Fig. 3.

Measurement of the α-angle and HNOR in multiple radial slices (clockwise system). (A) Reference plane for measuring the α-angle and HNOR. The dotted line is the axis through the center of the femoral neck. It is adjusted to be parallel to the femoral neck–shaft angle. The solid line is the reference plane for radial angle reconstruction. (B) The reconstructed three-dimensional image demonstrates superimposed radial reference lines at 15° intervals. R1 (3:00 o’clock) is the oblique axial slice (dotted line in A). R2 (2:30 o’clock) is a plane rotated 15° from R1. R3 (2:00 o’clock) is a plane rotated 30° from R1. R4 (1:30 o’clock) is a plane rotated 45° from R1. R5 (1:00 o’clock) is a plane rotated 60° from R1. R6 (12:30 o’clock) is a plane rotated 75° from R1. (C) Angle ø is the α-angle. The α-angle is the angle between the line joining the center of the femoral head and the center of the femoral neck, and the line from the center of the femoral head to the point where the spherical image of the femoral head is lost. The HNOR was calculated by dividing the anterior offset (asterisk) by the maximum diameter of the femoral head of each slice. The anterior offset is the thickness of the femoral head that lies anterior to a line passing the anterior wall of the femoral neck that is parallel to a line crossing the center of the femoral head and neck.