Anatomical Considerations in Pre-psoas and Transpsoas Interbody Fusion

Lumbar spinal fusion techniques have evolved over the past few decades, encompassing traditional, open surgical techniques and, more recently, minimally invasive surgical (MIS) approaches. MIS techniques offer potential benefits, including reduced blood loss, avoiding paraspinal muscle disruption, decreased postsurgical pain, and shorter hospital stays.

Lateral lumbar interbody fusion (LLIF) and oblique lumbar interbody fusion (OLIF) are 2 such techniques. They have emerged as 2 similar yet distinct solutions for achieving interbody fusion in both degenerative and deformity settings. The fundamental objective of both LLIF and OLIF is to achieve indirect neural decompression by restoring disc height and correcting coronal and sagittal alignment. In both techniques, the patient is placed in the lateral decubitus position, and a retroperitoneal corridor is exploited to access the disc space. However, each relies on different anatomical planes, leading to unique risk profiles and preoperative considerations.

The Transpsoas Corridor (LLIF)

In the LLIF technique, a flank incision is made, and the abdominal musculature is divided to access the retroperitoneal space.

Sequential dilators are then used to create a working channel through the psoas, followed by placement of a tubular retractor. The primary anatomical challenge is safely avoiding the lumbar plexus. The lumbar plexus consists of the ventral rami of L1–L4, with frequent contribution from T12. These rami unite within the psoas muscle belly, where the plexus gives rise to various nerve branches. At the upper lumbar levels, the plexus is situated relatively posterior in the muscle belly, providing a relatively safe window to access the disc space. However, as the approach moves caudally, the lumbar plexus migrates ventrally within the muscle, and the margin of safety diminishes. Anatomical studies have shown that the plexus is often at the center of the disc space at the L4-L5 level, requiring direct retraction.[1]

The most common approach-related neurological complication of LLIF is transient hip flexor weakness and thigh sensory changes, which may occur in up to 36% of patients according to a large systematic review.[2] These symptoms generally improve over time, often resolving by 3 to 6 months after the operation. Persistent neurologic complications were reported at a much lower rate of 4%.[2]

Intraoperative neuromonitoring (IONM), mainly triggered by electromyography (EMG), is essential for mitigating risk of neurologic injury. During dissection, a stimulating probe is used on dilators/retractor blades and the psoas muscle belly. If the tip is near a motor nerve, a muscle contraction (in quadriceps, adductors, etc) occurs at low stimulation thresholds. A high stimulation threshold (>10–15 mA) suggests the dilator is safely away from a major motor nerve. The addition of other IONM modalities to monitor the lumbar plexus, such as motor evoked potentials and saphenous somatosensory evoked potentials, may be superior to relying on EMG alone.[3]

There are other considerations besides the lumbar plexus when performing LLIF. The L5-S1 level is generally considered inaccessible due to the iliac crest, which prohibits a straight lateral approach. At the upper lumbar levels—namely L1-L2—the twelfth rib presents another physical obstacle; in this case, a subcostal approach is commonly used. The diaphragm, its attachments to the spine via the crura, and the pleura are carefully preserved during dissection, and the retractor is angled upward toward the disc space. A rib resection or an intercostal approach between the eleventh and twelfth ribs are also options at this level. Although the direct lateral approach is designed to avoid major blood vessels, their location relative to the psoas must be accounted for during preoperative planning, as anatomic variations exist.

There has been recent interest in the prone transpsoas (PTP) LLIF, which eliminates the need for an interoperative flip from the lateral decubitus (LD) to the prone position when adding posterior pedicle screw fixation. One distinct advantage of the PTP LLIF over the traditional LD LLIF is the improved ability to restore lumbar lordosis—a finding that has been reproduced across several studies.[4] In the prone position, gravity pulls down the weight of the abdomen, which helps to naturally increase lordosis as it hangs freely from the table. However, there are anatomic concerns unique to prone positioning that must be considered. While it may seem intuitive for gravity to pull the retroperitoneal organs away from the spine, this has not been shown to be the case. Paradoxically, the colon migrates posteriorly relative to the disc space and psoas by >50% compared to the LD position.[5] Moreover, prone positioning results in a longer surgical corridor, with the distance from the skin to the lateral disc surface being 134.9 mm in prone positioning compared with 118.7 mm in LD positioning.[5] These factors may combine to result in a more technically demanding procedure with a higher potential for visceral injury.

The Pre-psoas Corridor (OLIF)

Like the LLIF approach, the OLIF approach uses a retroperitoneal corridor. It exploits the anatomic plane between the psoas muscle and the great vessels—the aorta, inferior vena cava, and common iliac vein. In doing so, the psoas muscle is preserved, and the lumbar plexus is avoided entirely. However, this approach is not without its own inherent risks, with vascular injury being the most obvious. Major vascular injury, though uncommon, occurs at a higher rate in OLIF than LLIF, with a meta-analysis comparing the rates between both procedures and finding rates of 1.8% and 0.4%, respectively.[6] Vascular injury is not limited to the major vessels—the iliolumbar vein is also at risk during dissection, particularly just below the L4-5 disc or at the L5 vertebral body.[7] It can also be avulsed during retraction of the common iliac vein from which it branches, and so excessive retraction of the common iliac vein should be avoided.[8]

Preoperative planning and careful imaging evaluation are essential to identifying a safe corridor. One imaging study found that 11% of patients had no measurable oblique corridor at the L4-5 level on magnetic resonance imaging where the corridor is naturally at its narrowest, either due to vascular obstruction or a bulky psoas.[9] Variations in psoas morphology such as the “teardrop” psoas—also termed a “high-rising” or “Mickey Mouse” psoas—has also been described as precluding a safe corridor. A teardrop psoas is typically longer in longitudinal length and shorter in transverse length compared to a non-teardrop psoas and is associated with lateral and posterior migration of the iliac vasculature to the anterior third of the L4-L5 disc.[10] It is also associated with more anterior migration of the lumbar plexus.

