Type I

Is a mature hip with α angle >60˚. It is divided into two subgroups: type 1a, with β angle >55˚; and type 1b with β angle <55˚.

Type IIa

Is the physiologic immature hip in which α is between 50˚ and 59˚ in an infant younger than 12 weeks of age. If type 2a morphology persists beyond 12 weeks, it is termed as type 2b (acetabular dysplasia) where α is between 50˚ and 59˚.

Type IIc

Is a hip in the critical range α = 43˚ - 49˚. It is divided into two subgroups: Type 2c stable and Type 2c unstable.

In the Type D hip, the α angle is in the same range as in the Type 2c hip; however, the Type D is decentered and has a β angle >77˚.

Type III & Type IV

Hips are both decentered hips, with α <43˚ and β >77˚ in each.

Determination of the position of the cartilaginous roof is crucial for the differentiation of Type 3 and 4, which is pushed cranially in Type 3 hips, and caudally in Type 4 hips.

Type 3 hip is further divided into two subgroups according to the echogenicity of the cartilaginous roof. In Type 3a hips, the roof is hypoechoic, whereas, in the Type 3b hip, the hyaline cartilage is deformed, and appears hyperechoic.