-TLIF is an alternative option at L3–4 as well. However, given that the patient only required a lateral recess decompression (i.e., hemilaminotomy with medial facetectomy) at L3–4, did not have instability at L3–4, had a reasonably well-maintained disc space at L3–4, and had pre-existent cephalad lumbar degeneration, a facet fusion at L3–4 with only instrumentation to L3 is completed for a softer transition into the unfused, but degenerated, L2–3.

The additional L3–4 facet joints are visualized with the use of the 18-mm METRx tube. This is completed efficiently, leveraging CT-based intraoperative navigation. Once the intra-articular joint space is exposed with monopolar cautery, a high-speed burr is utilized to denude the superior and inferior articulating facets under direct microscopic visualization. After this is completed, graft can be packed into joint space. Care is taken to ensure the graft is tightly packed into the joint with the aid of a tamp and mallet. Leveraging Wolf’s law maximizes the opportunity for facet fusion.

Closure

Hemostasis is obtained. Vancomycin powder is placed. Then, the fascia is closed with a 0 Vicryl, the deep dermal layer with a 2-0 Vicryl, and the subcuticular closure with a 4-0 Biosyn suture. Skin glue is applied over both incisions. Telfa and Tegaderm are placed over the incision.

Postoperative considerations and tasks

Depending on surgeon comfort, a surgical drain can be left in place. No surgical brace is used. The patient is mobilized on the day of surgery. The Foley catheter is removed the morning after surgery. Pharmacologic deep venous thrombosis prophylaxis was started at 9 pm on postoperative day 1. A multimodality pain regimen is provided to the patient including around-the-clock acetaminophen, a muscle relaxant, a neuropathic pain medication, a lidocaine topical patch, and opioid medication only for breakthrough pain. Patients are discharged after a successful void and inpatient physical therapy evaluation.

Postoperative course

Within a few days of surgery, the patient’s right lower extremity radiculopathy resolved. The surgical site pain was tolerable without any medications. He was able to walk several hours a day. At 12-month follow-up, the patient was pain-free and had returned to baseline activities including swimming.

Tips and pearls

A few additional items should be discussed when considering MIS-TLIF for the treatment of the fractional curve. Firstly, patients with coronal imbalance with the trunk shifted toward the convexity of the fractional curve will be the most difficult to treat with a targeted-fractional curve only surgery. This situation, classified as an Obeid 1 coronal malalignment, will result in the worsening of global coronal imbalance unless the main lumbar curve is also addressed (Figure 3). The Obeid classification, proposed by Obeid et al. in 2019, builds on a previous categorization by Bao et al. of coronal malalignment based on convexity or concavity of the main curve, by adding specifications to guide surgical recommendations for adult spinal deformity patients (7,8). The value of this classification has been demonstrated in the deformity literature (9,10). Secondly, the tendency may be to add additional interbody levels to increase fractional curve correction. Although studies do show that interbody placement can improve curve correction and rates of fusion, placement also has risks such as nerve retraction, dorsal root ganglion injury, and increased operative time. Accordingly, focus may be placed on levels with instability. For example, in the case illustration, the patient had an L4–5 spondylolisthesis in addition to fractional curve scoliosis. However, L3–4 did not have spondylolisthesis nor disc space asymmetry. Because of this, a bilateral facet joint fusion alone was considered—in addition to instrumentation from L3 to L5. Furthermore, there is a concern about added stiffness with an anterior interbody graft at L3–4, especially when trying to mitigate the risk of adjacent segment degeneration at L2–3.

Figure 3 Representation of an Obeid type 1 coronal malalignment (A,B), where isolated revision of the fractional curve may worsen coronal alignment unless the main lumbar curve is also corrected. (B) The arrows indicate the rotation of the superior portion of the radiograph, illustrating the hypothetical outcome after correction of the fractional curve alone. The blue line indicates the C7 plumb line, and the red line represents the offset of the C7 plumb line from the central sacral vertical line.

