TABLE 1.

Cliff Sign Measurements

Measurement	Method
Cliff angle (Figure 2A)	Angle of the lines formed from the center of the femoral head to the proximal and distal edge of the empty space.
Reverse alpha angle (Figure 2B)	Angle of the lines formed between the femoral neck axis and a line from the center of the femoral head to the proximal boundary where the femoral head leaves the perfect circle.
Cliff/femoral neck Cobb angle (Figure 2C)	Cobb angle between (1) the line parallel to the slope of the cliff and (2) the line along the axis of the femoral neck.
Cliff/femoral shaft Cobb angle (Figure 2D)	Cobb angle between (1) the line parallel to the slope of the cliff and (2) the line parallel to the femoral shaft.
Cliff length/femoral head diameter ratio (Figure 2E)	The ratio of the cliff length to the femoral head diameter.