Intermuscular Fusion Between the Psoas Major and Iliacus Muscles: Anatomical and Histological Study

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Abstract
Introduction: The term iliopsoas has recently been questioned because some anatomical studies have revealed that the psoas major and iliacus have separate distal insertions. Additionally, some older references mention a more proximal intermuscular fusion. Therefore, the aim of the present study was to elucidate this anatomy. Methods: Eighty human adult anatomical donors (160 sides) underwent dissection of the psoas major and iliacus muscles in the abdomen and thigh. The relationships between the two muscles and their point of fusion were documented, and the findings were classified. Results: There was intermuscular fusion between the iliacus and psoas major muscles on all sides. The fusion was distributed as follows: 73 (45.6%) of the muscles fused proximally to the inguinal ligament (type I), 81 (50.6%) fused beneath that ligament (type II), and six (3.8%) (type III) fused distally to it. No sex-based differences were identified, while a significant side asymmetry was observed. Histologically, fascicular blending of psoas major and iliacus across a shared tendon-forming interface was seen. Additionally, connective tissue interdigitation and progressive interfiber continuity between these muscles were observed. Conclusions: Consistent anatomical continuity between the iliacus and psoas major muscles was found in all anatomical donors, justifying the continued use of the term “iliopsoas.” These muscles should therefore be considered a single functional unit regardless of their attachment to the femur. Our study indicates that in most cases, the iliacus and psoas major fuse at or proximally to the inguinal ligament.

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