The Prone-Angled Transpsoas Approach: Utilizing Position, Approach, Navigation, and Neuromonitoring in Prone Lateral Surgery

Abstract

BACKGROUND AND OBJECTIVES:

Lateral lumbar interbody fusion techniques have advanced toward minimally invasive, anatomy-preserving approaches that enhance sagittal alignment and reduce perioperative morbidity. The most common existing approaches—the prone transpsoas and oblique lumbar interbody fusion—offer distinct advantages with opposing limitations. This manuscript describes a novel prone angled transpsoas (PAT) technique that combines the ergonomic and sagittal benefits of prone positioning with a navigated oblique/angled approach and direct lateral working corridor. This technique aims to simplify lateral surgery and minimize plexus and vascular risk, while enabling single-position surgery with the help of image-guidance navigation and neuromonitoring.

METHODS:

The operative technique for a PAT approach is described. The rationale for this approach and a representative case example are reviewed.

RESULTS:

A 73-year-old woman with grade 1 L4-L5 anterior spondylolisthesis and bilateral radiculopathy underwent PAT lateral lumbar interbody fusion with concurrent percutaneous posterior fixation. With the use of neuromonitoring and navigation, PAT approach combined the advantages of prone transpsoas surgery (increased lordosis, more posterior lumbar plexus, single-position surgery, minimized risk of bowel and vascular injury) with the navigated anterior angled approach (oblique lumbar interbody fusion/anterior-to-psoas) that may minimize risk of plexus injury.

CONCLUSION:

The authors' early experience with the described PAT technique suggests that it is not only feasible but also offers advantages. This technique leverages prone techniques with an oblique angled approach while making use of navigation and neuromonitoring when creating a lateral corridor, which removes the need to work obliquely. The use of navigation and neuromonitoring improves the ability to safely work laterally. Further follow-up studies of this technique are ongoing.

ABBREVIATIONS:

EMG

electromyography

LLIF

lateral lumbar interbody fusion

OLIF

oblique lumbar interbody fusion

PAT

prone angled transpsoas

PTP

prone transpsoas.

Over the past 2 decades, lateral lumbar interbody fusion (LLIF) techniques have evolved toward less invasive approaches that minimize soft tissue disruption and approach-related morbidity. Compared with posterior fusion procedures, LLIF allows disk height and lordosis restoration while reducing blood loss, wound complications, and hospital stay.^1-4 It is widely used for degenerative disk disease, low-grade spondylolisthesis, adult spinal deformity, and degenerative scoliosis, especially when indirect decompression and sagittal correction are desired.^3,5,6 Candidates are identified radiographically by disk height loss, instability, or foraminal stenosis amenable to disk height restoration, with careful assessment of vascular and psoas anatomy.^5,7

In 2006, Luiz Pimenta and William Taylor introduced the extreme lateral interbody fusion, a minimally invasive retroperitoneal, transpsoas approach granting direct disk access without transperitoneal exposure or posterior muscle dissection.⁸ Electromyography (EMG) neuromonitoring was required to avoid lumbar plexus injury. The procedure was staged: lateral decubitus for interbody access and then prone for posterior instrumentation.

Building on Michael Mayer's 1997 anterolateral concept, Clément Silvestre et al described the oblique lumbar interbody fusion (OLIF) in 2012,⁹ establishing a distinct ante-psoas alternative to extreme lateral interbody fusion. OLIF may reduce reliance on EMG but risks compressive injury from retractor deployment and proximity to the sympathetic chain and great vessels. At upper levels, the aortic–caval confluence complicates access. Like LLIF, OLIF typically requires repositioning between interbody and posterior stages.

In 2020, Pimenta et al¹⁰ introduced the prone transpsoas (PTP) approach, enabling single-position prone surgery. PTP combines lateral access with simultaneous posterior instrumentation, eliminating repositioning and leveraging prone lordosis for sagittal correction. Because the interbody is still transpsoas, triggered EMG is required to minimize lumbar plexus injury.

