Location of the Femoral Nerve in the Lateral Decubitus Versus Prone Position

Abstract

Study Design

Cadaveric study.

Objective

To compare the position of the femoral nerve within the lumbar plexus at the L4-L5 disc space in the lateral decubitus vs prone position.

Methods

Seven lumbar plexus specimens were dissected and the femoral nerve within the psoas muscle was identified and marked with radiopaque paint. Lateral fluoroscopic images of the cadaveric specimens in the lateral decubitus vs prone position were obtained. The location of the radiopaque femoral nerve at the L4-L5 disc space was normalized as a percentage of the L5 vertebral body (0% indicates posterior location and 100% indicates anterior location at the L4-L5 disc space). The location of the femoral nerve at L4-L5 in the lateral decubitus vs prone position was compared using a paired t test.

Results

In the lateral decubitus position, the femoral nerve was located 28% anteriorly from the posterior edge of the L4-L5 disc space, and in the prone position, the femoral nerve was relatively more posterior, located 18% from the posterior edge of the L4-L5 disc space (P = .037).

Conclusions

The femoral nerve was on average more posteriorly located at the L4-L5 disc space in the prone position compared to lateral decubitus. This more posterior location allows for a larger safe zone at the L4-L5 disc space, which may decrease the incidence of neurologic complications associated with Lateral lumbar interbody fusion in the prone vs lateral decubitus position; however, further studies are needed to evaluate this possible clinical correlation.

Introduction

Lateral lumbar interbody fusion (LLIF) is a minimally invasive retroperitoneal approach that can improve patient reported outcomes while possibly reducing some of the risks associated with anterior or posterior interbody approaches to the spinal column.^1-4 Compared to posterior approaches, LLIF allows for reduced posterior muscle damage, and larger cage placement; larger cage placement may result in greater improvement in sagittal alignment, foraminal height, and a more favorable biomechanical environment for arthrodesis.^5-9 Compared to anterior interbody techniques, LLIF may be associated with less blood loss and a lower risk of vascular injury as the working corridor is further away from the major abdominal vessels. ¹⁰

While there are unique advantages associated with LLIF, one of the significant limitations is the need for intraoperative repositioning from the lateral to prone position to facilitate posterior decompression and or instrumentation. This position change may add 30 to 200 minutes to total operative time, resulting in increased time under anesthesia, healthcare cost, and resource utilization.^11-13 In an attempt to improve efficiency, performing LLIF and posterior percutaneous instrumentation in a single lateral position can decrease operative time, but may be technically challenging, particularly with regards to placing the downsided pedicle screws and image interpretation in the lateral position.

The prone transpsoas, or PTP, technique is a relatively new technique wherein transpsoas access to the lumbar disc space and posterior decompression or instrumentation are entirely performed in the prone position, avoiding some of the aforementioned limitations associated with lateral-then-prone or single position LLIF. ¹⁴ Initial results of the PTP technique have demonstrated improved clinical and radiographic outcomes.^15-18 In addition to the possible advantage of avoiding an intraoperative position change, proponents of the PTP technique have cited potential additional benefits such as improved intradiscal lordosis, facilitation of navigation setup, and more secure patient positioning when compared to traditional lateral decubitus LLIF. ¹⁸ Last, it has been purported that the lumbar plexus, traversing the intervertebral disc spaces within the psoas muscle, may be more posteriorly located during PTP compared to traditional lateral decubitus LLIF, which may increase the safe zone for access to the lumbar disc space, particularly at L4-L5.^14,16 The purpose of the present study was to compare the position of the femoral nerve within the lumbar plexus at the L4-L5 disc space in the lateral decubitus vs prone position, and test the hypothesis that the femoral nerve will be relatively more posterior in the prone position.

Methods

Five adult cadavers (average age 65 years, average BMI 29, 60% female) were used for this study. All cadaveric specimens were fresh frozen without chemical preservation. All cadaveric specimens included the torso, pelvis, and entire thigh. Each cadaver was evaluated for evidence of scoliosis (coronal cobb angle greater than 10°), compression fractures, and ≥ grade II spondylolisthesis from L1-S1. Cadavers with any of the aforementioned radiographic characteristics were excluded from the study.

In the lateral decubitus position, a midaxillary skin incision was made from the lower thoracic rib to the iliac crest. The abdominal wall muscle layers were identified and dissected using standard technique. The transversalis fascia was incised linearly to allow access to the retroperitoneal space. The psoas muscle was then identified and gently dissected to allow for identification of the lumbar plexus. The psoas muscle origin and insertion were not violated. The femoral nerve, formed most commonly from the L2, L3, and L4 nerve roots, was identified as the largest branch of the lumbar plexus within the psoas muscles, transitioning from posterior to anterior in a cephalad to caudad orientation (Figure 1A). Spinal needles were placed into the disc space and lateral fluoroscopic imaging was used to confirm lumbar levels (Figure 1B). The femoral nerve was then painted with a radiopaque paint (Figure 1C) and a repeat lateral fluoroscopic image was then obtained to confirm adequate visualization of the nerve on fluoroscopic imaging (Figure 1D).

