Fig. 3

Radiological characteristics of patients with long-term unreduced hip joint dislocation, secondary osteoarthritis and pseudoarthrosis. a On normal anterior-posterior view X-ray examination, a semicircular high-density area could be identified in typical cases. Because the area looks like the lunar aureole (pseudoacetabulum) around the moon (femoral head), it is called the “aureole” sign. When an aureole sign is identified together with a slightly upward shift of the rotation center of the hip joint, the diagnosis of long-term unreduced hip joint dislocation, secondary osteoarthritis and pseudoarthrosis must be considered to avoid the possibility of misdiagnosis. There is no objective quantitative index to describe the aureole sign. We estimate that the aureole sign can be observed in 13/16 patients (this is based on subjective results obtained by one observer). b. On axial CT, when the rotation center is identified, the anterior and posterior column and the internal wall of the acetabulum resemble a large letter “I” in healthy people. However, in patients with long-term unreduced hip joint dislocation, secondary osteoarthritis and pseudoarthrosis, the ilium, dome of the original acetabulum and the internal/posterior wall of the pseudoacetabulum will form a rhombus, which is called the “rhombus” sign. This sign might be helpful in avoiding the possibility of misdiagnosis. There is no objective quantitative index to describe the rhombus sign. We estimate that the rhombus sign can be observed in 11/16 patients (this is based on subjective results obtained by one observer)