Iliac hyperdense line: a new radiographic sign of gluteal muscle contracture

Background: A hyperdense line on the ilium that runs roughly parallel to the sacroiliac joint (we called it “iliac hyperdense line sign”) was frequently observed on pelvic radiographs of patients with gluteal muscle contracture (GMC). A literature search revealed no description of this sign. Objective: To determine the relationship between the iliac hyperdense line sign and GMC and to explore how this sign is formed. Materials and methods: Pelvic plain films of 103 cases of GMC and those of 200 control individuals were reviewed for the presence or absence of the iliac hyperdense line sign. Pelvic CT scans in 8 of 103 cases and 13 of 200 controls were analyzed with relation to the plain films. Results: The iliac hyperdense line sign was visualized in 85 of 103 (82.5%) cases of GMC and none of the 200 controls. In the GMC group, pelvic CT scans showed a deformity of the posterior ilium. The lateral cortex of the posterior ilium took on a partly or completely anteroposterior course, while in the control group the course appeared as an oblique orientation from posteromedial to anterolateral. Conclusion: The iliac hyperdense line on pelvic plain film can be used as a radiographic sign to suggest a diagnosis of GMC. This sign might be a result of the long and persistent pulling effect of the contracted gluteus maximus muscle, which deforms the lateral cortex of the posterior ilium from an oblique course to an anteroposterior course tangential to the X-ray beam.