Variability in the size of the retroperitoneal oblique corridor: A magnetic resonance imaging-based analysis

INTRODUCTION

The objective of this magnetic resonance (MR) morphometric study is to better characterize the oblique corridor between the vasculature and psoas muscle in neurosurgical patients undergoing lateral lumbar interbody fusion (LIF), anterior retroperitoneal transpsoas approach, and oblique lumbar interbody fusion (OLIF). Lateral LIF (LLIF) may be performed through an anterior or posterior approach or minimally invasively.

The anterior retroperitoneal transpsoas approach is another alternative but is associated with an 18% complication rate attributed to plexus injuries, sensory deficits, motor deficits, and anterior thigh pain.[3,4] The anterior retroperitoneal OLIF, another minimally invasive technique, avoids the lumbar plexus, but the corridor between the vasculature and psoas muscle can be variable and limiting.

MATERIALS AND METHODS

The study was undertaken with the approval of the Institutional Review Board of our hospital. We retrospectively reviewed 300 consecutive MR imaging (MRI) studies (slice thickness of 3 mm) from 2013 to 2015. The following variables were studied: age, sex, and location of the iliac bifurcation (defined as the first disc or vertebral body where two independent iliac vessels could be discerned on axial imaging) [Figure 1]. Most critically, the oblique working corridor, defined as the distance between the lateral border of the aorta or common iliac vessel and the medial border of the left psoas, was recorded in each case at the L2/3, L3/4, and L4/5 disc levels [Figure 2]. At L5/S1, the distance from the midpoint of the intervertebral disc to the closest vascular structure was measured and recorded (e.g., oblique working corridor at L5/S1) [Figure 3].

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Figure 1:: Axial magnetic resonance T2 sequence image demonstrating the bifurcation of the aorta into two iliac arteries. This landmark can be used to select the appropriate approach to the anterior spine.

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Figure 2:: Axial magnetic resonance T2 sequence image depicting the oblique working corridor between the aorta and psoas muscle. When narrow, this space can be expanded by retracting the anterior psoas muscle posteriorly.

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Figure 3:: Axial magnetic resonance T2 sequence image demonstrating the corridor at L5-S1. The midpoint of the intervertebral disc was measured to the nearest vessel, in this case, the left iliac vein.

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RESULTS

The 300 patients included 136 men and 164 women, averaging, 63 ± 14.2 years.

Size of oblique working corridor

The size of the oblique working corridor decreased in size from L2/3 to L5/S1. The size of the oblique working corridor at L2/3, L3/4, L4/5, and L5/S1 was 17.3 ± 6.4 mm, 16.2 ± 6.3 mm, 14.8 ± 7.8 cm, and 13.0 ± 8.3 mm, respectively [Table 1]. The percentage of an oblique working corridor sizes <1 cm at L2/3, L3/4, L4/5, and L5/S1 levels was 10.3%, 16.0%, 30.0%, and 39.3%, respectively.

Table 1:: The size of the oblique corridor was measured from L2-S1 and found to decrease in the caudal direction. A value of 0 was noted in the cases in which a vessel covered the entire corridor.

Level of aortic bifurcation

The level of the aortic bifurcation was most commonly found at the L4 vertebral body (158 patients, 52.67%). The second and third most common levels for the aortic bifurcation were the L4/5 intervertebral disc (74 patients, 24.67%) and L5 vertebral body (40 patients, 13.33%) [Table 2].

Table 2:: The level of the aortic bifurcation was most commonly found at the L4 vertebral body.

DISCUSSION

The LLIF has gained in popularity but is not without its own set of complications, such as 28.5% of patients experiencing hip flexor weakness in one study,[8] 36.07% of patients with transient neurological injury, and 3.98% of patients with persistent neurological injury. The rates of femoral nerve palsies are relatively low; one retrospective review found it present in 2.6% of 230 patients.[1] Rates of visceral and vascular injuries are also low in LLIFs.[7]

OLIF has similar benefits to the LLIF without the need to traverse the psoas muscles. Here, the oblique working corridor can be increased in size by sweeping the psoas muscle posteriorly. Woods et al.[9] found that the corridor can be dilated up to 26 mm. We found that the corridor decreases in size from L2/3 to L4/5, a finding corroborated by prior MRI studies. Of interest, the corridor is smallest at L4/5 (15 mm), and this space can easily be dilated to accommodate all instruments needed to complete a discectomy and place an interbody graft.[6]

Major complications include vasculature injury due to the more anterior trajectory compared to the LLIF. Woods et al.[9] reported 2.9% rate of vascular complication, all occurring at the L5/S1 level. Ureter injury and ventral dural tear have also been reported.[2,5] A large retrospective chart review found decreased rates of sensory changes and psoas weakness with OLIF versus LLIF.[4]

CONCLUSION

The OLIF fusion utilizes a natural corridor that exists between abdominal vasculature and the psoas muscle and avoids the lumbar plexus injuries seen with LLIF. MRI should be utilized to determine the size of the OLIF working corridor and to assess whether LLIF can be safely and effectively performed.