The most common cause of the loss of shadow of the psoas major muscle is pelvic and spinal fractures, which account for about 2/3 of the cases; followed by rupture of the retroperitoneal organs (kidneys, bladder, duodenum, and pancreas, etc.) and injuries to the great vessels and soft tissues. The psoas major muscle acts as a buffer between the kidney and the spine, separating the ureter from the tip of the transverse process of the lumbar vertebrae. The fascia encompassing the psoas major is the iliac fascia, which is part of the intra-abdominal fascia and fuses laterally with the anterior layer of the dorsal lumbar fascia that covers the anterior aspect of the psoas square muscle. The iliopsoas fascia attaches medially to the lumbar vertebral bodies, intervertebral discs, and the upper part of the sacrum, thus forming the psoas major sheath. The sheath of the psoas major muscle is very solid, and pus from thoracic and lumbar vertebral tuberculosis can flow downward into the sheath to the femur. Between this muscle and the iliopubic ramus and the hip joint capsule, there is a large synovial fluid sac called the iliopubic sac. This capsule is often connected with the hip joint capsule, so when the hip joint capsule is infected, the pus can spread to this capsule. When this muscle is contracted, the thigh can be flexed and rotated outward, and when the thigh is immobilized, the trunk is flexed forward by flexing the lumbar segment of the spine.