Two types of hip osteoarthritis (OA) are recognised: idiopathic, in which the underlying cause is unknown, and secondary, when the predisposing cause is well known.

Idiopathic OA is classified into two types: eccentric, which is characterised by a rapid deterioration, and concentric, which has a better prognosis because of its slow deterioration.

Femoroacetabular impingement (FAI) has also been referred to as a cause of early OA of the hip. However, clinical confirmation to support the hypothesis that FAI cause early OA is still lacking.

The most suitable term for the entire spectrum of deformities of the hip, already present at birth, is “congenital hip disease”.

We have recognised three main types of CHD of increasing severity: A, dysplasia; B, low dislocation; C, high dislocation.

Low dislocation is subdivided in B1 subtype (extended coverage of the true acetabulum by the false acetabulum) and B2 subtype (limited coverage). High dislocation is subdivided in subtypes C1 and C2, depending on the presence or the absence of a false acetabulum.