Unilateral Variant Psoas Major with Split Femoral Nerve

Background: The psoas major muscle originates from the transverse processes of lumbar vertebrae 1-5 and inserts into the lesser trochanter of the femur. The femoral nerve arises from ventral rami of lumbar spinal nerves 2-4 and passes through the psoas major in an inferolateral direction until it emerges on the lateral border of the muscle. Variations of the psoas major muscle are rare and can have clinical significance. Here we describe a novel variation of the psoas major muscle. Case Information: During a routine anatomical dissection of a 75-year-old female cadaver, a unilateral variant of the psoas major muscle was found amongst 30 donated bodies. The variant was observed on the left side as an accessory muscle to the psoas major. The variant lies posterior to and runs in parallel with the normal psoas muscle fibers. An anomaly of the femoral nerve was also noted. The femoral nerve arises from fibers that course both anterior and posterior to the variant muscle. These fibers recombine on the lateral border of the psoas major to make the femoral nerve. An additional peripheral neurovascular bundle to the accessory muscle was not observed during the course of dissection. Conclusions: The psoas major variant described has a number of clinically important features. An accessory psoas can result in issues with a lateral interbody fusion procedure. Its implication in femoral nerve injury is variable, and thus neurosurgeons must take particular caution when performing this procedure on patients with this variant. It has been suggested that variant psoas major muscles underlie neuropathies such as nerve entrapment and are often described as disrupting the course of the femoral nerve. Here the femoral nerve was divided by the variant psoas muscle. Such a division could cause compression of the femoral nerve leading to neuropathies and could explain these clinical pathologies. This condition can postulate hip pain. African-American men tend to have a thicker psoas than average, hence extra precautions should be taken for those with this variation in terms of risk of femoral nerve compression.