Robotic-Assisted Uniportal Full-Endoscopic Transforaminal Lumbar Interbody Fusion: A Technical Note on a Hybrid Form of Minimally Invasive Surgery

1. Indications

Indications for robotic-assisted FE-TLIF are degenerative spinal conditions with spinal instability. A standard workup for lower back pain to rule out other causes and the failure of conservative management after 6 months are routine prerequisites for surgery at our institution, but this varies on a case-by-case basis.

2. Instruments

Standard uniportal endoscopic instruments, including the uniportal endoscope, iLESSYS Delta system, radiofrequency (RF) probe, endoscopic drill, rongeurs and graspers (Joimax, Germany) were required. Standard C-arm fluoroscopy and a Jackson spine table were also required. We performed this operation under general anesthesia, and intraoperative neuromonitoring is imperative. A surgical robot, specifically the Mazor X Stealth Edition Robotic Guidance System (Medtronic Inc.), was used for robotic assistance during the surgery.

3. Room Setup and Patient Positioning

The patient is first brought in, anaesthetized under general anesthesia and then turned prone onto the radiolucent Jackson table (Mizuho OSI, USA). Intraoperative neuromonitoring is set up. The patient is positioned prone to open the lumbar interlaminar space and foramen. The Mazor X robot is then positioned caudally and contralaterally relative to the cage insertion site. Image intensifiers (namely, C-arm fluoroscopy) and endoscope video equipment are set up on the opposite side of the operating room, as shown in Figure 2.

Figure 2.

Operating room setup. (A) The operating room configuration is shown from the foot of the operating table. The surgeon (red highlight) is positioned on the right side, with the endoscopic tower (yellow highlight) directly opposite and within the surgeon’s line of sight. The image intensifier (blue highlight) is positioned immediately caudal to the operative field to facilitate intraoperative fluoroscopy. The Mazor X robotic system (green highlight) is secured to the Jackson table on the contralateral side. (B) A schematic representation of the operating room configuration is shown.

4. Setup of Robotic Assistance

Preoperative planning of pedicle screw placement can be performed after uploading the preoperative computed tomography (CT) scan to the Mazor X planning station. Rods, cage trajectory, and the range of sagittal alignment correction can be planned using the same software (Figure 3A). The Mazor X Stealth edition comprises a planning station and a robotic arm, which must be mounted on the patient (Figure 3B). After general anesthesia and positioning, the patient is cleaned and draped accordingly. The Mazor X robot is mounted on the surgical table frame. A stab incision is made over the contralateral posterior superior iliac spine point, followed by insertion of the Schanz screw (Figure 3C). An adapter (Figure 3D) will be attached to the Schanz screw [13] and it is subsequently attached to the robotic arm to complete the mounting (Figure 3E). Registration is performed using the Mazor X camera, which captures a snapshot to determine the location of the robotic arm and the reference marker on it. A Mazor X Software robotics image adaptor is attached to the C-arm image intensifier, and anteroposterior (AP) and oblique fluoroscopy images are acquired, with the localization tracker (Medtronic Inc.) attached to the robotic arm in both positions. These fluoroscopy images will be merged with the uploaded preoperative CT to enable navigation. Finally, all surgical instruments are registered and verified, concluding the registration process. Once registration is confirmed accurate, the robotic arm is moved into position for percutaneous pedicle screw placement. A sleeve is passed through the robotic arm to predict the skin incision, which is a paramedian incision to allow for muscle-dilating approaches for percutaneous pedicle screw placement [14] (Figure 3F). The skin incisions are marked with a skin marker.

Figure 3.

Robotic setup. (A) A screenshot from the Mazor X planning station illustrates the planned pedicle screw trajectories. (B) The Mazor X robotic arm is docked near the caudal end of the patient and operating table. (C) The posterior superior iliac spine (PSIS) is palpated, and a Schanz screw is inserted into the PSIS. (D) An adaptor is secured to the head of the Schanz screw. (E) The robotic arm is mounted onto the Schanz screw, and navigation registration is completed. (F) The robotic arm is then positioned for each pedicle screw trajectory, and a sleeve is inserted to mark the corresponding skin incision.