Figure 1.(.

A) Dissection of the lumbar plexus with the psoas muscle reflected anteriorly. Spinal needles were placed into the disc space to allow for identification of lumbar levels. (B) Identification on fluoroscopy of lumbar levels. (C) Radiopaque paint was used to pain the femoral nerve from approximately L2 to L5. (D) Lateral fluoroscopic image confirming adequate visualization of the radiopaque femoral nerve.

The cadaveric specimens were then taped to a flattop Jackson operative table in the lateral decubitus position with the hips flexed, as described for standard lateral decubitus LLIF positioning technique. ¹⁹ A lateral fluoroscopic image was obtained, centered on the L4-L5 disc space, with the perfect overlap of the L5 pedicles, inferior endplate of L4, and superior endplate of L5. The same cadaveric specimen was then placed in the prone position on a six post Jackson operative table, with the legs and hips extended, as described for standard technique when performing a prone transpsoas LLIF. ¹⁵ A lateral fluoroscopic image in the prone position was obtained, using identical technique as described for the lateral decubitus position. In one specimen, when transitioning from lateral to prone, the radiopaque paint dislodged anteriorly and partly coated the psoas muscles thereby obscuring radiographic imaging; this specimen had to be excluded from further analysis. A second specimen only allowed for a unilateral approach due to a prior dissection. In the remaining cadaveric specimens, bilateral dissections were performed, allowing for seven lateral decubitus vs prone radiographic comparisons.

All fluoroscopic images were analyzed using Surgimap software (Nemaris, New York). In all cadaveric specimens, a 100 mm radiographic marker was placed anterior to the psoas muscle to allow for uniform calibration during analysis. For lateral fluoroscopic images in both the lateral and prone positions, the L5 superior endplate diameter was measured (mm). The primary outcome was the location of the femoral nerve crossing the L4-L5 disc space which was measured as a percentage of the L5 superior endplate diameter from anterior to posterior (Figure 2), with increasing percentages representing a more anterior location of the femoral nerve. We secondarily categorized the location of the femoral nerve based on four different working zones as described by Uribe et al. ²⁰

Figure 2.

A representative lateral fluoroscopic image of the same cadaveric specimen in the lateral (A) vs prone (B) position. In this specimen the femoral nerve was located 41% anteriorly relative to the L5 vertebral body superior endplate in the lateral position and 30% in the prone position. A 100 mm radiographic marker (blue line) was used in all fluoroscopic images to allow for accurate calibration.

Statistics are reported as the mean and standard deviation. The data were assessed for normality using a Shapiro–Wilk test. Parametric data were compared using a paired t test. Categorical variables were compared using a Fisher’s exact test. A P-value < .05 was considered statistically significant.

Results

Seven dissections were included in the final analysis. In the lateral decubitus position the femoral nerve was located 28% ± 10% anteriorly from the posterior edge of the L5 superior endplate, and in the prone position the femoral nerve was relatively more posterior, located 18% ± 8% from the posterior edge of the L5 superior endplate (P = .037) (Figure 3). In five dissections, the femoral nerve was on average 15% ± 4% (range 10–20%) more posterior at the L4-L5 disc space in the prone position, compared to the lateral decubitus position (Figure 3). In two dissections, the position of the femoral nerve at the L4-L5 disc space moved more anterior in the prone position, an average 4% ± .6% (range 3–4%) (Figure 3).

Figure 3.

Location of the femoral nerve in the lateral decubitus vs prone position at L4-L5 across seven cadaveric dissections. An increasing percentage represents a more anterior location at the L4-L5 disc space.

Categorical analysis demonstrated that in three cadaveric specimens, the femoral nerve moved from Uribe Zone 2 to Zone 1 in the lateral decubitus vs prone position, and in the remaining four specimens, the location of the femoral nerve did not change Uribe zones (P = .27).

Discussion

The primary outcome of this study corroborates the hypothesis that the femoral nerve is significantly more posteriorly located at the L4-L5 disc space in the prone vs lateral decubitus position. On average, across the seven lumbar plexus specimens evaluated in this study, the femoral nerve was 10% more posteriorly located at the L4-L5 disc space in the prone vs lateral decubitus position. Pimenta et al., in two separate studies, suspected that the lumbar plexus, including the femoral nerve, may be more posteriorly located in the prone vs traditional lateral decubitus position when performing LLIF; however, this is the first study to validate this suspicion and objectively quantify the amount of posterior translation of the lumbar plexus, as identified by the femoral nerve, when in the prone vs lateral decubitus position.

