Two Cases of the Psoas Quartus Muscle Variant

Abstract

Background: The psoas major muscle originates from the thoracic and lumbar vertebrae, joins with the iliacus along the iliac crest, and attaches to the femur at the lesser trochanter. The iliopsoas muscle, formed from the joining of the psoas and iliacus muscles, acts as a powerful hip flexor. An additional psoas muscle, psoas minor, is commonly found in up to half of the population. Although rare, several other variations in this muscle have been reported that could influence the surrounding structures. Case Information: During routine dissections of 31 cadavers, two cadavers showed a variant of the psoas muscle that attaches superior to the iliac crest. A bilateral variant on a 71-year-old female cadaver and a unilateral variant on an 80-year-old female cadaver were discovered. Similar variants have previously described as the psoas quartus muscles. After reviewing the literature, this case appears to represent the third reported psoas quartus muscle, the first reported psoas quartus muscle without the presence of a psoas tertius variant, and the second reported bilateral psoas quartus muscle. Conclusion: Clinicians should consider the possibility of a muscle variant during surgical procedures, radiological interpretations, or evaluations of pain, especially in relation to the displacement of the femoral nerve.