A novel psoas muscle variant coexisting with femoral nerve bifurcation by psoas quartus: A case report

Anatomical variations of the iliopsoas complex can have clinical relevance, particularly those involving the femoral nerve. This report describes a previously undocumented psoas muscle variant and two additional muscular anomalies which may have implications for nerve entrapment and hip dysfunction, though further research is required to establish clinical significance. Routine dissection of the posterior abdominal wall was performed in 2024 on an adult female cadaver as part of an anatomy course at Liberty University. The dissection was carried out by a graduate teaching assistant (medical student) under the supervision of faculty anatomists. Detailed anatomical observations were recorded and compared to existing literature on iliopsoas variants. A bilateral muscle was identified originating from the medial aspect of the iliolumbar ligament and inserting into the posterior fibers of psoas major. It received distinct innervation from the femoral nerve. A review of literature revealed no prior documentation of this variant. Two additional variants were observed: prominent medial loops of the iliacus muscle and bilateral psoas quartus muscles dividing the femoral nerve. These variants may contribute to femoral nerve compression or snapping hip syndrome, especially in patients with idiopathic or recurrent symptoms. Recognition of such anatomical variations can aid in diagnosing unexplained groin pain or failed regional anesthesia and supports the need for more comprehensive documentation of iliopsoas morphology. • Psoas iliolumbalis was identified bilaterally during a cadaveric dissection. • It arose from the iliolumbar ligament and inserted into posterior psoas major. • The variant showed distinct femoral nerve innervation from nearby branches. • Bilateral psoas quartus muscles bifurcated the femoral nerve before rejoining. • Medial loops of iliacus were observed, supporting early but rarely cited reports.