5. Endoscopic Port Placement

We prefer to perform the endoscopic decompression and interbody fusion prior to pedicle screw placement to avoid the constraints imposed by the towers of percutaneous pedicle screws, which can hinder maneuverability. The same skin incision can be utilized for endoscopic port insertion and caudal screw insertion with planning using robotic navigation. A paramedian incision is performed over the target disc space, and dilating tubes are inserted to help retract the musculature until the tube is docked on the inferior aspect of the lamina superior to the disc space, in this case, the L4 lamina. This position is verified using intraoperative navigation on the Mazor X robotic system (Figure 4A). The endoscopic working port is inserted over the dilating tubes, with the screw threads of the endoscopic port helping to keep it in place (Figure 4B). Thereafter, we introduce the Joimax (Karlsruhe, Germany) Delta endoscope through the port and confirm the position with fluoroscopy.

Figure 4.

Full-endoscopic interbody fusion. (A) A navigation probe is used to confirm the position of the L4 lamina prior to insertion of dilating tubes and the endoscope. (B) The uniportal endoscope is introduced, and dissection, hemilaminotomy, facet resection, disc preparation, and interbody fusion are performed. (C) A radiofrequency probe is used to delineate anatomical structures by dividing muscular attachments, shrinking soft tissue, and achieving hemostasis. (D) Endoscopic visualization of the disc space following discectomy and endplate preparation is shown. (E) Direct endoscopic visualization of expandable cage placement demonstrates the thecal sac and traversing nerve root visualized medially, ensuring preservation of neural elements.

6. Insertion of Uniportal Endoscope, Hemilaminotomy and Flavectomy/Decompression

Under endoscopic visualization, the RF probe is used for soft-tissue dissection to reveal relevant anatomy and coagulate bleeding tissue (Figure 4C). The lamina is visualized with this technique, and the interlaminar space, spinous process, spinolaminar junction and facet joint are delineated. Ipsilateral decompression is first performed by performing a hemilaminotomy using a high-speed 4.5-mm uniportal diamond drill, followed by a Kerrison punch technique. We prefer drilling from a caudal-to-cranial direction, starting from the interlaminar window (Wu’s point) to reach the free edge of the ligamentum flavum. This is followed by lateral drilling to the pars interarticularis (Kim’s point). The inferior articular process (IAP) is harvested using a diamond drill and the Kerrison punch technique. This is followed by the removal of the superior articular process (SAP). Both IAP and SAP tips can be used as a bone graft. At this juncture, if severe bilateral and central canal stenosis is noted on preoperative imaging, over-the-top decompression with contralateral facet release can also be performed. This would greatly assist in correcting anterolisthesis.

Ipsilateral foraminal flavectomy to reveal the disc without complete exposure of the traversing nerve root would increase the safety during cage insertion. Complete flavectomy can be performed after cage insertion.

7. Discectomy, Endplate Preparation, and Expandable Cage Insertion

The phases of the discectomy and endplate preparation can be broken down sequentially as shown in Figure 5. The initial disc is visualized after laminotomy and flavectomy, with soft tissue overlying the disc space (Figure 5A). RF cautery is used to clear the soft tissue to better visualize the annulus fibrosus of the disc (Figure 5B). An annulotomy can be initiated with a diamond drill (Figure 5C) and followed by enlargement with an endoscopic Kerrison punch which helps to increase the annulotomy and remove disc material as well (Figure 5D). The endplate cartilage is identified (Figure 5E). Discectomy is performed using a combination of endoscopic instruments, including endoscopic graspers, Kerrison punches, Kerrison elevators, Endplate elevators, and an Endoscopic curette (Figure 5F). Contralateral discectomy can be performed using angled instruments and a flexible grasper in a similar fashion. Adequate disc and endplate preparation is ensured by direct visualization with the endoscope (Figure 5G).

Figure 5.

