Fig. 1

The double contour highlighted within the boxed area (a) on the pelvic AP radiograph comprises the most common aspect of one S-shaped line (outlined by red dots) and the second line with minimal curvature (green dots) (b). On a high-resolution CT scan (c), the origin of the contours are identified as the trochanteric crest (green arrow) and piriformis fossa (red arrow). In profile view, both lines are superimposed (d) and as the femur rotates, the two lines appear as separate elements (e)