Several prior cadaveric and radiographic studies have quantitatively evaluated the position of the lumbar plexus from L1 or L2 to L5 and have consistently demonstrated that the safe working corridor for access to the disc space is narrowest at L4-L5.^20-23 Within the spectrum of possible neurologic injury to lumbar plexus structures, femoral nerve injury may be the most feared complication due to significant morbidity-related to quadriceps weakness.^21,24 Therefore, this study, comparing the location of the lumbar plexus in the lateral decubitus vs prone position, focused on specific analysis of the femoral nerve at the L4-L5 level.

No prior cadaveric study has compared the location of the lumbar plexus within the psoas muscle in the lateral decubitus vs prone position. A single study by Godzik et al. analyzed two cadaveric specimens after performing a prone LLIF and demonstrated that dilation and interbody placement occurred anterior the lumbar plexus in both specimens. ¹⁸ They noted that the psoas muscle, and thereby the lumbar plexus, appeared to be shifted posteriorly, possibly due to hip extension in the prone position, as compared to hip flexion which typically occurs during lateral decubitus positioning for LLIF. ¹⁸ An MRI study by Buckland et al. ²⁵ compared supine vs sitting magnetic resonance imaging (MRI) of the lumbar spine and noted that the psoas muscle and lumbar plexus were more anteriorly displaced in sitting MRIs, where the hips were flexed, with greater effect noted at more caudad lumbar levels. Several prior studies have also assessed the safe interval between the iliac vessels or aorta and the anteromedial border of the psoas muscle during oblique lateral interbody fusion (OLIF) and these studies have shown significant changes in the safe interval, dependent both on patient positioning (supine vs lateral decubitus) and hip flexion vs extension.^26-28

Taking into consideration the results noted in the previously mentioned studies,^18,25-28 it is likely that the more posterior location of the femoral nerve in the prone position compared to the lateral decubitus position, as seen in our study, is primarily the result of hip extension in the prone position and hip flexion in the lateral decubitus position. Based on the results of this study, the safe working corridor at the L4-L5 level may be larger when performing prone, as opposed to laterally positioned, LLIF. However, it should be noted that tension on the lumbar plexus in the prone position may be increased due to hip extension, relatively stretching the lumbar plexus. As seen in Figure 2A vs 2B, the radiopaque femoral nerve in the prone position appears to be straighter and less redundant, possibly due to increased tension in the prone position compared to the lateral decubitus position; however, quantitatively evaluating femoral nerve tension was beyond the aim of this study.

Whether prone LLIF results in possibly less neurologic complications due to the relatively increased safe working at L4-L5, or more neurologic complications due to increased tension on the lumbar plexus has yet to be determined in clinical studies.^17,29 Previous retrospective studies assessing prone LLIF have reported a neurologic complication rate ranging from 37% to 57%,^16-18 which ranges within the neurologic complication rate associated with traditional lateral decubitus LLIF.^30-34 One prospective, non-randomized, comparative study assessed seven patients undergoing prone LLIF and ten patients undergoing traditional lateral decubitus LLIF. ¹⁷ In this study, both groups had three patients (42% prone and 30% lateral decubitus) with postoperative psoas weakness; four patients (57%) in the prone group and one patient (10%) in the lateral decubitus group had sensory changes postoperatively. ¹⁷

While this is the first cadaveric study comparing the location of the femoral nerve in the traditional lateral decubitus vs prone position for LLIF, there are several limitations. First, we only assessed the location of the femoral nerve at L4-L5; however, as previously mentioned, it is well-established that the safe corridor to access the disc space during LLIF is narrowest at L4-L5. Second, we only assessed the location of the femoral nerve and not the entire lumbar plexus. Third, we did not break the bed coronally in either the prone or lateral decubitus position, which is commonly done clinically, and may alter the position and mobility of the lumbar plexus. Fourth, we did not use an expandable retractor, which is used intraoperatively to dilate and retract the psoas muscle, and could affect the displacement characteristics of the lumbar plexus in the prone vs lateral decubitus position. Last, we did not have MRI results available to exclude patients with transitional anatomy or an anteriorly migrated psoas muscle.

In conclusion, the results of our study demonstrate that, on average, the femoral nerve is more posteriorly located at the L4-L5 disc space in the prone position compared to lateral decubitus. This more posterior location allows for a larger safe zone at the L4-L5 disc space, which may decrease the incidence of neurologic complications associated with LLIF in the prone vs lateral decubitus position; however, further studies are needed to evaluate this possible clinical correlation. This is the first study which confirms the hypothesis that the femoral nerve is relatively more posteriorly located in the prone than lateral decubitus position, and this change in position is most likely due to the relative increase of hip extension in the prone position.

ORCID iDs

Ram Alluri https://orcid.org/0000-0001-5919-707X

Nicholas Clark https://orcid.org/0000-0003-2111-7032

Karim Shafi https://orcid.org/0000-0002-6695-5593

Sheeraz Qureshi https://orcid.org/0000-0002-7177-1756