TABLE 1.

Description and definition of radiographic FAI signs evaluated

Radiographic sign	Description	Definition of positive test
Alpha angle	Angle formed by 1) the axis of the femoral neck, and 2) a line connecting the center of the femoral head to the point where the contour begins to stray from a spherical radius	>55° on Dunn view
Lateral Center Edge Angle	Angle between 1) a line through the center of the femoral head, perpendicular to the transverse axis, and 2) a line through the center of the femoral head, passing through the most superolateral point of the sclerotic weight-bearing zone of the acetabulum	>40° on AP radiograph
Crossover sign, focal acetabular retroversion	Anterior acetabular rim lies lateral to posterior rim in the cranial part of acetabulum, then crosses the posterior rim in the distal part of the acetabulum. Considered negative if the cross happens superior to the femoral head.	Present on AP radiograph