While OLIF has traditionally been performed using true AP and lateral fluoroscopic x-ray images to localize the disc space and aid in retractor placement, navigation may potentially increase accuracy and safety. Use of an intraoperative computed tomographic image allows the surgeon to be aware of the location of great vessels in real-time, allowing for a safer and more precise trajectory. In a large retrospective review of 214 patients who underwent navigated OLIF, there was a 0% rate of vascular injury.[11]

Another risk of the OLIF approach is injury to the lumbar sympathetic chain, located ventrally to the psoas muscle. Such an injury can manifest as symptoms such as unequal skin temperature in the lower extremities and, in male patients, retrograde ejaculation. A significant risk factor for this type of injury is prolonged retractor time, with one study identifying a time exceeding 31.5 minutes as a crucial thershold.[12]

Conclusion

While LLIF and OLIF may seem similar on the surface, they make use of very different anatomic corridors that carry unique risk profiles. LLIF requires a transpsoas approach, which carries a risk of injury to the lumbar plexus. OLIF makes use of a pre-psoas approach, which avoids the plexus but increases the chance of complications involving the great vessels or sympathetic chain. Thorough imaging analysis and careful patient selection based on individual anatomy are essential to reducing complications and achieving a successful outcome. Surgery at the L5-S1 level, obstruction from the ribs or iliac crest, a teardrop psoas, or the lack of a safe oblique corridor may make one approach preferable to the other—or necessitate a different interbody technique entirely. Emerging technology, such as navigation, will continue to improve surgical safety by offering real-time anatomic detail.

References

1. Park DK, Lee MJ, Lin EL, Singh K, An HS, Phillips FM. The relationship of intrapsoas nerves during a transpsoas approach to the lumbar spine: anatomic study. J Spinal Disord Tech. 2010;23(4):223-228.

2. Hijji FY, Narain AS, Bohl DD, et al. Lateral lumbar interbody fusion: a systematic review of complication rates. Spine J. 2017;17(10):1412-1419.

3. Alluri R, Mok JK, Vaishnav A, et al. Intraoperative neuromonitoring during lateral lumbar interbody fusion. Neurospine. 2021;18(3):430-436.

4. Rohde M, Echevarria A, Carrier R, Zinner M, Ngan A, Verma R. Prone single position approach to lateral lumbar interbody fusion: systematic review and meta-analysis. Int J Spine Surg. 2024;18(4):408-417.

5. Menezes CM, Andrade LM, Lacerda GC, Salomão MM, Freeborn MT, Thomas JA. Intra-abdominal content movement in prone versus lateral decubitus position lateral lumbar interbody fusion (LLIF). Spine (Phila Pa 1976). 2024;49(6):426-431.

6. Walker CT, Farber SH, Cole TS, et al. Complications for minimally invasive lateral interbody arthrodesis: a systematic review and meta-analysis comparing prepsoas and transpsoas approaches. J Neurosurg Spine. 2019;30(4):446-460.

7. Davis M, Jenkins S, Bordes S, et al. Iliolumbar vein: anatomy and surgical importance during lateral transpsoas and oblique approaches to lumbar spine. World Neurosurg. 2019;128:e768-e772.

8. Soh TLT, Lee HJ, Choi J, Oh JYL. Iliolumbar vein injuries in prepsoas lateral lumbar interbody fusion: strategies for prevention and management. Illustrative case. J Neurosurg Case Lessons. 2025;9(12).

9. Ng JPH, Kaliya-Perumal AK, Tandon AA, Oh JYL. The oblique corridor at L4-L5: a radiographic-anatomical study into the feasibility for lateral interbody fusion. Spine (Phila Pa 1976). 2020;45(10):E552-E559.

10. Louie PK, Narain AS, Hijji FY, et al. Radiographic analysis of psoas morphology and its association with neurovascular structures at L4-5 with reference to lateral approaches. Spine (Phila Pa 1976). 2017;42(24):E1386-E1392.

11. Xi Z, Chou D, Mummaneni PV, Burch S. The navigated oblique lumbar interbody fusion: accuracy rate, effect on surgical time, and complications. Neurospine. 2020;17(1):260-267.

12. Singhatanadgige W, Tangdamrongtham T, Limthongkul W, et al. Incidence and risk factors for lumbar sympathetic chain injury after oblique lumbar interbody fusion. Neurospine. 2024;21(3):820-832.

Contributors:

Jonathan Gabor, MD

Malik Scott, BS

Arash J. Sayari, MD

From the Department of Orthopaedic Surgery, Rush University Surgery, at Rush University Medical Center in Chicago, Illinois.