Stepwise depiction of discectomy and endplate preparation. (A) Initial dissection is performed to expose the disc space. (B) A radiofrequency probe is used to clear soft tissue from the annulus fibrosus. (C) Initial annulotomy is performed with a diamond-tipped high-speed drill. (D) Kerrison punches are introduced through the annulotomy to widen the opening and remove disc material. (E) Endplate cartilage is elevated to define the endplate margins. (F) Additional disc material is removed using curettes. (G) Completion of disc and endplate preparation is demonstrated.

The Joimax Delta scope port is exchanged with the Joimax fusion port to facilitate the insertion of the TLIF cage (Figure 6A). The Delta scope is inserted into the Joimax fusion port to ensure that the traversing nerve root is being medialized and shielded. Intraoperative neuromonitoring, which is utilized for all our cases, allows us to ensure that there was no nerve root irritation or injury during this procedure and the entirety of the cage insertion process.

Figure 6.

Radiographs of full-endoscopic interbody fusion. (A) The fusion port (solid red arrow) is visible and is substantially larger than the dilating tubes used for initial localization. (B) The fusion port is used to introduce the Catalyft trial cage (Medtronic, USA) into the L4–5 disc space to determine appropriate cage sizing; the asterisk (*) denotes the trial cage. The Schanz screw is visible inferior to the endoscope. (C) Fluoroscopy confirms the position of the bone graft funnel to ensure accurate delivery of bone graft into the anterior disc space; the hollow star denotes the bone graft funnel. The interbody cage has been inserted into the prepared L4–5 disc space, and its position is confirmed fluoroscopically; the solid star denotes the expanded cage. (D) Restoration of lumbar lordosis is evident following cage placement.

The Medtronic navigated Catalyft trial, is inserted into the disc space to correctly size the intended cage selection. This can be confirmed using C-arm fluoroscopy (Figure 6B). This is followed by insertion of an allograft into the anterior disc space through a bone graft funnel (Figure 6C). After confirmation of the cage size, the Medtronic Catalyft interbody cage implant is packed with demineralized bone matrix (DBM) putty and is inserted into the disc space. Expansion of the expandable cage is performed once the cage location is confirmed as satisfactory on fluoroscopic images (Figure 6D). The inserted expandable cage can be visualized with the endoscope to confirm placement and ensure that neural elements are free (Figure 4E).

During cage insertion, we recommend the use of neuronavigation, which is onboard the Mazor X robot, with constant communication with the neuromonitoring team, to avoid injury to the traversing nerve root. Neuronavigation is particularly useful in avoiding injury to the neural elements [15] and is recommended especially for new adopters of this technique. For new adopters, we also do recommend considering the use of cage gliders, which have been well described in the literature as a technique to guard against neural injury [16]. Whilst we do not use cage gliders routinely, due to familiarity with this technique, it is important to be cognizant of the various ways to ensure nerve root protection during this procedure.

8. Pedicle Screw Insertion

After the endoscopic interbody fusion is performed, re-registration of the navigation is repeated using a newly taken AP and oblique x-ray, with subsequent merging with the preoperative lumbar CT. The robotic arm will be moved to the desired trajectory to facilitate a stab incision, followed by the insertion of the robotic knife to cut sharply through the fascia (Figure 7A). A navigated drill is placed via robotic arm holder to drill a pilot hole according to the intended plan (Figure 7B). This is followed by navigated tapping and subsequent placement of a pedicle screw via the robotic arm holder (Figure 7C). The robotic arm is then moved into position for the next screw until all the necessary pedicle screws have been placed. The robotic arm is moved out of position, and the rods are placed percutaneously before final tightening is performed (Figure 7D). The C-arm is used for intraoperative fluoroscopy to confirm the appropriate positioning of all screws, rods, and interbody cages.

Figure 7.

Pedicle screw placement with robotic assistance. (A) The robotic arm is positioned for each screw trajectory, and a No. 22 blade is used to incise the fascia through the skin incision. (B) A navigated tap is introduced through the robotic arm. (C) The pedicle screw is inserted along the trajectory maintained by the robotic arm. (D) Percutaneous rod placement and final tightening are performed. (E) Anteroposterior radiograph of the lumbar spine obtained in the standing position demonstrates appropriate screw and interbody cage placement. (F) Lateral radiograph obtained during weight-bearing demonstrates appropriate implant positioning.

