Anatomical Variations of the Iliopsoas Muscle

Anatomical variations of the iliopsoas muscle are relatively uncommon but are clinically significant because of the muscle's relation to the femoral nerve (FN). Variants of the iliopsoas include accessory iliacus, iliacus minimus, psoas tertius, and psoas quartus muscles. This study describes anatomical findings of these variants. Dissections of the posterior abdominal wall in 95 Japanese cadavers at Kurume University School of Medicine (2015-2017) identified three cases of iliopsoas muscle variants. In the first case, a small variant muscle on the right side of a 91-year-old female cadaver originated from the iliolumbar ligament, pierced the FN, and joined the psoas major muscle (PMa). In the second case, a variant muscular slip was identified on the left side of a 78-year-old female cadaver. The slip originated from the anterior aspect of the middle part of the iliac crest and pierced the FN. The muscle joined the tendon of the PMa. Both cases were categorized as variations of the iliacus muscle (IM) and were called iliacus minimus muscles. In the third case, a bilateral variant muscle was noted in a 74-year-old male cadaver. The muscles mainly originated from the surface of the quadratus lumborum muscle. The variant muscle called the psoas quartus muscle, traveled posterior to the FN and united with the normal PMa and IM. This variation is seemingly very rare. A thorough understanding of these morphologic variations is essential for the accurate diagnosis and treatment of femoral nerve disorders and for safe lumbar and abdominal surgery.