We describe a novel prone anterior/angled transpsoas (PAT) technique that retains the single-stage prone advantages but employs a more ante-psoas, obliquely angled trajectory. This minimizes lumbar plexus risk while maintaining natural prone lordosis. The PAT requires intraoperative computed tomography-guided navigation and neuromonitoring for corridor visualization and safety. We also present an illustrative clinical case.

METHODS

The operative technique for a PAT approach is described. The rationale for this approach and a representative case example are reviewed. This research was approved by the medical center's institutional review board, the patient consented to the procedure, and the participant consented to publication of her image.

RESULTS

Technique Description

Patient Positioning

Under general anesthesia, the patient is placed prone on a radiolucent Jackson table with a free abdomen to accentuate lumbar lordosis and reduce venous pressure. The Alphatec frame provides lateral thoracic and pelvic bracing and allows coronal table bending (Figure 1). Of note, the bed remains parallel to the floor throughout the majority of the procedure, only rotated after the retractor system is fixated to the patient for the diskectomy and interbody placement portion of the procedure. Padding protects all pressure points, and the arms are positioned in the “Superman style.” Fluoroscopy confirms neutral alignment, and computed tomography navigation is registered via a posterior superior iliac spine pin. Table adjustments (extension or reverse Trendelenburg) may increase intervertebral space at challenging levels like L4-L5 or when the iliac crest limits exposure. Continuous neuromonitoring (triggered EMG and saphenous somatosensory evoked potential [SSEP]) is established for approach safety.

FIGURE 1.

Prone positioning for the prone-angled transpsoas approach in the Alphatec frame demonstrating ample exposure to the lateral working corridor as well as to the back for any posterior procedures to be performed.

Posterior Instrumentation

If posterior fixation is planned, navigated percutaneous pedicle screws can be placed first to prevent later disruption of navigation accuracy.

Incision Planning and Prone Angled Transpsoas Approach

Using navigation, a transverse incision is planned across the disk level. If multiple levels are considered including L4-L5, the incision is centered slightly inferiorly to the middle of the L4-L5 disk space, as it is the most difficult disk to reach. The incision is carried through subcutaneous tissue, and blunt dissection continues until the quadratus lumborum is identified. The tissue is pushed inferiorly where one can sweep cranially with a finger to feel the rib, and caudally to feel under the iliac crest, and helps confirm retroperitoneal entry. The anterior portion is gently dilated to facilitate the angled trajectory.

Trajectory planning for the retractor system is crucial—particularly at L4-L5, where the dilator is placed in an angled, anterior location within the anterior psoas and posterior to the vessels (Figure 2). This creates a safe corridor identified by navigation and EMG. In prone position, the lumbar plexus lies more posteriorly, enhancing safety. As the dilator advances, the trajectory angles slightly posterior to avoid anterior vasculature, targeting the anterior mid-disk zone or in the anterior-most aspect of the psoas muscle and posterior to the great vessels. Once EMG confirms a safe threshold (accepted working threshold >7-10 mA, typically >20 mA), larger dilators and retractor blades are inserted. The surgeon drops the hand 15° to 20° toward the floor to achieve the desired angle (Figure 3). Only the posterior blade is expanded—about 6 clicks (≈18 mm)—to displace the psoas posteriorly and preserve the anterior blade position.

FIGURE 2.

Intraoperative spinal navigation monitor with axial and coronal views. The blue trajectory demonstrates the superficial starting point involving the anterior-most aspect of the psoas muscle. The green outlined trajectory is the angled approach that should be taken as the dilator and retractor system is advanced through tissue and ends somewhere anterior to the midpoint of the disk and posterior to the vessels, depending on the electromyography signals. The pink trajectory shows a standard directly lateral approach. I, inferior; L, left; R, right; S, superior.

FIGURE 3.

