Morphological changes in hip dysplasia The dysplastic hip joint has a complex morphology characterised by a wide shallow acetabular cavity with an excessively oblique articulating roof. The acetabular cover of the femoral head is globally deficient4;5 and the acetabular rim is hypertrophied possibly due to excessive pull from the often hypertrophic labrum. Anteversion is normal5-7, but occasionally the acetabulum is retroverted8;9. The weight-bearing area between the acetabular roof and head is reduced and the articular cartilage is significantly thicker than normal10. Hip dysplasia is often associated with increased anteversion of the femoral neck5;11 and with valgus neck-shaft angle that results in a reduced abductor lever arm12. However the deformities vary from individual to individual and retroversion of the femoral neck has also been reported in hip dysplasia12. Patients with hip dysplasia are prone to developing osteoarthritis of the hip at a young age 13;14. The reasons for this are not fully understood, but an explanation could be that the reduced contact area between acetabulum and the femoral head as well as a reduced abductor lever arm increase the load per contact-area in the hip joint4. The increased load is a strain on the articular cartilage and believed to result in degeneration of cartilage and the subchondral bone and eventually osteoarthritis14-17. The purpose of periacetabular osteotomy (PAO) is to increase acetabular cover of the femoral head and thereby distribute pressures better over the available cartilage surface.