Psoas Quintus: A Novel Muscle Variant of the Posterior Abdominal Wall

Abstract

Anatomical variations of the psoas muscles are rare, with only a few examples reported in the literature. Typical musculature consists of two psoas muscles on the posterior abdominal wall. These include the psoas major, which is universally present in the human population, and the psoas minor, an inconsistent, but commonly occurring muscle. Other rarely reported psoas variants include the psoas tertius and psoas quartus muscles. Both are predicted to play a role in femoral nerve compression due to their proximity to the femoral nerve and their location on the posterior abdominal wall. Here we report the presence of both psoas quartus and psoas tertius muscles, as well as the discovery of a previously unreported psoas variant, which we have named psoas quintus. The muscle was discovered during a routine dissection of the left posterior abdominal wall of a 57‐year‐old Caucasian male. The psoas quintus had a proximal attachment via a slender tendon to the body of the L3 vertebrae and spread to the area of the anterior superior iliac spine by a relatively broad tendon. Its muscles fibers were oriented distinctly from those of the iliacus muscle below and the psoas quartus medially.