Another critical consideration is selecting the side for MIS-TLIF cage insertion. The factors mentioned above—including anatomical considerations of stenosis, spine stiffness, and fractional curve correction—should be considered. The side with worse lateral recess or foraminal stenosis is often the cage insertion side for the MIS-TLIF. This is because the approach—by definition—involves a direct decompression via facetectomy. Either bullet- or banana-style cages can be used for this purpose. Asymmetrically placed bullet-style cages may be more easily placed ipsilateral to the fractional curve concavity. Banana-style cages can be placed from either the convexity or the concavity. However, banana-style cages tend to find the path of least resistance and have a predilection to migrate towards the convex side of the disc space. For this reason, the surgeon should intentionally aim ipsilateral on the concave side to avoid asymmetric placement in the disc space. On the other hand if approaching from the convexity, the surgeon may have to intentionally aim more contralaterally to avoid asymmetric placement in the disc space. Finally, if the fractional curve is too stiff or the Cobb angle is marked—then placement on the convex side may be preferred, as Kambin’s triangle may be too small for cage insertion on the concave side.

Discussion

In general, open-TLIF and MIS-TLIF have similar outcomes (2,11). Most studies comparing the two in a retrospective fashion show MIS-TLIF is associated with less blood loss but longer operative times (12-14). The benefits of MIS-TLIF are highlighted in specific patient populations, i.e. obese patients. In this population, MIS-TLIF is associated with decreased blood loss, decreased length of hospitalization, decreased rates of postoperative wound infections, and even a lower incidence of dural tears (12). Fusion rates between open and MIS approaches are comparable, with modern studies showing fusion rates greater than 90% in both groups (15-17). Interestingly, MIS-TLIF is associated with decreased rates of adjacent segment disease and reoperation in multiple studies, likely stemming from decreased muscular dissection and minimal disruption of the posterior ligamentous complex (18). MIS-TLIF and open TLIF have comparable correction of radiological parameters such as lumbar lordosis, and pelvic tilt (19,20).

Under the broad umbrella of MIS-TLIF, there exist many technical variations. Herein, we highlight an MIS-TLIF approach that prioritizes correction of the lumbosacral fractional curve and uses O-arm navigation for pedicle screw placement, facet decortication and arthrodesis, and graft placement. MIS-TLIF approaches can lead to improved outcomes including improvements in hospital length of stay, blood loss, and decreased intraoperative/postoperative complications (1,5). We will highlight two keys to our operative technique: correction of the lumbosacral fractional curve and posteromedial fusion through facet decortication.

Importance of the fractional curve correction

A growing body of literature has identified the lumbosacral fractional curve as the key driver of patient symptoms in patients with lumbar degenerative disease. Patients symptomatic from their lumbosacral fractional curve via up-down foraminal stenosis on the side of the concavity will have radicular leg complaints ipsilateral to the concavity. The condition is disabling because upon standing, the neural foramen further collapses. The sensitive dorsal root ganglion is the structure compressed by the pedicle, leading to disabling pain. Given that restoration of disc height and increasing interpedicular distance is key to decompressing the nerve, a posterior decompression alone—without restoration of foraminal height—often insufficiently manages the patient’s complaints. Multiple studies have shown that correction of the fractional curve—by horizontalizing the fractional curve and increasing disc height via an interbody is associated with improvements in Oswestry Disability Index (ODI) and Visual Analogue Scale (VAS) outcomes (6,21). Even in long-segment scoliosis correction, failure to correct the lumbosacral fractional curve is associated with worse postoperative VAS back scores, highlighting the importance of fractional curve correction (22). Chou et al. highlighted this finding in comparing circumferential MIS surgery with open surgery in adult scoliosis correction (23). They found that circumferential MIS techniques could improve the lumbosacral fractional curve to a similar degree as open approaches and led to similar improvement in ODI and VAS leg/back symptoms. Importantly, the same group then studied isolated fractional curve correction in the adult scoliosis population. They found that in patients with a fractional curve of >10 degrees and radicular symptoms ipsilateral to the concavity, isolated fractional curve correction—in contrast to long-segment fixation (spanning the upper or lower thoracic spine to the sacrum)—was associated with fewer medical complications, shorter hospital length of stay, decreased blood loss and even decreased rates of discharge to rehabilitation (5). Notably, studies have also shown that TLIF and anterior lumbar interbody fusion (ALIF) cages can provide similar correction of lumbosacral fractional curves, showing that larger, more lordotic cages do not necessarily provide greater benefit depending on surgical technique and patient selection (24,25).