All incisions are then closed in layers with fascia closure with a braided, absorbable suture and skin closure with a monofilament, absorbable suture. Waterproof dressings are used to cover the wounds. Postoperative x-rays are obtained on postoperative days 1 and 2, once the patient can weight-bear and tolerate erect x-rays (Figure 7F).

9. Outcomes

Patients were discharged home on an average of 4 days after surgery. Follow-up imaging confirmed stable instrumentation, reduction of L4–5 anterolisthesis, and no hardware-related complications in all patients. There was subjective improvement of symptoms postoperatively and on clinical examination at 3 months, power was 5 on the Medical Research Council scale for all patients.

The average cumulative radiation exposure after the robotic FE-TLIF was 84 mGy as compared to standard MIS-TLIF with O-arm which averaged 90 mGy in our institution. The difference in radiation exposure is expected to increase as we become more efficient with the robotic FE-TLIF in our center.

The average VAS for back pain at preoperative assessment was 7, and it decreased to 4 at the 3-month follow-up, with an improvement of 42.8%. The VAS for leg pain at preoperative assessment was 8 on average, and it decreased to 2 at 3-month follow-up, an improvement of 75%. The Oswestry Disability Index (ODI) improved from an average of 16 preoperatively to 6 postoperatively and to 5 at the 3-month review, which was an 81.8% improvement. According to the Modified MacNab criteria, patient outcomes were all rated as good, reflecting successful symptom relief and return to functional independence.

1. Muscular Preservation

Endoscopic spine surgery, like other MIS techniques, aims to maximize the preservation of the natural muscular anatomy during surgery, reduce postoperative pain, and allow for faster postoperative recovery [18]. Nevertheless, musculature needs to be traversed to reach the intervertebral foramen and disc space. Natural planes exist between these muscles [19], and respecting these planes during placement of the endoscope can prevent injury to the erector spinae complex and specifically to the medial multifidus muscle, which has an important role in spinal stability and locomotor action [20]. Minimal damage is done to the muscles by introducing the endoscope with as small an incision as possible and with sequentially dilating tubes, which expand the surgical corridor by retracting the muscle away and preventing major damage to musculature [21]. In uniportal endoscopic surgery, only 1 incision is required for the endoscopic component compared to biportal surgery where 2 incisions are routinely required [20]. In the context of FE-TLIF this may not constitute a significant difference as one of the incisions made for the pedicle screws can be utilized as an entry point for the second port during biportal surgery. Whilst number of incisions are not a critical measure of success, in general, the uniportal approach helps to minimize muscle dissection from an additional port [22] and performing interbody fusion through the same incision preserves natural muscle planes, and consequently reduces bleeding [23]. Pedicle screws can also be inserted with robotic assistance, which enables preoperative planning based on preoperative imaging and intraoperative registration using matching technologies [24]. The use of a robot enables accurate placement in an efficient manner [22], whilst utilizing muscle-dilating paraspinal incisions [14], thereby avoiding unnecessary muscular dissection or division. Taken as a whole, robotic-assisted MIS pedicle screw placement complements FE-TLIF by maximizing muscular preservation, avoiding postoperative pain and future instability-related problems.

2. Benefits of Robotic Assistance

Robotic-assisted surgery has a proven safety record in pedicle screw placements and is superior to free-hand or fluoroscopic techniques, because the robot can determine the screw trajectory and placement based on preoperative CT scans, and line up the robot arm with the trajectory so that the surgeon can place the screws optimally [25]. This accuracy in placement has led to a plethora of benefits, such as reduced intraoperative blood loss without significant increases in operative time [26], lower screw loosening rates [27] and shorter hospitalization stays [26,28]. Robotic-assisted pedicle screw placement is especially useful in more complicated cases, such as lumbar spondylolisthesis [29] and in spinal deformity [30]. Having the robotic arm guided drilling and screws placement under navigation has taken away the toggling of the Jamshidi needle, which may contribute to unintended trajectory. This ability to simplify complex cases and improve efficiency is a hallmark of robotic-assisted pedicle screw placement

