Fig. 2

α-Angle goniometry. The proximal femoral epiphysis may exhibit a regular morphology (a) or a cam morphology (b). Since in a the femoral head center is in line with the neck axis, the 3-point method (a.1) is as suitable as the anatomic one (a.2), and both obtain α = 35° and AOR = AO/HD = 0.217. In b the center of rotation of the femoral head is slightly behind the neck axis, thus α = 60° according to the 3-point method (b.1, inappropriate), while the anatomic method provides the correct value of 65° (b.2). The AOR calculated in b.2 along the pure mid-neck axis (i.e., disregarding the head position) is 0.108, and belongs to the 2.5% left tail of the normal distribution of AORs (AOR anterior offset ratio, HD head diameter, AO anterior offset)