The importance of facet joint fusion

Long before MIS techniques, many studies highlighted the importance of posterior fusion in conjunction with interbody fusion for treating lumbosacral degenerative disease and achieving arthrodesis (26,27). The thought was that 20% of spinal load bearing occurred through the facet joints and posterior ligamentous complex, and 80% occurred through the vertebral body. Before interbody fusion was widely accepted, posterolateral fusion was often done without interbody fusion due to the increased neurological risk associated with pedicle screw fixation and interbody placement (26,27). Improvements in technology and technique have led to the acceptance of interbody placement, posterolateral fusion, and pedicle screw fixation as a common practice.

Posteromedial fusion through the facet joint is an important operative detail. Numerous studies have shown no significant differences in functional or clinical outcomes between posterior fusion alone versus with interbody, highlight a role for posterior fusion alone (28-32). This finding is likely driven by two key factors. First, motion through the facets at symptomatic levels can be a crucial pain generator for patients. Therefore, by limiting motion with a fusion—regardless of surgical technique—should help decrease pain. Second, although authors demonstrate high rates of fusion through the interbody, posterior arthrodesis—if performed meticulously—can also result in successful fusion. Biomechanical studies conducted in sheep demonstrated similar stiffness of facet fusion constructs (posteromedial fusion) versus posterolateral fusion across the transverse processes (33). Although stiffness was comparable, posteromedial fusion through facet joints had a 100% rate of fusion compared with a 50% rate of fusion seen with posterolateral fusion in this ovine model.

To this end, an MIS approach supplemented with posteromedial arthrodesis may decrease the need for revision surgery while also capturing the benefits of the MIS technique such as decreased hospital length of stay, surgical blood loss, and medical complications. We achieve posteromedial fusion through the facet joint through facet decortication and packing with bone morphogenetic protein and local bone autograft. In this case illustration, since the L3–4 level was without listhesis and disc height collapse, a facet fusion offers a reasonable alternative while avoiding the risks of cage placement.

Conclusions

With appropriate patient selection, this technique of MIS-TLIF can offer the benefits of an MIS approach (e.g., decreased musculoligamentous injury, less blood loss) with the benefits of a posteromedial fusion construct. Taken together, our experience and the literature show that a targeted MIS-TLIF approach with prioritization of fractional curve correction can effectively treat symptoms in patients with scoliosis.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the SUPER reporting checklist. Available at https://jss.amegroups.com/article/view/10.21037/jss-24-127/rc

Peer Review File: Available at https://jss.amegroups.com/article/view/10.21037/jss-24-127/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-24-127/coif). D.C. serves as an unpaid Editorial Board Member of Journal of Spine Surgery from December 2024 to December 2026. D.C. also reports consulting fees from Medtronic and Royalty from Globus. A.K.C. has received a grant from the Cervical Spine Research Society, consulting fees from Alphatec Spine, Inc., Isto Biologics, and honoraria from Spineart. The other 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 ethical standards of the institutional and/or national research committee(s) and with the Helsinki Declaration (as revised in 2013). Written informed consent was obtained from the patient for publication of this article and accompanying images. A copy of the written consent is available for review by the editorial office of this journal.

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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