3. Benefits of FE-TLIF

Similarly, endoscopic lumbar interbody fusion, FE-TLIF in particular, is an elegant method to achieve spinal fusion and has proven benefits over tubular methods due to the lack of collateral tissue damage [10], lower levels of blood loss [31], earlier postoperative pain relief [32] and shorter hospitalization stays [33]. One specific advantage of the FE-TLIF is the ability to carry out facet joint resection under direct visualization and, precisely, to prevent future spinal instability. In FE-TLIF, facet joint resection happens between 2 points well known to endoscopic surgeons, namely Wu’s point, at the junction of the cranial vertebral lamina to the IAP and Kim’s point, at the intersection between the SAP and the IAP [34,35]. Facet resection can either be done from Wu’s point towards Kim’s point or vice versa, with the surgeon docking on the relevant points to reveal the anatomy [34]. This precise and safe manner of facet resection allows access to the intervertebral disc without the need for excision of posterior structures, as compared to tubular or open surgery [36]. The FE-TLIF technique with facetectomy allows for spinal fusion with regular interbody cages, as compared to smaller cages, which are required in the trans-kambin approach [37]. Meticulous endoscopic disc preparation without damage to the vertebral endplate [36] as well as adequate end plate preparation and bone grafting [9,38] would also increase the fusion rate. The use of expendable cages is also more prevalent in FE-TLIF, owing to the smaller incision. Expandable cages have been shown to have increased disc height improvements and foraminal height improvements compared to static cages in TLIF [39] and have been associated with improved functional outcomes [40].

4. A Combination of Techniques to Reduce Radiation Exposure and for Efficient Surgery

The combination of robotic-assisted pedicle screw placement and FE-TLIF was conceptualized with patient safety in mind. Beyond the steep learning curve and the initial teething issues that naturally come with the combination of advanced techniques [41], robotic-assisted FE-TLIF promises to improve the efficiency and safety of the procedure. Robotic-assistance allows for pedicle screws to be placed under neuro-navigation, without the need for an intraoperative CT scan, which allows the patient to avoid another high dose of radiation [42]. The presence of an intraoperative CT scanner is also space-consuming, as is the endoscopic tower. Moving the CT scanner into the operative field periodically for real-time confirmation of the location of the bone graft tunneller and instrumentation is time-consuming and cumbersome. The robotic arm, which follows a planned trajectory for safe pedicle screw placement, also provides ease of mind for the surgeon [25]. Insertion of expandable cages take place in real-time with C-arm fluoroscopy, which is less cumbersome compared to using O-arm fluoroscopy (Medtronic Inc.), hence saving time and resulting in almost half the radiation exposure [43]. The real-time changes in the disc height during cage expansion can be observed with C-arm fluoroscopy, in tandem with intraoperative neuromonitoring, to achieve the ideal lordotic restoration, foraminal height and disc height [36,44]. Real-time expansion of the cage with fluoroscopic confirmation ensures that the cage is at the desired anterior-posterior position and allows for controlled expansion of the cage, which can also be adjusted accordingly, in a similar fashion as to MIS-TLIF cases [45]. Fluoroscopic guided expansion of the cage can be compared with the robotic planning images as well, to ascertain if the ideal height and lordotic restoration have been achieved [45].

This is evident in our practice thus far, with patients undergoing robotic-assisted FE-TLIF showing significant improvement in symptoms, including complete resolution of postoperative lower limb pain in some patients. Particularly, the improvements in ODI and VAS scores are comparable with reported literature for FE-TLIF cases [46]. Length of hospital stay averaged 4 days, comparable with or shorter than reported literature on endoscopic TLIF cases [47] and robotic lumbar interbody fusion (LIF) [48], or conventional LIF techniques [49].