Intraoperative spinal navigation monitor view of axial and coronal slices from a different case. The pink trajectory demonstrates the normal lateral approach for the prone transpsoas. The outlined teal trajectory demonstrates the prone angled transpsoas approach with a 15° to 20° angle from parallel. A, anterior; I, inferior; L, left; P, posterior; R, right; S, superior.

Interbody Preparation and Implant Placement

The interbody work is performed under navigation and continuous SSEP monitoring (Figure 4). After thorough diskectomy using standard instruments, the posterior annulus is left intact. The anterior half of the disk space is cleared for implant placement, and contralateral annular release is performed to allow a long cage spanning both apophyseal rings for endplate support. This anterior, bilateral position restores disk height and corrects sagittal and coronal alignment. Expandable cages further enhance lordosis restoration. Of note, the bed angle can be modified by the surgeon for ergonomic purposes during the interbody preparation after the retractor system is fixated to the patient, particularly if the surgeon prefers to stand for this portion of the case. Because the bed and retractor system will move as one unit, the approach remains fixed regardless of how the bed is rotated.

FIGURE 4.

Intraoperative spinal navigation monitor view of axial and coronal slices as well as 3-dimensional reconstruction of the intraoperative computed tomography scan. The blue navigated interbody device demonstrates once the retractor system is docked, the rest of the diskectomy and interbody work is done in the directly lateral position. A, anterior; L, left; P, posterior; R, right.

Closure

After retractor removal, a standard layered closure is performed. If additional posterior procedures are needed, the lateral incision may be temporarily covered until final imaging confirmation.

Illustrative Case

A representative case example is shown in Figure 5. A 73-year-old woman, with a history of prediabetes, hypertension, osteopenia, obesity (body mass index 31), presented with significant low back pain and bilateral, symmetrical radicular pain unresponsive to conservative therapies. She was diagnosed with a grade I L4-L5 anterior spondylolisthesis with significant central stenosis, severe facet arthrosis, and bilateral foraminal stenosis at that level. A PAT was performed at L4-L5 to correct the spondylolisthesis and help indirectly decompress the bilateral foramen. Separate posterior fascial incisions were used to place bilateral percutaneous pedicle screws from L4-L5 to provide posterior fixation and facilitate fusion. Simultaneous anterior and posterior column access was performed. The triggered EMG threshold remained greater than 20 mA for the oblique transpsoas approach docking the retractor system and saphenous SSEP monitoring remained stable throughout the rest of the surgery.

FIGURE 5.

A and B, Preoperative MRI demonstrating grade 1 L4-L5 anterior spondylolisthesis with severe central and bilateral foraminal stenosis at that level. C and D, Intraoperative lateral and AP fluoroscopy demonstrating adequate placement of hardware after an L4-L5 prone-angled transpsoas lateral lumbar interbody fusion. E and F, Postoperative standing lateral and AP radiographs showing good regional and global alignment with appropriate hardware placement. AP, anteroposterior.

The entire lateral and posterior minimally invasive procedure was performed in less than 2 hours, with a total of 20 cc of estimated blood loss and no intraoperative complications. Immediately postoperatively, the patient complained of minor left thigh pain and numbness, which resolved by discharge on postoperative day 2. Sagittal alignment and proper hardware placement were confirmed on postoperative standing radiographs along with resolution of her prior anterior spondylolisthesis (Figure 5E and 5F).

