Description and definition of radiographic femoroacetabular impingement signs
| RADIOGRAPHIC SIGN | DESCRIPTION | DEFINITION OF POSITIVE FINDING |
|---|---|---|
| Alpha angle | Angle formed by the axis of the femoral neck and a line connecting the centre of the femoral head to the point where the contour begins to stray from a spherical radius | > 55° on Dunn view and > 42° on frog-leg lateral view |
| Increased acetabular depth | Protrusion of the acetabular fossa medial to the ilioischial line (coxa profunda), or protrusion of the femoral head medial to the ilioischial line (protrusio acetabuli) | Findings on AP radiograph |
| Lateral centre edge angle | Angle between a line through the centre of the femoral head, perpendicular to the transverse axis, and a line through the centre of the femoral head, passing through the most superolateral point of the sclerotic weight-bearing zone of the acetabulum | > 40° on AP radiograph |
| Crossover (figure-eight) sign for focal acetabular retroversion | Anterior acetabular rim lies lateral to posterior rim in the cranial part of the acetabulum, then crosses the posterior rim in the distal part of the acetabulum. Considered a negative result if the cross happens superior to the femoral head | Presence on AP radiograph |
AP—anteroposterior.