A Crucial But Neglected Anatomical Factor Underneath Psoas Muscle and Its Clinical Value in Lateral Lumbar Interbody Fusion—The Cleft of Psoas Major (CPM)

Abstract

Objective To describe the anatomical feature positioned beneath the psoas muscle at the lateral aspect of the lower lumbar, and to create a new location system to identify the risk factors of lateral lumbar interbody fusion. Methods Six cadavers were dissected and analyzed. The anatomy and neurovascular distribution beneath the psoas major from L3 to S1 was observed and recorded, with particular focus on the L4/5 disc and below. The psoas major surface was divided homogeneously into four parts, from the anterior border of psoas major to the transverse process. The cranial‐to‐caudal division was from the lower edge of the psoas muscle attachment on the L4 vertebrae to the upper part of the S1 vertebrae, and was divided into five segments. Then a grid system was used to create 20 grids on the psoas major surface, from the anterior border of the muscle to the transverse process and from L4 to superior S1, which was used to determine the anatomical structures' distribution and relationship beneath the psoas major. Results A cleft was identified beneath the psoas major, from the level of L4/5 downwards. It was filled with loose connective tissue and neurovascular structures. We termed it the cleft of psoas major (CPM). The sympathetic trunk, ascending lumbar vein, iliolumbar vessels, obturator nerve, femoral nerve and occasionally the great vessels are contained within the CPM, although there is significant interpersonal variation. The grid system on the psoas major surface helped to identify the anatomical structures in CPM. There was a considerably lower frequency of occurrence of neurovascular structures in the grids of I/II at the L4/5 level where can be considered the “safe zones” for the lateral lumbar interbody fusion. In contrast, the distribution of neurovascular structures at the L5S1 level is dense, where the operation risk is high. Conclusion The CPM exists lateral to the vertebral surface from L4 and below. Although the occurrence and distribution of neurovascular structures within the CPM is complex and varies greatly, it can provide a potential cavity for visualization during lateral lumbar interbody fusion. Using psoas major as a reference, this novel grid system can be used to identify the risk factors in CPM and thus identify a safe entry point for surgery.

A three-headed psoas major muscle: a case report

“…During LLIF procedure neurovascular damage of the PM cleft often occurs. Mentioned neural and vascular injuries sustain during the penetration and retraction of the PM, therefore understanding of the anatomical complexity surrounding the PM is crucial in LLIF [8]. It is important to note, that variable morphology of the PM, like described in this study might also impair surgical procedures like LLIF and lead to further complications.…”

“…The cleft of PM is a potential cleft beneath the PM on the side of L5 vertebra [8]. Numerous anatomical structures, such as: great vessels, ascending lumbar vein, iliolumbar vein, obturator nerve and FN are distributed within described cleft [8].…”

“…The cleft of PM is a potential cleft beneath the PM on the side of L5 vertebra [8]. Numerous anatomical structures, such as: great vessels, ascending lumbar vein, iliolumbar vein, obturator nerve and FN are distributed within described cleft [8]. Jianfei et al [8] highlighted the cleft of PM as clinically relevant in lateral lumbar interbody fusion (LLIF) -technique that allows the surgeon to access the intervertebral space from a direct lateral approach either anterior to or through the PM.…”

Extreme lateral interbody fusion (XLIF) approach for L5-S1: Preliminary experience

“…According to Uribe et al ( 24 , 25 ), the femoral nerve , formed from branches of the L2, L3, and L4 roots, was found deep in the psoas muscle, descending in a gradual posterior-to-anterior trajectory at the L4–5 disc space and continuing downward between the psoas and the iliacus muscle. In a study by Jianfei Ji et al ( 26 ), the femoral nerve of all 6 specimens was located at the posterior middle quarter and the posterior quarter part at the L5-S1 level. The genitofemoral nerve travels obliquely in the psoas muscle from its origin, crosses the L2–3 disc space, and emerges from its medial border superficial and anterior at the L3–4 level, then lies on the anterior surface of the psoas at the L4–5 level, and finally descends along the surface of the psoas major ( 24 ).…”

“…The genitofemoral nerve travels obliquely in the psoas muscle from its origin, crosses the L2–3 disc space, and emerges from its medial border superficial and anterior at the L3–4 level, then lies on the anterior surface of the psoas at the L4–5 level, and finally descends along the surface of the psoas major ( 24 ). The obturator nerve, as one branch of the lumbar plexus, passes obliquely through the cleft of the psoas major, from the posterior border of the L4 vertebrae to the anterior border of the L5/S1 disc ( 26 ). Therefore, the probability of injury to the genitofemoral nerve and the obturator nerve, which are often located at the anterior quarter part of the psoas at the L5/S1 level, as well as the femoral nerve, which is located at the posterior half part of the psoas at the L5/S1 level , would be reduced when placing an operating corridor through the middle anterior quarter of the psoas and retracting the corridor under direct vision.…”