DISCUSSION

The refinement of minimally invasive lumbar fusion has produced the PTP and OLIF techniques, each with distinct advantages. The PTP adapts traditional lateral fusion to the prone position, allowing simultaneous anterior and posterior access, eliminating repositioning, and improving efficiency with navigation.^11-13 Prone posture increases lordosis and may enhance sagittal alignment compared with lateral LLIF.^14,15 It also shifts the psoas and lumbar plexus posteriorly, reducing femoral plexopathy risk and improving lateral access at L4-L5.^15,16 Early data show low complication rates and outcomes comparable with or better than LLIF or transforaminal lumbar interbody fusion.^13,17,18

The OLIF, or anterior-to-psoas approach, minimizes plexus manipulation and may reduce neurologic risk.^19-21 Like PTP, it achieves indirect decompression and height restoration while preserving posterior elements.^20,22 Studies show similar or better lordosis correction and less morbidity than minimally invasive-transforaminal lumbar interbody fusion,^22,23 although with potentially increased rates of cage migration, reoperation, or vascular injury compared with the transpsoas LLIF.²⁴ Because OLIF is done in lateral decubitus, it forfeits the additional lordosis benefit of prone positioning.

The PAT approach merges the key strengths of both. At upper lumbar levels (L2-L4), the great vessels are found more midline with the lumbar plexus significantly posterior in the psoas, which makes these levels amenable to both the PTP and OLIF approaches.¹⁶ At L4-L5, however, both move laterally, increasing complexity. OLIF loses prone lordosis, while PTP risks plexus compression. PAT uses an oblique anterior trajectory that maintains prone advantages, avoids repositioning, and reduces plexus contact. The approach-related concerns for this procedure are similar to what has been described in other lateral lumbar interbody studies.⁷ Before surgery, the surgeon should carefully review imaging to identify any transitional levels, lateral placement of vessels, prominent and entirely posteriorly placed lumbar plexus.

With navigation and EMG guidance, PAT docks anterior to the disk midpoint and posterior to the vessels. EMG confirms plexus distance; navigation defines the safe 3-dimensional path. These modalities in combination mitigate the need for rigid location of initial docking. Only the posterior retractor blade is opened, displacing the psoas posteriorly and enlarging the working corridor. Under continuous SSEP monitoring direct lateral working angle simplifies interbody placement while minimizing vascular or bowel injury—risks inherent in OLIF.

An essential tool that is required for the PAT approach is surgical navigation technology. The development of navigation for spine surgery has improved outcomes and techniques in spine surgery, primarily by enhancing implant accuracy, reducing complications, and facilitating minimally invasive approaches. In practice, intraoperative imaging—such as cone-beam computed tomography or O-arm scans—is acquired after patient positioning and exposure. This imaging is registered to the navigation system, which then allows the surgeon to visualize the anatomy and planned implant trajectory in 3 dimensions. Specialized navigated instruments are tracked in real time, enabling the surgeon to accurately access the disk space, perform diskectomy, and position the interbody cage according to preoperative or intraoperative planning, all while minimizing reliance on fluoroscopy and reducing radiation exposure.^6,25,26 Navigation is especially valuable in minimally invasive and lateral approaches, where direct visualization is limited and anatomic landmarks may be obscured. Studies have demonstrated that navigation-guided interbody placement results in high rates of accurate cage positioning within the optimal disk space zones, with a reduced risk of malposition and approach-related complications.^25,27,28 Navigation is essential for planning the angled transpsoas trajectory of the docking system and cannot be performed with only fluoroscopy. The imaging system allows the surgeon to visualize the pertinent bony, vascular, and muscular anatomy properly to do this approach successfully. While requiring navigation may be seen as a limiting factor for this approach, there has been a significant increase in the use of navigation in spine surgery over the past couple decades, and access to this type of technology is becoming easier. If there is any question about the accuracy of the navigation at any point in the procedure, particularly for multilevel cases, surgeons should be encouraged to use fluoroscopy as an adjunct at any point in time of the case as well as re-register the navigation system.

