Fig. 2
From: Limited angular remodelling after in-situ fixation for slipped capital femoral epiphysis
(A) The lateral head shaft angle (HSA) is the angle between the femoral shaft axis and a line perpendicular to a line connecting the femoral head’s physeal edges. (B) The 3-point α-angle method as defined by Nötzli (α3p). Place a circle over the femoral head’s bony contour and another circle over the femoral neck’s narrowest part. Draw a line connecting the centre of these two circles. Then draw a line from the femoral head’s centre to where the head-neck contour exits the femoral head circle. The α3p is the angle between these two lines. (C) The anatomic α-angle (αA) as described by Bouma. Determine the axis of the femoral neck by placing three circles touching the contour of the femoral neck. Draw a line through the centres of the neck circles to decide the anatomic femoral neck axis. The neck axis is translated to the centre point in the best-fit circle over the femoral head. A second line is drawn from the central point to where the head-neck contour first exits the femoral head circle. The αA is the angle between these two lines. (D) The anterior offset ratio. First, draw a line through the centre of the femoral neck. Then two parallel lines are drawn along the anterior edge of the neck and the femoral head. The distance between the latter (AO) divided by the femoral head diameter (d) gives the anterior offset ratio (AOR), also called the head-neck offset ratio (HNOR)