Fig. 2

a The double contour lines appear on the diagrammatic representation of the femoral head with red and green arrows indicating their origins on the trochanteric crest and piriformis fossa, respectively. On standard radiographs, the classic anatomical variant (b) comprises one S-shaped (red arrows) and one less curved line (green arrows); (c) superimposed lines are less common; and (d) the last and more rare variant is seen as two curvy lines. e Width measurement was defined at three levels perpendicular to the femoral axis (black hatched line). The measurements are defined between the levels of the neck (N) and maximum double line width (M) (f-h), where D1 = femoral neck level, D2 = minimum thickness between D1 and D3, and D3 = maximum double line thickness