The other essential adjunct for this novel approach is neuromonitoring. The development and integration of intraoperative neurophysiological monitoring—including EMG, motor evoked potentials, and SSEPs—have enabled real-time detection of neural compromise, allowing surgeons to adjust technique and minimize iatrogenic injury.^29-31 The importance of neuromonitoring is underscored by its role in enabling the full potential of minimally invasive lateral approaches, where direct visualization is limited and the risk to neural structures is heightened.^30,31 Although studies have shown that single modality intraoperative neurophysiological monitoring is less helpful because of varying levels of predictive value, sensitivity, and specificity, when used in combination, the joint efficacy significantly increases.³² In our experience, EMG and SSEPs have been essential in the PAT approach. We use both free-running EMG and triggered EMG. Gunnarsson et al³³ demonstrated that EMG has a low positive predictive value with a high sensitivity, thus making it a good screening tool in neuromonitoring as it provides a warning before actual neural injury. We use triggered EMG attached to the retractor system similar to a locating beacon for the femoral nerve root. We have a threshold of at least 7 to 10 mA, which suggests that the nerve root is at least 7 to 10 mm away from the retractor system. Thus, in our practice, we like to use EMG for proper docking of the retractor system with appropriate trajectory through the anterior-most portion of the psoas, while respecting both the lumbar plexus and the great vessels. If the triggered EMG provides an acceptable value, the use of navigation and EMG can help reposition the docking of the retractor system such that the triggered EMG provides a higher value. As mentioned previously, once the retractor system is appropriately docked with adequate EMG signal and appropriate oblique trajectory with neuronavigation assistance, the anterior blade remains at the same position, whereas the working corridor is expanded by opening the posterior blade roughly 6 clicks. Each click provides 3 mm of expansion, so 6 clicks on the retractor system will allow for the placement of 18 mm lateral interbody cage.

Once the retractor system is docked and expanded, the working corridor is repositioned to 90° or directly lateral. SSEPs are critical for monitoring femoral nerve injury the rest of the procedure. We use the continuous saphenous SSEP monitoring described in Tohmeh et al³⁴ when doing PTP LLIF. They found that this type of SSEP monitoring provides sensitive, real-time detection of femoral nerve compromise, which can allow for timely intraoperative countermeasures, such as repositioning of patient, retractor relaxation, and expedited completion, thus reducing the risk of postoperative femoral nerve injury. The surgeon is provided real-time alerts whenever the continuous SSEP monitoring had changes from baselines when the amplitude of any SSEP channel decreased by at least 50% or peak latency increased by at least 10%. Although no one surgery is perfect and there can be trade-offs with any single approach, the use of these 2 modalities along with navigation allow surgeons to have the safest approach for the prone angled transpsoas technique and mitigate intraoperative and postoperative complications related to neurovascular injury. The general guiding PAT principle is to use an angled trajectory targeting the 25 yard-line of the disk space to remain as anterior to the psoas while posterior to the important vasculature. However, navigation and neuromonitoring are heavily relied on to help make the real-time decision on the safest and most effective angled trajectory for this technique.

Limitations

This technical report includes a single illustrative case, limiting conclusions regarding generalizability and long-term outcomes. Follow-up is short-term, and fusion durability and delayed complications remain unknown. The PAT approach relies on advanced navigation and multimodal neuromonitoring, which may limit accessibility and introduces a learning curve. Although designed to mitigate plexus and vascular injury, this technique does not eliminate approach-related risks but modifies the risk profile, requiring careful patient selection.

CONCLUSION

Minimally invasive lateral spine surgery continues to evolve. Although lateral decubitus LLIF, prone transpsoas LLIF (PTP), and OLIF each offer benefits, limitations remain. The prone angled transpsoas approach, assisted by navigation and intraoperative neuromonitoring, combines their advantages—enhanced lordosis, single-position efficiency, posterior plexus displacement, and minimized psoas traversal—while reducing vascular and intra-abdominal risks.

With the integration of real-time navigation and neuromonitoring, PAT represents a safe and effective refinement of lateral fusion surgery. Ongoing follow-up studies will further assess its long-term outcomes and reproducibility.

Funding

This study did not receive any funding or financial support.

Disclosures

Dr Taylor is a consultant for ATEC. The other authors have no personal, financial, or institutional interest in any of the drugs, materials, or devices described in this article.