Abstract
BACKGROUND Psoas quartus is a rare anatomical variant of the psoas major muscle that can alter the course and branching of the femoral nerve as it descends from the posterior abdominal wall. Understanding its anatomical implications is crucial for clinicians and surgeons. CASE REPORT During a routine cadaveric dissection, we identified a unilateral psoas quartus muscle on the left side. This muscle was situated between the psoas major, psoas minor, and iliacus, originating from the transverse process of L5 and the quadratus lumborum. The femoral nerve exhibited an unusual branching pattern as it traveled around this muscle, bifurcating into medial and lateral branches before coalescing inferiorly. A review of the literature revealed only 4 prior reports of psoas quartus, suggesting it is a rare but significant variant. Its presence can contribute to nerve compression and altered biomechanics. CONCLUSIONS The presence of psoas quartus can contribute to femoral nerve compression, potentially leading to gait abnormalities and neuropathic symptoms. This case highlights the importance of recognizing such anatomical variations in clinical and surgical settings, particularly in procedures involving the lumbar plexus or femoral nerve blocks. Increased awareness of this muscle variant can aid in the diagnosis and management of femoral nerve-related pathologies, reducing the risk of iatrogenic nerve injury. Additionally, radiologists should consider this variation when evaluating imaging studies, as it can affect interpretations of femoral nerve entrapment or musculoskeletal abnormalities.