5. Comparing Robotic-Assisted FE-TLIF With Similar MIS Fusion Techniques

It is natural to compare our integrated technique with that which is being done more conventionally. These represent robotic-assisted MIS-TLIF, purely navigation-assisted FE-TLIF without robotic assistance, and FE-TLIF without robotic assistance nor navigation assistance

Robotic-assisted TLIF is well described in the literature [50], but we feel that our integrated approach combining robotic-assistance with full-endoscopic visualization has one key advantage – visualization with minimal surgical trauma. The use of the endoscopic during FE-TLIF provides unparalleled visualization of the neural elements and minimizes facet joint disruption, to prevent postoperative complications [36]. The ability to visualize key neural structures and anatomy without disrupting natural muscular planes and by minimizing surgical trauma has also been shown to be significant in postoperative outcomes and recovery [23]. It is for these key reasons that we feel the endoscopic approach is superior to the MIS approach, with the caveat that there is a large learning curve that needs to be surmounted.

Compared to purely navigation-assisted FE-TLIF, robotic-assisted FE-TLIF represents a more recent evolution to solve the issues with crowding in the operating theatre [51] and radiation exposure [42], without compromising on the accuracy of screw placement. Robotic-assisted navigation, which is based on preoperative CT scans rather than intraoperative CT scans with the O-arm, have been shown to be as safe and as accurate as intraoperative CT scans [52]. Bringing in the O-arm for FE-TLIF has a larger time cost and is cumbersome compared to robotic-assisted FE-TLIF in our experience. Moreover, O-arm based navigation also can have issues such as registration problems and may need additional C-arm fluoroscopy, which would add to operative time and crowd in the operative room [53]. Lastly, even with intraoperative CT scans and fully navigated cage insertion, confirmatory C-arm fluoroscopic images are still required, to confirm implant position [43] and bone graft placement – which is an additional step and an additional machine that needs to be present in the operating room. By integrating the use of C-arm fluoroscopy during cage placement already, our workflow allows for procedural efficiency on top of other advantages.

Comparing robotic-assisted FE-TLIF with FE-TLIF without navigation and robotic-assistance, the main differences are the higher dose of radiation involved with fluoroscopy-guided percutaneous screw placement and cage placement compared to robotic techniques [54], as well as the higher chance of suboptimal implant placement without the use of navigation [54]. We advocate for more precise surgery with radiation dosing being minimized as much as possible, and hence prefer the robotic-assisted FE-TLIF.

6. Reversal of Workflow With Insertion of Pedicle Screws Prior to Cage Insertion

Contralateral pedicle screws can be inserted with robotic assistance, followed by ipsilateral guidewire insertion into the ipsilateral pedicle. This would eliminate the constraints imposed by percutaneous pedicle screw towers, thereby improving maneuverability. Subsequent ipsilateral pedicle screws can be inserted after cage insertion. This workflow has also been shown to be safe and reasonably effective, with the benefit of avoiding the need to repeat robotic navigation registration after cage insertion.

7. Limitations

Robotic-assisted FE-TLIF has no strict contraindications, but like other spinal fusion procedures, it has limited benefits in patients with severe osteoporosis, extremely collapsed intervertebral space, patients with altered anatomy secondary to prior lumbar spine surgery, infections, tumours or injuries [55]. Patients with severe deformity for which there will be limited visualization of the disc space can make this procedure even more technically challenging, with the risk of inadequate endplate preparation, malposition of the cage/bone grafts and eventual inadequate fusion.

Compared to open and MIS techniques, robotic-assisted FE-TLIF has a steep learning curve. Endoscopic TLIF is already a technically challenging procedure. Although adding robotic-assisted techniques offers significant benefits, establishing a reproducible workflow requires substantial effort. Robotic-assisted FE-TLIF is also more expensive than open or MIS techniques due to the consumables used in endoscopic and robotic technologies. Less-resourced centers and surgeons in low- and middle-income countries may not have easy access to robotic-assistance, and this may prove a significant hurdle in the adoption of this technique globally [56].

Finally, whilst we do believe that the advantages in terms of reduced radiation exposure are significant in robotic FE-TLIF, this will have to be studied when the robotic-assisted FE-TLIF technique is well-established and at its most efficient, to ascertain the true differences in radiation dose exposure. Moreover, there is a need for comparison with MIS-TLIF, FE-TLIF without robotics and other comparable MIS techniques in a cohort study. Further studies are needed to corroborate our conclusions.